BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to lapping, polishing, finishing or smoothing of surfaces
with apparatus and processes which use abrasive sheeting. In particular, the present
invention relates to such processes and apparatus which use removable or replaceable
abrasive sheeting which operates at high surface speeds and secures the abrasive sheeting
to a platen on a flexible shaft which platen moves the sheeting at those high speeds.
The lapping system is capable of extremely smooth surface finishing at high speeds.
Background of the Art
[0002] The field of lapping or polishing traces it roots far back into time, even before
substantial technical developments. Early jewelry and decorations were provided by
minerals or materials (shells or wood) that had been smoothed by natural elements.
Stones smoothed by water currents or sand storms gave a much more pleasant look and
feel than unpolished stones or stones which had been roughly smoothed by available
means such as rubbing two stones together.
[0003] Early efforts at sharpening blades for plows or swords were amongst the first technical
advances in lapping and smoothing of materials, and these technical means are still
used in much the same way today. Swords and plow shears were sharpened by moving the
blade against a stone surface. The abrasive action of the stone against the blade
removed metal and thinned the blade at its edge. Grinding wheels, kitchen knife sharpeners,
and the like are not significantly different in function than the stone sharpening
tools, such as the grinding wheel which has been used to sharpen blades for thousands
of years.
[0004] In the 17
th and 18
th century, the combination of die casting and abrasive polishing enabled the manufacture
of interchangeable generic parts for equipment (especially the rifle and hand gun)
as opposed to the standard method of fitting individually made parts into a unique
piece of equipment with uniquely fitting parts. Each succeeding advance in the ability
of materials and processes to create smoother and more uniform surfaces advanced the
quality and capability of the resultant articles to perform whatever tasks for which
they were designed. Lenses with greater smoothness and uniformity advanced the degree
to which observation could be extended downward by microscopy and outward into space
by telescopes. Better fitting parts extended the longevity of equipment and increased
efficiency by reducing internal friction. The need for increasing efficiency, precision,
consistency and smoothness in lapping is as important today as ever. Each incremental
increase in the quality of lapping materials and processes advances many fields of
technology and industry, while at the same time offering the possibility of reducing
the cost of manufacture of goods.
[0005] Lapping and polishing are performed in many fields and industries. Metal and parts
polishing is the most obvious field, but smoothing of surfaces is extensively used
in lens manufacture, semiconductive wafer manufacture, gem polishing, preparation
of supports for optical elements, providing surfaces which can be joined or seamed
and the like. The smoothness and reproducibility of the processes and apparatus used
to create the needed levels of smoothness are critical to the success of products.
U.S. Patent No. 5,584,746 (Tanaka) describes a method of polishing semiconductor wafers
and apparatus therefor. The import of Tanaka is the physical control placed over the
wafer as it is being polished. The wafer is secured by a vacuum system on a wafer
mounting plate. The relative flexibility of the wafer is discussed as a method of
controlling uniformity of the wafer surface as is the uniformity of the vacuum applied
through the wafer support. The polishing of the wafer surface is accomplished by typical
means including a polishing pad which is mounted on a polishing surface (turntable).
It is suggested that the pad should not be subject to plastic deformation and may
be preferably selected from a group comprising close cell foam (e.g., polyurethane),
polyurethane impregnated polyester non-woven fabric and the like, which are known
materials in the art. No specific means of securing the polishing pad to the support
surface is described in Tanaka. No specific speeds of rotation for the operation of
the process are shown in the examples.
[0006] U.S. Patent No. 5,317,836 (Hasegawa) describes an apparatus for polishing chamfers
of a wafer. Hasegawa describes that in the manufacture of wafer materials from single
crystal ingots such as silicon, the wafer is produced by a combination or selection
of processes including slicing, chamfering, lapping, etching, buffing, annealing and
polishing. It is noted that chipping and/or incomplete surface polishing are a problem
in such ingot conversion to wafers. Hasegawa describes the use of a rotary cylindrical
buff formed with at least one annulargroove in its side describing a circle normal
to the axis of the cylindrical buff and a wafer holder capable of holding and turning
the wafer about an axis. The improvement is described as including at least the ability
of the cylindrical buff being adapted to freely shift axially, that the annular groove
has a width substantially greater than the thickness of the wafer, and that the apparatus
further comprises a means for axially biasing the cylindrical buff. No specific speeds
of rotation for the operation of the process are shown in the examples.
[0007] U.S. Patent No. 5,007,209 (Saito) describes an optical fiber connector polishing
apparatus and method. Saito describes a method and apparatus for polishing optical
fiber connectors with high accuracy. Saito indicates that the polishing is accomplished
by using an elastic polishing board rotating at high speed, but no specific speed
of rotation or method of attachment of the polishing board is described. Positioning
pins and other controls are provided in the system to accurately align the swing fulcrum
arm carrying the polishing material.
[0008] U.S. Patent No. 4,085,549 (Hodges) describes a lens polishing machine comprising
a lap tool holder and lens blank holder including independent means to provide linear
and rotary movement between a lens blank and lap tool. The machine is described as
useful for high speed grinding and polishing. The polishing element is gimbal mounted
on its lower extreme in a spherical bearing to allow a lens blank holder to follow
the contour of the lens during the polishing process. The movement between the rotary
drive and linear drive mechanisms independent of each other provides a balanced and
low vibration operation. No specific speeds of rotation are recited and the abrasion
is provided by a slurry.
[0009] U.S. Patent No. 4,612,733 (Lee) describes a very high speed lap with a positive lift
effect. The apparatus and method comprises a rotary lapping system which uses a liquid
slurry of abrasive particles. The diameters of the particles are shown to be from
about 1.5 to 5 micrometers, but may be outside this range. The system is described
as producing positive lift by presenting leading edge surfaces with a positive angle
of attack in the liquid abrasive slurry, the leading edge surfaces generating a positive
lift through hydrodynamic interaction with the slurry. Each of the positive lift tools
presents a grinding surface to said workpiece when it is rotated in the slurry. There
is again no specific rotational speed provided in the description, and the use of
liquid slurries would cause higher lapping/abrasive areas on the exterior of the grinding/lapping
face as the slurry would be at higher levels at the outside of the rotating grinding
area work surface.
[0010] U.S. Patent No. 4,709,508 (Junker) describes a method and apparatus for high speed
profile grinding of rotatably clamped rotation symmetrical workpieces. Rather than
the grinding element contacting the surface to be ground with a grinding surface which
is rotating within a plane, the edge of the grinding element (e.g., at the circumference
of a disk rather than on its face) is brought against the surface to be ground.
[0011] U.S. Patent No. 5,197,228 describes methods and apparatus for grinding metal parts,
especially with devices having a cooperative workpiece holder and a tool holder which
form a grinding station. The grinder table is reciprocally moveable along an axis
which is at right angle to the axis of travel of the workpiece. The grinder table
may also be equipped for controlled simultaneous movement along two axes. A microprocessor
is designed to send and receive signals to or from all of the moving parts of the
grinding machine for moving the workpiece table towards or away from the grinding
bit.
[0012] U.S. Patent No. 4,194,324 describes a carrier for semiconductive wafers during polishing
steps in their manufacture. An annular flange is present to receive pressure loading
from the polishing machine during the wafer polishing operation. The holder of the
polishing machine includes the ability to apply a vacuum to the carrier to maintain
the carrier selectively on the polishing machine. The arrangement on the equipment
allows release of the vacuum during polishing and enables simple intentional removal
of the carrier. Cam follower-slot arrangements permit tilting of the mounting head.
[0013] U.S. Patent No. 5,576,754 describes a sheet holding device for an arcuate surface
with vacuum retention. The sheet and device are described as useful for internal drum
plotters in imaging equipment. Vacuum pressure is applied to imaging film to keep
it securely positioned within the arcuate focal plane of the imaging equipment.
[0014] U.S. Patent No. 5,563,683 describes a substrate holder for vacuum mounting a substrate.
The holder is provided with two kinds of grooves or clearances in the supporting surface.
Circular support faces with multiple grooves and/or a plurality of pins to support
the work are shown. The device is generally described to be useful as a holder, with
such particular uses as in the manufacture of semiconductors and the support of photosensitive
substrate being shown. Similarly, U.S. Patent No. 4,943,148 describes a silicon wafer
holder with at least one access port providing access to the underside of the wafer
with vacuum pressure. U.S. Patent No. 4,707,012 also describes a method of applying
vacuum holding forces to a semiconductor wafer during manufacture in an improved manner.
U.S. Patent No. 4,620,738 shows the use of a vacuum pickup system for semiconductor
wafers. The wafers are placed into or removed from holders by the vacuum pickup.
[0015] Similarly, U.S. Patent No. 5,414,491 describes a vacuum holder for sheet materials
comprising a plurality of arrays of vacuum channels including a plurality of vacuum
plenums. Flow sensors are provided so that the system can indicate the presence and/or
size of the sheets being held. Specifically described are common types of imaging
materials using sheets of plain paper, photographic paper and photographic film.
[0016] U.S. Patent No. 5,374,021 describes a vacuum holding system which is particularly
useful as a vacuum table for holding articles. The holding table is particularly described
with respect to the manufacture of printed circuit boards. Controlled passageways
are provided which are supposed to control the application of reduced pressure and
to reduce the application of the vacuum when vacuum support is not required.
[0017] U.S. Patent No. 5,324,012 describes a holding apparatus for holding an article such
as a semiconductor wafer. At least a portion of the holder contacting the wafer comprises
a sintered ceramic containing certain conductive materials. The use of conductive
materials and fewer pores reduces the occurrence and deposition of fine particles
during use. The benefits of the materials are said to be in contributions to the cleanability
of the surface, insurance of mechanical strength, reduction of weight and increased
dimensional stability.
[0018] U.S. Patent No. 5,029,555 describes a holding apparatus and method for supporting
wafers during a vacuum deposition process. The apparatus is an improved system for
the angled exposure of at least one surface portion of a substrate supported on a
surface holder to an emission of a source impinging obliquely on the surface portion.
The device moves the surface holder to improve the uniformity of the emission received
on the surface portion. Wheel mechanisms are coupled together to provide maintenance
capability for predetermined positions of the surface. The substrate holder is moved
while its orientation to the source is carefully controlled.
[0019] U.S. Patent Nos. 4,483,703 and 4,511,387 describe vacuum holders used to shape glass.
Frames are shown with slidable members moving a deformable vacuum holder between a
shaping station and a mold retraction station. Pistons drive movable elements, such
as the vacuum holder, on a supporting frame.
[0020] U.S. Patent No. 4,851,749 describes a motor driven mechanical positioner capable
of moving an arm to any one of about 840 discrete angular positions. An infrared light
emitting device acts with a phototransistor to control the appropriate angular position.
Sensing devices also act on interdependent speed controls so as to increase the accuracy
of the positioning of the arm.
[0021] U.S. Patent No. 5.180,955 describes a positioning apparatus comprising an electromechanical
system which provides controlled X-Y motion with high acceleration, high maximum speeds,
and high accuracy, particularly for positioning an end-effector at predetermined locations.
A high speed mini-positioner is provided comprising a positioning linkage having a
changeable parallelogram structure and a base structure. A main benefit of the system
is the fact that the bars and bearings of the positioner are symmetrical about the
X-Y plane passing through the linkage height. The symmetry means that all actuator
forces and all inertial reaction forces act in vectors lying in the plane of symmetry.
[0022] U.S. Patent No. 5,547,330 describes an ergonomic three axis positioner. The positioner
is intended to move an article along three mutually perpendicular axes through a system
of interconnected slides and slide joints. Rack and pinions are also used to independently
move the slides. The device is suggested for use in the visual inspection of work,
particularly in the semiconductor industry.
[0023] U.S. Patent No. 4,219,972 describes a control apparatus for a grinding machine. A
revolution speeds changing means is provided which can selectively effect changes
at high speeds when grinding and changes at low speeds when dressing the article.
The relationship and control of the timing of the speed changes and the operations
detection circuits and timers.
[0024] U.S. Patent No. Re. 30,601 describes an apparatus and method particularly effective
in the positioning of a semiconductor wafer in a preferred plane with respect to a
photomask. Sensors regularly monitor the position of the wafer and a reference plane.
A photoalignment system is provided in which a wafer is not physically touched by
any portion of the photoalignment tool, thereby avoiding any contamination.
[0025] These systems have been described as providing benefits to particular technical and
commercial fields, but they have not been shown to provide any particular benefits
to truly high speed lapping/polishing systems and materials.
SUMMARY OF THE INVENTION
[0026] Lapping or polishing at high speeds with fine abrasive particles offer significant
advantages in the speed of lapping, savings of time in lapping, and smoothness in
the finished articles. Materials, processes, apparatus and specific features integrated
into the lapping processes and apparatus of the present invention can provide a unique
lapping effect with regard to both the quality (smoothness and uniformity of the produced
surface) and efficiency of the system. The present invention relates to a new field
of lapping technology with its own unique complexities due to the combination of high
rotational speeds on the abrasive platen and the use of sheets of abrasive material
rather than slurries. The combination of these two aspects creates dynamics and forces
which have not been addressed by previous lapping systems and requires an entirely
new background of engineering to address the problems.
[0027] One process of the present invention for lapping a surface comprises:
a) providing a work piece to be lapped, having at least one surface to be lapped,
b) providing a rotating platen having i) a back surface and ii) a flat surface which
can be adjusted to a position parallel to said at least one surface of said work piece,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said flat surface of said platen with the abrasive face of
said sheet facing said at least one surface to be lapped,
d) securing said sheet of abrasive material to said flat surface of said platen, and
- (1)
- rotating said platen at a rotational speed of at least 500 revolutions per minute,
and a surface speed at an outside edge of said sheet of abrasive material of at least
1500 surface feet per minute, and
- (2)
- contacting said abrasive face and said at least one surface of said workpiece to be
lapped.
[0028] One preferred lapper system for practicing the present invention comprises:
a) a shaft which is connected to a rotatable platen having vents for air on a front
surface of said platen, said platen having a back side to which said shaft is connected
and a flat front side on said platen to which can be secured an abrasive sheet by
reduced air pressure conveyed through said vents;
b) a frame having a total weight of at least 200 kg supporting a work piece holder;
c) said work piece holder is movable on said frame;
d) said work piece holder is attached to a movable element on said frame, said movable
element being capable of moving in a direction towards and away from said platen to
perform lapping of a work piece held on said work piece holder;
e) said work piece holder having at least one control element thereon which allows
for independent movement and alignment of said work piece holder along three perpendicular
axes so that a work piece on said work piece holder can be adjusted and oriented towards
parallelity with said platen so that a work piece can be lapped; and
e) said control elements having at least 50 settings per rotation, each setting moving
said workpiece holder along one of said three axes by a dimension less than 0.05 mm.
Another process for lapping a surface within the present invention may comprise at
least one of the following sequence of steps:
[0029] Sequence of steps A comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped,
b) providing a rotating platen having i) a back surface and ii) a flat surface and
providing a workpiece which can be adjusted to a position parallel to said platen,
said flat surface of said platen having openings therein through which air may flow,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said flat surface of said platen with the abrasive face of
said sheet facing said at least one surface to be lapped,
d) reducing gaseous pressure between said back side of said abrasive sheet and said
flat surface of said platen to secure said sheet of abrasive material to said flat
surface of said platen,
e) rotating said platen at a rotational speed of at least 500 revolutions per minute
and a surface speed at an outermost edge of said platen of at least 1500 surface feet
per minute, and
f) contacting said abrasive face and said at least one surface to be lapped on said
work piece;
[0030] Sequence of steps B comprising: a) providing a work piece to be lapped, having at
least one surface to be lapped, which can be adjusted to a position parallel to said
at least one surface of b) where
b) is a rotating platen having i) a back surface and ii) a flat surface said flat
surface of said platen having openings therein through which air may flow,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said flat surface of said platen with the abrasive face of
said sheet facing said at least one surface to be lapped,
d) wherein said sheet has an outer edge and an inner edge defining an annular distribution
of abrasive, said inner edge having a diameter which is greater than one-third the
diameter of said outer edge,
e) rotating said platen at a rotational speed of at least 500 revolutions per minute,
and
f) contacting said abrasive face and said at least one surface to be lapped on said
work piece;
[0031] Sequence of steps C comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped,
b) providing a rotating platen having a back side and a front side, said front side
facing said work piece and having a flat plateau which is continuous around the perimeter
of said front side of said platen and is elevated with respect to a central area on
said front side, thereby forming an annular region,
c) providing a sheet of abrasive material on said flat plateau, said sheet of abrasive
material having a front surface with an abrasive face and a back surface, with said
abrasive face facing said at least one surface to be lapped,
d) securing said sheet of abrasive material to said flat surface of said plateau,
and
e) rotating said platen at at least 500 revolutions per minute and contacting said
abrasive material and said work piece to remove material from said work piece;
[0032] Sequence of steps D comprising
a) providing a workpiece to be lapped, having at least one surface to be lapped,
b) providing a rotating platen having i) a back surface and ii) a flat surface and
providing a workpiece which can be adjusted to a position parallel to said platen
by rotation about a pivot joint of a workpiece holder supporting said workpiece, said
flat surface of said platen having openings therein through which air may flow, and
said back surface having a pivoting joint with a shaft which rotates said platen,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said flat surface of said platen with the abrasive face of
said sheet facing said at least one surface to be lapped,
d) reducing gaseous pressure between said back side of said abrasive sheet and said
flat surface of said platen to secure said sheet of abrasive material to said flat
surface of said platen, and
e) rotating said platen at a rotational speed of at least 500 revolutions per minute
by rotating said shaft, and
f) contacting said abrasive face and said at least one surface to be lapped on said
workpiece, and allowing said workpiece holder to pivot around said pivot joint so
that said abrasive sheet and said at least one surface to be lapped become more parallel
towards each other.
[0033] Sequence of steps E comprising:
a) providing a work piece with two surfaces to be lapped,
b) providing two rotatable platens, each rotatable platen having i) a back surface
and ii) a front surface,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said front surface of each of said two rotatable platens with
the abrasive faces of each said sheet facing the other sheet,
d) placing said work piece with two surfaces to be lapped between said two rotatable
platens, so that each abrasive face faces only one of said two surfaces to be lapped,
e) rotating said two platens at a rotational speed of at least 500 revolutions per
minute,
f) contacting each of said abrasive faces with said only one of said two surfaces
to be lapped, and
g) lapping said two surfaces of said work piece simultaneously.
[0034] Sequence of steps F comprising:
a) providing a work piece having two surfaces to be lapped to be lapped, having at
least one surface to be lapped,
b) providing two rotatable platens, each rotatable platen having a back side and a
front side, said front side facing a surface to be lapped on said work piece and each
of said two platens having a flat plateau which is continuous around the perimeter
of said front side of each of said platens and is elevated with respect to a central
area on said front side, thereby forming an annular region,
c) providing a sheet of abrasive material on said flat plateau on each of said two
platens, said sheet of abrasive material having a front surface with an abrasive face
and a back surface, with each said abrasive face facing only one of said two surfaces
on said work piece to be lapped,
d) securing said sheet of abrasive material to each said flat plateau, and
e) rotating said platen at at least 500 revolutions per minute and contacting said
abrasive material on said two platens and said two surfaces to be lapped on said work
piece simultaneously to remove material from said work piece;
[0035] Sequence of steps G comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped which
can be adjusted to a position parallel to said at least one surface of a rotating
platen,
b) providing a rotating platen having i) a back surface and ii) a front surface with
a periphery, said front surface of said rotating platen having a raised edge symmetrically
disposed about said periphery,
c) providing a sheet of abrasive material having an abrasive face and a back side
onto said raised edge to provide a symmetrical distribution of abrasive material on
said rotating platen, said back side being on said front surface of said platen with
the abrasive face of said sheet facing said at least one surface to be lapped,
d) securing said sheet of abrasive material to said front surface of said rotating
platen, and
e) rotating said rotating platen at a rotational speed of at least 500 revolutions
per minute, and
f) contacting said abrasive face and said at least one surface to be lapped on said
work piece; and
[0036] Sequence of steps H comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped which
can be adjusted to a position parallel to said at least one surface of a rotating
platen,
b) providing a rotating platen having i) a back surface, ii) a front surface, and
a periphery,
c) providing a sheet of abrasive material having an abrasive face and a back side
onto said rotating platen, with the abrasive face of said sheet facing said at least
one surface to be lapped,
d) securing said sheet of abrasive material to said front surface of said rotating
platen,
e) rotating said rotating platen at a rotational speed of at least 500 revolutions
per minute, and
f) contacting said abrasive face and said at least one surface to be lapped on said
work piece,
g) providing a first amount of liquid to assist lapping to said abrasive surface physically
in front of an area where work piece contacts said abrasive face,
h) providing a second amount of liquid to assist in washing solid material from said
abrasive surface physically after said area, and
i) directing air against said abrasive surface physically after providing said first
amount of liquid to assist in removing said first and second amounts of liquid from
said abrasive surface.
[0037] Each of the processes described above as including those sequences of steps within
the broader concept of the process invention preferably includes a sheet of abrasive
material comprising a circular sheet of material which is:
sufficiently non-porous as to be secured to a surface by reduced gas pressure with
a differential between a front side of said sheet and a back side of said sheet of
600 mm Hg, and
which sheet, if it has holes therein, has said hole(s) located so that said hole(s)
has both its center and outer radius within a first third of a radius of said sheet
as measured from the center of said sheet.
[0038] Another preferred aspect of the lapper system of the invention comprises:
a) a shaft which is connected to a rotatable platen having on a front surface of said
platen vents for air, said rotatable platen having a back side to which said shaft
is connected and a flat front side on said rotatable platen to which can be secured
an abrasive sheet by reduced air pressure conveyed through said vents;
b) a frame having a total weight of at least 200 kg supporting a work piece holder
and said shaft connected to a rotatable platen;
c) a work piece holder which is movable on said frame;
c) said work piece holder is attached to a movable element on said frame which is
capable of moving along said frame in a direction towards and away from said platen
to perform lapping of a work piece held on said work piece holder;
d) said work piece holder having control element thereon which allow for independent
movement and alignment of said work piece holder along three perpendicular axes so
that a work piece on said work piece holder can be adjusted and oriented towards parallelity
with said rotatable platen so that a work piece can be lapped; and
e) said control elements having at least 1000 settings per rotation, each setting
moving said shaft along one of said three axes by a dimension less than 0.005 mm.
wherein said lapper system includes a pivoting lapper platen system comprising:
f) a shaft which is connected to said rotatable platen, said rotatable platen having
a back side to which said shaft is connected and a front side on said rotatable platen
to which can be secured an abrasive sheet, said rotatable platen having i) a back
surface, ii) a front surface, and iii) a raised edge forming an abrading plateau on
said front surface of said rotatable platen, with an abrasive sheet secured to said
raised edge.
[0039] Another preferred lapper platen system according to the present invention comprises:
a) a rotatable platen having i) a back surface and ii) a front surface, wherein said
front surface of said rotating platen facing a work piece and said front surface has
a flat plateau which is continuous around a perimeter of said front side of said platen
and is elevated with respect to a central area on said front surface,
b) said front surface also having vents for air,
c) said platen having a back side to which a shaft is connected and a front side on
said platen to which is secured an abrasive sheet by reduced air pressure conveyed
through said vents,
d) said back side also being pivotally connected to a rotating joint which is in turn
connected to said shaft which rotates said platen;
e) a frame having a total weight of at least 200 kg supporting a work piece holder
and said shaft connected to a rotatable platen;
f) a work piece holder which is movable on said frame;
g) said work piece holder is attached to a movable element on said frame which is
capable of moving along said frame in a direction towards and away from said platen
to perform lapping of a work piece held on said work piece holder;
h) said work piece holder having control element thereon which allow for independent
movement and alignment of said work piece holder along three perpendicular axes so
that a work piece on said work piece holder can be adjusted and oriented towards parallelity
with said platen so that a work piece can be lapped;
i) said control elements having at least 50 settings per rotation, each setting moving
said shaft along one of said three axes by a dimension less than 0.005 mm;
j) a first liquid supply means upstream from said work piece holder with respect to
a direction of rotation of said platen;
k) a second liquid supply means downstream from said work piece holder with respect
to a direction of rotation of said platen; and
l) an air blowing means located downstream of said first liquid supply means.
A more preferred process and lapping system includes a pivoting lapper platen system
comprising:
a) a shaft which is connected to a platen, said platen having a back side to which
said shaft is connected and a front side on said platen to which can be secured an
abrasive sheet;
b) a pivoting joint comprising a gimbal joint,
c) said shaft being able to pivot about said pivoting joint relative to said platen.
The process may also comprise a sheet of abrasive material comprises a surface having
abrasive particles with an average diameter of from 0.1 to 100 micrometers and said
platen is rotated at a speed of at least 2,000 rpm and, during rotation of said platen,
a liquid is placed between said sheet and said work piece, said liquid forms a boundary
layer as it moves from an inner portion of said sheet to an outer portion of said
sheet, said sheet comprising abrasive particles which protrude by an average height
on said surface of said sheet, and said boundary layer is more than 50% and less than
150% of the average height of abrasive particles protruding from said sheet. A liquid
preferably is placed between said sheet and said work piece, said liquid forms a boundary
layer as it moves from an inner portion of said sheet to an outer portion of said
sheet, said sheet has abrasive particles which protrude by an average height on said
surface of said sheet, and said boundary layer thickness is within ±50% the average
height of abrasive particles protruding from said sheet.
[0040] Another aspect is a preferred process within the scope of the invention which comprises:
a) providing a work piece to be lapped, said work piece having a first surface and
a second surface which are parallel to each other, and at least one of said first
and second surface is a surface to be lapped,
b) providing a first and second rotating platen, each of said first and rotating platen
having I) a back surface and ii) a flat front surface which can be adjusted so that
said first platen is facing and parallel to said first surface of said work piece
and said second platen is facing and parallel to said second surface of said work
piece,
c) providing a sheet of abrasive material on at least said flat surface of said first
platen with an abrasive face of said sheet facing said first surface of said work
piece which is said at least one surface to be lapped,
d) securing said sheet of abrasive material to said flat surface of said first platen,
and
e) putting a liquid between both I) said first platen and said first surface of said
work piece and ii) said second platen and said second surface of said work piece,
f) rotating both of said platen at at least 500 revolutions per minute and contacting
said abrasive material and said work piece,
g) wherein contact pressure between said both I) said first platen and said first
surface of said work piece and ii) said second platen and said second surface of said
work piece are sufficiently similar that said work piece does not flex more than 0.1mm
at its exterior regions between said two platens.
[0041] A very important process aspect of the present invention includes the initial positioning
and contacting of the workpiece and the abrasive sheet material as in a process for
initiating contact between a workpiece to be ground and an abrasive surface comprising
abrasive sheeting on a rotatable platen, the process comprising:
a) supporting a workpiece on a workpiece holder,
b) supporting said workpiece holder on a linearly movable support,
c) advancing the workpiece into contact with an abrasive surface comprising abrasive
sheeting on a rotatable platen,
said process being further characterized by
d) determining a position at least approximating the position of contact between a
surface of said workpiece to be ground and said abrasive surface,
e) removing said workpiece from said position approximating the position of contact,
f) advancing the workpiece towards said abrasive surface while said rotatable platen
is rotating, and
g) controlling forces which advance said workpiece towards said abrasive surface and
into contact with said abrasive surface.
In this process, mechanical alignment of said workpiece and/or said workpiece holder
is effected to promote parallelity between a surface of said workpiece to be ground
and said abrasive surface after step c) but before step e). The controlling forces
provides a preferred contact force between 0.1 and 10 pounds per square inch between
a surface of said workpiece to be ground and said rotating platen during lapping of
said workpiece while said abrasive sheet is moving with at least 1,500 surface feet
per minute while in contact with said workpiece. This process and lapping system has
the workpiece holder supported by a pivot joint and said workpiece holder pivots upon
contact between said workpiece and said abrasive surface to hold a surface of said
workpiece to be lapped in a more parallel orientation with said abrasive surface.
[0042] Another desirable aspect of the process of the present invention is that pressure
is applied between the work piece and the abrasive sheet by a flexible joint or engagement
or gimbal supporting the work piece. The pressure applied between the workpiece and
the rotating platen may be from 0.1 psi to 100 psi, preferably from 0.1 to 25 psi,
more preferably from 0.1 or 0.5 to 5 psi.
[0043] Generally a particular improved process of the invention may be considered to comprise
a process for lapping a surface comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped,
b) providing a rotatable platen having a back side and a front side, said front side
facing said work piece and having a flat plateau which is continuous around the perimeter
of said front side of said rotatable platen and is elevated with respect to a central
area on said front side,
c) providing a sheet of abrasive material on said flat plateau, said sheet of abrasive
material having a front surface with an abrasive face and a back surface, with said
abrasive face facing said at least one surface to be lapped,
d) securing said sheet of abrasive material to said flat surface of said plateau,
and
e) rotating said platen at at least 500 revolutions per minute and contacting said
abrasive material and said work piece to remove material from said work piece.
[0044] This process particularly benefits when the plateau defines an annular shape on said
front face, and more particularly where the sheet of abrasive material comprises a
circular sheet or annular sheet of material. The sheet of abrasive material most preferably
comprises an annular shape in which a central open portion is at least three times
the radial dimension as the width of said annular sheet. A reduced gas pressure may
be applied against said back surface of said sheet between said sheet and said platen
through vents which are present at least or only on said flat surface of said plateau,
the reduced pressure securing the sheet against rotational movement relative to the
rotatable platen. A preferred abrasive sheet comprises an annular distribution of
abrasive material on a backing material, with a center area of said sheet being a
self-supporting structure which passes across said center area, contacting inner edges
of said annular distribution of abrasive material. That is, the central area may be
free of abrasive material, such as where said abrasive sheet comprises a continuous
substrate with a central area having no abrasive on said backing material, and an
annular zone of said backing material surrounding said central area having abrasive
material on a surface overlaying said plateau and facing away from said platen, or
where said abrasive sheet comprises an annular zone and said central area, said central
area being bonded to said annular zone, having less height than said annular zone
when said sheet is lying flat, and there being a seam or bond between said annular
zone and said central area.
[0045] A preferred lapper platen system according to the present invention may comprise:
a) a shaft which is connected to a rotatable platen having vents for air on a front
surface of said platen, said platen having a back side to which said shaft is connected
and a flat front side on said platen to which can be secured an abrasive sheet by
reduced air pressure conveyed through said vents;
b) a frame having a total weight of at least 200 kg supporting a work piece holder
and said shaft connected to a rotatable platen;
c) a work piece holder which is movable on said frame;
d) said work piece holder being attached to a movable element on said frame which
is capable of moving along said frame in a direction towards and away from said platen
to perform lapping of a work piece held on said work piece holder;
e) said work piece holder having control element thereon which allow for independent
movement and alignment of said work piece holder along three perpendicular axes so
that a work piece on said work piece holder can be adjusted and oriented towards parallelity
with said platen so that a work piece can be lapped; and
f) most preferably said control elements having at least 50 settings per rotation,
each setting moving said shaft along one of said three axes by a dimension less than
0.05 mm.
[0046] Movement and control of movement of the workpiece holder can be extremely important
in the performance of the present invention. The control of the movement is best effected
by the use of support systems for the workpiece which allow smooth motion of the workpiece,
especially by air pressure, hydraulic pressure, linear electric motors and the like.
[0047] Another improved process for lapping a surface according to the present invention
comprises:
using a lapper system comprising:
a) a frame having a total weight of at least 200 kg supporting a work piece holder
b) a rotatable platen having an abrasive surface comprising an abrasive sheet secured
to said platen;
c) a work piece holder which is movable on said frame;
d) said frame being movable in three dimensions, with controls for each of the dimensions
of movement (e.g., hinges, positioning screws, hydraulics, electric motors, etc),
e) walls may be present above a plane defined by a surface on said rotatable platen
which carries abrasive; and
f) said rotatable platen being surrounded on all sides by said walls which may be
angled (over said plane and towards said platen) to deflect impacting material downward
or at least preventing impacting material from ricocheting upwardly out of the impact
area (e.g., by using extensions or lips from the walls which overlay the impact area
and prevent vertical ricocheting off of the walls).
[0048] It is preferred that a safety box system is also included within the lapping system
which may include a means for introducing a first amount of liquid onto said abrasive
surface of said platen at a location before contact between a work piece held on said
work piece holder and said abrasive surface on said platen;
g) a means for introducing a second amount of liquid onto said abrasive surface of
said platen after contact between said work piece and said abrasive surface; and
h) means for directing air against said abrasive surface after introduction of said
second amount of liquid.
[0049] The second amount of water is larger than the first amount, the first amount providing
a function as a lubricant, coolant, or the like, and the second amount assisting in
washing away residue from the work piece and/or the abrasive sheet. The means for
directing air against the abrasive surface of the platen assisting in the rapid removal
of the liquid and the solid matter carried with it.
[0050] A work piece holder may be used which has a control element thereon which allows
for independent movement and alignment of said work piece holder along three perpendicular
axes so that a work piece on said work piece holder can be adjusted and oriented towards
parallelity with said platen so that a work piece can be lapped; and
a) said control elements having at least 50 settings per rotation (with as many as
1000 settings per rotation practiced), each setting moving said shaft along one of
said three axes by a dimension less than 0.05 mm.
wherein said lapper system includes a pivoting lapper platen system comprising:
b) a shaft which is connected to a platen, said platen having a back side to which
said shaft is connected and a front side on said platen to which can be secured an
abrasive sheet;
c) a pivoting joint comprising a spherical or torroidal element comprising a curved
outside surface, and said pivoting joint being located on the outside of said shaft,
said pivoting joint having an arcuate surface area and a receding surface area of
said outside surface of said pivoting joint, and said receding surface area is closest
to said workpiece holder;
d) said pivoting joint having a cross section with an effective center of its area,
said receding surface area of said pivoting joint being defined by a surface which
has average distances from said effective center which are smaller than the average
distances from said effective center to said arcuate surface area;
e) arcuate surface area of the pivoting joint is supported by at least one pair of
arcuate-faced bearings, said bearings comprising at least one upper bearing and at
least one lower bearing, said bearings being attached to a portion of said workpiece
holder, and allowing said pivoting joint to pivot between said at least one pair of
bearings;
f) said shaft being able to pivot about said pivot joint relative to said workpiece
holder.
[0051] Rotating of said platen is done at a rotational velocity sufficient to generate a
surface speed of at least 4,000 surface feet per minute (or even more than 20,000
surface feet per minute), which, depending upon the diameter of the rotating abrasive
may be at an angular speed of at least 500 revolutions per minute (which with a 15.2
cm or 6 inch diameter platen and abrasive sheet, equates to over 700 surface feet
per minute at the periphery of the abrasive surface), or even more than 3,000 revolutions
per minute (which with a 15.2 cm diameter abrasive sheet equates to over 4200 surface
feet per minute and with a 30.4 cm or 12 inch abrasive sheet equates to over 8400
surface feet per minute) and contacting said abrasive material with said work piece.
The process of the present invention allows the boundary layer of any liquid (e.g.,
coolant or lubricant) applied to the working surface of the abrasive sheet to be controlled
to improve the uniformity of the lapped surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] Figure 1 is a perspective view of a lapping apparatus according to the present invention.
[0053] Figure 2 is a perspective view of a lapping platen for supporting abrasive sheets
according to the present invention.
[0054] Figure 3 is a cross-section of a lapping system according to the present invention.
[0055] Figure 4 is a perspective view of an apparatus for applying liquid to the surface
of a lapping platen according to the present invention.
[0056] Figure 5 is a side view of a platen with raised peripheral edge portions.
[0057] Figure 6 is a perspective view of a platen with raised peripheral edge portions.
[0058] Figure 7 is a cutaway view of a platen with raised peripheral edge portions.
[0059] Figure 8 is a cutaway view of a different configuration of a platen with raised peripheral
edge portions.
[0060] Figure 9 is a cutaway view of a platen with a pivot connection to a rotary shaft.
[0061] Figure 10 is a perspective view of a single Bellview spring washer.
[0062] Figure 11 is a cutaway view of a platen with a pivot control mechanism within a shaft.
[0063] Figure 12 is a perspective view of an annular platen with a beveled edge.
[0064] Figure 13 is an edge view of a platen with a beveled edge and a workpiece being lapped
in a linear manner by said platen.
[0065] Figure 14 is an edge view of a workpiece and a platen.
[0066] Figures 15 are overhead views of abrasive platens with segments of abrasive sheets
thereon.
[0067] Figure 16 shows a workpiece holder with a vertical vibration damping element on it.
[0068] Figure 17 shows a platen with abrasive sheeting thereon with special surface features
to improve performance.
[0069] Figure 18 shows a workpiece holder with various orientations of gimbals to reduce
tilting torque on the workpiece holder under high speed lapping.
[0070] Figure 19 shows an overhead view of a platen and multiple part workpiece holder according
to one aspect of the present invention.
