FIELD OF THE INVENTION
[0001] The present invention relates to the machining of a bore in a workpiece and, in particular,
to the machining with an expandable tool of a thin wall defining a bore in a workpiece
and separating the bore from an adjacent passage.
BACKGROUND OF THE INVENTION
[0002] In the manufacture of engine blocks for vehicles, the cylinder bores are subjected
to a honing operation to provide bore dimensions within very close tolerances. In
the past, the maintenance of bore dimensions within the required tolerance range has
sometimes been difficult especially when the cylinder block is designed and cast to
have relatively thin walls separating the bores from adjacent coolant passages primarily
for weight reduction purposes. This difficulty has been compounded when the cross-section
of the wall varies considerably from one location to another as a result of engine
design considerations or core shift when casting the engine block. During honing,
the honing stones are expanded radially against the bore wall to exert an abrading
action thereon, a honing pressure in the range of 100 to 400 psi, typically 200-250
psi, being used. This tool expansion force has caused thin bore walls or thin portions
thereof to be distorted during honing, resulting in finished bore geometry out of
required tolerances.
[0003] U.S. Patent 4,117,633 issued Oct. 3, 1975 to C. L. Yother describes a honing plate
attachable to the cylinder block during honing to simulate the normal operating stresses
and distortions of an engine block caused by bolt-on components such as cylinder heads
and intake manifolds. Re-boring and honing are effected with the plate attached to
allegedly increase the accuracy of the boring and honing operations. However, this
patent does not deal with the problem of distortion of thin bore walls by the honing
tool itself, in particular by the tool expansion force or pressure needed for honing
engagement.
[0004] U.S. Patent 3,542,354 issued Nov. 24, 1970 to P. Fitzpatrick and U.S. Patent 3,751,050
issued Aug. 7, 1973 to I. W. Wades et al. disclose so-called workpiece chucks or fixtures
for holding a cylindrical cylinder liner during a machining operation such as honing
without distorting the cylinder liner. Each patent uses an expansible component to
grip and hold the workpiece around its exterior in response to fluid pressure exerted
on the expansible component. These patents seek to minimize distortion of the cylinder
liner workpiece caused by the clamping or fixturing means.
[0005] The aforementioned prior art workers did not deal with the problems of honing thin
bore walls in engine blocks or other articles wherein the honing tool itself causes
distortion of the thin walls and out of tolerance bore geometry.
SUMMARY OF THE INVENTION
[0006] The present invention relates to a method and apparatus for machining a workpiece
bore, in particular a thin bore wall separating a workpiece bore from another adjacent
passage such as a coolant passage and the like, with a tool expandable against the
thin wall wherein pressurizing means applies a fluid pressure to the adjacent passage
in the workpiece during machining acting to exert a force or pressure on the thin
wall counter to the distorting force of the expandable tool against the wall. Distortion
of the thin wall is thereby substantially reduced during machining to provide a more
accurate machined bore geometry.
[0007] In a particularly preferred embodiment of the invention, the coolant jacket or passages
of an engine block are fluid pressurized to a pressure level of for example 30 psi
during honing of the cylinder bores to substantially reduce distortion of thin cylinder
walls by the expansion force of the honing tool thereagainst. Considerable improvement
in bore geometry has been achieved in this manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008]
Figure 1 is a partial cross-sectional view through a four cylinder engine block showing
the walls separating the cylinder bores oil and #2 from adjacent coolant passages.
Figure 2 is a schematic end elevational view showing the apparatus of the invention
for fluid pressurizing the coolant passages of the engine block while the cylinder
bores are honed.
Figure 3 is a schematic side elevational view of the apparatus of Fig. 2.
Figure 4 is a schematic top view of the apparatus of Fig. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0009] Fig. 1 illustrates a partial cross-section through a typical engine block 10 including
cylinder bores 12 defined by walls 14 having relatively thick portions 14a at the
top end and relatively thin portions 14b therebelow. It is apparent that the walls
14 may vary in cross-sectional shape or dimension along their lengths. The thin walls
14b terminate at their bottom ends in thicker support walls 15 which are not honed.
As is apparent, the thin walls 14 separate the cylinder bores from adjacent coolant
passages 16 formed in the engine block.
[0010] Figs. 2-4 show an apparatus for use in the invention comprising a simulated cylinder
head 20 adapted to be bolted to the engine block. To this end, the head includes a
plurality of holes 22 for receiving standard bolts (not shown) and a bottom surface
24 to rest against the top surface 26 of the engine block and close off the coolant
passages in known manner where the passages open to the top surface 26. A resilient
gasket 28 is interposed between head surface 24 and block surface 26 to provide a
fluid pressure tight seal therebetween. The head 20 also includes a plurality of bores
30 coaxially aligned with the cylinder bores 12, the head bores 30 being slightly
greater in diameter than the cylinder bores to allow entry of the honing tools 34
to the cylinder bores for the honing operation. Honing tools used in the method of
the invention are commercially available and include a plurality of abrasive stones
35 or an abrasive sleeve expandable radially outward by various means such as tapered
arbors, to engage and contact the bore wall at a selected honing pressure to effect
honing, usually in the range of 100 psi to 400 psi, typically 200 psi to 250 psi.
