[0001] This invention relates to improvements in cylindrical grinding, and particularly
in centreless grinding.
[0002] Precision punches for sheet steel are produced from cylindrical blanks to very precise
dimensions, by centreless concentric grinding. Generally speaking the punching end
of the blank is ground to the desired diameter and for the desired length, the other
end of the blank providing a standard shape for gripping in a punch holder.
[0003] A CNC punching machine typically has a magazine with numerous punches of different
size. The breakage of a punch can seriously interrupt production, but the wide variety
of punch sizes means that stockholding of replacements is limited for economic reasons.
As a result it has become usual for punch suppliers to grind a replacement punch on
demand, and deliver the punch to the end user within twenty four hours. Each punch
is ground under manual control, and the production process is thus labour intensive
and somewhat expensive. Automated grinding of punches would be desirable.
[0004] Another difficulty in the production of such punches is that measurement of the diameter
of the workpart during grinding is essential in order to compensate for wear in the
grinding and control wheels. Typically the grinding wheel plunges generally horizontally
against the workpart whereas the measuring device acts vertically.
[0005] Existing measuring devices are however unsatisfactory. One such device uses scissor
like arms on the grinding wheel side to grip the workpart and therefore gauge diameter.
However since the arms do not contact the workpart on a diameter, the distance between
the scissor pivot and the centreline of the workpart is critical to accurate gauging.
In centreless grinding, where the control wheel provides a fixed reference, the workpart
centreline will move relative to the scissor pivot as the diameter of the workpart
reduces.
[0006] Another device uses vertically movable probes having anvils to ensure contact with
the workpart on a diameter. This arrangement reduces available space in the vicinity
of the workpart and may restrict movement. Also the anvils may not contact the workpart
exactly on a diameter, and this may introduce inaccuracy.
[0007] Both prior measuring proposals have components of significant length and are subject
to thermal expansion and contraction. Furthermore both prior proposals typically gauge
diameter at a fixed location, and cannot therefore measure taper of the workpart.
Accurate measuring of the workpart is essential in order to determine the extent of
a final grinding step to the desired finished diameter.
[0008] According to a first aspect of the invention there is provided a centreless concentric
grinder comprising a grinding wheel, a control wheel, a rolling clamp and workrest,
the clamp and workrest being adapted to hold a workpart against the control wheel
at a predetermined height, and the grinding wheel being arranged to plunge at one
side of the workrest to grind a workpart to a desired diameter, wherein the workrest
presents a flat face to the workpart, said face being at an acute angle to vertical,
and the workrest being mounted on a powered slide for movement towards and away from
the control wheel.
[0009] Such an arrangement permits the height of the workpart to be set as a function of
the lateral displacement of the workrest, and moreover for the workrest to be automatically
driven to the desired position in response to an input of the selected punch dimensions.
[0010] In practice the workrest is driven to the position which permits initial grinding
of the workpart to an oversize, at which point the precise diameter of the workpart
is measured. Following this the finish grinding step is performed to bring the component
to the desired diameter.
[0011] Owing to the fact that a finish grind is always required after measurement, the lateral
position of the workrest does not need to be absolutely true, a small variation in
workpart diameter being possible for a given workrest position. It is thus envisaged
that conventional motorized drives are suitable for positioning the workrest according
to the diameter of the punch to be ground.
[0012] In order to set the ground length of the workpart, it is also envisaged that the
usual end stop is motorized. This also permits the workpart to be positioned correctly
relative to the workrest and control wheel. The end stop may act on the underside
of an enlarged head of the workpart, or against the head, depending on which direction
the usual end forces are generated. The position of the end stop is fixed in relation
to workparts of a given length, and accordingly the position of the end stop does
not need to be changed for similar workparts which vary in diameter.
[0013] According to a second aspect of the invention a measuring device is provided for
a rotary machine tool having a powered carrier for plunge movement relative to a workpart,
the measuring device comprising an arm adapted for pivoting mounting on a said carrier,
and a calliper slidably mounted on the arm for inward and outward movement thereof,
the calliper being biased to an end stop of said arm, and having a fixed jaw and a
movable jaw, and the calliper further including means to determine the relative position
of the fixed and movable jaws, a first actuator being provided to pivot said arm from
an inactive condition to an active condition in which said jaws lie on either side
of a workpart, and said carrier being movable to draw the fixed jaw against a workpart,
and a second actuator being provided to move the jaws of the calliper together, so
as to grip a workpart on a diameter in order to determine the size thereof.