[0071] Figure 20 shows cross-sections of platens of an earlier but workable form (a) of
the present invention, and two improved configurations (b) and (c) according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0072] Apparatus and methods are needed for super high speed lapping at greater than 500
rpm, greater than 1500 rpm, higher than 2000 rpm, and even speeds of 2500, 3000 rpm
and greater, equating to surface speeds at the periphery of the abrasive sheet of
from about 500 to more than 25,000 surface feet per minute (sfpm, or sfm), depending
upon the diameter of the platen and sheet as well as the angular speed. In addition,
these higher speeds should be useable with finer and harder pre-made abrasive materials
without the use of liquid abrasive slurries. Some earlier attempts at using liquid
slurries at high rotational speeds were less effective than desired because of hydroplaning
of the liquid slurries, excessively rapid movement of the slurries out of the work
area, channeling of the slurry liquid and other effects. The different forces at the
different distances from the rotational center contributed to distributional difficulties
in the placement of the liquid. The different amounts of liquid slurry at different
radial positions caused variations in pressures and thickness at different radial
points. These effects in turn caused the lapping to be less even than should be the
capability of such lapping systems and materials.
[0073] A lapping apparatus according to the present invention comprises at least the following
elements:
1) a frame to support a rotatable platen and a workpiece holder;
2) a rotatable platen capable of rotating at least 500 revolutions per minute;
3) a workpiece holder; and
4) an abrasive sheet secured to a surface of the rotatable platen which faces the
workpiece holder. There are an extraordinary number of subtleties and issues which
combine to make the lapping system perform at its maximum efficiency, some of which
are independently unique contributions and inventions within the field of lapping,
and all of which that are known to the inventors in the best mode of practicing the
invention are described herein. The various areas and specific problems addressed
by these various methods are listed within this patent.
[0074] One process practiced in the present invention is a process for lapping a surface
comprising:
a) providing a work piece to be lapped, having at least one surface to be lapped which
can be adjusted to a position parallel to said at least one surface of a rotating
platen,
b) providing a rotating platen having I) a back surface and ii) a front surface with
a periphery, said front surface of said rotating platen having a raised edge (preferably
symmetrically) disposed about said periphery,
c) providing a sheet of abrasive material having an abrasive face and a back side
onto said raised edge to provide a (preferably symmetrical, but see non-symmetrical
distributions later described herein) distribution of abrasive material on said rotating
platen, said back side of said sheet of abrasive material being on (e.g., in contact
with) said front surface of said platen with the abrasive face of said sheet facing
said at least one surface to be lapped,
d) securing said sheet of abrasive material to said front surface of said rotating
platen, and
rotating said rotating platen at a rotational speed of at least 500 revolutions per
minute, and
contacting said abrasive face and said at least one surface to be lapped on said work
piece.
[0075] Another process practiced in the present invention may be described as follows:
a) providing a work piece to be lapped, having at least one surface to be lapped which
can be adjusted to a position parallel to said at least one surface of a rotating
platen,
b) providing a rotating platen within an area which is surrounded by walls on five
perpendicular planes (e.g., the four approximately vertical planes and a "floor" plane
underneath the rotatable platen) of six planes which would define a cube around said
platen to provide a safety box area, said five planes intersecting all extensions
of a plane of rotation of said rotatable platen; said platen having I) a back surface,
ii) a front surface, and a periphery,
c) providing a sheet of abrasive material having an abrasive face and a back side
onto said rotating platen, with the abrasive face of said sheet facing said at least
one surface to be lapped,
d) securing said sheet of abrasive material to said front surface of said rotating
platen, rotating said rotating platen at a rotational speed of at least 500 revolutions
per minute, and contacting said abrasive face and said at least one surface to be
lapped on said work piece, said walls intercepting any liquid or debris projected
from said rotating platen, and said intercepted debris falling to a lower section
of said safety area;
providing a first amount of liquid to assist lapping to said abrasive surface physically
in front of an area where work piece contacts said abrasive face,
optionally providing a second amount of liquid to assist in washing solid material
from said abrasive surface physically after said area, and
optionally directing air against said abrasive surface physically after providing
said second amount of liquid to assist in removing said first and second amounts of
liquid from said abrasive surface.
[0076] Still another process according to the present invention includes a process for initiating
contact between a workpiece to be ground and an abrasive surface comprising abrasive
sheeting on a rotatable plate, said process comprising:
a) supporting a workpiece on a workpiece holder,
b) supporting said workpiece holder on a linearly movable support,
c) advancing the workpiece into contact with an abrasive surface comprising abrasive
sheeting on a rotatable platen,
d) determining a position at least approximating the position of contact between a
surface of said workpiece to be ground and said abrasive surface,
e) removing said workpiece from said position approximating the position of contact,
f) advancing the workpiece towards said abrasive surface while said rotatable platen
is rotating, and
g) controlling forces which advance said workpiece towards said abrasive surface and
into contact with said abrasive surface.
[0077] This process may effect mechanical alignment of said workpiece and/or said workpiece
holder to promote parallelity between a surface of said workpiece to be ground and
said abrasive surface after step c) but before step e). The process may also have
said controlling forces providing a contact force between 0.1 and 10 pounds per square
inch between a surface of said workpiece to be ground and said rotating platen during
lapping of said workpiece while said abrasive sheet is moving with at least 1,500
surface feet per minute while in contact with said workpiece.
[0078] The process may also have the workpiece holder supported by a pivot joint and said
workpiece holder pivoting upon contact between said workpiece and said abrasive surface
to hold a surface of said workpiece to be lapped in a more parallel orientation with
said abrasive surface.
[0079] It is more preferred with respect to the protective walls that, rather than merely
having four essentially vertical walls intercept material which is expelled from the
work area by the rotational forces from the rotating platen (and often a rotating
workpiece holder in conjunction with a rotating platen), the surfaces (the walls)
which are intersected by the plane formed by the contact points between the platen
and the workpiece are angled (hereinafter referred to as the intersection plane),
sloped or curved so that impacting expelled material is deflected downward from the
point of contact by the angle of impact. This is a protective measure which can still
be improved by the provision of a lip, movable lip, fixed frame guard or the like
which extends from the walls (or continues from the walls as a continuous extension
of the walls) to provide additional protection from ricocheting materials. For example,
the walls may be curved, and the curve extends from above the intersection plane towards
the shaft supporting the workpiece to form an umbrella-like protective area. The extension
from the walls may be curved, flat, stepped, movable (e.g., on a rotating hinge so
that it may be lifted), slidable (so that it may be moved back and forth to open up
the work area if access to it is needed), and the like.
[0080] This guard wall or enclosure is neither a trivial matter nor a system which is relevant
to traditional lapping. In traditional lapping, much lower rotational speeds, such
as 200 revolutions per minute and/or smaller diameters (producing lower surface speeds,
e.g., less than 300 surface feet per minute) allow materials such as detritus, used
slurry, cooling liquid and the like the flow or stream off the surface at speeds which
are comparable to the rotational speeds of the platen. With the much higher speeds
used in the present invention, and the use of abrasive sheets, the dynamics, problems,
and failure of the system are unique and require differ considerations.
[0081] When high speed platen rotation is used with abrasive sheeting failure of the system
can occur for different reasons and with different results than in lower speed slurry
systems or lower speed abrasive sheet systems. For example, it must be remembered
that the clearance between the platen, sheet and workpiece are essentially non existent.
With the extremely high rotational speeds, events could and do occur as follows. In
one circumstance, the workpiece may be advanced into contact with the rotating platen
at less than perfect parallelity. If that difference from parallelity is too great,
the workpiece may grip and lift, fold, crinkle or crumple the abrasive sheet. Because
there is no volume within which the abrasive sheet may move (being confined by the
platen and the workpiece), the extremely high speeds of rotation cause extraordinarily
high forces to be brought to bear against the platen, the workpiece and the abrasive
sheet. The result of these extraordinary forces is an explosion created by the kinetic
energy from the high mass inertia and momentum of the platen, but usually also the
workpiece, and possibly the broken workpiece holder and the platen become muzzle velocity
shrapnel from the apparatus. These exploded fragments of materials do not merely fly
parallel to the intersection plane, but spray out of the work area, bounce off each
other, ricochet of the walls and floor of the work area, and can seriously injure
persons in the area or even damage the environment around the apparatus. This event
is unique to combination of the abrasive sheet and the high platen speed of rotation.
Neither the abrasive sheeting alone nor high speed rotation (with slurry or powder)
creates the forces effecting this explosive event. The guard system is therefore uniquely
necessary with the combined system of the present invention.
[0082] A process for lapping a surface according to this invention is also described wherein
a back surface of the workpiece is pivotally connected to a rotating joint which is
in turn connected to a shaft which rotates said workpiece, and said workpiece is allowed
to pivot around said pivot joint as contact is made between said abrasive surface
and said work piece so that said surface to be lapped becomes more parallel towards
said platen after said contact as compared to before said contact.
[0083] The process for lapping a surface according to the present invention may also comprise
an underlying process of:
a) providing a work piece to be lapped, having at least one surface to be lapped which
can be adjusted to a position parallel to said at least one surface of a rotating
platen,
b) providing a rotating platen having I) a back surface and ii) a flat surface, said
back surface having a pivoting joint with a shaft which rotates said platen,
c) providing a sheet of abrasive material having an abrasive face and a back side,
said back side being on said flat surface of said platen with the abrasive face of
said sheet facing said at least one surface to be lapped,
d) securing said sheet of abrasive material to said flat surface of said platen, and
rotating said platen at a rotational speed of at least 500 revolutions per minute
by rotating said shaft, and
contacting said abrasive face and said at least one surface to be lapped on said
work piece, and allowing said workpiece to pivot around said pivot joint so that said
abrasive sheet and said at least one surface to be lapped become more parallel towards
each other.
[0084] One particular advantage of one optional alternative of the present invention (the
vacuum hold-down of the abrasive sheet) is the ability of the apparatus to use preformed
sheets of abrasive materials at high speeds, and to rapidly and cleanly replace the
sheets without significant delays. During lapping and polishing processes, it is often
necessary to change the abrasive medium at various stages. In prior art usage of sheets
of abrasive materials, the individual sheets were secured to the chuck or rotating
face by an adhesive. The adhesive may have been precoated on the backside of the abrasive
sheet or applied as coating to the rotating support surface or the backside of the
sheet immediately before use. This adhesive coating adds another parameter or variable
which must be controlled in attempts to precisely lap surfaces. Even the best coating
techniques provide layers which have what are presently considered minor variations
in thickness in some fields of use. However, each variation, no matter how small,
is part of an additive effect upon the final article. The adhesive creates another
problem in that adhesives that are strong enough to secure the abrasive sheet to the
platen do not necessarily remove cleanly from the platen with the removal of the sheet.
Some adhesives build up on the platen surface, requiring washing or stripping to remove
them, if increasing variations in non-planarity of the surface are to be avoided.
This is time consuming, labor intensive, and expensive. Where the objective of the
system is to provide uniform flatness, even this additional minor variable component
becomes undesirable or limiting in the capability of the final article. This is particularly
true where the variations can cause uneven or non-uniform exposure of abrasive material
towards the workpiece, causing uneven grinding, polishing or lapping of that workpiece
surface. The use of rotational abrasive action, particularly at high speeds for short
duration, can quickly cause undesirable effects upon the workpiece. When sheets are
regularly changed with respect to their degree of coarseness in the abrasive grit,
subsequent variations because of the adhesive layers will not only fail to correct
the previous errors, but add further variations into the workpiece surface which were
not intended. Additionally, some adhesives remain liquid or pliable (e.g., pressure-sensitive
adhesives) and the centrifugal forces produced in high speed rotational abrasion can
cause the adhesive to shift, flow or shear, altering the thickness of the adhesive
layer even while the process is being performed.
[0085] One optional, but highly preferred aspect of the present invention therefore is to
support a sheet having at least one abrasive workface and a backside on a rotatable
support by vacuum forces, and to perform the abrading process with the vacuum forces
maintaining at least part, if not all of the contact between the support and the backside
of the sheet. Adhesive supplemental forces may be particularly used to advantage where
the adhesive contacts or adheres the abrasive sheet and the rotatable platen in a
region which will not place the abrasive sheet into contact with the workpiece. For
example, where an annular distribution of abrasive is present on the abrasive sheet
and the central area has no abrasive and is not brought into contact with the workpiece,
the use of adhesive between the platen and the abrasive sheet in this region is quite
acceptable, though still not preferred. Although vacuum forces have been used to support
or assist in the support of workpieces, there is not known to be any description of
the vacuum support of abrasive sheet materials in a high speed lapping process, nor
is their any indication of the potential problem with abrasive sheet thickness variations
because of the addition of adhesive coatings between the support and the sheet. The
references described above, even though they may refer to high speed in production
of materials, do not describe rotational speeds in excess of 1500, 2000, 2500 or even
3000 rpm, or expressed in other units, with surface speeds at the periphery of the
rotatable lapping platen of at least 550, at least 1,000, more preferably at least
1500 or at least 2,000 sfpm, still more preferably at least 2,500 or 3,000 sfpm, again
still more preferably at least 3,500 or 4,000 sfpm, and most preferably at least 8,000
or 10,000 or even 12,000 and more sfpm. Furthermore, it is usually the abrasive segment
of the apparatus and process of that prior art which is being rotated (although as
shown in U.S. 5,317,836, both a semiconductor wafer and the buff are rotated), while
the vacuum secured workpiece remains fixed. There is no teaching in the prior art
or consideration of the physical problems which could be encountered in attempting
to use vacuum pressure, and particularly only vacuum pressure to support an abrasive
sheet at high speed rotational lapping. For example, there is no consideration in
the prior art as to whether the vacuum forces could successfully restrain movement
of the abrasive sheet materials when forces (e.g., rotational) are applied to the
abrasive face. The shearing forces, especially if applied unevenly on the face by
non-symmetrical contact with the workpiece, could easily be envisioned to cause the
abrasive sheet to shift. This would be disastrous in a lapping system and could well
destroy all the earlier polishing steps performed or ruin the workpiece entirely.
Although adhesives provide problems as indicated above, a change from adhesive support
to vacuum support could have been considered to alter the system in unpredictable
ways. As adhesives can elongate with the rotational forces, there may have been some
benefit to the use of a somewhat elastic layer under the abrasive sheet, particularly
in removing any waves or irregularities in the original positioning of a sheet (although
this would not be technically desirable at low speed polishing or lapping since the
forces would be little likely to have a significant effect). The use of a vacuum would
not allow such elastic behavior in an intermediate layer, as there would be no intermediate
layer. This would be another unpredictable effect in such a change from adhesive to
vacuum support of an abrasive sheet material in high speed rotational lapping.
[0086] In the practice of the present invention, the abrasive sheets comprise sheets of
exposed abrasive grit as either a self-supporting sheet or film material or an adhered
layer on a support sheet. The sheets may have any type of abrasive material or surfacing
on the face which is to contact the workpiece. The preferred sheets are sheet abrasive
material manufactured and sold by Minnesota Mining and Manufacturing Company, St.
Paul, Minnesota, and comprises either a polymeric backing sheet with high Mohs hardness
abrasive particulates on a coated layer or a self supporting sheet of such high Mohs
hardness abrasive particulates. Preferred abrasive material comprises diamond particles
or particles comprising small diamond particles supported in a binding matrix (other
than any adhesive matrix forming the self-supporting layer or adhering the particles
to a support). The sheets may comprise a single layer of material (e.g., a binder
with abrasive grit therein or sintered abrasive grit without any other binder) or
multiple layers of materials. Such multiple layers could comprise one or more supporting
layers, intermediate layers (e.g., primer layers, vibrational damping layers, electrically
conductive or antistatic layers, magnetic layers, printed layers, sealer or barrier
layers to prevent migration of materials between other layers), and an abrasive outer
layer. The single layer, at least one layer in the combination of layers, or the interaction
of the combination of layers must be able to support a vacuum against the back surface.
Preferably the back surface (of the abrasive sheet) itself is non-porous or low porosity.
This is desirable as too much porosity would prevent the sheet from being held against
the rotatable support surface. The sheet does not have to be completely non-porous,
although this is the preferred method of making the sheets used in the present invention,
especially when combined with the vacuum draw-down of the abrasive sheets. In addition
to limiting the porosity of the sheets, the back surface should not have such a degree
of topography which would allow free air flow along the back surface when it is being
held against a surface by a pressure of at least 8, 9, 10, 11 or at least 12 lb/in
2. If there were raised channels, ridges or the like which would allow air flow from
the center of the sheet to its outer edges, the pressure would not consistently support
the sheet as air would more readily leak out from the region between the support surface
and the backside of the abrasive sheet. That construction would be useful, but less
preferred in the practice of the present invention.
[0087] The abrasive material may be any known abrasive material, depending upon the ultimate
needs in the process for grinding, polishing or lapping a particular finished article.
The abrasive particulate or raised particulate areas may comprise any solid, hard,
material such as silica, titania, alumina, Carborundum, boron nitride, homogeneous
inorganic oxides (such as metal oxides) or blends of inorganic oxides, diamonds (natural
or synthetic), or any other material which is harder than the solid surface to be
polished, ground or lapped. The abrasive surface may be abrasive particles bound in
a binder, either partially embedded, superficially bound to the surface, or initially
embedded so that the binder must initially wear away to expose the particles. The
abrasive surface may be a replicated surface structure of a pure abrasive material,
an etched abrasive surface, molded surface or the like. The abrasive surface may also
be deposited islands of abrasive material, with either physicalprocesses used to place
the abrasive (e.g., vapor deposition, screened application of powders which are fused,
powder arrays which are electrostatically deposited and bonded to the surface, impact
embedding of the particles) or chemical processes (e.g., electrochemical deposition,
chemical deposition at seeded sites) to form the particles in a random or ordered
manner. The preferred material is an abrasive sheeting manufactured by Minnesota Mining
and Manufacturing Co., known as Diamond Abrasive Disks (3M). These sheets are quite
effective for the high speed, fine finish lapping processes and apparatus of the present
invention. Also useful in the practice of the present invention are diamond particles
contained in a metal matrix on a sheet of plastic backing material (e.g., 3M Metal
Bond™ Abrasive). The only modification of the sheets which is essential for making
them completely compatible with the present invention is having the sheet converted
(cut) to fit the abrasion platen. The sheets may be cut into, for example, circular
shapes, with or without positioning holes or a centering hole in the sheet. This abrasive
sheet material has been able to provide an improvement at high speed lapping which
was not recognized at lower speed lapping, where the problem was not notice and/or
was not as significant. The 3M Metal Bond™ Abrasive has islands of the abrasive material,
as opposed to having a continuous matrix of binder with the abrasive particles therein.
The islands therefore allow swarf, debris and liquid to pass between the islands (driven
by centrifugal forces) and away from the contact area between the abrasive sheet and
the workpiece.
[0088] This prevents the moving material from forcing the workpiece out of alignment, creating
different grinding functions locally, or causing other mischief with the system.
[0089] The present invention may be further understood by consideration of the figures and
the following description thereof. Figure 1 shows a perspective of a basic lapping
apparatus
2 according to the present invention. The apparatus
2 usually comprises at least a main support frame
4 with a vibration absorbing surface
6 which may be a single layer
6 as shown in Figure 1 or multiple layers (not shown). The composition of the layer
may be thick metal, layered metal, composite, coated metal, and the like. Two thick
sheets of metal (not shown) is preferred, with one sheet fixed to the main frame
4 and the other sheet fixed to the frame top
8 at the arms
12 or which is removably attached to the first layer (not shown). There is also conveniently
a frame top
8 which may be removably or permanently attached to the main frame
4. An electrical enclosure
10 is shown over the vibration absorbing surface
6. A supporting frame
14 is shown for a workpiece spindle
16. A computer
18 is also shown in the lapping apparatus
2 to provide controls over the operation. The abrasive sheet (not shown) support platen
20 is located at a position on the vibration damping surface
6 over which the workpiece spindle
16 may be positioned. Various positioning systems (later shown) which operate to keep
the alignment of the workpiece spindle
16 and the abrasion support platen
20 can be preferred part of the apparatus
2. An abrasion platen drive motor
22 can be seen underneath the vibration damping surface
6. The size of the apparatus
2 is somewhat dependent upon the needs for the user. The length
24 of the base of the main frame
24 may be, for example, between about 3 to 8 feet (0.9 to 2.42m), the width of the main
frame may be, for example, between 1.5 feet and 4 feet (0.45 to 1.22m), and the height
of the main frame may be, for example, between 1.5 feet and 4 feet (0.45 to 1.22m).
Greater variations in the dimensions are of course possible, but the preferred dimensions
are within this range, and especially between 4.5 feet and 5.5 feet (1.64 and 2.0m)
in length and 2 to 3 feet (0.68 and 0.91m) in width and height. A heavy construction
is preferred, with at least 0.6cm thick steel plate in the arms
12, 30, 32, 34, 38, 40, etc. (collectively referred to as the arms
12. The arms
12 may be hollow with sheet metal of that thickness or larger, or may be solid. The
dimensions of the arms
12 may be, for example, from 2 to twelve inches (5 to 31 cm) a side (assuming a square).
This fairly massive composition will keep vibration to a minimum. A four wall box
19 is shown surrounding the platen
20 above its flat surface (e.g., the plane of rotation of the surface). A curved lip
21 is shown at the top of the four wall box
19 to prevent ricochet of exploded pieces and to deflect them down within the box
19, possibly to a collection area (not shown).
[0090] Figure 2 shows an abrasive platen
50 useful in the practice of the present invention. In the practice of the present invention,
a wide range of diameters is useful for such abrasive platens
50. Typical diameters are from 7.5 to 50 cm, more preferably from 7.5 to 40 cm in diameter.
The abrasive platens
50 of the invention are provided with a sufficient number of ports or holes (not numbered)
to enable a vacuum to be distributed against the backside of an abrasive sheet (not
shown). In Figure 2, three circular distributions of such holes
52, 54, 56 are shown distributed as a series of holes
58. The holes
58 are a convenient, exemplary distribution, but are not essential to the practice of
the present invention. Vacuum access to the backside of an abrasive sheet may be provided
in many different types of distribution. The distributions do not even have to be
symmetrical, but should be reasonably distributed so that sections of an abrasive
pad will not lift from the platen
50 during high speed rotation while other areas are secure. There is no need to have
an asymmetric distribution of holes
58, but it is a feasible construction. A circular distribution is convenient as the
abrasive sheets generally used tend to be circular to fit with the circular motion
of rotation and the usually circular shape of the platen
50. Other shapes may be selected, but they would tend to be prone to greater eccentricities
in their motion and therefore would be less desirable. The circular set
52 of holes
58 nearer the center of the top surface
66 of the platen
50 help to secure the center portion of an abrasive pad to the platen
50. Likewise, the circular distributions
54 and
56 tend to secure an abrasive pad to the surface
66 of the platen
50 along a radius
60. The number and spacing of holes on the platen surface
66 are designed to secure an abrasive sheet without the holes (e.g.,
58) being so large as to deform the sheet into the contours (not shown) of the holes.
Holes on the surface are preferably less than 5 mm in diameter, more preferably less
than 4 mm, still more preferably less than 3.5 or less than 3.0 mm, and most preferably
greater than 0.5 mm and less than 3 mm. The minimum size and number is determined
by that number and size which will support a vacuum against the backside of an abrasive
sheet. A minimum size of about 0.2 mm is a reasonable starting point for commercial
design. Smaller holes would clog too easily from materials produced during operation
of the apparatus. More preferred would be diameters of at least 0.5 mm, more preferably
at least 0.7, still more preferably at least 1.0 mm. These are average diameters,
and hole sizes that differ within each circular distribution or amongst circular distributions
are contemplated. Ranges of between 0.2 and 5 mm may generally be used. The circumferential
edge
68 of the platen
50 may have engaging grooves or cogs
70. These cogs
70 would be used to engage with driving gears
72 and
74. A motor (not shown) would drive these driving gears
72 and
74 to rotate the abrasive platen
50. It is also desirable to have the material around the edges of the holes hard or
abrasion resistant to avoid enlargement of the holes by abrasive grit being drawn
into the holes. Abrasion resistant coatings, sacrificial coatings, hardened metal
(e.g., hard chrome plating (Rc 80) and the like can be used to strengthen and harden
the holes.
[0091] Figure 2 shows an approximately 32.9 cm diameter (13 inch) platen
50 with a centering post
62 which may be a removable centering post
62 inserted into a hole
64 in the surface
66 of the platen
50. In Figure 2, the first circular distribution of holes
52 at a diameter of about 62.8 mm (2.5 inch) comprises 30 holes having diameters of
about 1.5748 mm (0.062 inches). The third circular distribution of holes
56 at a diameter of about 29.2 cm comprises 180 holes of about 1.5748 mm (0.062 inches).
The second circular distribution of holes
54 is at a diameter of 22.8 cm (9.0 inches). Radial, rather than circular patterns of
holes may be easily placed on the surface
66 of the platen
50. Designs or other patterns, or even random distributions of holes may be placed onto
the surface as long as a vacuum can be supported on the backside of an abrasive sheet.
[0092] Smoothness and flatness are two characteristics which are used in the art to measure
the quality of lapping and polishing performance. Smoothness can be measured by profilometers
(either, for example, confocal or stylus) and is measured in linear dimensions and
standard deviations or variations from uniformity. Flatness is conventionally measured
in terms of light bands, using equipment such as LAPMASTER™ Monochromatic Lights (e.g.,
Models CP-2 and CP-1) in combination with flat glass over the surface to be evaluated
for flatness. The use of light band units (e.g., the number of lightbands per unit
of horizontal dimension on the surface being evaluated, e.g., per inch) can measure
surface flatness within millionths of an inch. Curvature of radiating lines away from
a line of contact between the glass and the surface against which light is being projected
would indicate a degree of convexity to the surface and lines curving towards the
point of contact would indicate a degree of concavity. Straight, parallel, evenly
spaced lines indicate true flatness. Normal lapping procedures of the prior art are
able to achieve 1-2 lightbands of smoothness, but the process commonly takes hours,
depending on the material started with. Particularly when the material is hard (e.g.,
tungsten carbide or special alloys), conventional lapping is performed in hours, not
necessarily including the necessary cleaning time. The use of the apparatus, processes
and materials of the present invention can easily achieve 4-5 lightbands of smoothness
in minutes (e.g., 5 minutes or less), and with apparatus and processes combining all
of the improvements described in the present invention,. 1-2 lightband smoothness
has actually been achieved in less than an hour (e.g., 15 minutes or less, even at
10 minutes), which time included replacement of sheets at the various stages and time
for normal cleaning operations. Other conventional parameters of lapping have been
exceeded by practice of the technology of the present invention.
[0093] It is a standard assumption, proven consistently by reported data and analysis, that
lapping with abrasives causes fracturing within the workpiece to a depth which is
equal to the average diameter of the abrasive particles. That is, if the average size
of particles in a slurry or coated on a sheet are 50 micrometers, the workpiece, from
that operation, will show microfracturing on the lapped surface which is equal to
the average diameter of the abrasive particles used to lap the surface. Each successive
lapping operation (e.g., starting with 50 micron, then 10 micron, then 2 micron particles)
will leave successively smaller microfractures, but each will be approximately equivalent
to the average size of the abrasive particles used in the last lapping step. The amount
of material removed in each lapping step, however, will more nearly approximate the
degree of damage created in the previous step. Therefore, if 50 micron particles are
used in one step and 10 micron particles are used in a second step, the second step
will remove approximately 50 microns (the damaged depth remaining from the previous
step) and itself leave a damaged depth of about 10 microns. By operating at speeds
of at least 500 rpm (that is surface speeds of at least 1000 surface feet per minute),
diminished amount of microfracturing (where individual grains of material are broken
loose, resulting in "pick-out") has been reported in some cases in the practice of
the present invention. By using higher surface speeds, the micro fracturing continues
to be reduced until microfracturing pickout as little as or less than 90%, 80%, 70%,
60%, and even 50% of the actual average diameter of the abrasive particles occurs
in the work piece. This is a potentially improved characteristic of the lapping effect
of the present invention. No other lapping operation is known to provide this reduction
in pick-out. This is a definable aspect of a process according to the present invention,
and may be seen in many different materials, such as in tungsten carbide, blends or
alloys of metals (e.g., copper and tungsten), plastics, composites, etc. The process
also tends to smooth out non-homogeneous mixtures with less gouging of material, thus
leaving fewer holes or pits in the surface because lapping and polishing, rather than
gouging, is being effected. Even when performing conventional lapping processes using
slurries of individual abrasive particle material in liquid carrier, low speeds of
5-200 revolutions per minute (rpm) are normally used. Some processes do use higher
speeds with slurries up to 2500 rpm, for example, and the pressures used to hold the
rotating platen face and the work piece face together are perhaps 200 pounds with
a 10 cm by 10 cm work piece face (which is about 12.9 pounds per square inch contact
force). It is considered by abrasive technology researchers that a primary method
of material removal from the work piece is for the individual abrasive particles to
roll along between the piece part and the platen, rolling off or flattening high spots,
or the abrasive particles are dragged along by the moving platen and shear off high
spots. In either case, because the average normal clamping force is high, very large
localized forces are concentrated against individual grains or areas of the piece
part material at its surface. These localized forces are strong enough to weaken and
break the bond between the grain in the piece part and the main bulk of the piece
part at the grain boundary. Subsequently, the loosened grain will be forced out of
its original position and leave a void, pocket or pit where it was originally located.
These pits are referred to in the art as "pick-outs" and are very undesirable.
[0094] With high speed lapping according to the present invention, the normal (perpendicular)
force can be generally much lower than in lower speed lapping processes, being as
low as 10% of the forces normally encountered in lower speed lapping, such as only
20 pounds (8 kg) of normal force for a 10 cm by 10 cm work piece. As noted above,
the contact pressures in the practice of the present invention may range from 0.1
to 100 psi, but are more preferred between 0.1 and 10 psi, still more preferred between
0.1 and 5 psi, and most preferred between 0.1 and 3 or 0.5 and 3 psi. Because this
normal force is so much less, the localized forces on individual grains and abrasive
particles are reduced and much less fracturing of the piece part surface and grains
on the piece part surface occur. Pick-outs on the surface have been shown to be reduced
by from 10 to 90% as compared to surfaces with the same flatness, so that the smoothness
of the surface is improved even while the good flatness is preserved. This is particularly
important in the lapping of blends or composite materials where the surface to be
lapped is not uniform on a molecular scale (e.g., solid state solution), but rather
provides a surface with regions of different materials (e.g., particles in a matrix,
dispersed metal in a matrix, etc.), and where different responses to the action of
abrasive grains may be experienced in local areas of microscopic proportions. For
example, where blends of metals are present (e.g., tungsten and copper), high speed
lapping will tend to cut off both metals by impact fracture at the same level or height,
providing a superior surface finish (less roughness, more smoothness).
[0095] With the very high speeds of the abrasive particles in the practice of the present
invention, particularly at speeds above 7,500 or above 10,000 surface feet per minute,
as compared to 1,000 surface feet per minute, a completely different mechanism of
lapping appears to occur on the smallest levels of the materials. With the higher
speed lapping by particles on the abrasive sheet, the tops or high spots on the piece
part surface appear to be removed by impact fracturing in addition to involving the
normal mechanisms and effects of shearing and rolling down high spots. Removal of
excess tall material by the mechanism of impact fracturing results in lower levels
of disturbance to grain boundaries between grains in the piece part and reduces the
number of individual grains being broken loose.
[0096] Another significant advantage of the use of the abrasive sheets at high rotational
speeds according to the present invention is that wear on the platen surface itself
is greatly reduced. In slurry processes, the abrasive action works equally forcefully
against the platen face and can eventually wear off the surface of the platen to a
degree where the platen would have to be replaced. Even though the wear would of course
tend to be even, there is no functional reason to continually sacrifice or wear out
the platen. Some uneven patterns of wear may develop in the platen, and these would
be translated into uneven lapping of the piece part.
[0097] Other features of the lapping apparatus of the invention, problems addressed, and
solutions to these problems are also described herein. They are numerically listed
below.
1. FLEXIBLE PIVOT TOOL HOLDER
[0098] Problem: When grinding or lapping single or multiple piece parts held by a tool holder with
a typical diameter of 4 inches held by a center post, the tool holder is slowly (or
quickly) rotated as it is presented downwardly and vertically. This movement is intended
to uniformly contact the work piece and an abrasive surface rotating at very high
speeds of from 2000 to 3,000 rpm (this can effectively be equivalent to more than
9,000 surface feet per minute (sfpm), depending upon the diameter of the platen. During
this process, it is important that the piece part holder be "flat" so that the piece
parts which contact the abrasive first are not damaged. This would be the case if
the holder had one edge lower than another in its presentment to the abrasive sheet.
Furthermore, with high speed lapping and grinding, it has been found to be important
that the piece part holder assembly be held by a ball or gimbal pivot type of device
located as low as possible toward the high speed abrasive surface. This is the best
design found to align the total piece part assembly so all the individual parts (e.g.,
the platen carrying the abrasive sheet and the work pieces) are floated equally by
the thin boundary layer of coolant fluid on the surface of the disk which may be less
than 0.001 inch (0.0254mm) in depth. Boundary layers do not normally remain constant
as the distance from the leading edge (contact point or liquid introduction point,
or radial distance on the platen or circumferential distance along the tangential
distance on the workpiece). The changes in the thickness of the boundary layer cause
significant variations in platen separation distances from the work piece and effective
variations in penetration of the workpiece by abrasive particles on the sheet. With
this type of ball or gimbal pivot, the piecepart tends to lay flat with respect to
the platen abrasive and also this boundary layer thickness has a tendency to remain
uniform even with slight out-of-perfect-perpendicular alignment between the vertical
piece part holder shaft and the high speed abrasive platen. Foreign debris can be
accumulated in pivot joints and create unwanted friction.
[0099] Solution: A work holder device is created with the use of a special ball attached to a shaft
which ball and shaft combination provides a pivot action close to the bottom of the
work piece holder assembly. A sandwich of washers acts as a rigid base to transfer
downward a polishing normal force on the vertical shaft to push the piece parts into
the abrasive platen. The pivot action is restrained by encapsulating the whole assembly
with room temperature vulcanizing (RTV) silicone rubber or other elastomeric resin
(e.g., fluoroelastomers) which seals the unit from debris and also provides the function
on an elastic restraint that self centers the disk type part holder perpendicular
to the axis of the support shaft. Yet the elastic spring which centers the unit is
weak enough to allow conformal pivoting of the assembly during lapping action. Thus
when there is little side load present, as when lowering the piece part assembly,
the unit is flat aligned. But when the assembly is subjected to a normal force, the
unit is free to pivot. A piece part holder with the back stem and RTV resin was constructed
and used in a piece part assembly for lappingoptical connectors and appeared to function
well.
2. ABRASIVE METAL POLISHING MACHINE
[0100] Problem: The surfaces of metal objects are polished for many reasons including for optical
examination of metallurgical characteristics, to create a smooth, low-wear, tight
hydraulic or fluid seal and others. Usually this polishing is done at low speed (e.g.,
5-200 rpm), with rotating flat platen disk wheels of various types of construction
molding aluminum, steel, plastic cloth and others. The wheel surface is very flat
and the workpiece to be polished is held with controlled pressure by hand or work
holder. Water or other fluid, such a lubricant or wetted abrasive particles are introduced
as a slurry, or disks of fine abrasive sheets are "stuck" or bonded to the rotating
wheel. This process is slow to produce a highly polished surface, and it is labor
intensive if not automated. Inaccurate platen or shaft machining, loose bearings,
or weak machine structure and framework may cause polishing accuracy problems.
[0101] Solution: The present invention enables very high quality polishing which can be achieved
in a fraction of the conventional lapping time by using abrasive sheeting, such as
3M brand of micro abrasive disk sheets, for polishing at very high speeds of 2,000
rpm and more using disks about 8-10" in diameter. However, it is critical that the
rotating platen disk run very "true" and flat at the operation, speed range to provide
a mechanically stable moving surface against which the to-be polished workpiece is
held stationary with a controlled normal force or pressure (against the fine particle
wetted abrasive). Options also may change the pressure as a function of process time
or the workpiece rotated to distribute polishing across the surface.
[0102] A unique method to provide a very "flat" and accurate stable rotations platen disk
surface would be to mount the platen to a "weak" shaft which allows the rotating disk
mass to seek a true "smooth" center at speeds above its first rotating natural frequency.
The motor drive speed would be increased above its natural frequency, the workpiece
part presented in contact for polishing; then removed prior to reducing the disk RPM
below its critical harmonic speed.
3. REDUCTION OF HYDROPLANING
[0103] Problem: The presence of liquid on the abrasive surface adjacent the work piece has combined
with higher rotational speeds to generate significant hydroplaning of the liquid and
unequal forces on the face of the abrasive sheet and the work piece at differing positions
along the radial distribution from the center to the outer edge of the abrasive sheet
and also along the tangential contact length of the piecepart surface. The liquid
is often essential to control heat, friction and cleansing of waste materials, and
can not be easily removed.
[0104] Solution: The greatest needs for the liquid are 1) to control friction between the abrasive
surface and the work piece, 2) control the temperature of the sheet and the work piece,
and 3) to wash away residue of abrasive and abraded material from the work piece.
These effects do not have to be performed at the same location between the sheet and
the work piece and do not need the same amount of liquid (e.g., water, lubricant,
coolant, etc.) to accomplish the separate tasks. The inventor has recognized that
the amount of water needed to affect friction (a surface phenomenon, and essentially
two-dimensional [very thin] amounts of liquid may be effective) tends to be much less
than the amount needed to control temperature (a bulk, three-dimensional phenomenon)
and waste removal (a three-dimensional and mass flow process). With this recognition,
it has been found that liquid may be applied to the lapping process of the present
invention with controlled amounts, specified positions, and timed introduction to
perform the process with reduced likelihood of hydroplaning because of reduced amounts
of liquid between the abrasive ( as a sheet or other form) and the work piece. This
is accomplished in the following manner.