[0011] The apparatus also includes a cover 36 bolted over the water pump opening 38 in the
front side of the engine block. A resilient gasket 40 is placed between the cover
36 and engine block to provide a fluid pressure tight seal therebetween. In addition,
all other side or lateral openings 44 in the engine block extending to the coolant
jacket or passages are plugged fluid pressure tight by expandable plugs 46 or other
means.
[0012] To introduce fluid pressurizing means such as pressurized liquid or gas into the
coolant jacket including passages 16, a conduit 50 is attached to the engine block
at the engine temperature thermocouple opening 52. Of course, the conduit is connected
to a suitable source 54 of pressurized fluid such as pressurized liquid or gas. Honing
trials using the above-described apparatus were carried out using air pressure of
30 psi introduced into the coolant passages 16 from a conventional pressurized air
source and using a pressure regulator 56 in the air supply conduit 50 to the block
10. The coolant used in the honing operation could also be used as the pressurized
fluid (from source 54).
[0013] Table I shows the roundness deviation of bores in a 4-cylinder automobile cast iron
engine before honing by the invention and afterwards. Honing was carried out using
an expandable honing tool with 9 abrasive stones of 230 grit with approximately 200
psi hone pressure during a 33-second honing stage followed by a hone pressure of approximately
80 psi during a 5-second run-out stage. Roundness measurements were taken with a commercially
available "Talyrond" gage at the top, center and bottom of each cylinder bore in known
fashion.

[0014] It is apparent that a marked improvement in bore roundness is achieved by the honing
method of the invention. The increased out-of-roundness of bore #1 relative to the
other bores is attributable to the greater thickness of portions of the cast wall
defining this bore (see Fig. 1) compared to the other bores. Bore #1 would correspond
to the first bore at the front of the engine. The roundness of bore #I was nevertheless
considerably improved by the honing method of the invention. The present invention
is especially effective in improving roundness of bores with the relatively thinner
cast walls such as walls associated with the inner bores #2 and #3 and, depending
on engine block design, rear bore #4.
[0015] While there have been described what are considered to be certain preferred embodiments
of the invention, other modifications, additions and the like may occur to those skilled
in the art and it is intended to cover in the appended claims all such modifications
as fall within the spirit and scope of the invention.
1. A method for machining a bore in a workpiece wherein a thin workpiece wall defines
the bore and separates it from an adjacent passage and the like comprising engaging
an expandable tool against the wall for machining purposes and fluid pressurizing
the adjacent passage to reduce distortion of the wall by the expandable tool.
2. The method of claim 1 wherein the passage is pressurized with liquid.
3. The method of claim 1 wherein the passage is pressurized with gas.
4. A method for honing a cylinder bore in an engine block wherein a thin bore wall
separates the cylinder bore from an adjacent coolant passage in the block comprising
engaging an expandable honing tool in abrading contact with the wall for honing same
and fluid pressurizing the adjacent coolant passage to reduce distortion of the wall
by the honing tool.
5. An apparatus for machining a bore in a workpiece wherein a thin workpiece wall
defines the bore and separates it from an adjacent passage, comprising a means adapted
for releasable attachment to the workpiece to seal the passage in fluid pressure tight
manner, means for introducing fluid pressure into the sealed passage during machining
and tool means expandable against the wall and movable relative thereto to effect
machining, whereby the fluid pressure in the passage acts counter to any distorting
force exerted on the wall by the expandable tool means to reduce distortion thereof
during machining, providing a closer tolerance bore geometry.
6. The apparatus of claim 5 wherein the passage sealing means includes a tool-receiving
bore providing access of the tool means to the workpiece bore.
7. The apparatus of claim 5 wherein the means for introducing fluid pressure to the
passage includes a conduit connected in fluid flow relation to the passage and a source
of fluid pressure.
8. The apparatus of claim 7 wherein the source is a source of pressurized air.
9. The apparatus of claim 7 wherein the source is a source of pressurized liquid.
10. An apparatus for honing cylinder bores in an engine block wherein thin bore walls
separate the cylinder bores from adjacent coolant passage in the block comprising
a simulated cylinder head member releasably attachable to the engine block including
a plurality of openings slightly greater in diameter than the cylinder bores to provide
access to the cylinder bores, gasket means between the cylinder head member and engine
block to provide a fluid pressure tight seal therebetween, means for sealing other
openings to the coolant passages, means for introducing fluid pressure into the sealed
passages during honing, and honing tool means expandable against a respective one
of the walls and reciprocable relative thereto to effect honing, whereby the fluid
pressure in the passage acts counter to any distorting force exerted on the wall by
the expandable honing tool means to reduce distortion thereof during honing, providing
bores with a closer tolerance bore geometry.
11. The apparatus of claim 10 wherein the fluid pressure introducing means includes
a conduit connected in fuel flow relation to the passages and source of pressurized
fluid.