[0014] Plunge movement is generally perpendicular to the axis of rotation of the rotary
machine tool, and generally perpendicular to the rotational axis of a workpart in
the machine tool. The powered carrier is typically a motorized member drivable relative
to the machine tool frame under servo control, and may for example be drivable on
two mutually perpendicular axes so as to bring a machining member such as a grinding
wheel into contact with a workpart.
[0015] This measuring device can be pivoted from an inactive (vertical) position to an active
(horizontal position) for gauging workpart diameter. The calliper is slidable on the
arm for the purpose of indicating when the fixed jaw is in contact with the workpart,
the movable jaw being afterwards moved towards the fixed jaw to enable the precise
diameter to be determined. In the preferred embodiment a limit switch is provided
to permit contact between the fixed jaw and the workpart to be sensed, and to suspend
movement of the powered slide. Other means of suspending slide movement are however
possible, for example by making an electrical circuit between the fixed jaw and the
workpart.
[0016] The arrangement permits precise measurement of workpart diameter on the (horizontal)
grinding axis rather than at right angles thereto, and the measuring device is not
as susceptible to thermal effects since it is usually in the inactive condition away
from the heat generated by the grinding apparatus.
[0017] Other features of the invention are shown in the accompanying drawing of a preferred
embodiment shown by way of example only in the accompanying drawings in which:-
Fig. 1 is a schematic plan view of a centreless grinder;
Fig. 2 is a schematic plan view of the grinder of Fig. 1 in an alternative position;
Fig. 3 shows in section a workrest in two alternative positions;
Fig. 4 shows in side elevation a schematic measuring device;
Fig. 5 shows a measuring device in the inactive condition; and
Fig. 6 shows the measuring device of Fig. 5 in the active condition.
[0018] A conventional concentric centreless grinding arrangement is illustrated in Figs.
1-3. A workpart 11 is held against a control wheel 13 by a workrest 14 and a concentric
roller 15. A grinding wheel 12 is mounted on a carriage 20 and is driven towards the
workpart on axis X in order to reduce the diameter thereof. The workrest 14 and roller
15 act at one side of the portion of the workpart which is to be ground.
[0019] The carriage 20 is also movable on axis Y to permit initial grinding with a roughing
wheel 12a, followed by final grinding with a finishing wheel 12b. X-Y movement permits
periodic dressing of both grinding wheels against a fixed dresser 16, and allows a
carriage mounted measuring gauge 17 to be brought into contact with the workpart.
An end stop 18 prevents axial movement of the workpart due to the usual end forces
present in centreless grinding.
[0020] In concentric grinding the height of the workpart centreline is not varied with changes
in the diameter clamped. Thus up until now the workrest has been set to maintain the
workpart on the centreline of the grinding wheel, this being a function of the finished
diameter of the workpart. Setting and adjusting of the height and horizontal displacement
of the workrest has required manual intervention, which is time consuming.
[0021] In the present invention however, the workrest is mounted on a servo driven slide
for movement on the X axis, and by providing the workrest with an angled contact face
various diameters of workpart can be accommodated at the same height.
[0022] Typically the workrest contact face 19 is flat and at an angle of 45°, permitting
a variation of workpart diameter in the ratio 1:3 by moving the workrest on the X
axis. Thus workparts in the size range of e.g. 4-12 mm or 6-18 mm may be accommodated
by servo movement of the workrest to the desired position.
[0023] Fig. 3 is a somewhat schematic section through a device according to the invention
to illustrate the effect of the workrest. The drawing is not to scale, and certain
parts are exaggerated in order to clearly illustrate this aspect of the invention.
[0024] Fig. 3a illustrates a control wheel 13, the workrest 14 and concentric roller 15
in section. The workpart 11a is a relatively small diameter, and lies on the control
wheel centreline 21. The workrest has a 45° flat support face 22 and is close to the
control wheel 13, with the workpart supported close to the upper end of the support
face 22.