[0105] The abrasive sheet is of a sufficient size relative to the work piece that less than
fifty percent (50%) of the abrasive surface will be in contact with the work piece
surface during lapping. Preferably less than 40%, more preferably less than 25%, and
most preferably less than 15% of the total surface area of the abrasive sheet is in
contact with the work piece during lapping at any specific time. The area where the
abrasive and work piece are in actual contact is called the work area. In a zone or
area rotationally before the work area, water is placed on the surface of the abrasive
sheet. The amount of liquid (e.g., water) provided is preferably less than 120% by
volume of that amount sufficient to fill the valleys between the peaks of the raised
abrasive particles (100% essentially forming a smooth, continuous layer of liquid
over the abrasive material). More preferably it is less than 110%, less than 100%,
but at least 30% of that filling volume of liquid. Preferably the amount is between
30% and 120%, more preferably between 40 and 115%, still more preferably between 50
and 110%, and most preferably between 90 and 105% of the volume necessary to exactly
fill the valleys on the abrasive sheet so that an essentially flat film of liquid
appears although surface tension between the peaks and the film may distort the appearance
so that slight circular patterns may appear without dry exposure of more than 20%
by number of the particles. This approximately 100% volume amount is called the "leveling
amount of liquid" in the practice of the present invention.
[0106] At a zone which is rotationally before the work area, a first amount of liquid equal
to 30 to 120% of the leveling amount of liquid is placed on said abrasive surface.
The area where this is performed is called the wetting area. On the surface of the
abrasive sheet, rotationally after the work area, a second amount of liquid is applied
to said abrasive surface, said second amount being both sufficient to have the sum
of said first amount and said second amount equal to at least 120% of said leveling
amount of liquid, and equaling at least 30% of the leveling amount of liquid. Preferably
the total of said first and second amount comprises at least 150%, more preferably
at least 170% of said leveling amount. Likewise, it is preferable that the amount
of said second volume is equal to or greater than at least 50% of said leveling amount,
and more preferably at least 75% or at least 100% of said leveling amount. This second
volume will assist in carrying or washing the total residue on the abrasive sheet
(the residue abrasive and the swarf from the piece part). The second volume is applied
in what is referred to as a flood area on the abrasive surface. The high rotational
speeds will remove a significant amount of the liquid and total residue on the abrasive
surface, but because of the high quality sought in the lapping performance of the
present invention, this may not always be relied upon. To improve the removal of the
liquid carrying the total of the residue, air blades (e.g., hypodermic air knives)
can be positioned between the flood area and before the wetting area. The air blades,
in combination with the rotational forces, will remove a very high percentage of the
applied liquid and the total residue so that an essentially dry surface can be assumed
to enter the wetting area. To whatever degree it is found that not all liquid is removed
by the rotational forces and air knives, the first amount of liquid may be reduced
so that the appropriate percentage of leveling is provided.
[0107] The schematics of this apparatus and process are shown in Figure 4. A water controlled
system
340 according to the present invention is shown comprising a platen
342 having an annular distribution of abrasive sheeting
344. The annular distribution
344 is preferred, but not required in the practice of the present invention. A first
liquid (e.g., water) supply means
346 lays over said annular distribution
344. A second liquid supply means
348 is also shown to overlay the annular distribution
344. An air blowing means
350 is also shown to overlay the annular distribution
344 on said platen
342. A work piece
360 is shown over the platen
342. The rotation direction
370 of the platen
342 is such that liquid
362 deposited from said first liquid supply means
346 is upstream of the work piece
360. The liquid
364 provided by said second liquid supply means
348 is located downstream of the work piece
360. The air blowing means
350 is downstream of the second liquid supply means
348. The air blowing means
350 provides sufficient volume and intensity of air movement to assist in removing liquid
366 which had been on the platen
344.
4. PLATEN FLATNESS GRINDING
[0108] Problem: After a high speed 3,000 rpm, 12" (30.5 cm) diameter rotating abrasive platen has
been manufactured and used on a lapping machine, it does not remain perfectly flat
as originally machined. A platen which has been ground or damaged by wear or impact
away from a required or desired flatness is no longer effective for high precision.
For example, a platen should have a deviation in flatness of less than 0.0005 inch
(0.0126 mm) at the outer periphery with a need for the best performance to reach 0.0002
inch (0.00508 mm) or less than 0.0001 inch (0.00254 mm). The platen should be flatter
than the variations in thickness of the rotating abrasive disk surface. The platens
are ground to the above tolerances (e.g., less than 0.0126 mm variation in thickness
along an entire circle within the disk surface). These measurements can be made, for
example, with a micrometer or other linear measuring device. The flatness is measured
by reading the variations in thickness along such circles within the disk surface.
The abrasive sheet (e.g., the diamond sheeting) lays relatively flat on the surface
of the platen, but is expected to have some variations in thickness of the backing
material (e.g., plastic film, such as polyester) and the abrasive coating.
However, it is desirable to minimize variations and prevent additive deviations from
occurring. This measurement can be made by a dial indicator placed at the outside
diameter and the disk rotated by hand for one revolution to measure the maximum excursion.
Any deviation acts either as a "valley" where the abrasive does not contact the piece
part or a "high spot" which is the only area that contacts the piece part. When the
disk rotates at its normal high speed, the high spot will have a tendency to hit the
piece part and set up a vibration which will reduce the smoothness of the lapping
abrasive action. Localized distortions of the platen surface will also have a tendency
to penetrate the boundary layer of liquid between the platen (covered with a thin
sheet of diamond or other coated abrasive) and the piece part. This can produce a
localized scratch or track on the piece part surface. Any surface defect on the platen
structure is generally transmitted through the thin abrasive disk and produces a bump
or high spot on the disk.
[0109] Solution: An existing platen can be "dressed" as a machine by bringing it up to full high
speed RPM and lowering a heavy flat abrasive coated piece unit directly onto the bare
rotating platen and grinding or lapping off the bumps. High spots and even full out-of-flatness
surface variations can be removed by first using a coarse abrasive and progressively
using finer abrasive or lapping abrasive medium. A typical first abrasive may comprise
40 micron metal-bonded diamond and a final abrasive may comprise 3 micron or less
diamond or ceramic abrasive depending on if the platen surface is chrome plated, stainless
or base steel. The abrasive lapper disk could be oscillated back and forth across
the platen, it could be stationary or it could rotate at either slow speed or rotate
at a very high speed so the tip speed of the grinding disk will provide uniform removal
of platen material at the low surface speed of the inner radius of the platen. Different
geometries of adhesive disks could be used. Also a piece part holder already in use
for normal lapping could be used to perform this function.
5. LAPPER PLATEN SPIRAL SURFACE
[0110] Problem: When lapping or grinding at high speeds of 3,000 rpm on a 12" (30.5 cm) diameter
platen producing perhaps 8,000 to 12,000 surface feet per minute (sfpm) of surface
lapping speed by use of wetted plastic disks coated with thin layers of diamond or
other abrasive material, it sometimes is a disadvantage to have a uniform flat disk
surface in flat contact with precision piece parts. This is because the fluid boundary
layer of the wetting liquid has a tendency to draw the piece part down to the flat
surface of the rotating platen and create large fluid adhesion forces. These fluid
adhesion forces require more force to hold piece parts in combination with bigger
motors and require the use of larger and heavier holding devices for piece parts.
This may also create a lower rate of metal removal and the further disadvantage of
the grinding debris being carried along between the abrasive disk and the work piece
surface. This can produce scratching or other disturbances on the work piece surface.
[0111] Solution: A precision ground rotating platen can be fabricated with slightly raised spiral
surfaces having different shapes and/or patterns, these shapes or patterns varying
from the inside center of the platen toward the outer periphery of the platen. The
spiral patterns would create land areas at the top surface of the platen of the various
widths, shapes with areas between these land areas that are somewhat lower, perhaps
from 0.002 inch to .010 inch ( 0.051 to 0.254 mm) or more. Then a thin plastic coated
abrasive disk that is uniformly coated with precision fine abrasive (e.g., the 3M
diamond abrasive sheet material cut into disk form) would be mounted onto the round
platen and held in place by vacuum hold-down holes either on a raised land surface
or on the lower surface area or a combination of holes in both areas. The raised land
areas could be produced by manufacturing a precision platen and acid etching or photolithographically
etching land area geometry configurations. When the abrasive disk is mounted on the
platen, only some portions of the disk would be in contact with the piece part being
ground or lapped. The boundary layer of fluid coolant would be affected by the length
of the land area under the piece part, the direction the spiral, radial or circular
annular land shapes or a combination of the geometries. The effects on the boundary
layer thickness would be the rotation speed of the platen, as related to the vector
speed, including the direction of the surface relative speed between the two, the
viscosity of the fluid, and the normal force pressure of the piece part holding it
to the platen. The boundary layer thickness, which would vary over the surface of
the piece part, would affect how the individual particles of abrasive (normally protruding
about 1/3 of their size above the binding agent) effectively abrades a workpiece from
the surface of the abrasive disk. If more liquid is applied, the boundary layer would
tend to be thicker and less abrasive material removal is achieved. Thus the local
pattern of the surface of the abrasive contact area can be utilized for the optimum
grinding action using only one portion of the abrasive disk with the non-raised section
between the land areas of the abrasive allowing free passage of grinding debris. When
this surface area of the abrasive is worn, the disk can be unmounted by the vacuum
chuck, rotated to a "fresh" area of the abrasive, and then grinding would be continued.
The disk will remain uniform and strong throughout an extended service.
6. DOUBLE DISK GRINDING
[0112] Problem: Again, the problem to be addressed is hydroplaning, which distorts positioning of
the abrasive surface and the work piece relative to each other. Especially with relatively
thin or flexible work pieces (e.g., work pieces thinner than 10 cm, especially thinner
than 5 , 2, 1, or 0.5 cm), the worst distortion of the positioning occurs because
of bending or flexing of the work piece. This is because the flexible sheet may be
supported on a relatively inflexible support platen.
[0113] Solution: Two rotating platens may be provided, one each on opposite faces of the piece part
or work piece. The work piece is secured against movement between the two abrasive
surfaces (on the two rotating platens). The two rotating platens are rotated at the
same time, in the same or opposite directions, with similar amounts of liquid applied
between each platen and the work piece. The disks do not have to be rotated at the
same speeds, and when this is done, the volume flow rate of liquids used need not
be as similar since the respective hydroplaning forces are proportional to the speed
and the volume flow rate of liquid. The relative speeds of rotation and the relative
volume flow rates of water are selected so that the hydroplaning forces are fairly
similar at the opposite outer edges of the work piece. With similar forces pushing
against opposite faces or sides of the work piece at similar radial distances, there
is no effective flexing force applied to the work piece. The increasing forces along
the radial directions of each face of the work piece will be nearly equally balanced
by similarly distributed increasing forces on the opposed side of the work piece.
The two forces thus cancel each other out and there would be no flexing from hydroplaning.
The film of liquid between the abrasive surface and the work piece would then remain
essentially the same from where it was introduced to where it exits at the periphery.
The speed and volume flow of the liquid would actually decrease from the central region
to the exterior region at any given point along a radial line.
7. VACUUM CHUCK HOLDER
[0114] Problem: It is difficult to quickly load piece parts onto a piece part holder for use with
a high speed lapping and polishing system. Also, it is difficult to generate a flat
parallel system of polishing parts where .001" to .002" (approximately 0.025 to 0.051
mm) of material is removed from a surface to make the surface smooth, perhaps with
variations of no more than 4 lightbands in smoothness, while the surface remains flat
and parallel. Hot melt adhesives are presently used to fix piece parts onto the piece
part holder. The use of these adhesives is slow and cumbersome to apply. The residue
of the adhesives are also difficult to remove, and may contaminate the precision surface
of the piece part for later use. Typically, the piece part holder has a gimbaled spherical
ball end to freely allow the part to move about radially to self align the piece parts
(one or more) with the surface of the rotating abrasive platen.
[0115] Solution: A piece part holder can be constructed out of a heavy metal such as steel which
has substantial mass very close to the surface of the abrasive disk. The piece part
holder unit will be allowed to move freely with the surface by the ball-end holder.
A substantial hole can be made within the ball-end device which would allow vacuum
to be coupled to the piece part holder. Individual part pockets will firmly hold the
flat piece parts tightly against the individual tight fitting part pockets to create
and maintain a good vacuum. A thin layer of oil or grease can be applied to the piece
part to seal any leakage paths. By simply removing the vacuum applied by a rotary
union to the drive shaft open inside diameter, the part is released, it may then be
turned over. The opposite side may then be lapped to produce a high quality surface
which does not damage the already lapped side because intimate part-to-holder contact
is not made, the parts being separated by the film of oil. The part pocket is still
stiff enough for good polishing action.
8. ABRASIVE DISK WITH AN ANNULAR SHAPE
[0116] Problem: When using a diamond (or other fine and hard abrasive material) abrasive disk rotating
at very high surface speeds of 10,000 sfpm, most of the abrasive cutting action takes
place at the outer periphery of the disk. The inside area of the disk has low surface
velocity and low cutting action and also low wear rates. When a piece part traverses
the disk in a sweeping motion, to prevent wearing of tracks or grooves in the abrasive,
there is uneven wear at the outer and inner surfaces of the disk. There is typically
a small ¼, ½, or 5/8" (0.626, 1.27, or 1.58 cm) diameter hole at the inside of the
disk. The hole is usually centered to act as a positioning means to fix the abrasive
disk at the center of the platen to obtain good balance for the very high speed system.
A larger diameter round section could be removed from a disk to create an annular
ring of acting abrasive material somewhat larger than the piece part. This would eliminate
the inactive (and raised) uneven section but then the centering registration hole
for positioning the disk is lost.
[0117] Solution: A disk can be fabricated with abrasive coated or exposed on the entire surface of
the disk. The inside section of the abrasive disk, toward the center of the disk,
could be removed by grinding or peeling off the abrasive, leaving the backing material
intact with a raised section of the abrasive in an annular outer ring. The raised
area is only where the abrasive is raised above the surface of the carrier (by the
coating thickness). The disk backing material is usually plastic sheet, which may
be reinforced. Another way to construct an annular ring would be to punch out a center
disk section (e.g., a disk of 2 to 6 inches, 5.1 to 15.3 cm) of the disk for separate
use and then use a centering plug (e.g., a 5.1 to 15.3 cm thinner disk) with a small
locating hole. The plug could be centered on a platen center post and the annular
disk centered on the plug. When the disk or annular ring plus disk is fixed into place
by the vacuum grip platen, the plug is or may be removed to enable complete freedom
of movement of piece parts over an annular disk. This complete movement can be effected
since the centering post may also be removed after the annular disk has been positioned
and secured by the vacuum.
[0118] The process of using an annular disk element can be effected where the round sheet
has an outer edge and an inner edge defining a cut-out portion and comprises an annular
sheet, said inner edge having a diameter which is greater than one-third the diameter
of said outer edge. The process may also be performed where said sheet is round and
said round sheet has an outer edge and an inner edge defining a cut-out portion and
comprises an annular sheet, said inner edge having a diameter which is greater than
one-third the diameter of said outer edge.
9. VACUUM ADHESIVE HOLD-DOWN
[0119] Problem: When lapping or polishing at very high surface speed of about 10,000 surface feet
per minute, it is difficult to mount piece parts onto a rotating holder. The piece
part holders are used for contacting an abrasive disk mounted or constructed on a
rotating platen. The parts must be held in a sufficiently rigid manner that they are
not broken loose from their mount. It is also desirable to avoid a localized vibration
of the typically thin flat piece part (which vibration is induced by the high speed
contact with the rotating platen). Vibrations can cause patterns of uneven polishing
on the surface of the precision part. It is desirable for efficiency that one or more
piece parts are processed at the same time and that both mounting and unloading of
these parts can be done quickly and easily to provide cost effective polishing rates
of production. Furthermore, it is desirable to have a method of changing parts quickly
so that one side be lapped, that part turned over and the second flat side be lapped
to be very parallel to the first side. This must be done when typically .001" to .002"
or less is removed from each side.
Solution:
[0120] Thin piece parts of about 1"x 2"x .080" (2.54 x 5.08 x 0.23 cm) can be mounted onto
an individual piece of pressure sensitive adhesive (PSA) tape and this taped piece
part can then be held by a vacuum to a workpiece holder. The friction properties of
the non-adhesive side of the tape would be controlled by selection of tape backing
material or by surface conditioning of the backside of the tape to provide a sufficiently
high degree of friction which would resist lateral dynamic forces in a plane along
the surface of the thin workpiece as the nominal 14 pounds per square inch (psi's,
25 inches Hg vacuum, 6635 mm Hg) would apply a normal force holding the work piece.
A large section of adhesive tape could be used to hold a number of workpieces at the
same time. This would allow fast and easy installation of the workpieces by hand or
robot. This flexible assembly of pressure sensitive adhesive (PS) secured workpieces
could than be held in position against a precision flat surface of a workpiece holder
having random vacuum holes over its surface which would all be sealed by the wide
and complete expanse of tape covering the vacuum holes and at the same time firmly
holding the individual workpieces to the holder. To process the other side, the group
of workpieces would be removed, new tape would be applied to the lapped surface side,
and the tape on the unprocessed side would be easily peeled off. The tape would not
only fix the parts to the holder surface, but also would protect the precision lapped
side from any scuffing action or rubbing on the holder.
10. SPRING-CENTERED WORKPIECE HOLDER - Coiled Vacuum Hose
[0121] Problem: When holding piece parts on a rotating holder in contact with a rotating abrasive
coated platen rotating at a surface speed of 10,000 feet per minute, it is difficult
to create a gimbaled, free wobble motion which allows the contacting surface to be
continuously aligned by itself to the flatness of the rotating platen, while at the
same time the contacting surface of the piece part is held stiffly enough in a nominally
flat position. This is particularly true when first lowering the workpiece holder
to the abrasive surface while rotating the workpiece so as not to have one corner
of a workpiece contact the abrasive before other corners or surfaces. This would cause
the corner to be preferentially abraded away, thereby producing an uneven workpiece
surface. Vacuum piece part clamping hoses could also create problem forces.
Solution:
[0122] A coiled spring can be used to apply a self correcting force between the work piece
holder plate having a gimbaled spherical bearing and the rotating drive shaft of the
rotating piece part holder. This spring could be made of metal or plastic material
which would allow the straightening action to be applied but also would introduce
vibration damping for excitation vibrations set up by the high speed, contact abrasive
action. One or more solid plastic coupling bars could provide damped spring action.
Also, if a vacuum hose were to be used to provide vacuum clamping of the piece part
to the piece part holder through a hollow drive shaft, this type of hose could extend
from the shaft and be coiled to provide a spring support action (with perhaps less
than one complete turn, one complete turn or multiple turns which nominally lay flat
with the upper surface of the work piece holder, which would minimize the creation
of uneven "normal" turns).
11. ANGLED OR BEVELED SURFACE ABRASION
[0123] Problem: Many of the problems herein discussed for lapping with the flat surface of a platen
are also encountered with beveled edge lapping, where the edges of a platen are beveled,
and abrasive is on the face of the bevel. That abrasive face is then used to lap or
grind another surface.
[0124] Solution: There are two fundamental ways of addressing this issue. Both involve the use of
an annular abrasive sheet. The sheet has an outer edge and an inner edge (defining
the inner edge of the cut-out portion of the sheet, where it is cut-out from a circular
sheet, forming a central, round hole). The annular sheet should be placed on a platen,
which is either a) flat, with the outer periphery bent, or beveled, b) or the inner
annular section beveled, or both the inner and outer edge being beveled. The outer
edge should not extend significantly beyond the outer edge of the bevel or platen
(e.g., less than 1 mm, more preferably less than 0.5 mm, still more preferably less
than 0.1 mm). The inner edge should in likewise dimensions likewise not extend beyond
the interior edge of the bevel or the bend. If the annular disk is positioned on a
flat platen, the flat platen may be bent substantially (with the same or like dimension
tolerances) at the interior edge of the annular disk to form the lapping abrasive
edge on the platen. The only caution which must be exercised is to assure that no
folds or wrinkles appear in the annular disk. A preformed annular disk may be shaped
to fit on the angled or beveled element. The element may be molded or formed to fit
the shape of the platen surface (for example, by having a truncated conical sheet
segment with the inner, smaller diameter hole (formed by cutting the cone) fitting
the slope of the beveled edge, with the abrasive on the interior, upward facing surface
of the cone (within the original cone volume as opposed to being on the external surface
of the cone. The annular disk may be secured by adhesive, but the vacuum securement
of the present invention is preferred.
12. ABRASIVE LAPPER
[0125] Problem: Operation of the high speed lapping devices envisioned by the present invention
are at revolutionary or rotational speeds of at least 500 rpm, or at least 1,500 rpm,
and preferably at 2,000 to 3,000 RPM with a fine abrasive sheet, such as the preferred
3M diamond coated abrasive disk of about 12" (30.5 cm) diameter. These sheets are
normally held to a steel rotating platen by water film surface tension and positioned
by a ½" (1.27 cm) diameter hole at the center of the disks. These positioning holes
were used with a ½" (1.27 cm) diameter post at the center of the platen. When such
a rotational speed of operation was attempted with the disk secured by water film
tension, the disk lost its surface tension adhesion and was thrown off the platen
while polishing a tungsten carbide piece part. The forces on the disk were such as
to lift it off the ½" (1.27 cm) centering post and the whole disk was thrown off to
the side of the machine opening cavity at the top of the machine post.
[0126] Solution The ½" (1.27 cm) centering post could be made larger in diameter to perhaps 1" (2.54
cm) diameter or more. Also, the post could have a hexagonal shape or an oval shape
which would prevent the disk from rotating relative to the tangential surface of the
disk by having the apices of the hexagons (or other polygon) resist rotation against
a similar cut hole in the sheet or disk. The post could also be made higher so the
chance of the self-destructing disk climbing up the height of the post would be diminished
during this type of event. Another technique would be to employ a clamp type of device
to any of these round or non-round posts to clamp/hold the disk firmly to the surface
of the platen at the center areas of the disk which is not used for polishing. This
clamping force would be effective because of the slow lineal velocity in that sector.
The clamp could consist of a spring locked washer pressed on the disk surface with
a thread nut engaged with a top threaded post. Springs could also be used to control
the amount of force and to evenly spread the force uniformly. Ball insert or other
snap latch fixing devices could also be employed.
13. ABRASIVE LAPPER
[0127] Problem: Using round disks of minute particle coated sheets (e.g., abrasive particle sheets
and especially hard abrasive particles such as diamonds) of plastic film on 1,500,
2,000 or even 3,000 RPM spinning platens provides significant difficulties. It is
particularly difficult to hold the abrasive sheet in contact with the platen when
the lapping apparatus is operating in contact with stationary or semi-stationary workpieces.
When an abrasive disk becomes loose by breaking the conventional water filter "adhesive"
surface tension between the disk and the platen, the abrasive sheet has a tendency
to rip or bunch-up and wedge between the workpiece holder and the high inertia spinning
platen and can easily damage a workpiece part or can destroy portions of the workpiece
assembly with the possibility of great danger to the operator. This is a unique problem
due to the very high rotational speeds of 1,500, 3,000 or even greater RPM with a
platen of 15"(38.1 cm) diameter or more constructed of heavy steel which could generate
explosive type failures or at least high velocity projectile failure. As this equipment
is operated horizontally for the most part, the whole surrounding area around the
machine is susceptible to this danger. A previous attempt by applicants to reduce
the likelihood of this type of separation problem was to coat one side of the diamond
abrasive disk with a PSA, pressure sensitive adhesive film to temporarily bond the
disk to the platen. This adhesive created a flatness accuracy problem in that its
normal thickness accuracy varied greatly around the disk which causes high areas of
lapping contact for this super precision abrasive contact. Secondly, when a disk was
removed, some sectors or pieces of transparent PSA adhesive remained in the platen
and formed a bump when the next abrasive disk was installed on the platen. This then
destroyed the smooth vibration free abrasive lappings at high speeds.
[0128] Solution: Use a diamond or other abrasive disks without using PSA adhesive and first position
the disk at the true center of the platen by use of a center hole in the disk positioned
over a post positioned at the center of the platen (or by other centering means) and
then by holding the abrasive disk to the platen by use of vacuum by use of a rotating
union on the hollow rotating platen shaft. The preferred area to apply the vacuum
would be at the inner radius of the disk which would seal out air first as the disk
is installed at the platen center. Because this inner one-fourth or so of radius is
not used as much for lapping because of the slow surface lapping velocity, there would
be less direct forces applied at this portion of the disk. The second most preferred
vacuum area (e.g., the outermost edge region of the disk) would also not be used much
and would have large holding force.
14. SUPER HIGH SPEED LAPPER
[0129] Problem: It is difficult to quickly lap hand metal or ceramic or other materials with conventional
lapping techniques using disk platens which are 12" (30.5 cm) to 43" (109 cm) in diameter
operating at 200 to 300 RPM using loose abrasive paste media. The amount of time used
contributes to cost and time delays. Larger diameter platens are potentially dangerous
at high speeds and paste could be used in extremely large amounts as it would be difficult
to retain on the platen surface.
[0130] Solution: A high speed lapping system can be a sheet of abrasive material such as fixed diamond
abrasive coated or plated on a disk sheet of material. These sheets or disks may be
used on a rotating platen disk with a diameter of, for example, 12" (30.5 cm). When
operating at 500, 1,500, 2,000 or 3,000 RPM, the apparatus gives a surface speed of
about 9,000 to 20,000 feet per minute. If a larger diameter platen wheel of 15" inches
is used, the RPM can be lowered somewhat to perhaps 2,500 RPM to achieve the same
10,000 (or 9,000) feet per minute (fpm). Similarly, if the wheel diameter of the platen
is 18" diameter, then the speed can be further reduced to produce 9,000-10,000 fpm
at the outer periphery of the disk. Any reduction of angular or rotational speed created
by larger diameters is desirable because of the particular danger of a high inertia
wheel creating problems if a disk or part is damaged or comes loose. The higher speeds
used in the practice of the present invention, plus the controls shown for maintaining
accurate address between the abrasive surface and the workpiece allows for much faster
and therefore more economic lapping. Work that previously took hours, including intermediate
cleanup steps, can be performed in minutes using the apparatus and methods of the
present invention.
15. WATER FLOW RATE
[0131] Problem: The surface finish smoothness and flatness of hard parts made of metal or ceramic
or other materials vary as a function of the work force on the piece part as the workpiece
is held against the surface of a high speed 9,000 to 10,000 fpm abrasive lapping action.
Unexplained variations in the quality and accuracy of the lapping action were observed.
[0132] Solution: It was found that the amount of coolant, lubricating water or liquid applied to
the surface of the high speed rotating disk affects the quality of the lapping action.
If a reduced flow rate of water is applied, the abrasive cutting rate is increased
as the relative dimensions of the boundary layer and the total liquid thickness and
dimensions between the base of the abrasive disk and the piece part are increased.
This increase in the relative dimensions of the boundary layer and the decreasing
of the separation of the abrasive disk and the piece part by the liquid allows the
exposed diamond particles to be more active in removing material as they penetrate
deeper into the surface of the material. Also, if the water flow rate is reduced and
the piece part is more "flooded", then a thicker boundary layer of water or liquid
builds up between the part and the surface of the disk and the piece part. This keeps
the (e.g., diamond) abrasive particles away from the piece part and allows some fraction
of their normal penetration which results in a smoother and flatter surface on the
part. One method of utilizing this performance is to have reduced water flow at the
first portion of the lapping period for more aggressive material removal with an increased
roughness of the surface. Subsequently the water flow is increased somewhat during
the middle portion of the abrasive cycle to get better surface finish and yet have
a medium material removal rate and then to substantially increase the water flow rate
at the end of the cycle to produce a very smooth and flat surface with a low rate
of material removal. This could be easily done with an automatic water flow rate control
system. This would change the water flow rate automatically at various stages in the
abrasive cycle.
[0133] The liquid (especially water) introduced as a lubricant between the platen and the
work piece is normally filtered to eliminate particles which are 1 micron or larger
in their largest dimension. The use of a positive displacement pump such as a gear
pump or piston pump can be helpful in determining the optimum quantities of flow and
charge during operation of the system, at the beginning, middle and end of operation
of the lapping cycle.
16. SAFETY BOX FOR PLATEN
[0134] Problem: When performing abrasive lapping at high surface speeds of over 1000 fpm up to about
10,000 fpm on round platens rotating at 3,000 RPM with diameters of 12", 15" and 18",
there is substantial danger when a piece part is broken off its holder (as it normally
is held with a weaker adhesive or mounting system, and as uniquely effected in the
present invention with the use of abrasive sheeting and high speed platen rotation)
and the piece part being thrown off the platen or getting stuck on the platen and
ripping the diamond or other abrasive disk causing further possibility of fast destruction
of parts of the machine with parts thrown out and endangering an operator or others
or equipment due to large kinetic energy contained in the rotating disk.
[0135] Solution: The rotating platen is round in shape with about a 12" or 15" (30.5 cm to 43.5 cm)
diameter. A box is constructed which is rectangular in shape with "square" corners
(4 each) and with the walls some distance away from the round platen, typically 6"
or more. Also the box is desirable to be constructed of a soft plastic (or rubber)
such as ½" thick high density polyethylene which would tend to absorb impact from
a heavy metal part free flying, broken loose parts without ricocheting the part back
into contact with the rotating disk which would reinitiate this impact action. It
also prevents this reinitiated contact from damaging the part. Also, the "square"
corners provide a remote area to trap the part and to contain the part as it stopped
moving by being impacted in one or more rubber or plastic walls or lined metal walls.
Having a distance between the flat walls and the rotating disk which is somewhat larger
than the largest size of the piece part, centrifugal force would tend to drive the
part off the disk radially and allowing it to roll or move tangentially to a neutral
corner of the box away from the disk. At the same way, crumpled abrasive disks are
collected by the neutral open corners. Having a ledge over the inside portion of the
box also helps trap the parts.
[0136] The use of a safety box with at least 10% (of the diameter of the platen) clearance
on each side of the platen within the safety box area is quite effective. It is more
preferred to have the safety box with a clearance of 20%, 30% or even more than 50%
of the diameter of the platen (on each side of the platen within the box or at least
from at least one side of the platen) in the practice of this aspect of the invention.
It is particularly desirable to have the workpiece holder moving assembly lift the
workpiece holder out of the safety box so that the box may be cleaned without contacting
the platen. A removable bottom section may be constructed on the box for bottom cleaning
without having to significantly move the platen, but any openings or movable pieces
may add to vibration potential in the system and is therefore not the most desirable
engineering approach to the construction of the safety box.
[0137] The box may have a high center section and be angled or curved in the outer section
so that any loose parts or pieces would tend to drop below the rotating platen and
not be picked up by the platen and projected back toward the opening in an area above
the abrasive surface of the platen (e.g., towards the operator). As liquids are used
in the lapping action, a tapered bottom of the safety box area toward one or more
drain holes allows the expended liquid (and any carried particulates) to be easily
collected for disposal, even without opening of the safety box area. The angle of
the box bottom to obtain the best flow conditions for the liquid will be selected
to provide a washing action on the surface to minimize buildup of ground particles
on the surface of the bottom of the safety box. Grooves to concentrate water flow
or passage may also be provided.
[0138] A temporary cover may be provided over the opening of the platen top access hole
to provide additional safety to the operator from projectiles and also to contain
any mist formed by the high speed shearing and projection of liquids. Duct work can
also be installed in the box to withdraw air born vapor and particles as well as the
liquids, with reduced pressure removing the undesirable materials at a controlled
rate. Filter elements may also be associated with these removal systems.
17. COUNTERWEIGHT WORKPIECE HOLDER
[0139] Problem: When a heavy workpiece holder is held up by an air cylinder and controlled to provide
normal force on a workpiece against a high speed 10,000 fpm rotating disk by moving
vertically up and down to load parts and lap. Then there is potential great danger
if air pressure is lost due to air line leaks or electrical failure. If this load
of the disk rotating motor assembly which may weigh 60 lbs. (27.2 kg), drops on the
12" (30.5 cm) heavy rotating disk operating at 3,000 RPM, there is great danger in
that the abrasive disk can be torn or cut, jam up and create danger to the operator
or severely damage piece parts which may have great value.
[0140] Solution: The vertically moving piece part assembly can be mounted on vertical slide and a
chain or cable used with a counterweight which is perhaps 10 lbs. (4.54 kg) heavier
than the 60 lb. (13.6 kg) assembly. Upon loss of electrical power which would interrupt
power to the normally used suspension air cylinder or a line leak to the cylinder,
the piece part assembly would simply and quickly retract to the upper position, taking
it out of contact with the rotating platen and thereby reducing the chance of danger.
This could also be a more assured event by using an e-stop (emergency-stop) action
switch which would not require power to obtain safe action.
18. SECUREMENT OF WORKPIECES TO A SUPPORT
[0141] Problem When lapping parts, it is typically quite difficult to hold the lapped parts in a
fixture so that they are flat, stable and parallel when presented to, in contact with,
and when removed from the lapping platen wheel particularly when the platen is rotating
at high speeds of 3,000 rpm as compared to 200 rpm. Also a part which is fixed by
mechanism clamping is subject to be loose or compliant (soft), which results in ground
surface patterns or a lack of highly accurate surface finish such as (4) four light
bands is not attained. It is also difficult to quickly and accurately load and unload
parts. Also, for parts to be polished on both sides of the parts, the already polished
surface finish adjacent to the part holder side of the mounting may be disrupted or
destroyed when lapping the other side of the part.
[0142] Solution: Functional mechanical parts, which are typically 1 to 2 inches (2.54 to 5.08 cm)
in diameter (or shaped other than circular cross-section, such as rectangular) which
may be thin (.010inch, 0.254 mm) or thick (.500 inch, 12.7 mm) can be affixed to a
precision flat steel, other metal or other material plate by use of paraffin wax as
a bonding agent. Here the plate or part can be coated with wax or the wax simply melted
on the plate between the part and plate and the part placed on the plate, heat applied,
and the two pieces would have a fully wetted surface of molten wax. The parts could
be positioned by mechanical or other means of uniform pressure or force so that they
lay flat with a uniform and controlled thickness of molten wax. Upon cooling the part/plate
assembly, the parts would be positioned accurately and firmly for the plate ready
for lapping action. Then the plate could be attached to a piece part holding device
by use of a vacuum chuck or by use of a magnetic chuck if the plate were, for example,
steel. The piece part holder could have a ball type pivot close to the lapping action
surface. Plates could hold one or many individual parts. Upon lapping one side, the
plate/part assembly could be heated , the parts removed and, if desired, the parts
could be reassembled with heated wax on a plate with precise parallel alignment with
no danger of damage to the lapped surface because of separation from the plate with
no wax. And this way many plates could be preassembled for high production rates with
a single lapper.
19. OSCILLATING WORKPIECE LINKING SYSTEM
[0143] Problem: It is desirable to have a simple drive mechanism to position a stationary
or rotating workpiece on the outer periphery of a high speed rotating (3000 rpm) abrasive
disk so that for most of the processing time there is a small portion of the polishing
or lapping time spent at the inner radius portion of the abrasive disk where the surface
speed is reduced and the abrasive action is reduced.
[0144] Solution: A simple, eccentric harmonic motion, constant speed rotation can be provided by
a DC or AC gear motor hub used to drive a linkage system. This system will provide
a smooth continuous motion at a workpiece with most of the time in a given hub rotation
cycle being spent with the workpiece operating at the outer periphery of the abrasive
disk which has the highest surface speed and also grinding action. Only a very small
portion of the cycle time would be spent at the inner radius having a low surface
speed and reduced grinding action portion of the disk.
20. SUPPORT OF SMALL WORKPIECES
[0145] Problem: It is difficult to hold small hard parts which are thin (typical size: 1"
x 1" x 1/8", 2.54 x 2.54 x 0.318 cm) in such a fashion that surfaces (usually two)
with flat features can be polished with a lapping action by a high speed (e.g., as
high as 3000 rpm) rotating disk with a preferably diamond abrasive disk exerting substantial
lateral force by the moving platen powered by a (e.g., 2 HP) motor for a 12" (30.5
cm) diameter disk when subjected to about 10 (4.55 kg) pounds of normal clamping force
when subjected to surface water spray. This lateral force can separate the part from
the part holder.
[0146] Solution: These small parts can be affixed to a flat surfaced piece part holder or a holder
which has small shallow pocket areas just larger than the length and width of the
flat part so that an exposed surface of the part protrudes away from the holder. This
will allow the abrasive disk polishing action lateral force to be applied to the piece
part and not separate the piecepart from the holder, as it is trapped in the pocket
or is held rigidly in the part holder. A medium temperature wax, or other easily removable
adherent material can be melted and used to bond a rough surfaced part to the flat
smooth surfaced part holder plate. The flat plate in turn can be attached to a rotating
pivoting arm which is swept across a portion of the surface of the high speed rotating
disk until a smooth flat polished lapped surface is generated on one side of the piece
part. Then the part holder plate which would have 1 or 2 or many more parts attached
to it in a fixed mounting pattern could be brought into contact with another mounting
plate having a flat surface or a shallow pocketed surface pattern which matches the
first part plate. A higher temperature wax (higher temperature than the first wax)
could be melted at the surface of the parts already lapped and as they were held in
flat contact with the new plate, the original lower melting point wax would melt and
release the parts from the first plate. The parts would be transferred as a group
to the second plate ready to have the rough remaining side lapped as the first plate
is readily removed from this group of parts. High production rates at lapping flat
parts on both sides with good parallelism could be achieved.
