Machine tools abstract
Machine tools utilized in precision machine operations require
tool support structures which exhibit minimal deflection, thermal
expansion and vibration characteristics. The tool support structure
of the present invention is a graphite fiber reinforced composite
in which layers of the graphite fibers or yarn are disposed in a
0/90.degree. pattern and bonded together with an epoxy resin. The
finished composite possesses a low coefficient of thermal expansion
and a substantially greater elastic modulus, stiffness-to-weight
ratio, and damping factor than a conventional steel tool support
utilized in similar machining operations.
Machine tools claims
What is claimed is:
1. In combination with a machine tool comprising a machine tool
housing and a tool support carried by said housing for supporting
a tool in a desired working relationship with a workpiece, said
tool support comprising a hollow elongated tubular body formed of
a plurality of layers of graphite fibers with the layers and fibers
in each layer bonded together with a resin binder, the fibers in
the majority of said layers being disposed in a side-by-side relationship
and oriented so as to extend along a plane projecting between opposite
ends of the tubular body, the fibers in the minority layers being
wound about the tubular body in a side-by-side relationship and
projecting in a plane orthogonal to the fibers in the first-mentioned
plane, each of the minority of the layers being separated from one
another by a plurality of the majority of the layers.
2. The combination claimed in claim 1 wherein about 90 percent
of the total volume of the graphite fibers are disposed along said
first-mentioned plane.
3. The combination claimed in claim 1 wherein the graphite fibers
in the second-mentioned plane are circumferentially wound in situ,
and wherein each of the fiber layers in said first-mentioned plane
are provided by a plurality of gores disposed in a side-by-side
relationship, and wherein the seams of the gores are offset from
one another in succeeding fiber layers.
4. The combination claimed in claim 3 wherein five to twenty fiber
layers in said first-mentioned plane are disposed between each fiber
layer in said second-mentioned plane.
5. The combination claimed in claim 4 wherein the tool support
is of conical configuration, and wherein steel adapters are bonded
to the inner surface of said tool support adjacent to said opposite
ends for attaching said tool support to said housing at the larger
diameter end of said tool support and for carrying a tool at the
smaller end of said tool support.
6. The combination claimed in claim 4 wherein a rotatable shaft
extends through said tool support, and wherein bearing means are
disposed in said steel adapters at the ends of the tool support
for rotatably carrying said shaft.
Machine tools description
Precision machining requires the use of machine tool components
which introduce minimal error in the finished product. In order
to maintain the machine within desired operating parameters the
components utilized for fabricating the machine should exhibit minimal
deflection, thermal expansion, and vibration characteristics during
the operation of the machine tool, so as to reduce a substantial
source of dimensional errors in the finished product. In many machining
operations dimensional inaccuracies and surface defects in the workpiece
are usually produced by the weaknesses in the machine components
rather than in the machine program, such that any improvement in
the structural properties of the machine components would necessarily
result in an improvement in the dimensional accuracy and finish
of the machined product.
Accordingly, it is the primary aim or goal of the present invention
to provide a structure for supporting a machine tool which is particularly
characterized by a high flexural stiffness for minimizing machining
errors due to loadings applied against the machine tool during operation
thereof and also which has a high damping capacity for lessening
the effects of tool vibration on the surface of the workpiece. Another
desired property provided by the present invention is that the tool
support has a relatively low coefficient of thermal expansion so
as to minimize inaccuracies caused by temperature variations in
the environment and the tool support during the operation of the
machine. The goal of the present invention is achieved by employing
a novel tool support in combination with a machine tool which has
a machine tool housing with the tool support being carried by the
housing for supporting a tool in a desired working relationship
with a workpiece. The tool support comprises a hollow elongated
tubular body formed of a plurality of layers of graphite fibers
or yarn with the layers and the yarn in each layer bounded together
with a resin binder. The fibers in the majority of the layers are
disposed in a side-by-side relationship and so oriented as to extend
along a plane projecting between opposite ends of the tubular body.
The fibers in the minority of the layers are wound about the tubular
body in a side-by-side relationship and project in a plane orthogonal
to the fibers in the first-mentioned plane. Each of the layers of
graphite fibers forming the minority layers are separated from one
another by a plurality of the majority layers.
Other and further objects of the invention will be obvious upon
an understanding of the illustrative embodiment about to be described,
or will be indicated in the appended claims, and various advantages
not referred to herein will occur to one skilled in the art upon
employment of the invention in practice.
A preferred embodiment of the invention has been chosen for the
purpose of illustration and description. The preferred embodiment
illustrated is not intended to be exhaustive or to limit the invention
to the precise form disclosed. It is chosen and described in order
to best explain the principles of the invention and their application
in practical use to thereby enable others skilled in the art to
best utilize the invention in various embodiments and modifications
as are best adapted to the particular use contemplated. For example,
while the tool support structure of the present invention is shown
as a support for a precision contour grinder, it is to be understood
that the tool support of the present invention may be used in many
other machine tool supporting operations, such as associated with
boring bars, routers, and vertical milling operations.
In the accompanying drawing:
FIG. 1 is an elevational schematic view showing, in section, the
graphite fiber tool support structure of the present invention coupled
to a contour grinding apparatus; and
FIG. 2 is a schematic view of the tool support, partially broken
away, for showing details of the graphite fiber layers.