[0025] Fig. 3b illustrates a somewhat larger workpart 11b supported by the same workrest
at a greater distance from the control wheel 13. The workpart remains on the centreline
21 and is supported somewhat lower on the support face 22. The workrest 14 moves between
Fig. 3a and Fig. 3b in a horizontal plane only, in the direction of the axis 21.
[0026] Fig. 4 illustrates schematically a measuring device according to the invention.
[0027] An electronic micrometer 30 is provided on a frame 31 mounted for pivoting in a vertical
plane on the grinding wheel carriage, typically on the grinding wheel guard. The pivot
axis 32 is shown diagrammatically. An actuator of any suitable kind is provided to
swing the frame about axis 32 between end stops defining an advanced (horizontal)
condition, as illustrated, and a retracted (upright) condition. The fixed jaw 33 of
the micrometer is of sufficient length to lie on a diameter of the workpart when the
workpart is in the micrometer jaw. The micrometer body 35 is mounted on the frame
31 for limited sliding movement in the radial direction, and is biased inwardly to
an end stop by a light tension spring 36.
[0028] Figs. 5 and 6 illustrate a typically measuring device according to the invention.
Reference numerals in common with Fig. 4 are used where appropriate.
[0029] The micrometer 30 is mounted on a frame 31 by a cylindrical shaft 40 supported in
a linear ball bearing 41. An anti-rotation pin 42 extends from the micrometer to the
frame 31 to maintain the micrometer in the desired plane. The shaft 40 has a head
43 against which acts as a coil compression spring 36 grounded on the frame 31. The
spring 36 biases the micrometer inwardly of the frame 31.
[0030] The frame 31 is pivotable about axis 32 by a double acting ram 44 of any suitable
kind. The micrometer has a fixed jaw 33, and movable jaw 34 as illustrated. Two limit
switches 37 are provided to ensure continued operability should one switch fail.
[0031] Fig. 6 illustrates a damper and adjustable stop 45 to ensure that the micrometer
comes smoothly to rest on a diameter of a workpart 11 in use. Electrical leads 46,
shown schematically, connect the micrometer to a suitable display/recording device.
Circle 47 represents a typical grinding wheel.
[0032] In use measurement is performed by driving the grinding wheel carriage on the Y axis
until the micrometer is aligned with the diameter to be measured. The carriage is
driven in the X direction such that when the frame 31 is pivoted to the horizontal
position, the fixed jaw 33 of the micrometer is lowered to the opposite side of the
workpart. Necessarily the micrometer protrudes slightly more than the maximum diameter
of the grinding wheel(s) and the carriage is driven on the X axis to ensure clearance
between the workpart 11 and the fixed micrometer jaw 33.
[0033] From the position illustrated in Fig. 6, the frame 31 is driven away from the workpart
on the X axis (to the left) by movement of the grinding wheel carriage, until the
fixed jaw meets the workpart. At this point the micrometer body moves relative to
the frame against the effect of the spring 36, and a limit switch 37 is provided to
suspend movement of the carriage at a predetermined light load. The movable jaw 34
of the micrometer is then driven against the workpart so as to measure the diameter
thereof in a conventional manner.
[0034] After measurement, the procedure is reversed to bring the micrometer to the retracted
position.
[0035] The spring load at which carriage movement is suspended is not important provided
that the fixed jaw lies against the workpart and the load is not sufficient to introduce
distortion. It is envisaged that gross movement of the micrometer body 35 will be
of the order of 1 mm. Furthermore, the load at which the carriage ceases movement
due to operation of the limit switch is constant regardless of the diameter being
measured-it being merely necessary to ensure that the fixed jaw 33 clears the workpart
on dropping to the measuring position.
[0036] Typically a master component of known diameter would be loaded into the grinding
machine from time to time in order to permit the micrometer to be calibrated.
[0037] Although this measuring device has been described in relation to centreless grinders,
it is equally suitable for use on any machine tool having a traversable carriage.
For example the device could be mounted on the wheel guard of a between-centres grinder
or on the saddle of a lathe.