21. BOUNDARY LAYER CONTROL
[0147] Problem: When high speed lapping a 3000 rpm rotating flat platen with fixed abrasives attached
to the platen with adhesives or vacuum, water on the rotating platen abrasive surface
forms a boundary layer between the work piece and the abrasive media. The boundary
layer thickness and shape effect the flatness of the work piece. The work piece must
be allowed to "float" on the abrasive surface to achieve total flat contact even with
this water boundary layer. This is done with a gimbal mechanism which puts pressure
down on the rotating workpiece. It also allows the work piece to "gimbal" in the horizontal
plane while an independent driver pin drives the work piece around the center line
of the work holder shaft. The amount of down pressure also effects the boundary layer.
The work piece floating on the boundary layer of water allows the abrasive media and
the platen imperfections to be averaged out- high spots on the abrasive do the lapping
while the low spots are filled with water allowing the lapping action to take place
and produce a finished part (work piece) that is flatter than the media and platen.
The work piece will only be as flat as the boundary layer. The problem is how to control
or minimize the boundary layer thickness and control the shape on a work piece with
a small surface area that is not large enough to float on the boundary layer with
a minimum amount of down pressure, yet have enough water thickness for lubrication
and cooling.
[0148] Solution: Pump water (e.g., through the work holder) into controlled orifices or jets in strategic
locations that would encourage a controlled boundary layer to form between the work
piece and the abrasive media. The water would also stabilize the workpiece while presenting
it to the rotating platen initially and while lifting the work piece off after lapping
is complete. Water is injected or otherwise directed to an inside radial area of a
piece part holder which is holding a number of discrete piece parts at the same time.
This could be particularly helpful when an annular distribution of abrasive is used.
In this aspect of the invention, the inside portion of the water would develop a second
boundary layer under the trailing portion of the piece part holder which contains
a second piece part in contact with the narrow annular band of abrasive. Boundary
layer water entering under the leading edge of the holder would tend to lift up that
first piece part and tend to tilt the second piece part downward. This would cause
a ground cone shape to form on the piece part. A second boundary layer would also
develop under the second piece part at the trailing site of the holder and lift it
upward, which would compensate for the tilting of the first piece part. Collectively,
the whole piecepart assembly would tend to lay flat as it would be supported by both
boundary layers at the same time. There would be little tilting of the piece part
toward or away from the platen rotational center as the parts are in contact with
the (e.g., narrow) annular band of abrasive which would only effect a narrow strip
of grinding action. That is, the introduction of liquid between the piece parts (along
an arc [having the center of the platen as the center of the arc] connecting both
piece parts which are in contact with the annular abrasive areas), reduces any tilting
action which might normally occur because hydroplaning or boundary layer effects from
a liquid are introduced at the relative center of the abrasive sheet only.
22. BOUNDARY LAYER PROBLEMS WITH SMALL PIECE PARTS
[0149] Problem: When lapping or grinding a multiple number of small parts or single small parts
each having small surface areas and short surface dimensions in the approximate size
of 0.1 inch (2.54 mm) by 0.1 inch (2.54 mm) and these parts are positioned in contact
with a high speed rotating disk operating at 3000 rpm at perhaps 9000 sfpm speed,
there is not enough surface length to the part to build up a sufficient boundary layer
to float or support the part as it is making contact with the abrasive disk on the
high speed platen. The parts tend to dig into the abrasive disk and tear the disk
and prevent accurate polishing or lapping of the part.
[0150] Solution: Providing a system where an adequate boundary layer can be generated and maintained
while the individual piece parts are being lapped can easily be done by adding a secondary
device to the piece part holder device which would have sufficient surface area, and
dimensional length to develop a desirable boundary layer. The secondary device is
also ground down simultaneously with the piece parts in a sacrificial way. A typical
shape of this sacrificial contact device can be a disk of metal such as brass which
would be mounted on the inside annular position of a tool piece holder with the to-be-lapped
piece parts mounted inboard or outboard of this device on the periphery of a round
piece part holder. As the total exposed surface area is ground down, the piece parts
are held suspended above the high speed moving abrasive by the large surface area
of the sacrificial disk. A typical disk would be 4 inches (10.2 cm) outside diameter,
2 inches (5.08 cm) inside diameter and about 0.60 inches (1.52 cm) Thick. It could
be easily attached with vacuum chucking and/or adhesive tape and could be used over
and over by loading new piece parts with a partially ground disk. Other geometry sacrificial
plates could be used and combinations of materials including other metals such as
steel or ceramics.
23: CONTINUOUS SHEET WITH ANNULAR DISTRIBUTION OF ABRASIVE
[0151] Problem: The annular sheet provides significant advantages to the performance of many aspects
of the present invention, but as with advance, other issues may develop in performance.
Where annular sheets or disks are cut from sheets and applied to a flat face of a
platen, particulate grit and abraded material and/or liquid lubricant can work its
way under the inside edge of the annular section. Even in the small time periods when
the sheet is in use, which may be as short as ten to fifteen seconds, some particles
may lift an edge of the sheet and cause problems with the uniformity of the flatness
of the annular sheet. This would cause undesirable effects on the lapping process
and quality. Additionally, at extremely high speeds, the annular section becomes wobbly,
does not sit properly on the platen, may be difficult to lay down accurately, and
provide other structural difficulties in securing the annular sheet to the platen.
[0152] Solution: There are a number of ways in which a continuous sheet of abrasive material
may be provided, including a flat sheet having an annular distribution of abrasive
material and a continuous middle section without abrasive thereon. The most expensive
way of providing such a sheet would be to coat the abrasive out in an annular distribution,
as by roller coating, gravure coating or screen coating of the abrasive and binder.
An adhesive binder may be printed onto the backing and the surface dusted with the
abrasive grit to form an annular distribution on a continuous sheet. This type of
process would again require a new coating step rather than providing a means for using
existing sheet material. Another less preferred method of providing an annular distribution
of abrasive with a continuous sheet between the inner diameter of the annular distribution
would be to cut a circular element out of the abrasive sheet material and then abrade
away an interior section of only the abrasive particles (leaving the backing material)
to create an annular element. This would be a waste of significant amounts of abrasive
surface area, but would provide a useful annular sheet on a continuous backing.
[0153] The most preferred method according to the present invention is to cut out an annular
ring of material of the dimensions that are desired and then fixing or securing a
non-abrasive sheet material (hereinafter referred to as the center portion) within
the cut-out portion of the annulus. In providing such a construction, the following
concepts should be kept in mind. The joint between the annular sheet portion and the
center portion should not extend above the average height of the abrasive particles
with respect to the backing material. This can be done in a number of ways. A thinner
sheet material than the backing material may be used for the center portion. This
center portion does not have to provide any significant structural component to the
annular ring, but it can provide advantages as noted later if the center portion is
relatively stiff and strong (even stiffer and stronger than the annular sheet material
section). The presence of such material, stiffened or not, does tend to make the ring
easier to work with, avoids wrinkling, and makes the abrasive sheet easier to lay
down on the annular work zone. The center portion clearly provides a stabilizing influence
on the sheet as it is being applied to the platen. The material for the center portion
may be chosen from a wide range of materials because of the minimum physical and/or
chemical requirements for the material. Plastic film or paper is the easiest materials
to provide for the center portion. There may be a centering hole in the middle of
the center portion, or even a larger hole than is needed for centering. The larger
hole adds no significant structural advantage, and should not minimize the stabilizing
or edge protecting effect of the center portion, but some latitude is available in
the dimensions of the center portion with respect to the entire size of the annulus
without preventing some of the benefits of the present invention.
[0154] The center portion may be secured to the annular ring by any process which adheres
the center portion to the annular portion. This would include, but not be limited
to, butt welding, fusion of the sheet material to the annular segment, adhesive stripe
between the annulus and the center portion, thermal welding, ultrasonic welding, hot
melt adhesive, etc. The application of an adhesive may be the most likely to cause
raised areas which could be avoided, but existing process technology makes controls
over the dimensions of the adhesive very effective. Additionally, since the adhesive
would be much softer than the abrasive material, some sacrificial abrading on the
inner edge of the annulus could be performed to lower any edges. Therefore, some conditioning
grinding or lapping at the inner edge of the annulus could be performed before the
abrasive sheet is used for its primary effort at lapping.
[0155] Another method for forming such a sheet would be to cut out an annular ring of abrasive
sheet and lay it over another plastic circular sheet having an outside diameter approximating
that of the annular cut-out (it may be somewhat smaller or larger). This sandwich
could be joined together by any method which would maintain a consistent thickness
to the abrasive sheet. since the highest quality coating methods could be used in
joining these layers (the circular and annular disk), even adhesive securement is
useful, where because of process limitations in the application of adhesive to the
platen to secure the abrasive sheet, adhesive securement would not be desirable between
the abrasive sheet and the platen. Securement might also be made between the annular
ring of abrasive and a backing sheet by thermal welding, ultrasonic welding, or any
other method, particularly those which seal the entire circumference of the joining
line between the annular sheet and the backing sheet to prevent liquid and particles
from entering the seam. A poor seam closure would allow edges to lift or pull and
would be undesirable.
[0156] An annular disk provided with a natural raised outside area of abrasive could be
easily used on a flat platen surface. Other structures of abrasive sheets with attached
central areas, where the sheet has a height of the central area and the abrasive area
relatively equally may need a platen with a raised annular area on the outside of
the platen to take the greatest advantage of the annular configuration. It is to be
noted that if the central area were minimally abrasive or minimally hard (or a later
described, completely free of abrasive), contact between the central area and the
piece part during lapping would have negligible or even beneficial (buffing) effects
and the sheet could be used on a flat platen.
[0157] The annular band or sheet with an annular distribution of adhesive may be secured
to the platen by a number of different means. Positioning of vacuum holes or ports
or vents in the platen can be effectively arranged. For example, vacuum holes may
be located exclusively inboard of the annular band to assure that no imprint of the
hole is transmitted across the abrasive sheet to the abrasive surface. With the use
of appropriately sized holes, this potential effect has not occurred, but this positioning
of the holes allows for such a distribution of relatively larger holes or vents if
desired. Rows of holes directed relatively radially through the underside of the sheet
from the radial portion into or towards the center area may be used. Concentric circles
of vents or ports may be located, some or all in the center area or under the abrasive
annular distribution. Pressure sensitive adhesive may be used in limited areas, such
as in the center area only, where there would be no possibility of adverse affects
on the consistent level of the abrasive or buildup effects. The adhesive could be
used alone or in combination with vacuum retention in that area or with the vacuum
in areas not secured by adhesive. Pressure sensitive adhesive could be located outside
the annular area of the abrasive, and thereby not affect the level or evenness of
the abrasive surface. It is possible to have some adhesive under the annular ring
of abrasive, but this would, of course, detract from the evenness and ease of replacing
the sheets.
[0158] High friction, rough surfaces may be provided on the platen to assist in the draw
down of the abrasive sheet. When an entire disk (rather than just an annular ring
with no center portion), the vacuum holes or vents are sealed by the disk, particularly
at the inboard portion of the sheet. It is therefore important that all holes underneath
the sheet be in vacuum tight relationship with the sheet to prevent debris from entering
the holes, clogging them, and providing deformities on the surface of the sheet. The
debris can also grind away portions of the holes or vents, later disturbing the disk
surface. The pattern and distribution of the holes can therefore be important. The
best distribution to date appears to be with a completely continuous sheet (not even
a centering hole) and concentric circles of holes predominating in the center area
and minimized (or even absent) from the annular abrasive distribution area. A problem
with the use of a centering post is related to this phenomenon, in that debris may
enter underneath the sheet around the centering post and gradually cause adverse changes
in the holes or platen surface. Also liquid flow variations and different volumes
and sizes of particulates may be flung outwardly, underneath the sheet, if such materials
enter the space between the platen and the sheet through access around the centering
post.
24. VIBRATION DAMPING IN THE LAPPING APPARATUS
[0159] Problem: The motor driving the platens and/or work piece holders (if they move) apply vibration
to the entire lapping system. The rotation of the platen itself provides vibration,
as does the movement of the abrasive over the face of the work piece. The flow of
liquid over the lapping contact zone (between the platen and the work piece), especially
where there is any hydroplaning or uneven distribution of the liquid over a moving
surface, also creates pressures and forces which can add vibration into the lapping
system. These vibrations in the system can cause minor instantaneous variations in
the relative positions of the platen and the work piece. These variations, of course,
show up in reduced lapping quality in the product and are undesirable.
[0160] Solution: The weight of the frame an the individual elements (the platen and any moving or
stationary work piece holder must be designed to minimize vibration. The joints between
elements and attachments of moving parts must also be controlled to minimize vibration.
The primary method of reducing or damping vibration is to add mass to the frame and
to strategic portions of the apparatus. The frame of the system should weigh a minimum
of 100 kg. Also, an energy-absorbing member or layer (e.g., a viscoelastic layer)
may be present between concentric tubular structural beam members and between flat
plates where a first of the two flat plates is merely a flat mass unit which tends
to remain stationary in space while the second plate integral to the frame has vibration
excitation induced in it. The thin elastomer layer mutually bonded to both plates
and is sheared across the thickness and, due to its very high viscosity, will absorb
the vibration energy and dissipate it into heat. All of the vibration damping systems
would be designed for a specific portion of the machine, especially with respect to
localized natural frequency, its expected amplitude multiplication (which can easily
exceed fifteen times the oscillation excursion of the excitation source), the design
and characteristics of the vibration damping/absorbing device, and the different multiple
frequencies expected. Secondary spring-mass systems can also be utilized by positioning
masses with spring supports tuned to the excitation frequency by the formula Wn =
the square root of k/m where Wn equals the natural frequency in Hz, k equals the spring
constant in pounds/inch, and m equals the mass in pounds, with the necessary constants
required for equation units (e.g., such as gravity acceleration of weight in pounds
to mass in slugs). The secondary spring mass tends to oscillate at the same frequency
as the excitation frequency, but out-of-phase, so as to cancel out the excitation
frequency force.
[0161] Another vibration prevention device is the use of a large, thick, heavy flat plate
weighing 90 kg or more mounted horizontally in the same plane as the platen at about
the same level as the platen. This mass tends to absorb any vibration due to imbalance
of the platen/abrasive sheet combination assembly. This prevents the vibration motions
from exciting the machine frame in such a way as to oscillate the piece part being
ground or lapped. Adhesively bonding a viscoelastic layer to this flat mass plate
and bonding another large mass flat plate to it can very effectively reduce the buildup
of vibration oscillations,
[0162] Some other vibration excitation sources can be the platen system being out of balance,
the piece part spindle being rotated when out of balance, oscillations being generated
by the stick-slip conditions between the abrasive sheet and the work piece, hydrodynamic
fluid-induced vibrations at the moving fluid boundary layer interface between the
piece part and the platen, sudden motion of machine elements, electrical pulses, etc.
Vibrations should be prevented from entering the system, wherever their source. Adding
a large mass ring of heavy, dense material to the outboard diameter of a (typically)
round workpiece holder in a fashion which allows the center of gravity as close as
possible to the moving abrasive surface is a very effective method of minimizing vibrations
in the work piece. The mass attenuates vibration excursions and oscillatory vibration
forces generated at the abrasive surface contact area. The same mass will also interrupt
vibrations originating from the machine motor drive, and platen imbalance (insofar
as it would travel down to the workpiece support mechanism).
[0163] To minimize vibration, it tends to be more preferable that the mass of the frame
comprise at least 200 kg, still more preferably at least 350 kg., and most preferably
at least 500 kg., with no maximum weight contemplated except by the limitations of
reasonableness. The weight of the actual intended commercial embodiment of the frame
of the present invention is about 600 kg. The platen, at a revolutionary speed of
3000 rpm with a twelve inch (30.2 cm) diameter, has a natural frequency of about 50Hz.
The frame should be designed with a natural frequency above the frequency of the highest
useful speed of the platen (and motor) to avoid the frame being vibrationally excited
by the motor as it is brought up to specification during operation. For example, with
the maximum designated speed of a lapping apparatus with 30.2 cm platen and abrasive
sheeting being 3000 rpm with a frequency of 50Hz, the natural frequency of the apparatus
frame should be at least 2% above this operating frequency. Greater differences between
the operational frequency (the Hz equivalent of the rotational speed of the platen)
and the natural frequency of the frame would provide additional levels of vibrational
avoidance at the higher speeds, so that natural frequencies more than 3%, more than
5%, more than 10% or more than 20% of the operational frequency are desirable. Operating
equipment used by Applicant in the practice of the present invention has been made
with 3000 rpm operational speeds (50Hz) and 76 Hz natural vibration frequency. This
enables the frame of the machine to be operated at higher speeds and higher frequencies
(e.g., 3600 rpm and 60Hz, and 4200 rpm and 72 Hz) by increasing the capability of
the motor, replacing the motor, but not significantly modifying the frame. If need
be, weight and mass may be added to the frame after construction to improve vibration
resistance. Damping material, such as elastomeric materials may also be added at strategic
sites within the frame and apparatus, such as at joints, between a work frame and
the main frame, over bolts and nuts (if present), between legs on the frame and the
floor, etc. The purpose of these features being to mask the vibration or dampen it,
as by increasing the natural vibration frequency of the frame to a meaningful level
(e.g., at least 2 Hz or at least 2%) above that of the operational frequency of the
lapping apparatus.
25. LAPPER PIVOT CRADLE PIECE PART HOLDER
[0164] Problem: When a piece part is ground or lapped on a high speed (e.g., diamond) abrasive
disk with surface speeds of about 9,000 sfpm or higher, with a 12 inch (30.5 cm) diameter
platen rotating at 1,500 rpm or 3,000 rpm or more, there can be an uneven grinding
action due at least in part to the liquid boundary layer between the piece part and
the abrasive surface. There can be a thinner layer at the outer periphery of the circular
boundary layer due to the high relative surface speed at that outer region. The relatively
much slower surface speeds at the inner radial region of the disk will conversely
have a thicker boundary layer because of the slower speeds and the fact that the same
volume of liquid is moving over a smaller area (the area defined by the smaller radius)
at a slower speed. Typically abrasive particles at the outer radius of the rotating
platen more easily penetrate the thinner boundary layer at the outer periphery of
the disk and effect material removal more efficiently in that region than where the
boundary layer is thicker. Therefore, the abrasive activity is affected not only by
the differential in surface speeds between the inner region and the outer region,
but also there is another effect because of the variation in the thickness of the
boundary layer between radially related regions. Thus the abrasive particles integratly
attached to the abrasive sheet may be held away from the work piece and not remove
material as efficiently. This causes uneven wear and lapping on the piece part due
to the boundary layer effect which has not been previously considered in this technical
field.
[0165] Solution: The use of an annular ring, with the inner and outer radius of the center opening
and external edge, respectively, being sufficiently close in dimensions that the relative
velocity of the two surfaces, and more importantly the thickness of the boundary layer
at both of these radial positions, are within a narrower variation than previously
used. It is important to note that this effect is important for the high speed lapping
process of the present invention, and would have had an insignificant effect at the
5 - 200 rpm rotational speeds common to previous grinding processes. The high rotational
speeds create the dramatic boundary layer changes for which this invention is important.
Even if annular disks had been used with slower speed grinding, polishing or lapping
processes, the benefits of this aspect of the present invention would not have been
noted, even if the benefit was provided by such lower speed annular disk usage. It
would be desirable to have the boundary layer thickness approximate the average height
of the abrasive materials protruding from the support surface (e.g., from at least
about 0.1 micrometers, and for example from about 1 to about 100 micrometers). It
is desirable that the boundary layer thickness approximate that height with a variation
of no more than ± 50% of the average abrasive particle height, more preferably ± 30%,
still more preferably ±20%, yet more preferably ±15%, and most preferably within +10%
of the average protrusion of the abrasive particles from the average height of the
substrate (e.g., the valleys formed by the binder). The process may be performed with
two piece part holders, each rotating in a direction opposite (clockwise versus counterclockwise)
from the other. Both holders may be mounted on a common pivot arm. each piece part
holder would tend to stabilize the other and would also allow each of the piece part
holders to stabilize the other across the width of the platen. A special wobble joint
at each piece part holder would allow each to conform to the slightly uneven boundary
layer on the platen. Rotating each piece part holder would provide the same amount
of abrasive material removal to the exposed surfaces of the piece parts. The normal
contact force, surface speed, liquid flow rate, viscosity, etc. would all be optimized
in the entire assembly. The assembly pivot cradle would be oscillated to obtain even
surface wear.
[0166] This aspect of the invention can be considered with respect to cutaway Figure 9.
A lapper platen system
130 is shown which comprises a shaft
132 is connected to a rotation source (e.g., an engine, not shown), a platen face
134 on which will be secured an abrasive sheet (not shown). The platen face
134 contains ports
136, 138, 140, 142, and
144 through which reduced pressure may be provided to the platen face
134. A spherical or torroidal element
146 (hereinafter referred to as the "ball
146") with a flattened or flat beveled bottom portion
148 is secured by a flat internal face
150 to the lower portion
152 of the shaft
132. The rounded outer surface of the ball
146 is supported by pairs of spherical-faced bearings
154, and
156, and
158 and
160, which may also be a pair of torroidal bearing elements with concave spherical faces
contacting ball
146. Over said upper spherical faced bearings
154 and
158 are flexing elements
162 and
164. This may be any spring-like elements, coils, or spring washers which provide a cushioning
effect or spring effect between said upper spherical bearings
154 and
158 and bearing securing means
170 and
168 which help to secure the upper bearing elements
154 and
158 against movement and provide a stabilizing and positioning force to the ball
146. A convenient securing means may be a circular nut with spanner wrench holes, with
threads on the sides to fix into the platen neck
172. A cushioning material
174 and
176 are provided between the shaft
132 and the interior surface
178 of the platen neck
172. If a force is applied to the face of the platen
134 and the force is slightly uneven distributed against the face
134, the face of the platen may adjust to the force and level itself by pivoting through
ball
146. The degree of pivoting is cushioned by internal resistance of the ball
146, and the elastic resistance of the cushioning materials
174 and
176. A lubricant (not shown) may be provided in any cavities
180 and
182 which exist between the cushioning material
174 and
176 and the ball
146. The lubricant may be any preferably liquid lubricant such as an oil. The cushioning
material
174 and
176 may be any flexible composition, such as, but not limited to, natural or synthetic
rubber, silicone or fluorine containing elastomers, spring elements, or the like.
Lubricant may be provided by syringe injection into the cavity
180 and
182 or may be provided through a replaceable cap (not shown).
[0167] Figure 10 shows a preferred flexing element for use with the present invention, a
Bellview spring washer
190. This element is no more than a standard washer whose outer periphery has been bent
down to form a truncated cone shape. These Bellview spring washers may be stacked
to form a spring-like element.
[0168] It is desirable to limit the degree of pivoting which this aspect of the invention
may undergo. During an emergency, a limitation on pivoting, beyond that provided by
friction and the cushioning materials
174 and
176. One method according to the present invention is shown in Figure 11. A platen-shaft
system
198 may comprise a platen
200 with a front face
202 and an internal anti-pivot shaft
204. The anti-pivot shaft
204 is separated from the inside face of the platen shaft
206 by a distance of
A. The platen
200 may not pivot any angle greater than that which would cause the anti-pivot shaft
204 to contact the inside face of the platen shaft
206. By adjusting the dimensions of the respective elements (e.g., the length and thickness
of anti-pivot shaft
204, dimension
A, etc.), the limits on the degrees to which the platen may pivot can be preset.
[0169] This aspect of the invention may be described as a pivoting lapper workpiece holder
system comprising:
a) a shaft which is connected to a platen, said platen having a back side to which
said shaft is connected and a front side on said platen to which can be secured an
abrasive sheet;
b) a pivoting joint connected to a shaft attached to a workpiece holder, the connection
of the shaft comprising a spherical or torroidal element comprising a curved outside
surface, and said pivoting joint being located on the outside of said shaft, said
pivoting joint having an arcuate surface area and a receding surface area of said
outside surface of said pivoting joint, and said receding surface area is closest
to said workpiece holder;
c) said pivoting joint having a cross section with an effective center of its area,
said receding surface area of said pivoting joint being defined by a surface which
has average distances from said effective center which are smaller than the average
distances from said effective center to said arcuate surface area;
d) arcuate surface area of the pivoting joint is supported by at least one pair of
arcuate-faced bearings, said bearings comprising at least one upper bearing and at
least one lower bearing, said bearings being attached to a portion of said workpiece
holder, and allowing said pivoting joint to pivot between said at least one pair of
bearings;
e) said shaft being able to pivot about said pivot joint relative to said workpiece
holder.
The workpiece holder system may have over said at least one upper bearing a space
between said shaft and a neck of said workpiece holder, said shaft being restrained
within said space by a cushioning means between said shaft and an interior surface
of said neck, said cushioning means being selected from the group consisting of flexible
compositions and springs.
[0170] The workpiece holder system may have said cushioning means comprise a flexible composition,
and may have said cushioning means comprises an elastomeric composition, as previously
described. As previously noted, said elastomeric composition preferably comprises
a silicone elastomer or a fluoroelastomer. The workpiece holder system, between said
flexible composition and said at least one upper bearing may have a spring element,
and above said spring element and below said flexible composition may be a securing
element, said securing element being capable of being adjusted in a direction parallel
to said shaft to increase force upon said spring element, said force on said spring
element in turn increasing force of said at least one upper bearing to press said
bearing against an arcuate surface of said pivoting joint.
[0171] The workpiece holder system may have at least said flexible composition, spring element,
shaft, at least one upper bearing and pivoting joint creating a cavity with said workpiece
holder system. The cavity preferably contains a liquid lubricant.
[0172] To restrict non-lapping (out of plane) rotation of the workpiece holder, the workpiece
holder system may have an elongate element which is associated with said workpiece
holder so that movement of said workpiece holder, out of its natural symmetric rotation
plane as is used during lapping, causes movement of said elongate element, said element
extending from said back side of said workpiece holder through an interior channel
of said shaft so that said movement of said elongate element when said workpiece holder
pivots will cause said elongate element to contact an interior surface of said shaft,
restricting the amount of pivoting which said workpiece holder can perform. The elongate
element will contact said interior surface of said shaft when said workpiece holder
is turned less than 30, preferably less than 20, more preferably less than 15 degrees,
and most preferably less than 10 or 5 degrees.
[0173] The workpiece holder system may use a spring means or spring element which comprises
a stacked array of truncated hollow cone elements stacked upon each other.
[0174] This system is a great advantage over a simple ball bearing type of design for a
number of reasons. Fine abrasive grit can easily get into a ball bearing, while the
pivot center of this design is fully enclosed. Even if some grit does enter the system,
the oil can support it, wash it out, and remove it almost completely with replenishment
of the lubricant. A spindle holder (or the workpiece holder shaft) is never uniformly
and consistently perpendicular to the workpiece holder. A perfect ball bearing would
be very loose and could cause the workpiece holder to contact the platen in a manner
to cause abrasive damage from the first contact, while the cushioning material (the
elastomer) used in the present invention stabilizes the workpiece holder direction
and tilt within a more controllable range. The use of an elastomer is preferred over
spring support of the shaft because it also provides an added measure of vibration
damping.
26. ANNULAR DISK ON A RAISED PERIPHERAL PORTION OF THE PLATEN
[0175] Problem: Sometimes the extreme liquid pressures and forces can drive the liquids under an
interior edge of an annular disk. Once the edge is lifted, many undesirable events
can occur. The annular abrasive disk presents an uneven face, since one edge is deformed
from planarity. Residue from the abrasive disk and swarf material from the work piece
can embed themselves under the raised edge. Each of these distortions of the abrasive
surface are undesirable and can damage the workpiece.
[0176] Solution: There are a number of solutions to this problem. One basic consideration is to provide
an abrasive sheet which does not have any openings in its surface. This can be done
by having a circular sheet with no holes therein coated with an annular ring of abrasive
material. A circular abrasive sheet may have the core circle of abrasive scraped or
abraded off to leave an annular distribution of abrasive on an impervious sheet backing.
An annular disk with an opening in the center may be provided with a 'plug" or circular
piece that completely fills the central area. As shown in Figure 5, an annular disk
112 having annular, flat support area
114 with abrasive on the upper surface
116 may have a plug
118 which abuts (and is preferably secured to) the inside edge
120 of the annular ring
112. An area
122 between the flat annular surface support area
114 and the inside edge
120 is shown with a bevel, but this is not essential. Securement between the plug
118 and the interior edge
120 may be effected by direct fusion (by heat or solvent) of the two pieces, adhesive
or the like.
[0177] Figure 6 shows a platen
90 with a depressed region
92 and a wall
94 between the flat upper annular support area
95 and the depression
92. A number of means are available for providing an annular abrasive disk or annular
abrasive work surface (not shown) on this flat portion
95. Figure 7 shows one of these methods. The platen
90 has an abrasive sheet
100 on its surface. The sheet
100 comprises a backing layer
102 and abrasive material
104. A vacuum port
96 (or other securement means) retains the back surface
98 of the sheet
100 against the flat annular surface
95. The reduced pressure will be passed along the back surface
98 press the sheet
100 against the flat surface
95. The reduced pressure will also secure the sheet
100 against the wall
94 and the depressed area
92. The wall
94 is shown with an arcuate slope, but may be more sharp or smooth in the transition
from flat area
95 to depressed area
92. For example, the transition may be by two right angles or by an S-shaped curve or
other form. Figure 8 shows a platen
90 with a plug
93 which is secured to the backside
98 of the annular sheet
106 with abrasive
106 on it. The location of the abutment
110 between the backside
98 of the sheet
106 and the plug
93 is shown at an approximately right angle, rather than the edge-on abutment of Figure
5. The abutment
110 of Figure 8 may be by means similar to those described for the joining of the plug
118 and the flat annular support
112 at the abutment
120 in Figure 5.
27. RAPID WEAR IN PARTICULAR AREAS OF THE ABRASIVE SHEET
[0178] Problem: Abrasive sheets, even in annular form, tend to wear in a specific pattern. The precise
positioning of the sheets or ring against a work piece causes the same radial portion
of the abrasive surface to be in contact with the work piece. This tends to cause
the abrasive surface to wear down in specific circular lines or annular areas. As
the abrasive surface is not as useful where there is a discontinuity in the abrasive,
the remaining sheet may have to be discarded because of the absence of abrasive over
only 10-20% of the sheet work face.
[0179] Solution: Working at high rotational speeds, the centering of the sheet or annular disk on
the platen was assumed to be very important, mainly because the radial forces would
have been thought to be sufficient to create significant damage to the sheets, literally
ripping them apart with the force, or the creation of vibrations which would effectively
distort the relative face of the abrasive sheet. It has been surprisingly found that
not only would the off-centering of the sheet or annular disk not create damage, but
such off-centering could prolong the life of the abrasive work surface. By positioning
the center of the sheet or annular disk at least 1%, preferably at least 2-5% (even
up to 10-20% of the radius, off-center) of the radius of the sheet or annular disk
away from the center of the platen, the work surface of the sheet or the annular disk
would effectively oscillate, rather than present the exact same radial dimension to
the work piece. This oscillation, since it is unlikely to repeat in a single rotation
of the platen, would expose different areas of the abrasive work surface to the work
piece. Abrasive material would be removed in broader (wider) annular patterns, as
compared to the more narrow annular patterns that would be worn in the work surface
of a perfectly centered abrasive sheet. The degree of off-centering useful or tolerable
in the system is related to the rotational speed and the density of the abrasive sheet.
The greater the rotational speed, the heavier (higher weight per unit surface area)
the abrasive sheet, the less off-centering which may be tolerated. It is also quite
useful to provide a massive (heavy) support for the work piece and platen. The heavy
apparatus pieces will help to dampen vibrations that may occur by the eccentric rotation
of the sheet or annular disk.
[0180] Additionally, the abrasive disk could be either intentionally repositioned at its
exact original position or a different position by use of a marker system. Even a
felt-tip writing implement could be used to mark on the abrasive disk and/or the platen
where it was exactly located on the platen relative to the mark, or a permanent marking
system on the platen. An abrasive disk may then be removed and reinstalled at nearly
the identical radial and tangential position on the platen without requiring the disk
to be redressed each time that it is used. Furthermore, the abrasive disk could be
sequentially or progressively or randomly moved tangentially to align "low" wear areas
of the disk with "high" elevation areas of the platen which would better utilize all
of the expensive abrasive particles of the disk. Small increment tangential repositioning
of the disk would reduce the requirement for re-dressing the disk as many of the causes
which require re-dressing - platen high spots, thickness variations in the abrasive
disk, etc. - tend to then be distributed in areas rather than at specific points which
is more tolerable within a lapping system.
[0181] The abrasive disk can also be preconditioned so that high defect spots or areas are
reduced in height to reduce the possibility of local scratching on the work piece
surface. A hard material can be held stationary against the disk surface (particularly
at an edge) or the hard material may be oscillated slowly and radially to knock off
or wear down high spots. Another abrasive material could be rotated with its own high
(or slow) velocity against the surface of the abrasive disk to remove high spots or
loose materials. Any loose or weak abrasive materials at the inner or outer radius
of the disk would be broken loose by this initial conditioning treatment and would
be eliminated from the system prior to actual lapping of the work piece.
28. AVOIDING DAMAGE FROM FLYING DEBRIS
[0182] Problem: Because of the higher rotational speeds that can be used in the present invention,
liquids, swarf, removed abrasive and the like is hurled at extremely high velocity
away from the platen. With linear velocities of 20,000 feet per minute, debris is
constantly projected from the surface at over 200 miles (280 km) per hour. This projectile
material can cause serious damage to person around the machine, and upright box-like
protective enclosures (particularly with flat upright surfaces at right angles to
the path of the projected materials) are readily worn away by the projected matter,
much of which can be abrasive material. Additionally, the particulate waste can accumulate
against surfaces and the liquid will also run over any flat surfaces.
[0183] Solution: The platen may be enclosed in a sunken box or walled area, with significant space
below the platen to a lower surface for the containment area. The surface of the platen
and the surface which is contacted by the abrasive sheet should be below the upper
edge of the protective walling-in enclosure. Preferably the plane formed between the
work piece and the abrasive sheet should intersect the wall element at least 1 cm
below the highest part of the wall. Preferably there should be at least 2 cm of such
clearance, more preferably at least 4, 5 or even 10 cm of wall above that plane. The
distance below that plane to the floor of the containment area should be at least
5 cm, more preferably at least 10 cm, and may be 20-50 below the plane. Abraded material
may harmlessly collect in the floor area, and the area cleaned out from above (around
the sides of the platen or by moving or removing the platen) or from below (by an
access panel or regular drainage system). The collected materials may be more readily
disposed of and collected in this manner. The walls of the enclosing elements may
be metal, coated metal, composite, abrasion-resistant coated material, or sacrificially
coated materials, high friction materials, or energy absorbing materials. The walls
may be sloped outwardly so that impacting material may be reflected down towards the
floor/collecting area. The entire enclosing structure may be removable most easily
down from the bottom of the work area, there may be constant or sporadic drainage
allowed through the floor area, and the like.
29. LINE CUTTING. LAPPING OR POLISHING WITH AN ANNULAR FACE OF ABRASIVE
[0184] Problem: It is often desirable to control the application of the abrasive material to a substrate
so that a specific pattern and particularly a straight line of lapping is effected
on the work piece. This type of polishing could be done with a rotating beveled cup
abrasive wheel with the beveled side edge coated with abrasive so that the abrasive
action is directed against a plane parallel to the axis of rotation of the workpiece
or piecepart. Sheet material is not naturally thought to be applicable to such a process
unless the sheet material were applied along such an outer edge. The flat front face
of a platen could not create a straight line contact between the abrasive and a workpiece.
Unless a beveled face as shown in U.S. Patent No. 4,219,972 was used for the abrasive
grinding wheel, there could be no such possibility for any line or flat surface lapping
unless an entire surface were to be treated. That type of configuration would not
be expected to be amenable to abrasive sheet material, as the potential for wrinkling
in fitting the sheet to the outer edge would seem to be significant. Additionally,
there has been no disclosure of the use of sheet applied materials on beveled edges
of lapping or polishing materials as only flat sheets in rectangular and round facial
patterns have been provided.
[0185] Solution: A platen
220 is provided with an upper surface
222 (which is shown in Figure 12 as a flat surface with ports
226 for securing sheets to the surface. On the beveled side edge
224 are additional air vent ports
230 for securing subsequently applied abrasive sheet material
228 to said edge
224. A circular sheet of abrasive material (not shown) or an annular sheet of essentially
two dimensional conformation
228 may be applied to the upper surface
222 of the platen
220. A flat abrasive sheet (not shown) would be secured by reduced air pressure through
ports
226 on the upper surface
222 of the platen
220. It is to be noted that because of the beveling of the edge
224 of the platen
220, it is not necessary that the upper surface
222 of the platen
220 be flat. That surface may be rough, smooth, arcuate (e.g., spherical segment), or
any other shape, with or without features, since the lapping surface is no longer
a face of the platen but is the beveled edge
224. The edge is beveled at an angle between 1 and 89 degrees away from the top surface
222 of the platen
220; preferably the angle is between 5 and 45 degrees, more preferably between 5 and
30 degrees. When an essentially two dimensionally formatted abrasive sheet
228 is applied from above the platen to the upper face
222 of the platen, pressure (and/or heat) may be used to conform the sheet
228 to the beveled surface
224. The pressure from reduced air pressure through ports
230 may not be sufficient to form the sheet
228 and additional pressure as from a mold overlay (not shown) which match the shape
of the beveled platen
220 may be needed. It has been surprisingly found that the sheet
228 may be formed over the surface without distortion of the configuration of the sheet.
No wrinkles are formed in this fitting procedure. As one of ordinary skill in the
art knows, normally when an annular sheet-like object in sheet form is fitted over
a truncated conical form, the sheet distorts and forms wrinkles when attempting to
conform to the surface. The sheet material backing on commercial abrasive sheeting
has been found to be able to conform without wrinkles when pressed onto the beveled
shape. This is believed to be in part caused by elastic or inelastic give in the backing
material itself. What is additionally surprising is that with the stretching or reconfiguration
of the backing material, the essentially uniform abrasive surface of the abrasive
sheet is not adversely disrupted. This is particularly surprising since the uniformity
of the distribution of the abrasive material on the surface is so important to the
quality of the lapping process, and the amount of elastic conformation at the lower
edge of the platen may be 10% or more.
[0186] The beveling of the edge provides a geometry to the edge that when, as shown in Figure
13, a workpiece
240 is addressed by the beveled edge
224 of a platen
220, the beveled edge
224 is parallel to a surface
232 of the workpiece
240. Additionally, a relatively clean line contact is made between the beveled face
224 and the face of the workpiece
232 so that a relatively flat lapping contact is made. The shape of the area removed
234 by extended contact with the edge
224 of the platen would be nearly rectangular (for most purposes), and only if the lapping
were used in more of a grinding fashion would an angularity in the wall
236 be noticeable while there was only a right angle configuration on the distal wall
238 of the area
234. An angularity or pitch in the wall
236 while the distal wall
238 was relatively perpendicular to the face
232 of a ground area
234 would be a fingerprint of the practice of the present invention.
[0187] The use of the annular ring with the beveled edge geometry has numerous benefits
and improvements over a cylindrical section or disk element for the grinding wheel.
Systems of grinding wheels with abrasive on the outside periphery of the wheel (not
on the flat face) are known for systems where the abrasive is part of the wheel material
itself (e.g., a grindstone) or coated onto the edge. An abrasive sheet material does
not lend itself to facile application or use on such an outer edge, both for technical
and mechanical reasons. There are basically three ways in which a sheet material could
be applied to the outer edge of a grinding wheel: 1) coat abrasive on a cylindrical
sheet and cut continuous sections from the sheet which fit the grinding wheel diameter;
and 2) cut strips of abrasive sheet material and adhere them to the surface of the
edge. The first method would involve a specific new manufacturing process and technique
to manufacture such a continuous circular element, and the tolerances for good fit
to the wheel would be quite small. It is possible to have the backing layer of the
circular cut element shrinkable to fit the article more tightly to the wheel, but
adhesive would have been desirable, and this leads to disuniformity. The vacuum hold-down
of the present invention would have helped in this format, but the new manufacturing
procedure would have still been needed.
[0188] The second manner of providing an abrasive edge to the wheel would have required
that the strip be attached at its ends to form a circular element. This would require
the formation of a joint or weld, which would be likely to provide a weak spot, an
elevated patch, a wrinkle, or other aspect which would not lend itself easily to use
in the fitting of pre-made abrasive sheeting to the end of grinding wheel.
[0189] The use of the completely beveled edge on the platen in this aspect of the present
invention provides a mechanism for providing a continuous strip of abrasive sheeting
made by existing technology and available as a staple in the market place as an abrasive
surface on a high speed lapping system which can provide linear lapping and polishing
as well as complete surface lapping. It is an attribute and fingerprint of this aspect
of the present invention to provide a platen with a beveled exterior edge and a continuous
strip of abrasive sheet material on at least the beveled edge. The particle distribution
in the abrasive sheet may well result in a gradient of slightly lesser density of
particles in the upper, smaller diameter region of the beveled face than in the lower,
larger diameter beveled face. This particle density may be as slight as 1, 2, 5, or
10 % depending upon the angle of the bevel and the degree to which the underlying
support sheet has been shaped by the fitting process. This minor particle density
variation has not been noted as providing any adverse effects on the lapping quality
provided by this configuration, and the important fact is that the shaped annular
disk conforms well to the beveled face and provides a very consistent and smooth orientation
of the abrasive sheet upon the beveled edge.
30. UNEVEN WEAR ON THE SURFACE OF THE PLATEN WITH AN ANNULAR ABRASIVE AREA
[0190] Problem: Because of the high rotational speeds of the platen and the abrasive sheet material
on the lapping face of a platen, there is uneven wear between a radial outer area
of the abrasive material and a radial inner area of the material. There are difference
in the linear speeds at the two areas, the amount of surface area each incremental
area of the abrasive addresses, and therefore there is more rapid the wear in the
abrasive surface towards the outer edges and likewise more rapid wear on the workpiece.
[0191] Solution: In Figure 14, a workpiece
254 and a platen
250 with an abrasive surface
252 address each other. The workpiece
258 has an effective center line
A - B. The workpiece
254 is moved so that the center line
A - B spends more time inside the outer edge of
260 of the platen
250 while the abrasive surface
252 of the platen
250 and the workpiece
254 are in contact during lapping. By distributing or shifting the majority of the time
of contact between the abrasive face
252 and the workpiece
254 towards this interior region, there is less wear on the outside edge
260 of the platen
250. As the most serious wear and damage to the workpiece
254 can occur with excessive wear on the outside edge (as cracking, flaking, and sharp
edge features can more easily develop, this is an important improvement in the wear
performance of the abrasive sheet material
252. Figure 13 shows that the direction of rotation
256 of the platen
250 is opposite the direction of rotation
258 of the workpiece
254. This aspect of the invention works even better where the workpiece is rotated at
the same time that the platen is rotated, to more evenly distribute the time and position
of orientation of the workpiece and the abrasive surface. Even if uneven wear does
occur, the dual rotation of the workpiece and the abrasive sheet on the platen will
reduce any linear effects or artifacts on the workpiece surface. The rotation
256 258 does not have to be in opposite directions, but this is the preferred mode of practice.
[0192] The time when a workpiece is in contact with an abrasive sheeting is referred to
as the total contact time Tc. The time when the center of the workpiece is inside
(not merely directly aligned with) the outer edge of the abrasive surface must be
at least 50% Tc when operating at a constant speed. That is if the speed of rotation
of the platen decreases, the Tc must be weighted according to the surface area fanned
or covered by the workpiece. Operating at a constant speed, it is preferred that the
workpiece center be within the outer edge at least 60% of the time, more preferably
at least 75% of the time, still more preferably at least 80 or 90% percent of the
time, and it is most preferred and most convenient to have the center of the workpiece
aligned within the outer edge of the rotating platen at least 95% and even 100% of
the Tc.
[0193] The combined effect of moving the center of the workpiece inward of the outer edge
and the rotation of the workpiece not only reduce uneven wear on the abrasive surface,
but provides a synergistic effect in reducing the potential unevenness of lapping/polishing
on the surface by both improving the consistency of the abrasive surface addressing
the workpiece and reducing any linear effects that any unevenness in the abrasive
surface could cause in the workpiece. Additionally, by having an eccentric or non-repetitive
movement of the workpiece with respect to the radial position of the abrasive surface,
there is even less likelihood of any linear uneven lapping effects upon the workpiece
surface.
[0194] In the system where the center of the work piece is off-set so as to be located predominantly
inside of the annular ring center line of the abrasive sheet, the lapping set-up may
include multiple workpieces. As the platen carrying the abrasive sheet is rotated,
a workpiece will normally cover or be in contact with only a very small fraction of
the surface of the abrasive sheet. This leaves space or areas on the abrasive sheet
available for additional lapidary work. It is convenient to have multiple workpieces
distributed about the periphery of the platen carrying the abrasive sheet. At least
one workpiece should be oriented as described above with respect to the relative position
of the center of the workpiece and the annular ring center line of the abrasive sheet.
Preferably more than one of the workpieces and most preferably all of the workpieces
are so oriented. To increase the effect of reduced uneven wear according to the practice
of the present invention, at least two of the multiple workpieces should be rotating
in opposite directions with respect to each other. That is, when viewed from one direction
perpendicular to a platen face, at least one workpiece will be rotating clockwise
and another will be rotating counterclockwise. It is preferred that with an even number
of workpieces, clockwise and counterclockwise rotation is evenly distributed and alternative
between the workpieces, and with an odd number of workpieces, the numerical distribution
would be n+1/2 and n-1/2 for clockwise and counterclockwise workpieces, with only
one pair of adjacent workpieces rotating in the same fashion.
[0195] This format of distribution with respect to a lapping surface is useful in the practice
of the present invention whether an entire platen surface is covered with abrasive
sheeting or whether an annular distribution of abrasive sheeting is provided. The
problem of uneven wear occurs in both type of systems, the potential for damage is
present in both types of systems, although it may be somewhat magnified in the whole
sheet system since there is a large variation in the radius and thus the surface speed
of the disk, and so any degree of uneven wear provides greater likelihood for that
uneven portion to contribute to damage to the workpiece surface. This is simply a
matter of probability in that any damaged area has a greater probability of being
in contact with a workpiece when it constitutes a larger percentage of the total abrasive
surface area.
[0196] It is also a consideration in the operation of a lapping apparatus using the conformation
of work piece positioning and the outer edge of the abrasive sheeting to assure that
at least some of the contact time of the work piece and the abrasive platen positions
the workpiece over the outer edge of the abrasive sheet, and if an annular distribution
of abrasive, over the inner edge of the abrasive distribution. The passage of the
work piece over the edges of the abrasive distribution avoids the formation of ridges
on unused portions of the abrasive surface. By rotating the work piece while the platen
is spinning, differing areas of the work piece are presented to areas of the abrasive
sheeting. More importantly, however, buildup of ridges are avoided by the extension
of the edges of the workpiece over the outer (or inner with an annular configuration)
edge of the abrasive distribution. The extension should cover at least 1%, more preferably
at least 3%, still more preferably at least 5%, and most preferably at least 10% of
the effective diameter of the piecepart. (Note that the piecepart should be somewhat
larger than the width of the ring, which is 100% Tc.)
[0197] Another operation which proves to be of benefit in the operation of the lapping apparatus
is to precondition the outer edges of the abrasive sheeting before actual lapping
of a work piece. Such sacrificial lapping on the outer edge for a brief period of
time (e.g., less than 50%, preferably less than 25% or 10% of the actual Tc for the
next intended work piece, e.g., for 1-5 seconds) can remove manufacturing or conversion
(cutting) deficiencies in the outer edge. This has been found to assist in reducing
the occasion and occurrence of particulates being dislodged in the outer area and
wedging themselves between the abrasive sheet and the piece part.
31. GIMBALED WORKPIECE HOLDER
[0198] Problem: In initial work with high speed lapping systems, a gimbaled workpiece holder had
been used. This provided unsatisfactory results in that relatively cone-shaped surfaces
were produced. This effect was primarily due to the fact that the interior region
of the lapping abrasive surface is moving slower than the outside region (radially
outside) of the lapping abrasive surface. Less grinding per rotation was being performed
on the interior region, less material was being removed, and so the interior region
of the workpiece was higher in the relative topography of the surface, producing the
cone-like structure. Hydroplaning effects of liquid between the platen and the workpiece
also contributed to an unevenness in surface smoothness, as did uneven wear in the
different regions of the abrasive sheet surface. The basic system of the platen covered
with abrasive sheet material, rotated at high speeds (e.g., 2,000+ rpm) and a gimbaled
workpiece would produce surfaces with light band uniformity of at best 4-5 light bands
smoothness, and this was attainable only through constant and severe control of the
system.
[0199] Solution: The combination of a platen surface with an annular ring of abrasive material (e.g.,
with the non-abrasive inner region comprising at least 20% of the total area of a
circle defined by the outer circumference of the annular abrasive sheet) when used
in combination with a gimbaled workpiece holder has been found to improve surface
flatness as compared to a continuous surface of abrasive material. The light band
flatness is reduced to 1-2 light bands. With the annular abrasive sheet with a gimbaled
workpiece, lapping times of from 15-30 seconds at 3,000 rpm are used to with a twelve
inch diameter annular disk with comparable times of 60-100 seconds at 1000 rpm.
[0200] The gimbaled workpiece holder is desired in more conventional lapping apparatus as
it is difficult to align the upper workpiece holder perfectly perpendicular to the
abrasive platen surface. Even if it is initially aligned, it becomes even more difficult
to retain that alignment with disturbance from hydroplaning forces and other machine
factors, such as uneven bearings, other dynamic forces, and the like. The combination
of the gimbaled workpiece holder with annular sheets of abrasive material attenuates
or substantially eliminates some of these effects and problems.
32. RIGID WORKPIECE HOLDER AND POSITIONABLE ABRASIVE PLATEN
[0201] Problem: It is desirable to be able to provide a system where only one of the workpiece and
lapping platen are needed to be moved during operation of the system. There has been
no effective lapping apparatus which has been able to provide the complete control
over positioning of the platen face and the workpiece face during lapping which would
produce high quality smoothness at high speeds. Because of the high speed component
of the present lapping apparatus, the ability for accurate and fast alignment of the
surfaces (lapping and workpiece) is much more important than in previous systems.
The lapping process for slurries of abrasive or lower speed lapping with abrasive
sheet materials (especially in combination with adhesively secured sheets) would take
hours. The amount of material removed from surfaces with maximum rotational speeds
of 200 rpm was very small and took a large amount of time. In the lapping process,
it is often is not always necessary to replace abrasive material during the complete
procedure. The abrasive had to be changed because first coarser than finer abrasive
material had to be sequenced to rough grind, then polish, then lap the surface. The
slow rotational speeds increased the amount of time needed for each step. The need
to remove abrasive sheets secured by adhesive was especially slow and unwieldy because
of the need to strip the adhesively secured sheet from the platen, remove excess adhesive,
and reposition a new sheet with new adhesive. Additionally, even with adhesive removal
between sheets, there was a likelihood of adhesive buildup.
[0202] Solution: A heavy support frame for the workpiece and lapping platen (including rotation engine
or motor) is provided in combination with a preferably fixed workpiece holder secured
to the heavy frame. The lapping portion of the system (the motor and lapping platen)
is carried on a heavy frame. The workpiece support or workpiece platen (along with
gearing or in combination with the motor) is positionable in three axes (the x, y
and z axes). Each axis is separately controllable, with an extensive amount of positioning
being capable in the axis controlling the linear spacing between the abrasive platen
and the workpiece (the Z axis), e.g., can be measured in full meters. However, in
addition to any gross maneuverability of the workpiece platen along these three axes,
there may also be a control system in place for at least the y and x axes (which define
the piecepart position parallel to the abrasive platen surface. The fine controls
on the system would require that there be at least one hundred (100 ) positions available
within any centimeter of movement along either axis, more preferably at least 250
positions, still more preferably at least 500 or 750 positions available within any
cm of movement, and most preferably that there be at least 100, 250, 500 or 750 positions
available for every millimeter of movement of the platen face along anyone of and
all of the three axes of movement of the platen face. The degree of control may also
be measured as with respect to the rotation of a control element. That is, there may
be 36, 72, 120, 144, 180, 200, 240, 300, or 360 individual positions within a single
rotation position of a control or switch. These numbers have been selected merely
because of their relationship to 360°, which is the basic unit for a rotation, but
any other unit or number may be selected, as between 1 and 100,000. The actual construction
the best working model of the present invention uses position control with a stepping
motor having 50,000 step increments per revolution, which divides the forward motion
from a single rotation into 50,000 units of travel. Units of more 5,000, more than
10,000 and more than 25,000 are particularly desirable. Each revolution of the control
means may have as little movement of the directed portion of the platen (e.g., one
edge moving along one axis) as less than 0.05 mm, preferably less than 0.005 mm, still
more preferably less than 0.001 mm, and the like.
[0203] Positioning along these axes can be effected by any means which can move the platen
face with accuracy. Screw pins and screw drives have proved easy to configure into
the system because the pitch of the screw can be adjusted to control the amount of
linear movement along an axis with respect to any particular amount of screw rotation.
For example, with a screw drive having 1 thread per cm, a 360° turn would advance
the screw and any part attached thereto by one cm. A 36° rotation would advance the
screw 0.1 cm. Similarly, with 5 threads per cm., a complete rotation of the screw
head would advance the screw and any attached workpieces or platens 0.2 cm., and a
36° rotation would advance the screw 0.02 cm. Thus the sharpness or fineness of the
control can be designed by the threading of screws.
[0204] The mass of the frame also has a beneficial effect upon the performance of the system.
As the system is subjected to vibration forces, it is desirable to minimize these
forces. This can be done in a number of ways, but the easiest way to have a major
impact on controlling vibration is to increase the mass of the support system and
the connectors of the workpiece holders and the abrasive platen. The frame of the
system should weigh a minimum of 100 kg. For a lightweight, small manufacturing model.
More preferably at least 200 kg, still more preferably at least 350 kg. And most preferably
at least 500 kg., with no maximum weight contemplated except by the limitations of
reasonableness. The weight of the actual commercial embodiment of the present invention
is about 600 kg.
[0205] The apparatus described in this section would generally be a lapper platen system
comprising:
a) a shaft which is connected to a rotatable platen, said platen having a back side
to which said shaft is connected and a flat front side on said platen to which can
be secured an abrasive sheet;
b) a frame having a total weight of at least 200kg supporting a work piece holder
assembly and said shaft connected to a rotatable platen;
c) said workpiece holder is attached to a movable element which is capable of moving
along said frame in a direction towards and away from said abrasive sheet,
d) said workpiece holder assembly having control element thereon which allow for independent
movement and alignment of said workpiece holder assembly along three perpendicular
axes so that said flat face of said platen can move towards parallelity with said
work piece to be lapped; and
e) said control elements having at least 50 settings per rotation, each setting moving
said workpiece holder assembly along one of said three axes by a dimension less than
0.05 mm.
33. ADDITION OF FINE SLURRY BETWEEN THE ABRASIVE SHEET AND THE PIECE PART
[0206] Problem: It desirable to increase the speed of the material removal, obtain better flatness
and surface finish smoothness with a fixed abrsive disk.
[0207] Solution: A slurry of abrasive particles can be added to the lubricant, coolant (e.g., water)
which can be used with the coated diamond abrasive sheets. These loose particles could
be larger or smaller than the average diameter of the fixed diamond particles, and
have a controlled size distribution to enhance the performance of he abrasive disk.
Different types of chemical additives could also be added to the liquid composition
provided between the disk and the work piece, such as surfactant, viscosity modifying
(reducing or thickening) agents,, or acidic or basic solutions, etc. Some selectively
chosen foreign matter could also be added to the slurry mix, such as glass beads,
plastic beads, fibers, fluorescent materials, phosphorescent materials (for examination
of the face of the work piece by other means). The different solid or abrasive materials
in the slurry could perform a surface separation effect to obtain flatter contact
between the work piece and the abrasive sheeting and also additional material removal
mechanism effects. The other additives would have to be considered on an individual
basis as a function or relationship of the type of abrasive used in each portion of
the grinding cycle and the make-up of the work piece and its compatibility with the
chemical make-up of the additives. The combination of different abrasive particles
with the diamond sheeting can provide unique lapping effects and intermediate effects
between traditional lapping with slurry compositions and the high speed abrasive sheet
grinding of the present invention.
34. LIFT MECHANISM FOR LAPPER PART HOLDER
[0208] Problem: When a piecepart is brought into contact with a moving abrasive surface, the amount
of material that is removed in lapping can be extremely small, perhaps only 0.1 micron
(micrometer) while the typical distance the piecepart is moved from a typical "start"
position to the abrasive is relatively larger, perhaps 4 to 6 inches. It is desirable
to traverse the travel distance for part loading or unloading rapidly in perhaps 1
to 5 seconds as the actual lapping or grinding action may last only 10 seconds after
contact with the high speed 10,000 sfm abrasive.
[0209] Typically the thickness of the material abraded away during one step of a grinding
or lapping process is equal to the thickness or diameter of the abrasive media particles
used in the previous step. A process lapping may start with 50 micron abrasive for
the initial grind and be followed with 3 micron particle abrasive which removes approximately
50 microns of material (although as noted above, the practice of the present invention
may beneficially reduce this amount of removal to less than 90% of the abrasive particle
size). Next 9 micron abrasive will remove 3 microns of material, 1.0 micron abrasive
would remove 1.0 microns of material and 0.1 micron abrasive would remove 1.0 microns
thickness.
[0210] Trying to control the contact of the piecepart with the abrasive surface positionally
through the use of geometric advancement devices such as motor driven screws is very
difficult to these very small distances. A fine pitch screw system with the capability
to be moved in 0.1 micron or less increments does not have the capability to be moved
through large distances for initial part loading or mounting in the machine whereas
many other devices which have micro motion capability such as piezoelectric actuators
or thermal expansion actuators are not capable of large excursions of 4 inches.
[0211] A further problem exists with screws in that those using recirculating ball bearings
with inherent large pitches of 3 to 5 threads per inch tend to have significant position
errors relative to accuracies of 0.1 micron or less due to out-of-roundness of the
balls and non-perfect pitch variations of the lead screws used in conjunction with
the balls to advance a carriage when the lead screw is precisely rotated. These rolling
balls result in low drive friction.
[0212] Use of a servo motor to drive a lead screw provides fast continuous motion of the
lead screw and the carriage to which the part holder is mounted, but when the servo
motor is stopped at the desired contact position it has a natural tendency to "dither"
or oscillate mechanically and positionally due to its control system electronics which
corrects for the position error sensed. First it will move past the target, create
an error, and then move back again past the target making a new error and correction.
[0213] If a stepper motor is used to drive a screw, then very significant accuracies can
be achieved with micro stepping control architecture where a motor can be moved in
increments of 50,000 steps per revolution. The accuracy of these micro steppers with
ball screws having typical pitches of 3-5 threads per inch of travel is marginal with
respect to the requirements of lapping with 1 micron or less abrasive media.
[0214] Using linear electrical motors directly on a carriage slide device has problems in
that these motors again have a limited number of magnetic poles which results in minute
speed and force variations along the length of travel of the moving portion of the
motor device. Also they exhibit "dither" problems at a fixed position, similar to
rotating servo drives.
[0215] An inherent problem of great significance is trying to achieve a smooth analog progressive
grinding event with incremental or digital movements. Material is progressively ground
away from the surface of the piecepart on a continuous basis as the part is brought
in contact with the moving abrasive. The total amount of material removed is expected
to be at a steady fixed removal rate over a period of time with a constant contact
force between the piecepart and abrasive. However, if a piecepart is moved incrementally
by a stepper motor or an "over-shooting" servo drive, the piecepart will be driven
into the abrasive at initial contact with too much resultant force and therefore excessive
and probably low quality or harmful grinding initially will occur as the piecepart
is ground away during this time period when the part holder is advanced this one step.
As time goes on in this period of the incremental positioning step, material is removed
and the contact pressure is reduced to less than desired until another incremental
step or position change is made in this positional control system. Typical CNC (computer
numerical control) machine tools operate with small or fine increments of motion and
a cutting tool is driven by the strong machine into the piecepart along a prescribed
path with the surface finish and accuracy outcome a function of the size of the incremental
steps and the speed of the mill cutter. Damage of a submicron layer of the piecepart
is not generally a concern with a CNC positionally driven machine.
[0216] Over-aggressive grinding action on a typical lapped part for 1 second or less can
cause considerable submicron damage to the grain structure of these pieceparts which
are usually of great hardness being of such substance as tungsten carbide, alumina,
ceramics, silicones, glass, titanium, carbide and others. Interstitial grain cracking
at grain boundary layers is a common effect as is localized thermal stress heat cracks.
[0217] It is critical that the pressure contact force between the workpiece and the moving
abrasive surface is held at a ;level determined to be best for a given piecepart material,
abrasive type, geometry, etc. The pressure on a given piece which is defined by the
total normal force divided by the surface area would be quite consistent which means
the normal force needs to be changed when the surface area size of the part is changed
to achieve optimal grinding on lapping. Each piecepart material would have a unique
pressure force that results in faster grinding or better surface finish. This problem
would change also as a function of the period of the grinding cycle. Typically a higher
pressure is used early in a period for greater material removal rates and a lower
pressure is used late in the period for improved smoothness.
[0218] Determining the exact position at which a new part of unknown size or thickness initially
contacts a moving abrasive surface is desirable for controlling grinding process parameters
during the grinding process. This initial contact position changes in a potentially
significant amount each time a new sheet of abrasive is installed for a series of
grinding events with progressively finer abrasive media having a different sheet thickness
used for a smoother ground surface.
[0219] Also, it is very important to know how much material is removed from critical parts
and the rate of material removal. The rate of material removal indicates directly
the condition of the abrasive media and indirectly the expected quality of the surface
finish. It is extremely difficult to successfully use an exclusively position control
system to present a workpiece for contact with a high speed abrasive surface such
as the abrasive sheeting used in the present invention. About 10 micrometers of material
from a workpiece surface is typically removed in about 15 seconds, and machine tool
component parts (such as bearings) typically have fitting gaps larger than those dimensions,
and the high friction that would exist with tighter fitting components would have
too high a level of friction for the smooth movement of equipment necessary for the
best practice of the present invention. An excellent criteria for good grinding or
lapping action is control of the pressure force (which is difficult to measure) by
incremental position steps which are used to create the desired contact force.
[0220] Solution: It is necessary to provide a precise, controlled contact pressure force between
the piecepart and the high speed abrasive surface during the whole abrasive grinding
or lapping event. Once the piecepart is removed from surface contact with the abrasive,
then less precise or different means can be employed to move the piecepart to another
more remote location on the machine. A force based design (as opposed to a purely
position-based design) is preferably used within the lapper system. The contact pressure
between the workpiece and the abrasive surface is controlled by force controlled (and
measurable force devices) devices such as pressure controlled cylinders (as herein
described) acting as a piecepart slide carriage which present a workpiece to be ground
to the moving abrasive.
[0221] This aspect of a process of the present invention may be summarized as follows. A
workpiece holder is supported on a linearly movable support (usually vertically with
respect to the abrasive surface). The workpiece is advanced into contact with the
abrasive surface (while the surface is static or while it is rotating, preferably
at a speed that does not cause immediate significant abrasion ( e.g., less than 10
microns grinding in 15 seconds). The parallelity of the workpiece surface to be lapped
and the abrasive surface is preferably adjusted at this point, as by appropriate adjustment
of positioning screws or other alignment elements, particularly mechanical, position
oriented, linearly oriented elements (e.g., such as those herein described with at
least 50 positions settings per rotation with no more than, for example, 0.05mm linear
movement per setting, preferably no more than 0.01 mm, and more preferably no more
than 0.005 mm per setting) to place the workpiece surface to be lapped in good parallel
alignment with the abrading surface of the sheet. The position is indicated (e.g.,
a program setting, position setting, etc. is indicated within the system, as on a
computer) and the workpiece is retracted and removed from contact with the abrasive
surface. The workpiece is then advanced towards the rotatable surface of the platen
with the abrasive sheeting thereon, with the surface rotating, preferably at the grinding
speeds desired (e.g., greater than 500 rpm with a 12 inch diameter outside diameter
platen). The advancement is done with a low friction carriage so that the movement
of the workpiece is relatively slow (e.g., less than 0.5m/sec., preferably less than
0.4m/sec., and more preferably less than 0.3m/sec. or less than 0.2 or 0.1 m/sec.)
and smoothly progressing. This is best accomplished by a system of elements herein
described. This system of elements basically operates in a preferred mode by providing
both vertical support forces (e.g., lifting forces as by air pressure, hydraulic pressure,
pneumatic pressure, electromechanical pressure, magnetomechanical pressure, etc.)
and vertical downward (advancing) forces (gravity, air pressure, hydraulic pressure,
pneumatic pressure, electromechanical pressure, magnetomechanical pressure, etc.).
The system may also be inverted, with gravity operating as a "lifting" force with
respect to the vertical movement between the workpiece and the platen (that is with
the platen at a higher elevation than the workpiece and the vertical "downward" force
being a vertical upward force (provided, for example by air pressure, hydraulic pressure,
pneumatic pressure, electromechanical pressure, magnetomechanical pressure, etc.).
The difference between the to forces (the lifting and descending force) controls the
contact pressure between the workpiece and the abrasive surface at the moment of contact
and thereafter. By accurate measurement and control of these controllable (relatively
controllable, as gravity will be fixed for a workpiece/workpiece holder system) forces,
the contact and lapping operation pressure can be accurately controlled.
FIRST METHOD - SCREW DRIVE
[0222] One method of solving this positioning and force application problem is to use a
screw drive system to move the piecepart from its remote initial mount installation
position to a new position close to the moving abrasive sheet and then change the
method of controlling the movement of the piecepart from a position based system to
a pressure or forced based system for the grinding event only. After the grinding
event cycle has been completed, then the piecepart would be removed from contact with
the abrasive and then control would be transferred back to the position based control
for a "large distance" physical move of the part while the next grinding or lapping
event is being prepared. An example of this lapping event change would be to change
from a 9 micron abrasive disk to a 3 micron disk to be used in the next lapping event.
[0223] The lapping machine would require a number of other functional devices (e.g., at
least two distinct systems) to allow the easy transition from a positional mode to
a force mode. These functional devices would be used as a part of the grinding procedure.
First System - Motor Driven Lead Screw
[0224] A motor driven lead screw would be used for the first positional mode system. The
preferred type of lead screw is not a large pitch acme screw with ball bearings but
rather a standard bolt type 50 pitch per inch of screw length which gives about 10
times the linear resolution as a 5 pitch (threads per inch) ball screw. Also by using
a standard threaded nut with this screw, there is little or no variation in the nut-to-screw
location at any position because the third contact element which creates variations,
the balls, are eliminated.
Second System - Air Cylinders, Bellows
[0225] Also flexible bellows can be used as short, low friction cylinders for the second,
force or contact pressure based mode. Low friction air cylinders or hydraulic cylinders
are mounted at one end on the screw nut assembly and are connected on the other end
to the piecepart holder lift mechanism. Thus the piecepart holder can be put into
place (e.g., into contact with the non-rotating, slowly rotating, or high speed rotating
platen) by the screw drive and at that time the cylinders can be activated to lift
the part holder up a small distance of 1/8 to ½ inch before significant lapping has
been effected. Then the screw drive can be lowered again until the piecepart is nearly
touching the moving abrasive. The pressure is then appropriately reduced in one of
a number of cylinders which may be used to support the piecepart holder, sometimes
one but usually at least two cylinders, preferably at lest three or four, and up to
six offer definite advantages. In this case, with four air cylinders present, pressure
in three of the cylinders would support most of the weight of the workpiece carriage
assembly and independent pressure to the fourth cylinder can be used to raise and
lower the carriage with a nominal low force of only one fourth of the weight of the
carriage. When pressure to the fourth apparatus cylinder has its pressure reduced,
this allows the piecepart to come into contact with the moving abrasive at a controlled
rate and pressure. The cylinder pressure was changed by a voltage-to-pressure (E/P)
transmitter to provide a very low initial contact force, which increased as the lapping
event progressed, decreased at the end of the event, and was then changed more to
lift the piecepart away from the surface of the abrasive sheet. There would be a nominal
weight of the piecepart assembly acting down against the force of the cylinders. The
force of the workpiece against the surface of the abrasive surface can be seen as
a combination of three possible forces. There is a support force component (in a relatively
vertical direction) provided by the force mode system (e.g., the air cylinders) and
there is a gravity component (in a generally negative or downward vertical direction).
There may also be a third component (either a separate supporting component or a driving,
downward component) to control the force or position of the workpiece as it contacts
the platen.
[0226] After the piecepart is raised adequately from the surface of the abrasive sheet by
the cylinders, then the driven screw lift would be raised which will allow the cylinders
to be lowered to their bottom or home position without the piecepart contacting the
moving abrasive. Non-typical air cylinders such as AIRPEL brand cylinders with limited
air leakage around rigid non-seal inside rod glass tubes provide very low sliding
friction. The process may be generally described as follows. A workpiece holder with
a workpiece thereon is moved from a first position to a second position which places
the workpiece into a second position comprising contact with or at a distance of less
than 2mm from the surface of an abrasive sheet on a rotatable platen. This second
position is registered within the system which moves or controls the movement of the
workpiece holder (e.g., a computer registers the specific position of the second position).
Movement towards the second position may be done with the platen fixed, the platen
slightly rotating, or the platen fully rotating, but only a very small amount of material
removal is allowed, such as lapping of more than 10 microns for a 50 micron average
diameter abrasive particle into the surface of the workpiece should be avoided in
this step. While in the second position, adjustments in general parallelity between
the workpiece and the abrasive sheet on the platen may or may not be made. After the
second position has been reached, the workpiece is removed from the second position
to a third position. This third position may or may not be the same as the first position,
but is a position which does not afford contact between the abrasive sheet and the
workpiece. This distance may be essentially any distance as the second position has
been registered by the workpiece moving system. The workpiece holder is then moved
from the third position to a fourth position which may be selected by the operator
as approximately before the second position (before with respect to the workpiece's
path of movement from the third position towards the abrasive sheet surface), to the
second position, or where the second position was before contact had been attained,
slightly beyond the second position. The fourth position is selected so that the actual
contact forces between the abrasive sheet and the workpiece have a maximum pressure
of between the desired range of 0.25 and 100 psi, and more preferably within the other
ranges of preferred pressures desired in the lapping process. It is again most preferred
that the pressure control mode used for the movement of the workpiece into contact
with the abrasive sheet surface assures that the contact pressure is within the desired
range. This is effectively done by assuring that the difference in forces (between
the supporting upward vertical forces and the lowering downward vertical forces is
the same as or preferably less than the intended contact force. The chosen difference
forces might have to be smaller than the desired contact force to avoid the additional,
but temporary force that would be added because of the momentum of the workpiece and
the workpiece holder. That momentum would be absorbed, in part by compressive activities,
but the momentum would definitely tend to momentarily add to the contact force between
the abrasive sheet and the workpiece. By carefully controlling the relative forces
(e.g., the weight is a constant and the air pressure or hydraulic pressure, for example,
may be measured instantaneously or controlled), the contact force, even in the initial
moments of contact can be accurately controlled. The contact forces during lapping
can be accurately controlled by using stress gauges or the like to indicate the level
of forces that must be provided in the support or driving force system provided in
the movement of the workpiece holder.
Dashpot
[0227] A hydraulic or pneumatic dashpot or damper or snubber can be used along with the
air cylinders. This device could be spring loaded to raise its plunger or cylinder
rod cylinder into an up position toward the piecepart lift mechanism arm. When the
arm is lowered by reducing pressure to the cylinders which act against the weight
of the piecepart assembly, the dashpot will control the speed at which the piecepart
contacts the abrasive. The dashpot can be adjusted for fast travel or slow. This can
be used to control the momentum in the moving piecepart and piecepart holder.
Force Sensors
[0228] Force sensors can be mounted on the end of the lifting cylinders (e.g., the air cylinders,
hydraulic lifters, electronic or electrostatic lifters, etc.) and also be attached
to the piecepart assembly arm. As the force sensors are mounted in series with the
air cylinders, they would sense and indicate the actual pressure that the piecepart
arm is experiencing. If the cylinders are deactivated, the sensor would still indicate
the force that the arm is experiencing directly from the screw drive. These force
sensors are typically strain gauges mounted on bending beams but may also be piezoelectric
or other type devices. The force gauges may be integrated with the force control and
position control devices through a computer with a program set up to perform specific
levels of contact pressure during each, every or any lapping stage.
[0229] This same force sensor can be used to sense the force between the piecepart and the
abrasive disk. As the piecepart arm is lowered onto the moving disk some of the force
supplied by the driven screw on the air cylinders supporting the piecepart assembly
is now supplied by the contact force. The net result is a reduction in the force on
the sensor. If all of the weight of the assembly were on the abrasive, the force gauge
would read zero.
[0230] If an additional force were to be applied downward for extra high grinding force,
then the sensor would change signs (if the sensor were initially in a tension mode)
and the total force would be the weight of the assembly plus the new applied force.
This additional force could be used where the differential between the lifting (supporting)
force and the downward force were intentionally kept small so that the amount of contact
force could be actively controlled by a driving force applicator. This driving force
applicator would be any system which could apply a downward vertical force in controlled
amount onto the workpiece holder. Electric, electronic, hydraulic, magnetic, air pressure
or any other force supply could be used.
[0231] The force sensor can be used to establish the location or position of the piecepart
as it just makes contact with the abrasive disk. Here, the abrasive disk is stopped
(and if desired, a piece of paper, etc. of known thickness is laid on the stationary
abrasive) and the piecepart assembly is lowered until it is just in contact, at which
time the force sensor will change its reading to correspond with the amount of force
now being applied to the piecepart. Contact is now used as a mechanism to establish
the position by use of a precision position scale attached to the piecepart slide
arm, or by programming into computer operated controls on the system.
[0232] The force sensor can be a single readout device or multiple units. Use of multiple
units increases the reliability of accuracy in the sense that each of the sensors
should give the same reading for a given equally shared load, so one bad sensor should
give a different reading which can trigger a sensor accuracy review. Using three sensors
mounted in a tripod arrangement gives a "three-point" natural contact for equal loads
to each device. Also, any defective device would disagree with two others which increases
the redundant reliability factor. The part contact force can be easily read out by
"taring out" the weight of the part holder assembly. Three force sensors reduce the
offset deflection of the bending beam used for mounting an electrical strain gauge
sensor.