[0038] Measurement of the diameter of the workpart may be repeated at several axial positions,
in order to check for taper, by simply traversing the carriage on the Y axis to the
desired location and repeating the measuring procedure.
[0039] It will be understood that the location of the pivot and the extent of movement of
the carriage on the X axis are not critical to the dimension measured by the micrometer;
these merely bring the micrometer into a position from which an accurate measurement
can be made in order to determine the amount of material to be removed in a final
grinding pass.
[0040] As an alternative to the limit switch, movement of the carriage in the X direction
may be suspended by making an electrical circuit between the fixed jaw 33 and the
workpart. A low voltage circuit would be suitable, and connected to any conventional
means for causing the traversing motor to cease.
[0041] A third actuator, or releasable catch may be provided to maintain the micrometer
jaws in a fixed position during movement to and from the measuring position, and whilst
in the retracted condition. Such a device will prevent chattering due to machine vibration
and during movement about the pivot 32.
[0042] In use it is envisaged that a typical automatic machining sequence will be as follows.
1. Unload previous component.
2. Adjust position of workrest and end stop to suit required dimensions of next component,
by reference to a look-up table.
3. Load unfinished blank.
4. Drive head of workpart against end stop.
5. Rough grind workpart in one or more plunges to the finished size +0.2 mm on diameter,
using roughing wheel.
6. Semi-finish grind workpart with finishing wheel to finished size +0.05 mm on diameter.
7. Traverse grinding wheel away from workpart and advance micrometer to measure ground
diameter.
8. Calculate remaining depth of material to be removed by reference to actual and
required diameter.
9. Finish grind to desired diameter.
10. Optionally, re-measure diameter as confirmation.
[0043] This sequence provides unmanned grinding of punches, and can record measurements
at several grinding stages in order to provide inspection and quality control data.
[0044] Automatic loading and unloading by gripping the punch head is envisaged. Such loading
is typically by robot arm from a pallet of punch blanks, the finished punch being
stored in a separate pallet with known identity and location. For an average punch
having a blank diameter of 12 mm, a punching diameter of 5 mm and a punching length
of 20 mm, a cycle time of less than 150 seconds is envisaged.
1. A measuring device for a rotary machine tool having a carrier (20) adapted for plunge
movement relative to a workpart (11), the measuring device comprising an arm (31)
adapted for pivoting mounting on a said carrier (20), and a calliper (30) slidably
mounted on the arm (31) for inward and outward movement thereof, the calliper (30)
being biased to an end stop of said arm (31), and having a fixed jaw (33) and a movable
jaw (34), and the calliper further including means to determine the relative position
of the fixed and movable jaws (33,34), a first actuator (44) being provided to move
said arm from an inactive condition to an active condition in which said jaws lie
on either side of a workpart (11), and said carrier (20) being movable to draw the
fixed jaw against a workpart (11), and a second actuator being provided to move the
jaws of the calliper together, so as to grip a workpart (11) on a diameter in order
to determine the size thereof.
2. A measuring device according to claim 1 and including an actuator to urge said calliper
in one of said inward and outward directions.
3. A measuring device according to claim 1 or claim 2 and including a spring (36) to
urge said calliper in one of said inward and outward directions.
4. A measuring device according to claim 3 wherein said spring (36) is a coiled compression
spring.
5. A measuring device according to claim 3 or claim 4 wherein said spring is adapted
to urge said calliper inwardly of said arm.
6. A measuring device according to any preceding claim wherein said arm is adapted to
pivot from the inactive condition to the active condition.
7. A rotary machine tool having a carrier adapted for plunge movement relative to the
rotary axis of the machine tool, and the measuring device of any preceding claim mounted
thereon.
8. A centreless grinder having a grinding wheel adapted for plunge movement relative
to a workpart, and a measuring device according to any of claims 1-6 mounted thereon.
9. A centreless grinder according to claim 8 wherein said measuring device is substantially
horizontal in the active condition and upright in the inactive condition.
10. A centreless grinder according to claim 8 or claim 9 wherein said measuring device
is mounted on a wheel guard of said grinding wheel.