Precision Position Scale
[0233] A linear encoder device such as a Hindenhain brand scale or a LVDT (linear variable
differential transformer) can be used to establish the position of the piecepart as
it is processed by the machine during the lapping process. The position sensor allows
control of the amount of material removed by the grinding process by comparing the
position of the piecepart assembly relative to its fixed height slide mount to the
changing position as the piecepart is ground or lapped. The Hindenhain brand linear
encoder has the ability to determine position changes of 0.1 microns or less, and
therefore is quite useful within the objectives of the invention. Another device which
could be used to accurately determine position as an alternate to the Hindenhain device
is a LVDT device.
Edge Finder Switch
[0234] An edge finder device used by machinist to physically locate the edge of a part to
be machined for reference input for a CNC machine controller or for manual machining
control may be used to determine that the air cylinder has lifted the assembly off
the bottom home position. Another similar unit may be used to confirm that the assembly
is in a fully raised position. These units typically are able to locate within 0.001".
An edge finder switch can be used to sense liftoff of piecepart contact with abrasive
- establishing the "second position".
Auxiliary Lift Cylinders
[0235] Small pneumatic or hydraulic cylinders can be used either to independently counteract
part of the weight of the work holder assembly or be adjusted to exactly counteract
the weight of the assembly or to provide more lift than the assembly. This last arrangement
would then require a downward force to push the workpiece against the abrasive table.
Cylinder Pressure Sensor
[0236] An electronic pressure sensor can be used with the force lifting mode (or the position
sensing mode) such as with the air cylinders to be used to calculate the theoretical
lifting force of the cylinders.
Slides
[0237] A variety of slides can be used, including Thompson brand balls on single rods, Daedal
balls on four small rods, and air bearing slides to obtain low friction forces which
act against the piecepart holder. Friction slide forces of typical slides are generally
greater than the desired grinding contact forces which can be very low, in the 1 to
20 lbs. range for most parts.
SECOND METHOD - LINEAR MOTOR
[0238] A second method of providing pressure force control during lapping or grinding would
be to use a linear motor operated in a position mode control for moving the piecepart
about the machine and then changing the mode of the motor control just before the
part makes contact with the moving abrasive. As the motor current on a direct current
DC motor represents force for a linear motor (or torque for a rotating motor) the
control mode change can be made very quickly by modern controllers.
[0239] The linear motor position mode system would be used with other functional devices
much the same as for the FIRST METHOD using a screw drive system.
[0240] Of particular note is the above described precision position scale which can be used
to establish the position of a piecepart starting the lapping or grinding process
and to follow the size change as material is removed. Here, the initial position of
the piecepart in contact with the abrasive wheel can be determined by observing a
change in the current of the linear motor upon making contact between the piecepart
and the abrasive platen as less force is required to sustain the weight of the workpiece
assembly when part of the weight is borne by the contact force.
[0241] Other combinations of devices may be utilized such as a lead screw; air cylinders
both of traditional design and AIRPEL low friction design; a dashpot to control descent
speed; a force sensor system; or an edge finder switch; or auxiliary lift cylinders.
THIRD METHOD - HYDRAULICS
[0242] A third method that can effect a solution is the use of hydraulics to both move the
piecepart precisely to different positions and also to effect a pressure or force
based contact with the moving abrasive media. A single low friction cylinder would
be used which would have a number of common input fluid sources which are coupled
or decoupled with the use of solenoid valves. The cylinder would be either connected
directly to the work holder lift assembly or connected in series with a force sensor.
[0243] The cylinder and work holder assembly would be positioned very accurately by the
use of high pressure low leakage gear pumps such as those with the Zenith brand name.
The nominal pressure would typically be less than 100 psi even though the pumps would
have the capability of generating more than 1,000 psi. A large capacity gear pump
would be used for fast travel and a very small gear pump would be used to make precise
minute incremental changes in position. Here, the gear pump would be operated by use
of a stepper motor which will allow a fixed increment of fluid to be injected into
the cylinder which would raise in proportion to the surface area of the cylinder piston.
Generally, a 1 inch (2.54cm) diameter cylinder would be used with a pump which has
a volume output of 1 cc or less per revolution and a step motor which has 50,000 incremental
steps per revolution to obtain very small changes in position per step increment.
[0244] When a desired position is reached, then the solenoid valves are closed, which prevents
leakage back through the pumps and holds the part holder assembly in place.
[0245] A precise position measurement device such as a Hindenhain scale or a LVDT is used
to indicate position of the assembly In the event of significant leakage of hydraulic
fluid past the cylinder rod end cup seals, a change of position is sensed and a corresponding
corrective amount of fluid is injected into the cylinder by an activated gear pump.
Large diameter cylinders preferably would be used to reduce cylinder friction so that
the cup seal lips are not held too firmly against the cylinder wall because the hydraulic
pressure is low due to the large surface area providing adequate lifting force to
raise a typical work holder assembly weighing, for example 30-100 pounds, such as
approximately 60 lbs.
[0246] To apply a controlled downward pressure to hold the piecepart to the abrasive surface,
the downward force may be controlled by an air/oil (pneumatic pressurized oil container)
source. After the piecepart is positioned very closely above the abrasive surface,
perhaps only 0.050 inches away, the solenoid valves are controlled so that the hydraulic
pressure applied to the cylinder is from an air/oil source. The air pressure is reduced
and the cylinder starts to drop but the speed is held in control by a separate adjustable
dashpot or by orifice flow restrictors. Contact abrasive pressure during the grinding
event is then controlled by an E/P voltage controlled pressure transmitter such as
supplied by Wats Co. or Rosemount Co. to change it as desired over the duration of
the grinding cycle event. After the grinding event, the air/oil device can be used
to lift the piecepart from the surface of the abrasive and then through the use of
solenoids, transfer can be made back to the gear pump based position control system.
[0247] A ball check valve can prevent formation of fluid bubbles when a vacuum is generated
by reversing a gear pump when a cylinder is bottomed out and can't move. Mechanical
stops can be used to limit the motion of the cylinder. A load cell force sensor system
can also be used in series with the cylinder to obtain an independent reference of
the force which can be compared with a calculated force based on the pressure readout
device sensor which gives the pressure of the fluid in the cylinder at all times.
35. POSITIONING HOLES ON THE DISK OR SHEET
[0248] Problem: When using disks of abrasive coated material in lapping or grinding operations,
especially when using thin disks of diamond coated plastic which are round (e.g.,
circular or annular in shape), there is a problem of positioning and maintaining the
position of the disk, especially during high speed operation (e.g., at perhaps 2,000
to 3,000 revolutions per minute). In the past, these disks have been either positioned
with a casual surface tension bond of a water film or also by use of a aggressive
or nonaggressive PSA (pressure sensitive adhesive) layer which allows disks to be
removed and used again. In the probable event that the disk would be installed even
slightly off-center on the rotating platen when it is stationary, there would be mass
out of balance. This would be a significant problem with high speed rotation of the
disk due to the center of gravity not being positioned at the exact center of the
rotating platen. When the platen is increased in angular velocity, the eccentric centrifugal
force due to out of balance mass is progressively increased by the square of the rpm
speed. This force would have a tendency to move the abrasive disk sheet even further
out of balance with the ultimate possibility of the disk setting up vibrations which
would affect product surface quality or perhaps leaving the platen with potential
operator danger.
[0249] Solution: The disk needs to be positioned initially accurately on the platen when installed
and then maintained in that position by at least one mechanical means. One technique
for initial accurate positioning would be to punch a small or larger hole at the center
of the disk and have a corresponding pin or post located at the center of the platen.
By placing the disk on the pin or post, the disk would be centered and restrained
at its true balance position. The disk could be easily prebalanced with respect to
the hole without the necessity of placing it on an active platen. The existence of
a pin or sub post would not materially affect the use or utilization of the expensive
disk or affect the processing techniques of lapping or polishing as the linear velocity
vector at the center of the disk area is quite small. The center of the disk is seldom,
if ever, actively used in polishing. Competitive techniques using slow rpm (approximately
200 rpm) methods employ platens with large holes at the inside center and radius.
Larger holes, e.g., greater than 3 cm, may actually be used also. Another technique
for proper initial positioning would be to use a slightly raised outside edge about
the thickness of the disk at the outer periphery to capture the disk and position
it. In both cases, water or water plus PSA or PSA can still be used to temporarily
secure the disk to the platen surface.
36. LIVING HINGE ALIGNMENT OF THE PIECEPART HOLDER
[0250] Problem: The alignment of the part holder with the rotating platen is critical to achieve
precision flat and parallel grinding of pieceparts which are vertically positioned
in contact with the abrasive and moved laterally in "X", "Y" patterns along the surface
of the rotating abrasive.
[0251] Solution: A simple, inexpensive, stable and adjustable mechanism is to mount the vertical
piecepart assembly mounting plates, each of which has a "living hinge" on one end
and 1 or 2 adjusting screws on the "free" end. The adjusting screws allow the free
end of the plate to be pivoted nominally in a pure axis rotation about the semi-fixed
hinged end which creates the ability to adjust the position of a mounted apparatus
in one axis. The use of a second similar living hinge plate mounted at a position
90 degrees to, but flat to the first plate, allows the nominal adjustment of the plate
about the second axis perpendicular to the first. By adjusting both plates independently
and together as a system, it is possible to easily align an apparatus precisely perpendicular
to a reference plane. Simple mechanical screws could be used, differential thread
mechanical screws could be used for fine adjustment, wedge slide blocks could be used,
as well as could thermal expansion bolts or other similar devices. In all cases the
flat plates remain flat but some twist out-of-plane could be effected by independently
adjusting two bolts at opposed ends of the free end of the plate. Adjusting could
be done mechanically by hand or by motor driven screws, electrical heat supplied to
thermally expanding bolts or piezoelectric actuators. Adjustments could be made to
achieve precision flatness or perpendicularity or to provide slight contact angles
to create unique grinding efficiencies by closed loop controllers also.
37. INCREASED SIZE OR MODIFIED ACIRCULAR SHAPE OF A CENTERING POST TO STABILIZE THE
SHEET
[0252] Problem: Operation of the lapper of the present invention is typically at 3,000 rpm with
a 3M Diamond Coated Abrasive disk having a twelve inch diameter. The disk is held
to the steel rotating platen by water film surface tension and positioned by a 0.5
inch (1.27cm) diameter hole at the center of the disk used with a 0.5 inch (1.27cm)
diameter post at the center of the platen. At the high speeds, the disk lost its surface
tension adhesion and was thrown off the platen while polishing a tungsten carbide
piecepart. The forces on the disk were such as to lift it off the centering post,
and the whole disk was thrown off to the side of the machine, opening a cavity at
the top of the machine post.
[0253] Solution: The 0.5 inch (1.27cm) centering post was made larger in diameter to a 1 inch (2.54
cm) diameter or more post. Also, the post could have a non-circular shape with at
least one surface positioned against a center post which would resist rotation, such
as a hexagonal shape or an oval shape which would prevent the disk from rotating relative
to the tangential surface of the disk. The post could also be made higher so the chance
of the destructing disk climbing up the height of the post would be diminished during
this type of event. Another technique would be to employ a clamp type of device to
any of these round or non-round posts to clamp/hold the disk firmly to the surface
of the platen at the center area of the disks which is not used for polishing because
of the slow lineal velocity in that sector. The clamp could consist of a spline locked
washer pressed on the disk surface with a thread nut engaged with a top threaded post.
Springs could also be used to control the amount of force and to evenly spread the
force uniformly. Ball detent or other snap latch fixturing devices cold also be employed.
As previously noted, since this section of the abrasive sheet would not be in lapping
contact with a workpiece, adhesive could be used in this area to secure the sheet
while vacuum was used in the other area to improve planarity.
38. DISTRIBUTION OF VACUUM OB BACK SURFACE OF THE SHEET
[0254] Problem: Round disks of minute diamond particle coated sheets of plastic film on 3,000 rpm
spinning platens are difficult to hold in contact with the platen when running in
contact with stationary or semi-stationary workpieces. When an abrasive disk becomes
loose by breaking the water film "adhesive" surface tension between the disk and the
platen, it has a tendency to rip or bunch-up and wedge between the workpiece holder
and the high inertia spinning platen and can easily damage a workpiece part or can
destruct portions of the workpiece assembly with the possibility of great danger to
the operator. This is a unique problem due to the very high rotational speeds of 3,000
or greater rpm with a platen of 15 inch diameter (38cm) or more constructed of heavy
steel which could generate explosive type failures As this equipment is operated horizontally
for the most part, the whole surrounding area around the machine is susceptible to
this danger. One method to reduce the likelihood of this separation problem is to
coat one side of the diamond abrasive disk with a PSA (pressure sensitive adhesive)
film to temporarily bond the disk to the platen. This adhesive creates a flatness
accuracy problem in that its normal thickness accuracy varies greatly around the disk
which causes high areas of lapping contact for this super precision abrasive contact.
Secondly, when a disk is removed, some sectors or pieces of transparent PSA adhesive
remains on the platen and forms a bump when the next abrasive disk is installed in
the platen which then destroys the smooth vibration free abrasive lapping at high
speeds.
[0255] Solution Use diamond or other abrasive disks without using PSA adhesive and first position
the disk at the true center of the platen by use of a centerhold in the disk positioned
over a post positioned at the center of the platen (or by other centering means) and
then hold the abrasive disk to the platen by use of vacuum by use of a rotary union
on the hollow rotating platen shaft. The preferred area to apply the vacuum would
be at the inner radius of the disk which would seal out first as the disk is installed
at the platen center and also because this inner one fourth or so of radius is not
used much for lapping because of slow surface lapping velocity. The second most preferred
vacuum area would be the outer ½ inch (1.27cm) of disk radius at the periphery of
the disk as this would also not be used much and would have large holding force.
39. INDEX LOCATION MARK ON ABRASIVE DISK
[0256] Problem: Fast removal, remounting of disks (10-15 second intervals of typical use) need to
be replaced in the "original" position. When a disk is installed on a platen it can
be held by double stick adhesive tape or by vacuum. A typical disk is a thin layer
of plastic film which is coated with abrasive diamond or other ceramic type coatings
which wear off with use - presenting new fresh sharp material for fast accurate material
removal. Also diamond particles are captured with metal plating on a film and an additional
backing material is adhesively bonded to this plated film. If the finished product
abrasive disk is attached to a rotating platen with adhesive, the adhesive is usually
coated on both sides of another thin film, all of which have dimensional tolerances
so one area of a disk may be thicker than another and result in non-uniform abrasive
wear. All the variations in thickness of the sticky adhesive can be eliminated by
use of the vacuum hold-down holes of the platen.
[0257] Solution: When either the platen or the disk is uneven, only the high spots of the abrasive
disk will wear down first. When a disk is removed after typically 15 seconds usage
(because 10,000 sfpm abrasive cutting is 20-30 times faster than conventional grinding)
and a new finer grit disk is used, there needs to be a method to accurately relocate
the disk the next time it is used. A disk typically can be used ten to hundreds of
times.
[0258] By marking a disk with color pen or mechanical cut-outs, notches, etc. and positioning
this disk mark on a corresponding mark on a platen, a disk is reinstalled at a location
where it "fits" and does not have to be reground to size for the next operation, saving
time and disk wear costs.
40. ANNULAR DISKS
[0259] Problem: Using hold-down vacuum holes, adhesive annular disks at the outer periphery platen
of a high speed rotating platen have special problems of lifting at the inner radius
due to surface water and grinding particles being driven under the annular film disk
by the high rotational speeds. Once lifted slightly, the raised edge gathers even
more water/debris which raises the edge further and presents this structurally weak
disk edge to a stationary piecepart having a typical sharp edge - which has a tendency
to catch or cut the disk edge. Because of the high speed of the platen, running at
from about 1,000 to 10,000 surface ft/min, the disk can become damaged and crumpled
and tear and then either be thrown off the platen or wedge between the platen and
the piecepart holder which can create large dynamic forces which result in dangerous
flying shrapnel. If a vacuum hold-down is used, the vacuum would have a tendency to
suck the abrasive debris particles into the vacuum holes, eroding the hold edge and
enlarging them, which would locally distort the working surface of the abrasive disk.
Also centrifugal force from the 500 to 3,000 rpm 12 inch (30.5cm) diameter disk would
have a tendency to curl or raise up the inside disk edge.
[0260] Solution: It is desirable to provide a full circular disk with a method of "raising" the outboard
annular section so water and debris particles can't get under the inside radius to
start the curl-up. A uniform disk with no annular cutout or even an inner radius hole
would be best because no water or debris can get under the disk. Because of the high
costs of the disk material, an annular ring of abrasive disk could be adhesively bonded
to another uncoated circular (not annular) disk. This could be done by adhesive securement
at the meeting edges of the central disk and the annular disk, butt welding, sonic
welding and any other form of attachment between the two sheets that provideds a barrier
for water or abrasive grit flow under the annular sheet. The inboard circular disk
would be thinner than the outboard annular abrasive sheet disk.
41. Simplified Drive Motion
[0261] Problem: It is desirable to have a simple drive mechanism to position a stationary or rotating
workpiece on the outer periphery of a high speed rotating (approx. 3,000 rpm) disk
abrasive for most of the processing time with a small portion of the polishing or
lapping time spent at the inner radius portion of the abrasive disk where the surface
speed is reduced and the abrasive action is reduced.
[0262] Solution: A simple, eccentric harmonic motion, constant speed rotation as provided by a DC
or AC gear motor hub can be used to drive a linkage system will provide smooth continuous
motion of a workpiece with most of the time in a given hub rotation cycle with the
workpiece operating at the outer periphery of the abrasive disk which has the highest
surface speed and highest grinding action and a very small portion of the cycle time
spent at the inner radius, low surface speed, and reduced grinding action portion
of the disk.
42. BELLOWS SANDWICH BALL PIECEPART HOLDER
[0263] Problem: A piecepart may need to be rough ground flat which requires a rigid (non-pivoting)
piecepart holder, but then may need to be processed on a spherical ball piecepart
holder to achieve extreme flatness of 1 to 2 light bands or less. It is desirable
to do this on one single machine using coarse grinding media of 40 micron particle
on the rough finish using the rigid holder and 3 micron particles using the pivot
holder.
[0264] Solution: A precision rigid piecepart spindle piecepart holder system can be constructed with
vacuum holding of the piecepart for rough grinding the piecepart flat. Then a flat
sandwich construction spherical ball pivot piecepart holder can be constructed with
an internal vacuum chamber to allow the piecepart to be held or mounted with the same
vacuum source and utilize an internal spherical ball for allowing the piecepart to
"float" on the abrasive surface rotating in contact with the piecepart holder.
43. LAPPER PLATEN
[0265] Problem: Constructing a high speed lapper platen rotating at 10,000 SFM velocity or 12inch
(30.5cm) diameter wheel at 3,600 RPM is difficult where the annular edge of an abrasive
disk is raised for use with an annular ring of abrasive disk. It is necessary to avoid
water or debris getting under the inboard radius. Also when abrasive particles are
drawn into the vacuum holddown holes on the platen, they tend to wear the edges of
the holes and enlarge them, which results in distortion of the flexible abrasive disk
sheet at he hole locations.
[0266] Solution: The platen can be constructed with an outboard raised circular land area and have
a lower inboard area to avoid contact with the piecepart but yet have a further recessed
(depressed) lip or edge so the inner radius of the annular abrasive disk is below
the inboard area of the platen so that water or debris on the surface of the platen
travels above or on the top surface only of the abrasive disk and does not raise the
inner radius. This is shown in Figure 25, with platen 1400, abrasive sheet 1402, inboard
area 1404, and the distance of the inner radius of the annulus below the inboard area
shown as 1406.
[0267] It is desirable to make the platen out of hardened stainless steel about Rockwell
"C" hardness 40 minimum or plate with a hard chrome of Rockwell C 65 or harder on
steel to reduce the wear of the vacuum holes.
44. PIVOT BALL SANDWICH
[0268] Problem: For high speed lapping, it is desirable to quickly convert from lapping with a rigid
piecepart holder to a pivot type holder, particularly when utilizing a vacuum to hold
the piecepart to the holder for both the rigid mount and the ball pivot mount.
[0269] Solution: A piecepart holder can be constructed as a sandwich of two flat surfaced plates
with a single ball at the center. This ball will transfer downward abrasive contact
pressure force to the piecepart and yet allow the surface of the piecepart to move
freely in contact with the moving abrasive surface so that it is in alignment with
this non-perfect perpendicular mounting between the holder axis and a normal right
angle with the platen surface.
[0270] The vacuum present at the surface opening port holes of the rigid spindle holder
can be transferred through sealed internal passages in the sandwich holder to the
piecepart contact surface simply by clipping a flat pancake sandwich holder to the
rigid holder. Because both the rigid holder surface and the matching piecepart surface
is very flat and smooth, an effective vacuum seal is effected between the two surfaces
upon contact. Surfaces need to be cleaned to obtain a good seal. The ball can be sealed
with RTV (room temperature vulcanizing rubber), sealants or grease or other material.
Two concentric rings of plastic or elastomer can be positioned so as to form a passageway
for vacuum transfer from one surface to another and yet seal the passageway from leakage
to outside the sandwich. The outer ring can be attached to the sandwich by adhesive
or other mechanical or cast-in-place means. The elastomer can flex with a controlled
stiffness to allow angular motion centered about the ball. Both sandwich plates can
be precision aligned perfectly parallel to each other before attaching the elastomer
rings and they would tend to maintain this parallelism for presenting the piecepart
to the abrasive surface. Radial pins in a controlled slot length will prevent over
travel on the spherical ball pivot and also prevent tangential rotation of one sandwich
disk relative to the other for torque input to the holder unit.
45. BREAK-UP OF THE BOUNDARY LAYER AND HYDROPLANE PREVENTION
[0271] Problem: Pieceparts tend to hydroplane when they are held in contact with high speed platens
using a water film that develops a boundary layer between the platen and the piecepart.
The resultant piecepart is not ground flat because the boundary layer pressures tip
the part upward at the leading edge.
[0272] Solution: It is desirable to break up this boundary layer by having abrasive disks coated in
striped patterns such that only short land areas, as measured perpendicular to the
direction of travel, with grooves or spaces in between these land areas are present
to relieve this hydrodynamic pressure. The land areas could be formed by spiral patterns,
by islands of abrasive or other patterns.
46. ESTABLISH RELATIVE POSITION BETWEEN PIECEPART AND MOVING ABRASIVE IN LAPPING
[0273] Problem: It is difficult to establish the precise distance for moving a partially ground
piecepart down to contact the moving surface of an abrasive disk of unknown thickness
when initially starting to process a piecepart or when changing to a new abrasive
disk of finer grit without damaging the piecepart or approaching too slow. When using
coarse abrasive, a few mils are removed in 10 seconds but when using fine 3 micron
abrasive, only a few microns are removed in 10 seconds. The speed of contact used
to start new grinding with a finer grit abrasive is important, so as not to lose set-up
approach time.
[0274] Solution: A piecepart can be processed, the abrasive disk changed and the piecepart brought
into close proximity to the moving abrasive disk, perhaps 1 to 10 mils (0.001 to .010")
away. At that time, an excessive amount of water lubricant can be applied to the surface
of the disk which would tend to hydroplane the piecepart without having contact with
its abrasive particles. A force sensing device can indicate when this physical contact
has been made with the water wetted surface. A correlation can be established with
the amount of force sensed and the exact water flow rate to determine the precise
distance between the piecepart and the abrasive sheet. Then the water flow can be
reduced progressively while the piecepart is lowered to the abrasive part surface
until grinding or lapping action starts to take place. In this way the water film
would act as a protective barrier at first contact and allow an algorithm estimate
be made of the necessary vertical action required to remove very limited amounts of
piecepart material, perhaps 0.1 micron per second or less. This whole procedure could
be automated and computer controlled with the parameters of force, flow rate, rotational
speed (or any combination thereof need) correlated to separation distance.
48. ADHERENCE OF PIECEPARTS BY NON-AGGRESSIVE ADHESIVE
[0275] Problem: When lapping parts, it is typically quite difficult to hold the lapped parts in
a fixture so that they are flat and parallel when presented to and in contact and
when removed from the lapping platen wheel, particularly when the platen is rotating
at high speeds of 3,000 rpm as compared to 200 rpm. If a part is fixtured by mechanical
clamping it is subject to being loose or compliant and patterns or lack of highly
accurate surface finish such as (4) four light bands is not attained. It is also difficult
to quickly and accurately load and unload parts. Also the surface finish of the part
holder on the mounting side may disrupt or destroy the surface already polished when
lapping the other side.
[0276] Solution: Individual parts, typically 1(1.27cm) to 2 inches (5.08cm) in diameter or rectangular
which may be thin (.010 inch, (0.0254cm)) or thick (0.500 inch, 1.27cm) can be fixtured
to a precision flat steel, other metal, or other material plate by use of paraffin
wax as a bonding agent. Here the plate or part can be coated with wax or wax simply
melted on the plate between the part and plate and the part placed on the plate, heat
applied and the two would have a fully wetted surface of molten wax. The parts could
be positioned by mechanical or other means of uniform pressure or force so they lay
flat with a uniform and controlled thickness of molten wax. The mechanical alignment
pressures should equal or exceed the pressures to be encountered during lapping to
assure that there is no movement under the lapping pressure. Upon cooling the part/plate
assembly, the parts would be positioned accurately and strongly to the plate ready
for lapping action. Then the plate could be attached to a piecepart holding device
by use of a vacuum chuck or by use of magnetic chuck if the plate were steel. The
piecepart holder would have a ball type pivot close to the lapping action surface.
Plates could hold one or many individual parts. Upon lapping one side, the plate/part
assembly could be heated, the parts removed and if desired, the parts could be reassembled
with heated wax on the plate with precise parallel alignment with no danger of damage
to the lapped surface because of separation from the plate with the wax In this way
many plates could be preassembled for high production rates with a single lapper.
49. SUPPORT OF THIN WORKPIECES IN POCKETS
[0277] Problem: It is difficult to hold small hard parts which are thin (typical size: 1 x 1 x 1/8inch,
2.54x2.54x0.32 cm) in such a fashion that both surfaces of the flat part can be polished
by lapping action by a high speed 3,000 rpm rotating disk with a diamond abrasive
disk exerting substantial lateral force by the moving platen powered by a 2 HP motor
for a 12inch (28.5 cm) diameter disk when subjected to about 10 lbs. (4.3kg) of normal
clamping force when subjected to surface water spray.
[0278] Solution: These small parts can be fixtured to a flat surfaced piecepart holder or a holder
which has small shallow pockets, just larger than the length and width of the flat
part so that the exposed surface of the part protrudes away from the holder. In this
way, the abrasive disk polishing action is applied to the piecepart and not the holder.
A medium temperature wax can be melted and used to bond a rough surfaced part to the
flat smooth surfaced part holder plate. The flat plate in turn can be attached to
a rotating pivoting arm which is swept across a portion of the surface of the high
speed rotating disk until a smooth flat polished lapped surface is generated on one
side of the piecepart. Then the part holder plate which would have 1 or 2 or many
more parts attached to it in a fixed mounting pattern could be brought in contact
with another mounting plate having a flat surface or a shallow pocketed surface pattern
which matches the first part plate. A higher temperature wax (than the first wax)
could be melted at the surface of the parts already lapped and as they were held in
flat contact with the new plate, the original lower temperature melting point wax
could release the parts from the first plate and upon cooling somewhat, the parts
would be transferred as a group to the second plate ready to have the rough remaining
side lapped as the first plate is readily removed. High production rates of lapping
flat parts on both sides with good parallelism could be achieved.
50. VACUUM CHUCK HOLDER
[0279] Problem: It is difficult to quickly load pieceparts on the piecepart holder for use with
a high speed lapping and polishing system. Also it is difficult to generate a flat
parallel system of polishing parts where .001 to .002 inch (0.025 or 0.05 mm or so)
material is removed from a side to make them smooth, perhaps to 4 light bands, flat
and parallel. Much of the time, hot melt adhesives are used which are slow and cumbersome
to apply and also difficult to remove because of contaminating the precision surface
of the piecepart for later use of the part. Typically the piecepart holder has a gimbaled
spherical ball end to freely allow the part to move about radially to self align the
pieceparts (one or more) with the surface of the rotating abrasive platen.
[0280] Solution: A piecepart holder can be constructed out of a heavy metal such as steel which has
substantial mass very close to the surface of the abrasive disk. The unit will be
allowed to move freely with the surface by the ball-end holder. A substantial hole
can be made within the ball-end device which would allow vacuum to be coupled to the
piecepart holder individual part pockets to firmly hold the flat pieceparts up tightly
against the tight fitting pocket. To create and maintain a good vacuum, a thin layer
of oil or grease can be applied to the piecepart to seal any leakage paths. In this
way, by simply removing the vacuum applied to a rotary union to the driven shaft open
inside diameter, the part is released, can be turned over and the opposite side lapped
to produce a high quality surface not damaged on the already done side because intimate
part-to-holder contact is not made because of separation by the film of oil, yet is
stiff enough for good polishing action.
51. ABRASIVE DISK ANNULAR SHAPE
[0281] Problem: When using a diamond (or other) abrasive disk rotating at very high surface speeds
of 10,000 fpm, most of the abrasive cutting action takes place at the outer periphery
of the disk. The inside area of the disk has low surface velocity and low cutting
action and also low wear rates so that when a piecepart traverses the disk in a sweeping
motion to prevent wearing of tracks or grooves on the abrasive, there is uneven wear
between the outer and inner surfaces of the disk. There is typically a small 1/2 inch
(1.27cm) diameter hole in the inside of the disk at the center to act as a positioning
agent to apply the abrasive disk at the center of the platen to obtain good balance
of this very high speed system. A larger diameter round section could be removed from
a disk to create an annular ring of active abrasive material somewhat larger than
the piecepart which eliminates the inactive (and raised) uneven section but then the
centering registration hole for positioning the disk is lost.
[0282] Solution: A disk can be fabricated with abrasive coating covered on the whole surface of the
disk. Then the inside section of the abrasive toward the center of the disk could
be removed by grinding or peeling it off leaving the backing material intact with
the centering hold. Here the piecepart could be in contact with the raised section
of the abrasive on an annular outer ring only as the abrasive is raised (by coating)
from the disk backing material (usually plastic sheet). Another way would be to punch
out the center ring of the disk for separate use and then use a centering plug with
a small locating hole so the plug could be centered on a platen center post and the
annular disk centered on the plug, become fixtured by the vacuum grip platen and the
plug removed for complete freedom of movement of pieceparts over a disk as the post
could be removed from the platen also.
52. LAPPER WOBBLE PLATE FREE BALL
[0283] Problem: When a wobble plate is used for polishing, grinding or lapping, a piecepart must
be presented exactly parallel to the moving abrasive surface without a leading edge
hanging down where it will be the first section to contact the abrasive. This could
tend to jam the piecepart into the abrasive and damage the outside edge of the piece
part. This problem is made worse by having a heavy piecepart mounted off-center with
the mass center of gravity outboard of the center axis of the wobble plate. This would
tend to dip the heavy side down and create an out-of-parallel presentation to the
moving abrasive. Also any friction on the wobble plate ball or an out-of-balanced
spring center system will result in dipped edges of the part.
[0284] Solution: A ball is used to support the applied contact force of the wobble plate. The ball
is constrained in a cylindrical hole such that the ball is free to fall loose with
the weight of the lower movable section of the wobble plate and the weight of the
piecepart combined. There may be 3 adjustable screws at 120 degrees apart which act
as parallel location stops to hold the lower piecepart parallel to the wobble plate
spindle bottom flat surface. This results in the piecepart being parallel to the moving
abrasive surface. The loose ball will allow the free lower section of the piecepart
and holder to be held accurately by the 3-point screws. Then when the piecepart is
lowered into contact with the moving abrasive, flat contact is initially made but
the free motion slack in the ball holder is then taken up (perhaps .010 inches, 0.25
cm) so that the wobble plate is free to move in an angular fashion and the ball surface
is in contact with a hard flat surface which results in very low friction. An anti-rotation
leg is used also. One, two or three legs can be used for anti-rotation with clearance
for gimbal wobble angle action.
53. HIGH SPEED SUPER ABRASIVE
[0285] Problem: It is difficult to quickly lap hard metal or ceramic or other materials with conventional
lapping techniques using disk platens which are 12(28.5 cm) to 48inches (114 cm) in
diameter operating at 200 to 300 rpm using loose abrasive paste media. Larger diameter
platens are potentially dangerous at high speeds.
[0286] Solution: A high speed lapping system can use fixed diamond abrasive coated or plated on a
disk sheet of material and be used on a rotating platen disk with a diameter of 12
inches (28.5 cm) when operating at 3,000 rpm which gives a surface speed of about
9,000 feet per minute. If a larger diameter platen wheel of 15 inches (38.1 cm) diameter
is used, the rpm can be lowered somewhat to perhaps 2,800 rpm to achieve the same
10,000 (or 9,000) feet per minute (fpm) and if the wheel diameter is 18 inches (47.7
cm) diameter, then the speed can be further reduced to produce 9,000 - 10,000 fpm
at the outer periphery of the disk. Any reduction of rotational speed for large diameters
is desirable because of the potential danger of a high inertia wheel creating problems
if a disk is damaged or comes loose.
54. WATER FLOW RATE
[0287] Problem: The surface finish smoothness and flatness of hard parts made of metal or ceramic
or other materials vary as a function of the work force on the piecepart as the workpiece
is held against the surface of a high speed 9,000 to 10,000 fpm abrasive lapping action.
[0288] Solution: It was found that the amount of coolant and lubricating water or liquid applied
to the surface of the high speed rotating disk affects the quality of the lapping
action. If a reduced flow rate of water is applied, the abrasive cutting rate is increased
as the boundary layer of water is decreased between the piecepart and the rotating
disk, better allowing the tips of the exposed diamond particles to be in more direct
contact with the piecepart and thus are more active in removing material as they penetrate
deeper into the surface of the material. Excessive water covers the abrasive particles
and keeps the abrasive from contacting the piecepart surface. Here if the water flow
rate is increased and the piecepart is more "flooded", then a thicker boundary layer
of water or liquid builds up between the part and the surface of the abrasive disk.
A moderate amount of water will tend to keep the diamond abrasive particles away from
the piecepart some fraction of their maximum penetration which results in a smoother
and flatter surface on the part. One method of utilizing this performance is to have
reduced water flow at the first portion of the lapping period for more aggressive
material removal, but with a resultant increased roughness of the surface. Then the
water flow is increased somewhat during the middle portion of the abrasive cycle to
get better surface finish and yet have a medium material removal rate. Finally the
water flow rate is substantially increased at the end of the cycle to produce a very
smooth and flat surface with a low rate of material removal. Changing of the water
flow rate to alter the material removal rate and to change the surface smoothness
could be easily done with an automatic water flow rate control system which varies
the flow rate during an abrasive cycle.
55. EXTENDED PLATEN BOX
[0289] Problem: When doing abrasive lapping at high surface speeds of 9 - 10,000 fpm on round platens
rotating at 3,000 rpm with diameters of 12, 15 and 18 inches (28.5, 38.1 and 47.7
cm), there is substantial danger when a piecepart is broken off its holder (as it
normally is held with a weaker adhesive or mounting system) and the piecepart being
thrown off the platen or getting stuck on the platen and ripping the diamond or other
abrasive disk causing further possibilities of fast destruction of parts of the machine
with parts thrown out and endangering an operator or others or equipment due to large
kinetic energy contained in the rotating disk.
[0290] Solution: The rotating platen is round in shape with about a 12 or 15 inch (28.5 or 38.1 cm)
diameter. A rectangular corner box is constructed as described earlier to deflect
explosively propelled pieces downward into a collection area. The deflection may be
from a vertical surrounding surface coupled with a lip or partial cover which reduces
the amount of shrapnel which can move vertically out of the work area, as described
above. The box is desirably constructed of a soft plastic (or rubber) such as ½ inch
(1.27 cm) thick high density polyethylene which would tend to absorb impact from a
heavy metal free flying broken-loose part without ricocheting the part back into contact
with the rotating disk which prevents it from being thrown again or damaging the part.
Also, the "square" corners provide a remote area to trap the part and to contain the
part as it stopped moving by being impacted on one or more mutual walls. Having a
distance between the flat walls and the rotating disk which is somewhat larger than
the largest size of the piecepart, centrifugal force would tend to drive the part
off the disk radially and allowing it to eventually roll or move tangentially to a
neutral corner of the box away from the disk. In the same way, crumpled abrasive disks
are collected by the neutral open corners. Having a ledge over the inside portion
of the box also helps trap the parts.
56. COUNTERWEIGHT WORKPIECE HOLDER
[0291] Problem: When a workpiece holder is held up by an air cylinder to provide normal force on
a workpiece against a high speed 10,000 sfpm rotating disk by moving vertically up
and down to load parts and lap them, there is potentially great danger if air pressure
is lost due to air line leaks or electrical failure. If this load of the disk rotating
motor assembly, which may weigh 30 lbs. Or more, drops on the 12 inch (28.5 cm) heavy
rotating disk operating at 3,000 rpm, there is great danger in that the abrasive disk
can be torn or cut, jam up and create danger to the operator or severely damage pieceparts
which may have great value.
[0292] Solution: The vertically moving piecepart assembly can be mounted on vertical slides and a
chain or cable used with a counterweight which is perhaps 10 lbs. (4.6 kg) heavier
than the 30 lb. (13.8 kg) assembly. Upon loss of electrical power which would interrupt
power to the normally used suspension air cylinder or a line leak to the cylinder,
the piecepart assembly would simply and quickly retract to the upper position, out
of contact with the rotating platen and thereby reducing the chance of danger. This
would also be more assured when using an E-stop (Emergency Stop) action switch which
would then not require power to obtain safe action.
57. VACUUM ADHESIVE HOLDDOWN
[0293] Problem: When lapping or polishing at very high surface speeds of about 10,000 surface feet
per minute, it is difficult to mount pieceparts to a rotating holder for contact with
an abrasive disk surfaced rotating platen in a way to hold the parts rigid enough
they are not broken loose from their mount. Also it is desirable to avoid a localized
vibration of the typically thin flat piecepart (which vibration is induced by the
high speed contact with the rotating platen) as patterns of uneven polishing takes
place on the surface of the precision part if it vibrates during grinding. It is further
desirable that one or more pieceparts be processed at a time and that unloading these
parts and remounting new parts is done quickly and easily to provide cost effective
polishing rates of production. Additionally, a method of changing parts quickly so
that one side of a piecepart can be lapped, the part turned over and the second flat
side be lapped to be parallel to the first side. Typically .001 inch (0.025mm) to
.002 inch (0.050 mm) or less is removed from each side.
[0294] Solution: Thin pieceparts of about 1 x 2 x 0.080 inches (2.5x5.1x0.16cm) can be mounted on
to an individual piece of pressure sensitive adhesive (PSA) tape and this taped piecepart
can then be held by a vacuum to a workpiece holder. The surface characteristics of
the nonadhesive side of the tape would be controlled by selection of tape backing
material or by surface conditioning to provide a high friction which would resist
lateral dynamic forces in a plane along the surface of the thin workpiece as the nominal
14 psig (25 inches Hg vacuum) would apply a normal force holding the workpiece to
a rotating holder. A large section of tape could also be used to hold a number of
workpieces at once which would be fast and easy to install by hand or with a robot.
This flexible group assembly of PSA bonded workpieces could then be held into position
against a precision flat surface of a workpiece holder having random vacuum holes
over its surface which would all be sealed by the wide and complete expanse of tape
covering all the vacuum holes and at the same time firmly holding the individual workpieces
to the holder. To process the other side, the group would be removed, tape applied
to the lapped surface side and the tape on the unprocessed side would be easily peeled
off. The tape would not only fixture the parts but would protect the precision lapped
side from scruffing action of rubbing on the holder.
58. SPRING CENTERED WORK PIECE HOLDER COILED VACUUM HOSE
[0295] Problem: When holding pieceparts on a rotating holder in contact with a rotating abrasive
coated platen rotating at a surface speed of 10,000 sfpm, it is difficult to create
a gimballed, free wobble motion so the contacting surface can continuously align itself
to the flatness of the rotating platen and yet be held stiffly enough in a nominally
flat position when first lowering the workpiece holder to the abrasive surface while
rotating so as not to have one corner of a workpiece contact first and be preferentially
abraded away thereby producing an uneven workpiece surface. Vacuum piecepart clamping
hoses could also create problem forces.
[0296] Solution: A coiled spring can be used to apply a self correcting force between the workpiece
holder plate having a gimbal spherical bearing and the rotating drive shaft of the
rotating piecepart holder. This spring would be made of metal or plastic material
which would allow the straightening action to be applied but also would introduce
vibration damping for excitation vibrations set up by the high speed contact abrasive
action. One or more solid plastic coupling bars could provide damped spring action
also. If a vacuum hose were to be used to provide vacuum clamping of the piecepart
to the piecepart holder through a hollow drive shaft, this type of hose could extend
from the shaft and be coiled with perhaps one or less on multiple turns which nominally
lay flat with the upper surface of the workpiece holder which would minimize the creation
of uneven "normal" direction workpiece contact forces as the workpiece holder turns.
59. LAPPER PERPENDICULAR ALIGNMENT OF UPPER PIECEPART HOLDER AND PLATEN - PIVOT POST
ADJUSTMENT
[0297] Problem: It is difficult to adjust the small diameter upper piecepart holder surface to be
precisely parallel to the platen large diameter surface and thus the finished ground
pieceparts may have a coned surface if outside edges of the piecepart are ground more
than inboard areas..
[0298] Solution: The abrasive sheet carrying platen is mounted on a thick heavy steel support plate
with leveling jack screws on the four (or three) outer corners to get a nominal axis
alignment of the platen with the axis of the piecepart holder to be coincident with
the axis of the platen abrasive spindle. Then a swing arm is mounted on the piecepart
holder which is rotated slowly about the stationary platen. The swing arm is extended
out to the surface of the platen. This measurement indicates the "Z" axis error perpendicular
to the surface of the platen at different "X" and "Y" coordinate positions on the
horizontal surface of the platen. Adjustments are then made to align the lower platen
mounting plate to the upper piecepart axis. An upper frame can also be constructed
for the pivot arm lapper by attaching the bottom portion of the stiff pivot vertical
post to a round solid steel rod which in turn is attached to the base of the machine
frame. Then two long arms are attached to the upper portion of the post at 90 degrees
to each other, aligned with the "X" and "Y" axis. These arms can be fixtured with
threaded screws on the outer ends and both "X" and "Y" can be adjusted independently
with these screws which are in effect bending this rigid post at the base. Mechanical
clamps hold the post in place after adjustment. This alignment adjustment could be
automated with stepper motor driven screws, piezoelectric actuators, etc.
[0299] There are a variety of different adjustment actuators which can be used. These include,
but are not limited to a threaded bolt, motor driven threaded bolt, piezoelectric
actuator, and a thermal expansion bolt (e.g., electrically heated thermal expansion
bolt). A stepper motor, servo motor, DC or AC gear motor, and the like can be used
motor to drive the alignment arms to different positions and make corrective adjustments
to align both axis of piecepart and platen as indicated, for example, by an out-of-plane
gap sensor.
60. ANNULAR ABRASIVE DISKS
[0300] Problem: When flat circular disks having diamond or other abrasive media are used on a high
speed platen rotating at 3,000 rpm or more to produce surface speeds of above 2,000
sfpm and even about 10,000 sfpm, the outer periphery of the abrasive sheet at the
outside diameter has a high speed with good abrasive action but the inner diameter
of the disk has a lesser velocity proportional to the radius and less abrasive action.
Most of the abrasive grinding or lapping material removal from a piecepart is removed
by the outer diameter of the disk which tends to wear down the abrasive media at the
outer diameter more than the inner radius which results in an uneven flatness of the
abrasive disk. It typically is a cone shape with a higher section at the circle center
of the disk which prevents a piecepart from being ground or lapped flat across its
surface which is critical to part surfaces having good enough surface flatness or
surface roughness finish for pump seals, computer chips, hard disk computer components
and for other parts. The unworn inside of a disk is not utilized and therefore there
is inefficient use of the abrasive sheet material which is quite costly.
[0301] Solution: An annular ring disk can be used on a flat rotating platen which is made from the
original circular disk of abrasive media by cutting out these rings in a cookie cutter
fashion. Typical rings may be 18 inch OD (47.7 cm) x 15 inch (38.1 cm) ID; 15 inch
(38.1 cm) OD x 12 inch (28.5 cm) ID; 12 inch (28.5 cm) OD x 8 inch (20.3 cm) ID. A
piecepart which is presented in contact with the rotating ring abrasive disk typically
would be swept across both the inside diameter portion of the disk progressively to
past the outer diameter of the annular ring where both the inner and outer radius
of the disk would have diameters and surface speeds and abrasive action and disk wear,
fairly constant across the full surface of the disk ring thereby reducing the cone
effect wear on a given disk which would produce better flatness and more uniform roughness
surface finish on a piecepart. In this way, expensive diamond particle type of abrasive
disks can be fully utilized for good cost savings and efficient use of the abrasive
media. A pivot arm could be used to sweep the workholder back and forth across the
annular abrasive disk ring with a preferred contact to occur in a quadrant of the
abrasive sheet which provides a stabilizing friction contact force directed away from
the rotating axis of the pivot arm. Also an X-Y table can be used to sweep the width
of the annular ring. A single solid circular disk could be cut into multiple annular
rings and the core center circle could also be all sold and used as separate units
with no manufacturing waste. The disks could also be cut into ellipse or oval shapes
with an annular ring shape where the outer and inneredges of the disk would be "moving"
relative to the piecepart and not have a tendency to produce nonuniform abrasive wear
tracks on the piecepart as much as a true circular abrasive disk.
[0302] To increase the efficient use of the annular rings, the piecepart is also rotated
as it is presented to the abrasive sheet surface and is being lapped. This assures
even lapping address by the surface of the piecepart to various radial portions of
the annular abrasive distribution.
61. ANNULAR RING DISKS VACUUM SEALS
[0303] Problem: When using annular ring disks of various sizes on a given circular high speed rotating
platen having a vacuum hold down system, any inboard vacuum holes are exposed or non-sealed
for large sized ring disks and thus the vacuum hold down system doesn't work. The
same is true for using smaller ring abrasive disks with exposed outer vacuum holes.
[0304] Solution: If an 18 inch (47.7 cm) or smaller platen is constructed with concentric paths of
vacuum holes spaced at various radius of the platen, or if scattered holes are positioned
to not create a circular track and to avoid making abrasive "track" patterns, the
exposed holes would be sealed with a pressure sensitive adhesive thin plastic film
on either or both the inside or outboard portion of the vacuum holes left exposed
when applying the nonadhesive backed abrasive disk material having an annular ring
shape with an inside and outside radius, either circular, oval or other shape. This
adhesive backed sealing disk or ring can be left on the platen for a duration of time
and it can be used to register or accurately position guide the annular abrasive disk
onto the true center of the platen for achieving good dynamic balance of the very
high speed rotating assembly operating at perhaps 3,000 rpm or 10,000 surface feet
per minute. Safety is very much enhanced by good balance and the quality of surface
grinding or lapping is also enhanced by good circular location and strong reliable
vacuum hold down of the abrasive disks which may be constructed using fine diamond
particles or other media. The inboard non-abrasive disks described above to reduce
lifting of the annular abrasive sheet by grit, slurry or water would also solve this
problem.
62. ANNULAR RING DISKS ANGLED CONE SURFACE
[0305] Problem: Some specialty grinding techniques can be improved by having an abrasive media disk
with a slightly angled surface relative to the normal typical flat plate surface for
high speed (e.g., above 500, above 1000 or about or above 3,000 rpm, e.g., up to and
beyond 10,000 sfpm) use of abrasive sheeting such as fine abrasive particle coated
disks such as diamond coated disks.
[0306] Solution: Annular rings of diamond or other media coated abrasive disks are generally fabricated
in thin disks with thin metal or plastic 0.005 inches (0.12mm) thick, more or less,
that is locally elastically conformable to a hard surface. A flat rotating platen
can be constructed with a portion of the surface raised somewhat from the flat circular
surface and a cone angle created on this surface to which an abrasive annular ring
is adhesively bonded or held in position by vacuum holes to this angled raised ring.
A piecepart can then be presented to this cone shaped surface at an angle to the platen
perpendicular which is approximately the same as the abrasive disk cone angle. The
piecepart presentation angle may either be more or less than the abrasive angle to
control the portion of the piecepart surface that is in contact with the rotating
abrasive for optimized grinding/lapping action.
63. HIGH SPEED LAPPING IN A MILLING MACHINE
[0307] Problem: Achieving ultra flat and smooth surfaces in a milling machine operation process without
subsequent grinding and lapping type steps.
[0308] Solution: In a milling machine, CNC horizontal or vertical, a conventional milling cutter
can produce a relatively flat surface with a 16 rms finish. A special media holder
can be clamped in the spindle which has a flat precision surface perpendicular to
the machine spindle centerline. A flat abrasive with a pressure sensitive adhesive
would be attached to the special media holder The abrasive could be die cut into an
annular ring, for example 6 inch (14.3 cm) OD and 4 inches (10.2 cm) ID. With the
spindle running at, for example, 6,000 rpm and about 9,000-10,000 sfpm, the surface
of the machined part can be "high speed lapped" with the special holder and abrasive
media. The abrasive should be in contact with the work piece. The machine table moved
in a crossing pattern to evenly distribute the lapping action. A supply of coolant
fluid should be used to keep the work piece cool. It could be pumped through the spindle
and special holder if available. A typical material removal piece pass would be 0.0001
- 0.0003 inches (0.025 mm to 0.076 mm) in the "Z" direction. Using this technique
and starting with 125 micron diamond abrasive media and stepping down to lapping films,
1 micron for example, surface finishes and flatnesses of very high quality can be
achieved in one machined part set-up, eliminating subsequent grinding and lapping
operations with a substantial part handling and cost savings.
64. FLEXIBLE PIVOT TOOL HOLDER
[0309] Problem: When grinding or lapping single or multiple pieceparts held by a tool holder with
a typical diameter of 4 inches (10.2 cm) held by a center post and the tool holder
is slowly (or fast) rotated as it is presented down vertically to uniformly contact
an abrasive surface platen rotating at the high speeds of the present invention, it
is important that the piecepart holder be "flat" so that the pieceparts which contact
the abrasive first are not damaged because the holder has one edge lower than another.
Further, with this type of lapping and grinding it is important that the piecepart
holder assembly be held by a ball pivot type of device located as low as possible
(as close as possible so that the central point of rotation of the pivot is as close
as possible to the abrsive sheet surface when contact is made. It is also best to
align the total piecepart assembly so all the individual parts are floated equally
by the thin boundary layer of coolant fluid on the surface of the disk which may be
less than 0.001 inch (0.025 mm) in depth. With this type of gimbal pivot, this boundary
layer thickness has a tendency to remain uniform even with slight out-of-perfect-perpendicular
alignment between the vertical piecepart holder shaft and the high speed abrasive
platen. Foreign debris contaminates pivot joints and create unwanted friction. It
is also important to control the water boundary layer thickness and shape between
a workpiece surface and the abrasive surface for a small workpiece with a correspondingly
small surface area that is not large enough to be positioned flat on the abrasive
surface with a minimum amount of down pressure.
[0310] Solution: A work holder is created with the use of a spherical ball attached to a shaft which
provides a pivot action close to the bottom of the workpiece holder assembly. A sandwich
of washers (between the piecepart holder housing and the ball) act as a rigid base
to transfer polishing normal force downward on the vertical shaft to push the pieceparts
onto the abrasive platen. The washers apply only a small to prevent slack between
the ball and the holder, or the resultant ball friction would prevent free pivot action
on the ball. The pivot action is restrained by encapsulating the whole assembly (the
ball post, ball washers and ball socket) with RTV silicone rubber which seals the
unit from debris and also provides the function of an elastic restraint that self
centers the disk type part holder perpendicular to the axis of the support shaft,
yet the elastic spring which centers the unit is weak enough to allow conformal pivoting
of the assembly during the lapping action. Thus when little side load is present,
as when lowering the piecepart assembly, the unit is flat aligned, but when subjected
to a normal force, the unit is free to pivot. A piecepart holder with the ball stem
and RTV was constructed and used for lapping of a piecepart assembly for optical connector
devices and appeared to function well.
65. BOUNDARY LAYER CONTROL
[0311] Problem: When high speed lapping, a rotating flat platen with fixed abrasives attached to
the platen with adhesives or vacuum, water on the rotating platen abrasive surface
forms a boundary layer between the work piece and the abrasive media. The boundary
layer thickness and shape effect the flatness of the work piece. The workpiece must
be allowed to "float" on the abrasive surface which is partially covered with a boundary
layer of water.
[0312] Solution: The work piece must be allowed to "float" on the boundary layer. This is done with
a gimbal mechanism which puts pressure down on the rotating work piece. It also allows
the work piece to "gimbal" in the horizontal plane while an independent driver pin
drives the work piece around the centerline of the work holder shaft. The amount of
down pressure also effects the boundary layer. The work piece floating on the boundary
layer of water allows the abrasive media and platen imperfection to be averaged out,
so high spots on the abrasive do the lapping while the low spots are filled with water,
allowing the lapping action to take place and produce a finished part (work piece)
that is flatter than the media and platen. The work piece will only be as flat as
the boundary layer.
[0313] Water is pumped through the work holder and into controlled orifices or jets in strategic
locations that force a boundary layer to form between the work piece and the abrasive
media. The water stabilizes the work piece while presenting it to the rotating platen
initially and while lifting the work piece off after lapping is complete.
66. LAPPER SACRIFICIAL DISK
[0314] Problem: When lapping or grinding a multiple number of small parts or single small parts
each having small surface areas and short surface dimensions in the approximate size
of 0.25 inch by 0.25 inch (0.63 cm) and these parts are put in contact with a high
speed rotating disk, there is not enough surface length to the part to build up a
sufficient boundary layer to float or support the part as it is making contact with
the abrasive disk on the high speed platen and the parts tend to dig into the abrasive
disk and tear the disk and prevent accurate polishing or lapping of the part. This
problem is again uniqely felt in the high speed lapping process of the present invention
with abrasive sheets secured to the platen.
[0315] Solution: A system is provided to effectively extend the too short surface contact length
dimensions of the pieceparts to allow them to be presented flat to the abrasive surface.
Here an adequate boundary layer is generated and maintained while the individual pieceparts
are being lapped by adding a secondary device to the piecepart holder device. This
sacrifical device, which would have sufficient surface area and length would be mounted
outboard of the piecepart on the piecepart holder device. It would also be ground
down simultaneously with the pieceparts in a sacrificial way. A typical shape of this
can be a disk of metal such as brass which would be mounted on the outside annular
position of a tool piece holder with the to be lapped pieceparts mounted inboard of
these on the periphery of a round piecepart holder. The sacrifical piece should have
a susceptibility to grinding which is within about 50% of the workpiece (either greater
of lesser, preferably lesser) to assist in more uniform grinding. The susceptibility
to grinding can be readily measured by grinding identical surface areas of the materials,
with similar initial roughness, for the same period of time, at the same speeds and
pressures, with the same abrsive sheeting and comparing the amount (e.g., weight)
of material removed from each sample by the lapping. As the total exposed surface
area is ground down, the pieceparts are held suspended above the high speed moving
abrasive by the large surface area of the sacrificial disk. As the sacrificial device
lays outboard of the piecepart, it is contacted first by the abrasive when the piecepart
is tilted and initially brought into contact. Contact with the piecepart is prevented
until the entire assembly lies flat. A typical disk would be 4 inches (10.1 cm) outside
diameter, 2 inches (5.1 cm) inside diameter and about 0.060 (1.58 mm) inch thick.
It could be easily attached with vacuum chucking and/or adhesive tape and could be
used over and over by loading new pieceparts with a partially ground disk. Other geometry
sacrificial plates could be used and combinations of materials such as steel, ceramics.
67. PLATEN FLATNESS GRINDING
[0316] Problem: When a high speed rotating abrasive platen is manufactured and after repeated usage
of the machine, the platen is not perfectly flat as it had been originally machined
or ground (having been damaged by wear or impact) to a required or desired flatness
of less than 0.0005 (0.00127 mm) inch at the outer periphery with a need for the best
performance to reach 0.0001 inch (0.00065 mm) as measured by a dial indicator placed
at the outside diameter and the disk rotated by hand for one revolution to measure
the maximum excursion. Any deviation acts either as a "valley" where the abrasive
does not contact the piecepart or a "high spot" which is the only area that contacts
the piecepart. When the disk rotates at its normal high speed, each high spot will
have a tendency to hit the piecepart and set up a vibration which will reduce the
smoothness of the lapping abrasive action. Localized distortions of the platen surface
will also have a tendency to penetrate the boundary layer of liquid between the platen
(covered with a thin sheet of diamond or other coated abrasive) and the piecepart
and produce a localized scratch or track on the piecepart surface. Surface defects
on the platen structure may be generally transmitted through the thin abrasive disk
and produces a bump or high spot on the disk.
[0317] Solution: An existing platen can be "dressed" on a machine by bringing it up to full speed
rpm and lowering a heavy flat abrasive coated piece unit directly onto the bare rotating
platen and grinding or lapping off the bumps, and high spots. Even full out-of-flatness
surface variations can be removed by first using a coarse abrasive and progressively
using finer abrasive or lapping abrasive media. The platen, in effect, becomes the
workpiece and the workpiece becomes an abrasive surface or sheet. The typical first
abrasive may be 40 micron metal bonded diamond and ending up with 3 micron or less
diamond or ceramic abrasive depending on if the platen surface is chrome plated, stainless
or bare steel. It is important that the surface area of the abrasive lapper disk be
large enough to cover the total area of the platen with a slight overlap and it could
be oscillated back and forth across the platen, could be stationary or rotating at
either slow speed or rotating at very high speed so the tip speed of the grinding
disk will provide uniform removal of platen material at the low surface speed of the
inner radius of the platen. Different geometries of adhesive disks could be used.
Also a piecepart holder already in use for normal lapping could be used to perform
this function.
68. ABRASIVE METAL POLISHING MACHINE
[0318] Problem: The surface of metal objects are polished for many reasons including the optical
examination of a metallurgical characteristic, to create a smooth low wear tight hydraulic
or fluid seal and other uses. Usually this polishing is done on low speed 5-200 rpm
or so rotating flat platen disk wheels of various types of construction may be used,
such as aluminum, steel, plastic, composite, cloth and other materials. The wheel
surface is very flat and the workpiece to be polished is held with controlled pressure
by hand or work holder against the rotating wheel with water or other fluid wetted
abrasive particles introduced as a slurry or disks of fine abrasive sheets "stuck"
or bonded to the rotating wheel. This process slowly produces an accurate, highly
polished surface and it is labor intensive and expensive if not automated. Inaccurate
platen or shaft machining or loose bearings or weak machine structure frameworks may
cause polishing accuracy problems.
[0319] Solution: It has been found that very high quality polishing can be achieved at a fraction
of the expended time by using microabrasive sheeting, such as 3M brand microabrasive
disk sheets for polishing at the very high speeds of this invention described above.
The process is especially useful with disks about 8 to 10 inches (20.3 cm to 25.4
cm) in diameter. However, it is critical that the rotating platen disk run very "true"
and flat at the operating speed range to provide a mechanically stable moving surface
against which the to-be polished workpiece is held stationary at a controlled normal
force or pressure (against the fine particle wetted abrasive). Options also may change
the contact pressure (between the abrasive sheet and the workpiece during lapping)
as a function of process time or the workpiece rotated to distribute polishing across
the surface. A unique method to provide a very "flat" and accurate stable rotating
platen disk surface is to mount the platen to a "weak" shaft which allows the rotating
disk mass to seek a true "smooth" center above its first rotating natural frequency.
The motor drive speed would be increased above the natural frequency of the rotating
platen with abrasive sheeting thereon, the workpiece part presented in contact for
polishing, then removed from contact prior to reducing the disk rpm.
69. LAPPER PLATEN SPIRAL SURFACE
[0320] Problem: When lapping or grinding at high speeds producing as much as perhaps 5,000 or even
at least 8,000 to 10,000 sfpm of surface lapping speed using plastic disks coated
with thin layers of diamond or other abrasive material, it is sometimes a disadvantage
to have a uniform flat disk surface in flat contact with precision pieceparts. This
is due in part because the fluid boundary layer has a tendency to draw the piecepart
down to the flat surface of the rotating platen (by the effects of Bournoulli's principle)
and create large fluid adhesion forces requiring more force to hold pieceparts (e.g.,
with bigger motors) and the need for larger and heavier holding devices for the pieceparts,
and the need for more frequent variations in the holding forces because of the variations
in the adhesion forces from fluid flow rate changes. This may also result in uneven
material removal resulting in non-flat parts. Furthermore, when a liquid boundary
layer builds up, it has a tendency to increase in thickness along its length, which
has the effect of tilting the surface of the piecepart relative to the abrasive.
[0321] Solution: A precision ground rotating platen can be fabricated with slightly raised spiral
surfaces having different shape patterns from the inside center of the platen toward
the outer periphery of the platen. These spiral patterns would create short land areas
at the top surface of the platen of varying widths and shapes with areas between these
land areas that are somewhat lower, perhaps from 0.002 inch (0.05 mm) to .010 inch
(0.25 mm) or more. Then a thin plastic coated abrasive disk that is uniformly coated
with precision fine abrasive would be mounted to the round platen and held in place
by vacuum hold-down holes either on the raised land surface or on the lower surface
area or a combination of holes in both areas. The raised land areas could be produced
by manufacturing a precision platen and acid etching the land area geometry configurations
of the lands. When the abrasive disk is mounted on the platen, only some portions
of the disk would be in contact with the piecepart being ground or lapped. The boundary
layer of fluid coolant would be effected by the length of the land area under the
piecepart, the direction of the spiral or radial or circular annular land shapes or
a combination of these geometries. The effects on the boundary layer thickness would
be the rotating speed of the platen, as related to the vector speed, including direction
for the surface relative speed between the two, the viscosity of the fluid, the normal
force pressure of the piecepart holding it to the platen. The boundary layer thickness
which would vary over the surface of the piecepart would affect how the individual
particles of abrasive normally sticking 1/3 of their size about the bonding agent,
either metal plating or plastic bonding, surface of the abrasive disk. If more liquid
is applied, the boundary layer would tend to be thicker and less abrasive material
removed is achieved. Thus the local pattern of the surface of the abrasive contact
area can be utilized for the optimum grinding action using only one portion of the
abrasive disk with the non raised section between the land areas allowing free passage
of grinding debris. When this surface area of the abrasive is worn, the disk can be
unmounted by the vacuum chuck, rotated to a "fresh" area of the abrasive and grinding
continued. The disk will remain uniform and strong through service. This can be done
in at least two different ways. A grooved pattern with a preselected distribution
of islands on the surface of the platen is created by molding, etching or the like.
[0322] When a thin backing abrasive sheeting (as used in aspects of the present invention)
is applied and secured to this textured platen, the backing of the sheet conforms
to the pattern. Continuous boundary layers will be broken up by the predesigned variations
in the surface of the conforming abrasive sheet, which is very desirable. Since the
pattern is chosen (with the highest areas on the platen being fairly uniform and constant),
a planar area of contact between the abrasive and the workpiece can be maintained,
with areas of non-contact or light contact provided which will break up the boundary
layers. It is better to have a flat platen with a groove pattern existing on the abrasive
sheet or by using segments of abrasive sheet, as described herein. Abrasive sheets,
even with diamond abrasive, are now available from 3M with abrasive islands (e.g.,
diamonds within a matrix) having paths where swarf, liquid and the like may flow between
the islands without disturbing the contact between the workpiece and the abrasive
on the sheet.
70. LAPPER PIVOT CRADLE PIECEPART HOLDER
[0323] Problem: When a piecepart is ground or lapped on a high speed diamond or other coated abrasive
platen rotating at high surface speeds, there is an uneven grinding action due in
part to the boundary between the piecepart and the abrasive surface being uneven with
a thinner layer thickness at the outer periphery being thinner due to the high surface
relative speed at the outer diameter and much less at the inner radius of the platen
which is subjected to liquid water or other fluids. Typical abrasive particles at
the outer radius of the rotating platen penetrate the thinner layer of the boundary
layer and provide material removal quite aggressively there. At the inner radius,
the boundary layer is thicker, the abrasive particles don't penetrate as well through
the boundary layer which "floats" or hydroplanes the piecepart, with the result of
significant material removal at the outer radius of the platen and reduced removal
at the inner radius. This produces uneven wear on the piecepart which is subjected
to both extreme areas of the platen radius and the piecepart is not flat or the surface
is not uniform in surface damage.
[0324] Solution: An annular ring of abrasive mounted on a platen is used so the relative surface
velocity at both the inner and outer radius is close enough that the boundary layer
is about the same relative to the height of the coated abrasive (from above 0.1 or
from about 1 to 100 microns). There may be two or more piecepart holders, both rotating
in reversible directions if desired for special grinding effects, with both mounted
on a common pivot arm (either straight with two piecepart holders or branched with
three or more piecepart holders. Each piecepart holder would tend to stabilize the
others across the platen. A spherical wobble joint at each piecepart holder would
allow each to conform to the slightly uneven boundary layer on the platen. Rotating
each piecepart holder would provide the same amount of abrasive material removal to
all the exposed surfaces of the individual pieceparts. The normal force, surface speed,
liquid flow rate, viscosity, etc. could all be optimized The whole assembly pivot
cradle could be oscillated to obtain even surface wear.
71. ABRASIVE HIGH SPEED LAPPER
[0325] Problem: It is often desirable to have a narrow annular ring of abrasive material on the
outside periphery of a rotating platen to effect fast high quality lapping action.
Production of a narrow annular abrasive disk as a continuous ring of material from
a linear web results in removal of the inner diameter disk of a large diameter which
is very expensive. This inner disk of material may be 8 inches (20.3 cm) in diameter
when producing an annular ring with an ID of 8 inches (20.3 cm) and of 12 inches (30.5
cm) is also constructed of the same web coating of fine diamonds or other expensive
abrasives. These smaller disks are not readily sold in the marketplace.
[0326] Solution: Cut annular segments having circular curvature from a web and join these end-to-end
in a pattern to form a continuous annular ring. These annular segments can be adhesively
attached or, even better, fused to a common base material of strong plastic such as
polyester or other materials such as hard thick plastic or metal disks. The long ends
of these segments can be butted directly adjacent to each other, butt welded together
or prescribed gaps can be left between the ends of the segments to allow water/lubricant
to better carry away swarf. Different shapes can be given to the annular rings which
may promote the abrasive lapping such as serpentine shapes or curved radial segments.
All of these shapes can be cut out of linear web material with very little yield loss
or throw away. Short or long segments can be used.
72. ACOUSTICAL SENSOR PIECEPART CONTACT SENSING DEVICE
[0327] Problem: It is difficult to determine if a piecepart has been brought into contact with a
high speed moving abrasive surface when it is initially presented for grinding as
it is not easy to calculate positionally when this would occur when first using an
unknown sized (thickness) part and when using abrasive disks of unknown thicknesses
and other machine variables.
[0328] Solution: The apparatus can have Fast Fourier Transformation spectrum analysis pattern recognition
controls used with an annular ring of abrasive. These characterize vibration by amplitude
as a function of frequency. It has been found that when piecepart materials such as
ALTIC (aluminum tungsten carbide or aluminum titanium carbide) are brought in contact
with high speed platens using the abrasive sheeting (such as the 3M diamond abrasive
disks) operated at high surface speeds, especially such as about 10,000 sfpm, that
a characteristic significant sound is produced which is quite audible to the human
ear at the very first contact between the piecepart and the abrasive surface. At the
time of the onset of this audible sound, it is possible to very precisely determine
the relative location of the piecepart to the machine frame with the use of a Heidenhain
linear scale and then to commence to remove a fixed amount of the piecepart surface
of about 0.005 inches (0.0064 mm) by motor driving a threaded screw actuator device
which forces the piecepart into contact with the abrasive surface.
[0329] The audible signature allows the piecepart to be moved quite rapidly up to the surface
of the abrasive and then to be slowed or stopped for restart to allow a very slow,
controlled motion approach by driving the piecepart into the moving abrasive surface
at a slow prescribed rate with optimized controlled flow of lubricants for a specific
abrasive particle size over a fixed period of time. With this technique, a piecepart
surface will not be damaged by too sudden contact due to excessive heat generation
or impact.
[0330] It is difficult to determine if a piecepart has initially made contact with a highspeed
abrasive moving platen surface and also to control the normal (right angle) pressure
between the piecepart surface and the abrasive surface to optimize the removal rate
of grinding. The goal of producing a smooth ground surface with 2 lightbands or less
flatness is difficult to accomplish. A square piece of ALTIC material about 2 x 2
inches (5.1 by 5.1 cm) was stepper motor driven in small increments to where the contact
force between the workpiece and the abrasive moving, at 3,000 RPM for a 12 inch (30.5
cm) diameter platen with about a 1.5 inch (3.77 cm) wide ring of annular shape had
an initial contact force of about 2-20 pounds (0.9kg to 9 kg), usually around 9 lbs
(4.1 kg). The first portion of the grinding period of about 1 minute removed surface
material quite rapidly, but as time went on, the force sensor showed a progressive
decrease in contact force with an unchanging machine incremental position. Also the
swarf of ground debris visually was quite heavy, but decayed in some proportion to
the contact force. A typical amount removed was about 0.005 inches (0.13 mm) over
this 1 minute period. The finished surface of the part was very smooth in surface
roughness, producing a mirror finish and the flatness was better than 1 lightband
as measured by a green optical light flatness measuring instrument. As the machine
was not advanced during this period, the spring compliance of the machine members
produced this very successful fast initial removal of ground material with a proportional
or exponential decay of force which resulted in a progressively more gentle contact
at the last portion of the period, resulting in the desired surface.
73. LAPPER PART HOLDER
[0331] Problem: When a piecepart is initially brought into contact with a high speed rotating (or
linear) high speed moving abrasive surface, there exists the possibility of one portion
of the piecepart contacting the surface of the abrasive in such a way that it will
get caught or impact the high speed abrasive and either harm the piecepart due to
uneven grinding or jam it into the moving abrasive surface which generally has very
high inertia and momentum which can then cause a virtual explosion with fracture of
the piecepart, the holder, and the abrasive media, either in sheet form or bonded
abrasive. This can result in great danger to the machine operator or significant damage
to expensive parts being precisely ground to size, finish or flatness. Also perfect
alignment between piecepart and the moving abrasive surface is difficult to achieve.
[0332] Solution: A multiple piecepart holder can be constructed such that the piecepart is held rigidly
and precisely on a flat surface by vacuum or other means such as adhesive, melted
wax or be established by mechanical measuring equipment and process techniques so
the piecepart can be lowered (vertically) so it is just barely within 0.001" of the
moving abrasive surface and then when contact is made by further motion, the piecepart
holder then is allowed to move freely by use of weak springs which allows perfect
flat alignment between the piecepart surface and the grinding surface. For rigid grinding
to obtain initial flatness of the piecepart surface, small air cylinders can be used
to clamp the piecepart mechanism by driving a lower wobble plate portion of the piecepart
(workpiece) holder against adjustable mechanical stops. These stops align the piecepart
adequately parallel for the initial grinding contact and/or activity. These small
air cylinders are strong enough to overcome the weak springs. The weak springs are
used primarily only as the wobble plate is allowed to pivot. The air cylinders prevent
the wobble plate from pivoting. In this way the "floating" piecepart holder device
can be used to initially rough grind a piecepart by cylinder clamping and then use
the floating springs to continue grinding or lapping to produce typical mirror finishes
with flatness better than 1 or 2 light bands. The air (or hydraulic) cylinders are
only activated during rigid grinding but they could also be used to apply a varying
pressure to hold the piecepart against the abrasive depending on the grinding process
cycle events.
74. LAPPER PIECEPART HOLDER
[0333] This entire section relates to a combination piecepart holder which allows spherical
pivoting (for finish grinding) and is also able to be supported in a rigid position
(for initial grinding). The piece part does not have to be changed, so there is no
set-up time needed for changing from these grinding modes.
UP/GRIND POSITION
[0334] When the pivot workpiece holder is used for rigid grinding of a part, the free moving
spherical section is moved against mechanical stops which rigidize the unit. Moving
this portion of the pivot part (workpiece) holder can be effected, for example, by
a variety of devices which include (but are not limited to) springs, flash cylinders,
electric solenoids, linear electric motors, thermal or electrical screw devices, and
the like. The important function is to hold the piecepart holder against local stops
to rigidize it, and then the entire rigidized assembly is lowered to present the piecepart
in rigid contact (non-pivotable contact) with the abrasive surface (e.g., the abrasive
sheet on the platen). This rigid piecepart holder can be rotated axially, but does
not have a spherical pivoting action at this time. When a piecepart has been initially
ground, it can then be followed by conformational spherical grinding without changing
to a different lapping apparatus. It is very important with these relatively thin
sheets of coated abrasive material that the piecepart be presented to and contact
the abrasive with controlled pressure and force rather than attempting just a position
controlled presentation. The following equipment and procedures may be used to effect
this result.
[0335] A center slide (may be spring retained or activated by a cylinder or an electric
solenoid). Pressurize bottom of cylinder to lock part holder "up" against ball for
rigid grinding. Spherical joint for cylinder
DOWN/LAP WOBBLE POSITION
[0336] Can use frictionless "air pot" brand cylinders with small air or oil gap between
cylinder wall and piston which allows fluid leakage but no stick (friction break away).
Center ball - can be held in a fixed position or allowed to slide vertically. Multiple
metal flex bellows with vacuum applied to draw "up" against ball stud for initial
grinding to flatten piecepart parallel or to initiate presentation of piecepart to
abrasive platen. Hollow metal of plastic flexible disk bellow stack. Bellows can also
be given a positive pressure to hold piecepart flat against the abrasive platen surface
with controlled contact force or pressure. Metal bellows disk can be single annular
unit or a multiple number such as three each at 120 degree increments.
75. LAPPING MACHINE AND PROCESS PROCEDURES
[0337] Problem: When lapping at high speed with a rotating platen it is very difficult to align the
rotating piecepart holder precisely perpendicular to the platen abrasive surface and
to accurately bring a piecepart into contact with the high speed moving abrasive without
impact.
[0338] Solution: Construct a lapping machine which has the possibility to micro-align the axis of
the piecepart holder mechanism and the abrasive lapper platen. Also use a fine pitch
(40 threads/inch) screw to move the piecepart down into contact with the abrasive
with a stepper motor having 50,000 steps per revolution. Further, the screw is attached
to an in-line force gage which senses when the piecepart comes in contact with the
abrasive surface and this position is sensed very accurately with a precision linear
encoder device. A linear actuator with a stepper or other motor is used to position
the piecepart holder on the annular ring of abrasive of the platen in the quadrant
of the platen where the grinding or lapping force action is the most stable depending
on the direction of the platen rotation.
SET-UP PROCEDURE FOR IMPROVED ALIGNMENT
[0339] One method is to first align the platen baseplate with 4 corner jack screws then
align the pivot post, then align the pivot arm.
PIECEPART PROCEDURE
[0340] Then mount the piecepart, find its contact position with stationary abrasive platen,
grind flat, finish lap with wobble plate.
76. LEVEL INITIAL PIECEPART CONTACT WITH PLATEN PIECEPART DOWNWARD PRESSURE
[0341] The use of a sacrificial outer ring, square, segment pieces or ring with water inlet/outlet
slots, with the sacrificial parts made of various different materials: plastic, metals,
ceramics and metal/other composites, combinations, can assist in assuring that the
initial piecepart contact with the platen is level. By having the sacrificial parts
at a higher elevation with respect to the approach path to the platen abrasive surface
(usually by being outboard of the piecepart), the sacrificial material will contact
the abrasive surface of the platen before the piecepart. This initial contact with
the sacrificial part will level out the workpiece while the sacrificial part is being
lapped, without any damage to the workpiece. This causes a touch down on the outer
ring of sacrificial material first, to "level" the workpiece part. Examples of sacrificial
material could be substantially anything that would not interfere with the lapping
(e.g., explosive materials, highly abrasive material that would destroy the abrasive
surface, etc.), such as porous material filled with lubricant. This technique may
be used rigid mounts or spring mounts on the piecepart holder.
[0342] Flooded Wedge Angle: One can also present the piecepart at angle tipped to raise
an edge toward incoming abrasive and water. Water will develop a high pressure under
the back (downstream) portion of a flat workpiece and lower the workpiece flat. This
will keep the piecepart from being presented with the leading edge contacting first
and "camming in" due to friction or water pressure which destroys the leading edge
as the piecepart is ground or lapped.
[0343] Boundary Layer Lifting: The use of a finite element dynamic fluid flow computer program
(FIDAP, by Fluent Company) shows that where a boundary layer of water is uniformly
flat under the full downstream length of the piecepart, there is little tilting force
on the piecepart. However, if excess water pushes up to form a "dam" at the leading
edge of the piecepart, a dynamic pressure head is created under the first portion
of the piecepart which tends to tilt the part on the abrasive surface. A leading ramp
knife edge can be used to reduce the dam pressure build-up effect. Large leading edge
pressure head lifting results if there is a raised front edge or a big dam head of
water on front leading edge of the piecepart.
[0344] The tapered ramp knife edge is used at the front to cut off the water dam by lifting
it up (as with a snow plow), forcing the front of the piecepart down due to reactive
forces. The best procedure is to only use enough lubricant to wet the valleys in abrasive
mountains plus a little extra.
[0345] Change Down Pressure: By using speed control, downward normal force is a function
of surface speed, with greater downward force being used with greater speeds to counteract
the lifting or tilting force of hydroplaning of piecepart.
[0346] One should use very small down pressure at first contact, then increasing the pressure
after contact has been made, then again reducing the pressure very fast with lift
off from the moving platen.
[0347] Stationary Platen Start-Up: The platen is started only after the piecepart is in
contact with the abrasive sheet surface, using a start slow acceleration, then a quick
ramp up to full speed. The platen would normally be brought from a stationary position
(zero speed) to a full 3,000 rpm in about 15 seconds, or at least about 100 or 200
rpm/sec. acceleration.
[0348] Option 1: Have the piecepart stationary until some minimum platen speed (e.g., at
least 200 rpm) is reached.
[0349] Option 2: vary the speed of piecepart rotation before the platen start-up and also
during processing of grinding event. The piecepart could be rotating or stationary
at the time of the piecepart removal. Removal could be made with platen at full speed,
partial speed or slowed to a stationary state. The piecepart will tend to stay conformed,
flat to the platen at low speeds or stationary and therefore it will not damage the
leading edge of the workpiece.
[0350] Water or lubricant can be varied during the process, with large excess amounts used
during start-up initial contact or during removal at low platen speeds or stationary
platen. In the case where it is desired to intentionally tilt the piecepart spindle
relative to the abrasive platen to produce a slight cone shape on the piecepart surface,
the platen can also be started from a stationary position after the piecepart is placed
into contact with the abrasive. An initial "motor mat" tilt angle can also be used
with stationary start-up or lift off.
[0351] Add a loose material as a contact initial barrier such as powdered plastic, abrasive
particles or other materials. These would be used either as pre-coating on piecepart
surface or as constant flow input with water lubricant source during initial contact,
but stopped or eliminated during normal grinding. Their addition can be restarted
prior to lift off to develop a film or layer between the piecepart and platen. The
material could also be a thick liquid, such as a polymer solution, grease, etc.
77. PIECEPART DOWNWARD PRESSURE
[0352] Problem: It is desirable to prevent tipping of the piecepart of a wobble pivot part holder
as it first contacts the abrasive which grinds of the leading edge of the piecepart.
[0353] Solution: Use a sacrificial contaminant ring surrounding piecepart so that the outboard sacrificial
ring makes the first contact with the abrasive. Also the piecepart could be potted
in an adhesive, epoxy-like device which encompasses the piecepart.
[0354] Piecepart pressure from high speed air jets across the top surface directed under
the surface to create an air film under the piecepart. Water jets impinging around
the piecepart on top of the piecepart surface to provide uniform pressure across the
piecepart surface to form a water film under the piecepart.
[0355] A stationary hollow holding ring can be held in a fixed position above the abrasive
surface and a piecepart which matches the ring opening can be dropped into the ring
to be in contact with the abrasive.
[0356] A dead weight may be placed on the piecepart top surface. A dead weight with spring
between weight and top surface may be used. One may also use a dead weight with resilient
spring material which is filled with vibration damping material to reduce vibrations.
Damping can be from liquid in foam or from motion induced shear action within foam
material itself where high local velocities from vibration of piecepart introduced
by unstable hydrodynamic forces are alternated by local damping. It is also possible
to use diaphragm pressure on vacuum pistons to produce uniform pressure across free
weight by use of conformal diaphragm membrane in contact with piecepart top surface.
Floating Piecepart Holder
[0357] Use heavy or light piecepart ring with open center hole to mount piecepart and have
an extended outer portion with a low outboard bearing contact ring having a spherical
shape.
[0358] Two or more stationary standard roller bearings would be mounted to contain the piecepart
ring as it is forced against the bearings by the forces induced by the moving water
coated abrasive. The low position of the extended spherical portion results in reactive
forces kept low toward the abrasive surface and minimizes upward tipping forces on
the piecepart. A spherical surface on the extended portion assures only point contact
with the support bearing outer-flat surfaces.
[0359] Another variation is to use support bearings with spherical surfaces to get point
contact. This point contact feature minimizes lifting or tipping forces on the piecepart
ring.
[0360] Gear teeth can be used on the outer edge of the piecepart ring so the ring can be
turned by a motor driven gear matching contact with the ring gear.
[0361] Other mechanical ring rotation drive mechanisms can be employed such as engagement
pins with contact "dog" arms, universal joints, magnetic couplers, roller drive wheels,
air or fluid contact impingement jets, inductive magnetic electrical fields.
[0362] Another drive mechanism is the differential speed of the outer periphery of the rotating
platen abrasive having a greater contact force than the inner radius abrasive contact
thereby setting up a relatively slow differential rotating velocity of the piecepart
ring.
78. LAPPER ABRASIVE PATTERN
[0363] Problem: When a piecepart is ground or lapped using an annular ring which is less wide than
the piecepart, there is a center portion of the piecepart which is in constant grinding
contact with the abrasive, while other parts of the piecepart are not in contact with
an abrasive surface. This central area receives more grinding action than the outboard
portions of the pieceparts (which are typically rotated) that leave contact with the
abrasive. This center section typically has a circular shape as the piecepart is rotated.
If the piecepart is not rotated, then a groove would be ground into the piecepart
and it would have a width equal to the width of the annular ring. The heat which would
be generated by the friction contact force with the abrasive is at a greater amount
at the inside circle, and this also tends to swell and raise this circle due to greater
thermal expansion in the inboard (central) area than in the outboard areas which leave
contact with the abrasive and are water cooled. When the raised, thermally swollen
surface is ground level and cools off and shrinks, the circle will be a "low" spot
on the piecepart.
[0364] Solution: The annular ring can be changed from an essentially uniform (evenly distributed
particles over any given significant area) surface to one of smaller, parallel, concentric
rings with free space grooves between the raised abrasive which is flooded with water
coolant. All portions of the piecepart then would leave contact with the raised abrasive
as it is rotated. The annular ring could be made with raised tangential abrasive segments
with gaps between staggered adjacent inner concentric rings to grind-cool-grind a
given area. Also the piecepart rotating axis can be moved sideways during the grinding
so that a selected area can be moved out of contact with the abrasive surface.
79. LAPPER PIECEPART WOBBLE GIMBAL PLATE
[0365] Problem: When a lapper wobble spherical ball gimbal pivot plate is used to hold a piecepart
in intimate flat contact with a high speed rotating abrasive surface to compensate
for small minute misalignment between the piecepart support rotating shaft and the
platen shaft (collectively called the spindles), there is generally sufficient friction
in the antirotation mechanical device used to keep the lower part holder portion of
the wobble plate from torsionally rotating relative to the upper portion which is
attached to a spindle. As these two portions of the piecepart wobble plate must move
freely in a spherical pattern, rotating about the spindle center, any friction from
an outboard antirotation device will impede the free spherical movement of the piecepart
as it attempts to align itself perfectly flat to the abrasive surface with a small
nominal downward contact pressure force which holds the part surface to be ground
in flat contact with the moving abrasive. A typical piecepart is ½ to 8 inches in
diameter, typical downward contact force is 0.5 to 20 lbs. and more, and the amount
of ground off material is typically 0.0001 (0.0025 mm) inch to 0.003 inch (0.0077
mm) to obtain a flatness of typically 1 optical lightband or less. Usually a post
with a stationary ball on one end is used where the ball spherical surface is in rubbing
contact with a flat surface and the frictional contact force between the ball and
the flat surface increases with increasing piecepart rotational torque. This friction
prevents easy movement of the ball against the flat surface which is required to allow
the spherical movement of the piecepart, and this friction is further increased when
the flat wall is contaminated by grinding debris or swarf.
[0366] Solution: The stationary ball post is replaced with a roller bearing, either a low friction
needle bearing, ball bearing, roller bearing or air bearing and this bearing is constrained
between two round stationary posts mounted on the opposing plate which act on either
side of the bearing so the piecepart can be torsionally rotated in either direction.
The outer cylindrical surface of the bearing will be self cleaning as there is only
point contact between the bearing surface and the posts during sliding oscillations
of each piecepart revolution.
81. WOBBLE PLATE ANTIROTATION DEVICE
[0367] Problem: A wobble spherical pivot plate that is made in two plate sections attached to each
other by use of a free floating trapped spherical ball needs to be restrained or have
the two plate sections coupled to each other to transmit rotational torque from the
upper plate to the lower plate. A typical "dog" type of system where a post on one
plate contacts a surface on the other provides rotational torque, but has the disadvantage
of having sliding friction on the ball post to flat surface area which impedes the
free pivoting action of the wobble plate which is moving in an oscillating motion
to maintain the wobble plate piecepart surface flat to the moving abrasive surface
as the wobble plate is rotated during a grinding or lapping action. This friction
can create undesirable patterns of uneven ground surfaces in the piecepart, as the
spherical pivot action will tend to stick, break loose or stick again due to changing
from the high forces of static friction and lower forces of dynamic sliding friction
which occurs at each piecepart revolution.
[0368] Solution: A linkage bar with pin pivots at each end can be used to couple the upper plate
with the lower plate to obtain good torsional coupling with free motion of the spherical
pivot action of the wobble plate. The pins would be solid with a small diameter which
are periodically lubricated or they may have ball, roller or sliding bearings at the
pivots. The longer the bar and the more horizontal the bar, the less incremental rotation
of the lower plate relative to the upper plate with the pivot action. Another method
to accomplish the reduction in "stiction" (jumpy dynamic friction) is the use of a
hinge linkage system or a living hinge solid flexible spring that is wide to be stiff
for rotational forces but weak for spherical pivot.
[0369] Figure 3 shows some of the features of apparatus of the present invention in a segmented
view of the apparatus
1200. This apparatus
1200 comprises a rotatable platen
1205 with an annular ring of abrasive
1201 located on an upward face of the platen
1205. The workpiece holder assembly
1230 comprises a rigid shaft
1232 and an assembly housing
1234. Two of three air cylinders
1202 and
1203 (the third is removed by the segmentation of the figure) are attached to the housing
1234 by pivoting connections
1236 and
1238. The air cylinder
1202 is shown by further segmentation to be a spring air return cylinder. The cylinder
1202 is connected through a shaft
1240 to an intermediate plate
1242. An "up" stop screw
1244 with a ball end
1208 is positioned below the intermediate plate
1242. A "down" stop screw
1206 is positioned at another position on the intermediate plate
1242. The rigid shaft
1232 which is driven by shaft bearings
1204 is rigidly attached to the inside surface
1246 of the housing
1234. A second rigid shaft element
1248 is rigidly connected to the underside
1250 of the housing
1234 to slide or telescope within the first rigid shaft
1232. This creates a rigid connection from above the housing
1234 to the pivot ball sleeve bearing
1212 below the housing
1232. A sleeve bearing
1212 for a pivot ball
1211 radially restrains the second rigid shaft element
1248. The sleeve bearing
1212 is connected to or at least associated with a piecepart holder
1252. The ball nut
1214 is adjustable to allow the telescoping gap distance to be set. This connection or
association may be accomplished in many different ways, the requirement being that
the piecepart holder
1252 spherically rotates around the pivot ball
1211. A piecepart
1209 is fixed on the bottom of the piecepart holder
1252. There is preferably an antirotation ball pin and stop
1215 limiting the ease of rotation of the piecepart holder
1252 with respect to the bottom surface
1250 of the housing
1234. A spring element (not shown) may be used with the ball nut
1214 to control the axial gap movement. A segment of a spherical mass of elastomeric material
1213 such as a room temperature vulcanizing rubber can effectively perform the function
of sealing the ball joint from grinding debris and also seal in a ball lubricant.
This configuration allows for the solution of a uniquely difficult problem in alignment
of the lapping apparatus
1200.
[0370] To be optimally effective in performing the function of proper alignment of thee
workpiece or piecepart
1209 to the abrasive annular ring
1201, the piecepart holder
1252 must first act in a wobble or adjustable mode to place the piecepart
1209 into alignment with the abrasive ring
1201. To assure the best high speed lapping, during the actual lapping process, the piecepart
1209 is best held in a more rigid alignment with the abrasive annular ring
1201. The configuration in Figure X allows this adjustment in modes. When the piecepart
1209 is placed into contact with the abrasive annular ring
1201 in a non-lapping contact according to a preferred method of the practice of the present
invention, the initial contact is made between the piecepart
1209 and the abrasive annular ring
1201, the force on the top surface of the piecepart holder
1252 is provided by the two air cylinders
1202 and
1203 and the ""up" stop screws
1207 and
1244 with the ball end
1208. These "up" stop screws
1207 and
1208 (the third or more is not shown because of segmentation of the drawing) are able
to move independently and are allowed to move independently to allow the piecepart
holder
1252 to wobble or move spherically about pivot ball
1211 with the air cylinders
1203 and
1202 mount pivoting connections
1236 and
1238 and find proper alignment with the abrasive annular ring
1201. The pressure on the contact is minimal as the air cylinders
1202 and
1203 are precisely controlled. When this first, non-lapping contact controlled by the
"up" stop screws
1207 and
1208 is made, further force is applied to the housing
1234 by lowering shaft
1232 so that it drops further. The piecepart holder
1252 moves towards the bottom surface
1250 of the housing
1234. Contact is made between the ball end
1208 and the piecepart holder
1252. The bottom end
1256 of the "down" stop screw
1206 makes contact with the top surface
1246 of the piecepart holder
1252 to equal the axial gap between the pivot ball
1211 and the ball nut
1254. Each individual "down" stop screw (e.g.,
1206) is adjusted so that in this static position of contact between the piecepart
1209 and the abrasive annular ring
1201 in a non-lapping contact, the "down" lock screws
1206 are in the exact alignment position desired when the piecepart
1209 is eventually brought into contact with the abrasive annular ring
1201 during lapping. Therefore, the initial contact between the piecepart
1209 and the abrasive annular ring
1201 during the lapping process, when the platen
1205 is rotation at greater than 500 or more revolutions per minute and at high surface
feet per minute speeds, the piecepart holder
1252 will be rigidly held in place in proper alignment by the rigid support between the
bottom
1256 of the "down" stop screw
1206 and the top surface
1246 of the housing
1234 as the housing
234 is pushed down by the air cylinders
1202 and
1203. If the air cylinders
1202 and
1203 are deactivated, then the piecepart holder
1252 is allowed to wobble with the pivot ball
1211 in contact with a hardened contact plate
1210. Vibration of the piecepart
1252 is prevented by insertion of a vibration damping agent or damping device
1261 which provides a connection between the piecepart holder
1252 and the housing
1234. In this manner, the apparatus will be able to shift from a wobble or floating mode
to a rigid lapping mode during the rapid operation of the equipment. This configuration
is best performed with three sets of "up" and "down" stop screws and three sets of
air cylinders. Two, four or more can be used, but three has been found to provide
the best results to date.
[0371] Another issue which may have to be addressed is the fact that when annular rings
are cut from round sheets of abrasive disks, there can be significant waste of material
from the central round area cut from this disk. This is one reason why printing of
patterns of abrasive on a sheets is desirable. However, because the sheets of abrasive
are most commonly available in round sheet form, the cutting out of annular rings
is the most likely source of the annular rings. For this reason, this invention also
describes an annular distribution (to be included within the meaning of the term "annular
rings") of abrasive sheet material which can use the residue of the process where
a single piece, continuous annular ring was cut from a round sheet of abrasive. As
shown in Figure 15, segments or pieces of abrasive sheeting may be lain in an annular
distribution within the abrading surface area of a rotating platen. In Figure 15(a),
two segments
1301, each of which is a half of an annulus, have been cut from the remaining material
from the original round sheet of abrasive material (not shown) and then placed end
to end to form the annular shape. The vacuum hold down of the platen (not shown) can
secure the individual piece
1301 into a secure position onto platen
1320. The individual pieces
1301 may be secured together at their intersection 1304 by adhesives, fusion, butt welding
or the like. The center area
1306, as with a single piece annular ring, may be left open or may be filled with a central
round sheet (which may also be physically joined to the two segments
1301 to prevent flow of material under the segments
1301 and add support. Figure 15(b) shows a multiple number (5) of arcuate segments
1308 aligned around the platen
1320 in an annular distribution. Any number of segments may, of course be used, but the
fewer the number of segments, the less work is needed to align them.
[0372] Figure 15(c) shows a number of distinctly different shapes of abrasive sheet segments
on a platen
1320. There are three sets of abrasive materials, each with distinct shapes, grouped as
multiple wave forms
1322, kidney shaped
1325 and smaller arcuate
1324. An important feature of this configuration is the fact that there are physical gaps
1326 between one of the pairs of segments
1324. One of the problems previously discussed was the effects of removal and passage of
detritus, swarf and liquids away from the lapping contact area, especially the problems
associated with boundary layer thickness changes, channeling of liquid flow (with
or without swarf included), and other effects on the alignment or pressure or exposure
of particulate abrasives to the workpiece. This Figure 15ⓒ shows another benefit of
the use of non-butted and non-smoothly joined segments form a residual cut-out sheet.
Because the segments allow spaces
1326) to exist between the abrading or lapping surfaces (e.g.,
1324), natural run-off areas are provided which can carry away material without its moving
completely within the lapping contact area (e.g., on the surfaces of the segments
1322, 1324 and
1325). The dimensions of this gap
1326 are defined by the surface of the platen 1320 and the height of the segments (e.g.,
1324).
[0373] Figure 15(d) shows other configurations of segment areas which provide fluid or swarf
removal capability. The platen
1320 may have many various configurations of abrasive sheet segments on the platen
1330. For example, segments
1331 have holes
1332 in them which can trap material, rather than just letting it flow away in the gap
1334. Segment
1336 has serpentine paths
1338 without abrasive thereon to form the flow paths. Segment
1340 has both central open areas and an outlet area
1342 in a single design. This enables both some collection and a flow path for material.
As the most significant area of potential damage from material on the surface of segments
(e.g.,
1340) is on the outer areas, this configuration is very efficient. Segment
1344 has straight open lines
1346 between the areas of abrasive
1348. The segments radially curved
1350 are smaller arcuate pieces which provide a significant flow area
1352 between the arcuate pieces. It is to be noted that the segments may be touching (as
in (a)) or not touching (as in Figure 15 (d)) or combinations of these may be used.
By having non arcuate segment elements such as segments
1336 and
1342 contact each other, flow passages which allow the movement of material from the center
of the equivalent annular abrasive ring to the outside of the ring would be provided.
[0374] Another significant problem in the design of the equipment is the effect of vibration
on the workpiece holder and workpiece. As the finished piecepart dimension specifications
desired for the lapping process are so small, anything which dynamically moves the
abrasive sheet, the platen, the workpiece or the workpiece holder, or shifts their
relative positions is undesirable. As the platen is quite massive, there is seldom
any significant vibration in that element (especially since designing the weight and
construction of the assembly have made considerations for that problem). However,
the workpiece may vary from job to job, the workpiece and workpiece holder do not
have as great a mass as does the platen and its housing, and vibration is much more
likely to occur with the workpiece holder, especially when in contact with the abrasive
material rotating at the high speeds of rotation of the present invention. Figure
16 (a) and (b) shows mechanisms for reducing vibration on the workpiece holder and
consequently the workpiece. A shaft
1360 is shown attached to a workpiece holder
1362 with a workpiece
1364 attached thereto. A vertical vibration damping assembly
1366 is shown on the workpiece holder
1362. A leaf spring
1370 comprising a sandwich dual spring
1368 with a viscoelastic damping layer
1372 is shown. A mass
1374 is on the outer edge of the vertical vibration dampening assembly
1366. The natural frequency of the unwanted natural frequency vibration can be ascertained
and a secondary spring mass vibration absorber can be designed and installed to combat
these vibrations. In Figure 16, a spring constant for the leaf spring vibration damping
assembly is designed and installed to combat these vibrations. The spring constant
is selected to be matched with the discrete mass
1374 so that its natural frequency, as described by

is equal to the undesired natural frequency oscillation, wherein Wn is the natural
frequency, K is the spring constant, and M is the mass. This secondary spring-mass
will vibrate 180 degrees out of phase with the unwanted natural frequency of the workpiece
holder in a direction which is perpendicular to the abrasive surface (this is why
it is referred to as a vertical vibration dampening element) and will not be affected
by the rotation of the workpiece holder. This is because when a flat spring is used,
it flexes in only one direction, which is substantially perpendicular to the abrasive
surface. It is desirable that at least two, preferably three, and possibly more of
these units would be installed, most preferably approximately symmetrically around
the piecepart holder circumference. When the most preferred arrangement of three vibration
dampening elements are used, they would be installed circumferentially with about
120 degree spacing between the elements. The most preferred element construction,
primarily from a cost and convenience standpoint, is the use of two metallic layers
(e.g., lead spring layers) with a vibration dampening material (e.g., a viscoelastic
material) acting as a dampening agent between the two springs.
[0375] Figure 17 shows a configuration, previously discussed herein, for reducing swarf,
detritus and liquid movement problems within the system while it is lapping at the
high speeds of the present invention. A lapping system
1400 is shown which comprises the workpiece holder
1401, a workpiece
1410 and the high speed rotatable platen
1403 with an abrasive sheet
1405 secured onto the platen
1403. The abrasive sheet
1405 makes contact with the workpiece
1410 in a narrow region of contact
1403. The surface of the platen
1414 after a significant flat area of contact
1403 has been effected, slopes away from this contact area to a lower region
1422. This lower area
1422 has a ledge indentation distance
1406 which is the difference between the level of the lowest point
1422 and the interior surface
1416 of the platen
1402. The abrasive sheet is shown to be secured to the platen
1402 by vacuum passages
1404. Debris and liquid
1408 move over the interior surface
1416 towards the contact area
1403 between the abrasive sheet
1405 and the workpiece
1410. The level of this surface
1422 is preferably lower than the height of the surface of the abrasive sheet
1405 and more preferably below the height of the platen
1402 within the contact area
1403. The liquid and debris
1408 move radially over the surface
1416, but are propelled to due centrifugal forces to jump over the ledge indentation's
distance
406 gap and continues on radially to contact the top surface of the abrasive sheet
1405 and thus avoid the inside radial edge of the annular abrasive sheet
1405 and prevent lifting of this inside radial edge of the abrasive sheet
1405. Even the high centrifugal forces will not force the liquid and debris between the
abrasive sheet
1405 and the platen
1402. Figure 14ⓒ shows a sharply stepped ledge indentation distance
1406 which prevents liquid and debris from being forced by centrifugal action under the
abrasive sheet
1405. Figures 17 (a), (b) and ⓒ all show how contact with the inside radius cuts off the
annular abrasive sheet
1405 which potentially has loose particles from the platen, the center of the surface
area of the workpiece does not align with the geometrical center of he curved annular
segment of he abrasive which contacts it. However, the vacuum removal passage
1420 is a desirable assurance against such movement.
[0376] Because of the use of an annular distribution of material on the rotating platen,
previously unknown geometrical effects have been introduced into the system which
have been first addressed in the practice of the present invention. When a workpiece
is being lapped, it is natural to place the geometric center of the workpiece within
the center of the rotating abrasive surface. It has been found in the practice of
the present invention that this natural positioning is somewhat less preferred than
another orientation. Because of the arcuate nature of the annular ring of abrasive
where the portions of the annular section which in contact with the piecepart surface
"break away" to the center of the platen, the center of the surface area of the workpiece
does not align with the geometric center of the curved annular segment of abrasive
which contacts it. Because these two centers are not perfectly aligned and a contact
force is applied to bring them together for lapping, there is a subtle tendency for
the piecepart to tilt out-of-flat-contact to the radial outside of the platen. This
happens because there is less contact area support under the workpiece at the outside
portion and more contact area on the inside portion. This deficiency can be corrected
by a slight radial repositioning of he workpiece area center relative to the center
line of the annular ring. It is therefore desirable to shift the position of the workpiece
towards the inboard area of the annular abrasive sheet. This shift of the geometric
center of the workpiece should be at least 1%, preferably at least 3%, more preferably
at least 5% of the theoretical matching radial dimension location of piecepart area
center and the area center of he contacted segment of the annular abrasive sheet dimension
of the workpiece which addresses the abrasive sheet surface. The exact percentage
of shift of the geometric center of the workpiece can be precisely calculated by simple
arithmatic means, but has not been done so here as it would have to be done for each
annular shape (e.g., ID and OF considerations). The speed of rotation does not by
itself affect this calculation.
[0377] Another factor in the movement effects of the workpiece holder (and consequently
to the workpiece) shifting during the high speed lapping of the present invention
is the forces being applied to the workpiece (and consequently to the workpiece holder
) by the high rotational speeds of the workpiece holder. The forces caused by debris
and liquid flow under the workpiece also contribute to this effect. These forces can
cause the workpiece holder to want to swivel about the ball pivot joint, or other
pivoting joint, which secures the second rigid shaft member to the workpiece holder.
This problem is again unique to the high speed rotation of the lapping system, particularly
in combination with the abrasive sheet which is less forgiving to shifting of the
workpiece than a liquid slurry on a slower speed rotating platen. The extent and seriousness
of the problem can be reduced by making at least one geometric reconfiguration of
the relationship of elements. It has been found that to correct for out-of-balance
swiveling of the workpiece holder due to rotation of the workpiece holder with a mass
center of gravity located below (or above) the pivot can be reduced by moving the
center of the pivot joint closer to the center of gravity of the workpiece holder.
It has been found that to correct for out-of-alignment problems due to the dynamic
abrasive contact friction forces on the surface of the workpiece that it is desirable
that the location of the workpiece gimbal axes be located as close as possible to
the surface of the abrasive sheet.
[0378] Figures 18 and 19 show constructions which address solutions to this problem and
which move the center of gravity of the workpiece holder closer to the rotational
center of the pivot connection to the shaft. Figure 18 shows a lapping assembly
500 which addresses this problem. The shaft
501 is connected to a primary support plate
502 having X and Y axis pivoting connections such as gimbal bearings and pivot shafts
506 and
508 connected to downwardly extending arms
504 on the primary support plate
502. A pivoting second support plate
510 is connected to the workpiece holder
512. The workpiece
516 is connected to the workpiece holder
512 and is in contact with the abrasive sheet
520 on the rotating platen
518. The abrasive sheet happens to be shown in this configuration as larger than the
workpiece, but that is not required. In many instances the abrasive sheet
520 may be the same or smaller in the radial dimension or radial direction (with respect
to the platen) than the workpiece
516. The workpiece holder
512 is shown with arms
514 which carry mass upwardly, even beyond the line of the pivot shafts
506 and
508. This mass distribution keeps the center of gravity closer to the plane of the gimbal
bearings
506 and
508 than using a workpiece holder which was flat on all sides (e.g., a slab with rectangles
on all sides). Another configuration that would work is shown in perspective in Figure
19. In this configuration, the lapping assembly
530 is shown with a shaft
532 attached to a first external gimbal arm
534. The first external gimbal arm
534 is attached through gimbal bearings and pivot shaft
536 to a second external gimbal arm
538. This second external gimbal arm
538 is connected through gimbal bearings and pivot shaft
540 to a piecepart holder
542. The piecepart holder
542 holds the workpiece
544. By having the piecepart holder sitting within a volume of space created by the combination
546 or
534 of the first external gimbal arm
534 (and the second external gimbal arm
538), the center of gravity of the piecepart holder is maintained in a position which
is relative close to the line of rotation of the gimbals
534 and
538 through the gimbal bearings
536 and
540 to reduce tilting of the workpiece holder
542 due to the rotating speed of the workpiece. In addition, this configuration also
demonstrates a method for lowering the plane of the axes of the pivot gimbal running
through the gimbal bearings
536 and
540 close to the abrasive contact surface of the workpiece
544. This geometric orientation reduces the tilting torque on the workpiece and assists
in the maintenance of proper alignment within the lapping system.
[0379] Another benefit of the present invention, particularly with the use of annular rings,
is the ability to lap multiple pieces and even use multiple piecepart holders at the
same time. Figure 20 provides a description of this aspect of the invention. A lapping
system
550 is shown with an annular abrasive sheet
552, an arm
554 carrying two piecepart holders
556 and
558. Each of the piecepart holders
556 and
558 support a multiplicity of pieceparts
560 and
562. The piecepart holders
556 and
558 rotate so that the individual pieceparts
560 and
562 are exposed to the abrasive sheet
552. Each of the piecepart holders
560 and
562 are aligned on wobble plates (not shown) and are operated by the processes described
above in the practice of the present invention. The arm
554 may also have alignment mechanisms associated with it to assure proper alignment
with respect to the annular ring
553 and the rotatable platen (not shown). In this system, the different pieceparts
560 and
562 do not even need to be of the same size or cross section. For example, one set (e.g.,
560) could be round, and the other set (e.g.,
562) could be square or triangular in cress-section. It is equally useful to have a three
arm central support piece for three separate workpiece holders. It is desirable to
process each piecepart for an equal amount of time to make the surface treatments
equivalent. Therefore, pieceparts located at the center of the piecepart holder, such
as pieceparts
566 and
564 may be eliminated in this grouped set-up of pieceparts. If this were not done, pieceparts
566 and
564 would be continually lapped over the process, while other parts located in a ring,
such as shown for parts
560 and
562 would be processed only intermittently.
[0380] In positioning an abrasive sheet material in platen with an annular raise area on
the outboard edge of the platen, it is often convenient to use a sheet with larger
dimensions (especially with respect to the radius) than the raised annular area. When
the support layer (and even when it is a continuous sheet of abrasive with polymeric
or other binder) is position over the flat central area of the platen (or a part thereof)
and then fitted over the annular raised area, the sheet of abrasive shows a tendency
to crinkle and lift at the transition from the central area to the annular area. This
is shown in Figure 20(a), shown with the platen
600, raised annular area
602, vacuum hold down holes
604, abrasive sheet
606, and central area
608. As the abrasive sheet
606 moves up the step-up distance
610 with section
612 of the abrasive sheet
606, a crinkle or fold
614 forms at the point
616 at the raised annular area
604. Figures 20(b) and ⓒ show alternative platen shapes
620 and
622 which provide sloped transitions
624 and
626 from the central areas
628 and
630 to the flat raised areas
632 and
634. The slopes should never present an angle that would bend the abrasive sheet past
an angle of 65 degrees (e.g., forming an apex of less than 65 degrees by bending it
more than 25 degrees away from horizontal), preferably not past an angle of 70 or
75 degrees, and most preferably not past an angle of 75 or 80 degrees, or more than
85 degrees. By reducing the angle that the abrasive sheet must be bent, the possibility
of any crinkling is avoided. As the placement of abrasive sheets over an annular raised
area is another unique aspect of the invention, this solution is unique to the field
of the invention.
[0381] In Figure 3, two separate supports
1253 and
1252 (the housing) form the substance of the wobble plate. To further reduce vibration,
a cushioning, compressible element
1261 is provided between the wobbling piecepart holder
1252 and the bottom
1250 of the housing
1234. The compressible element
1261 should make contact between both the wobbling piecepart holder
1252 and the bottom
1250 of the housing
1234. Viscoelastic material, springlike elements, elastomers, rubbers, and layered structures
may be used. In the Figure 3, double sides polymer backed adhesive tape was rolled
into a tube and cut to the proper length. The tube was placed between the wobbling
piecepart holder
1252 and the bottom
1250 of the housing
1234. As they are brought together, the two surfaces compress and flatten the cushioning,
compressible element. This element assists in reducing the vibration within the wobble
plate element and the piecepart assembly.
[0382] In the movement of the workpiece holder and the workpiece towards and into contact
with the rotating abrasive sheet covered platen, the contact force application has
been repeatedly identified as a desirable focus of control within the practice of
the invention. An additional aspect of this control is the speed with which the workpiece
(and the workpiece holder ) approaches the rotating platen. As initial contact forces
tend to be higher because of momentum, reactive forces from the stationary surface,
and elastic forces, control of the speed of the movement of the workpiece and work
piece holder are desirable ways of controlling or moderating the initial contact force.
Thus, as generally mentioned herein, velocity control devices, such as fluid dampers
(oil dampers preferred, but other fluids, including gases, may be used). These velocity
control devices may be used with the cylinder contact force system to prevent the
workpiece from 'slamming' into the abrasive at a speed which would cause an undesirable
level of contact force initially. Therefore, a somewhat distinct or auxiliary speed
control or speed dampening system should be overlaid on the cylinder contact force
system to provide a second aspect of control to the contact force aspects of the present
invention. This speed control or speed dampening system may also be used to lock the
workpiece holder at a desired vertical position at any time during the process (as
for example after the removal of the workpiece from contact with the abrasive sheet
and platen element).
[0383] While the abrasive sheet and platen are rotating at the high speeds of the present
invention, it has also been found to be desirable to rotate the workpiece (usually
by rotation of the entire workpiece holder, although with multiple workpieces in a
group holder, the individual workpieces may also be easily rotated). It is desired
and has been proven to be beneficial to the flatness and especially the smoothness
of the work piece to have the workpiece rotated during the lapping process. The workpiece
should be rotated at least 1 or 2 full rotations during 10 seconds of active grinding,
especially at the point where the finer abrasive particles are being used. The workpiece
be rotated at a rate of at least about 100 rpm, preferably at least 150 rpm, and more
preferably at least 200, at least 300 rpm, which for a 30.8 cm diameter disk at 500
rpm, there should be at least 3 to 4, and preferably more than 4 rotations of the
workpiece during 10 seconds of lapping. It is preferred that the workpiece be rotated
at least 3 or 4 times in a 10 second interval during lapping in the practice of the
present invention. The work piece may be rotating as it is brought into contact with
the abrasive sheet surface.
[0384] As has been previously noted, it is desirable to only fill the valleys between the
peaks of the abrasive particles (the peaks protruding rom their binder support on
the backing sheet) by from 50% of the protruded height to perhaps 110 to 150% for
an abrasive sheet with an essentially continuous (uniform) coating or covering of
abrasive particles. However, where the provided abrasive sheet is provided with island
areas of abrasive or other broken or less continuous or less uniform distribution
of abrasive particles, then part of the water or coolant flow will lie in the river
valleys which are relatively lower than the protruding mountains of the abrasive islands.
The water will therefore be much deeper (a thicker boundary layer) than with a continuous
and uniformly coated abrasive sheet, and the piecepart will not hydroplane. In fact,
the more water that is present, the better is the grinding, as more heat is also carried
away by the larger volume of coolant water.