Described generally, the present invention is directed to a machine
tool in which the tool employed for precision work upon a workpiece
is supported by the machine tool by a graphite fiber-resin binder
composite. This tool support forms the basis of the present invention
and comprises an elongated tubular body formed of graphite fiber
or yarn within a resin binder system in which the majority of the
fibers are oriented in the longitudinal direction so as to provide
a high flexural stiffness, a high damping capacity, and a low coefficient
of thermal expansion. The tool support structure of the present
invention is characterized by an elastic modulus between steel and
tungsten carbide while possessing a stiffness-to-weight ratio of
approximately seven times better than steel and approximately four
times better than tungsten carbide at only one-tenth of the weight
of the latter. Also, the tool support has a damping factor, relative
to steel, of approximately twenty times better than a conventional
steel tool support employed for the same purpose.
The pattern of the fibrous graphite yarn utilized in the fabrication
of the tool support structure is a 0.degree. and 90.degree. pattern
wherein the longitudinally oriented layers, that is, the layers
extending along the 0.degree. plane projecting between opposite
ends of the support, are provided by manually applied gores of parallel
fibers with these gores being cut to size and shape from a unidirectionally
wound "fiber blanket." The layers of yarn in the 90.degree.
pattern are formed in situ on the composite by circumferentially
winding the yarn in a side-by-side relationship upon a mandrel utilized
in the fabrication of the tool support structures. The yarn and
resin binder are subjected to several compactions in a matched die
assembly during fabrication to provide a relatively dense, high-fiber
content structure.
Described more specifically and with reference to the accompanying
drawing, the graphite-fiber tool support 10 of the present invention
is shown afixed to a contour grinder 12 comprising a frame 14 to
which a spindle bracket 16 is attached thereto, slots 18 are provided
in the frame 14 for pivoting the grinding head during various contour
grinding operations. Pins 20 extend through the spindle bracket
16 and are utilized for effecting the pivotal movement of the tool
support 10 in the frame 14. A drive motor 22 is shown carried by
the spindle bracket 16 and is coupled to a drive shaft 24 which
projects through the tool support 10 to a grinding wheel 25. Within
the tool support 10 there are metal adapters 26 and 28 for respectively
securing the tool support 10 to the spindle 16 by bolts or the like
(not shown) and for rotatably supporting the drive shaft within
the tool support 10. These adapters which are tubular and cone-shaped
to conform with the inner wall surface of the support 10 are placed
against and bonded to these inner wall surfaces by any suitable
epoxy adhesive. A metal sleeve 29 is shown disposed about the upper
end of the tool support 10 for providing an additional coupling
of the tool support 10 to the adapter 26. The adapters 26 and 28
are provided with suitable bearings 30 and 31 for rotatably supporting
the drive shaft 24.
The fabrication of the tool support 10 is provided by employing
a 0/90.degree. fiber pattern with five to twenty longitudinally
oriented layers of fibers for each circumferentially disposed fiber
layer. The graphite fibers as described herein are in the form of
a yarn with about 1440 graphite monafilaments. The number of layers
of the yarn or fibers required for fabrication of the tool support
depends upon the particular application, normally a thickness of
about one inch provides the rigidity necessary for surpassing that
provided by a similar steel structure. With an average thickness
of each fiber layer being about 6 mils, the total number of layers
desired in the tool support may be easily calculated.
As best shown in FIG. 2 the tool support 10 is of a generally
tubular hollow conical configuration. This conical configuration
is of an angle of 3.degree. to 4.degree. with respect to the longitudinal
axis and provides the best flexural stiffness due to increasing
section moment of inertia in approaching the mount end.
In forming the tool support 10 a right angled cylindrical mandrel,
not shown, having a circumference at least as great as the length
of the tool support 10 is wound with graphite fibers in a side-by-side
relationship. These fibers are preferably passed through a bath
of a suitable epoxy resin binder prior to winding the fibers about
the mandrel. When the winding is completed, the winding is removed
from the mandrel and laid out as a single solid sheet or blanket.
From this sheet gores of a generally triangular configuration are
cut by using a suitable triangular template. The number of gores
32 utilized for the construction of the tubular support is not critical
in that, for example, three triangularly-shaped gores provide adequate
coverage of the tool support surface. The cutting of the gores from
the blanket is such that the orientation of the graphite fibers
in the gores when applied to the tool support 10 is parallel to
the longitudinal axis of the tool support, that is, the fibers project
along a plane extending between opposite ends of the tool support
10.
To apply the gores 32 during the buildup of the tool support 10
a mandrel having a generally conical configuration is first provided
with a release agent of any suitable type, e.g., polytetrafluoroethylene
spray coating or mold release wax on the surface, then covered by
a polyester film, and then the gores are placed in position thereon
and held in place by the tackiness of the gore surface. As successive
layers of the gores 32 are placed upon the mandrel, their joints
are offset from those of the preceding layer of gores to assure
that the seams are staggered about the circumference of the tool
support. After a desired number of layers, e.g., about five to twenty
layers, of the gores 30 have been placed on the mandrel, the mandrel
is placed in a conventional winding machine so that a circumferential
layer 34 of a continuous graphite yarn may be applied in situ. This
circumferential layer of graphite fibers is provided over the entire
length of the tool support with the fibers being disposed in side-by-side
relationship along a plane orthogonal to the longitudinal axis of
the tool support. The yarn employed for forming the circumferential
layer is preferably passed through the resin binder prior to winding
it about the mandrel. Satisfactory results have been achieved by
orienting approximately 90 percent of the graphite fibers in the
longitudinal direction and about 10 percent of the fibers in the
circumferential direction. The purpose of positioning the majority
of the graphite fibers in the longitudinal directon is that the
elastic modulus is greater with the majority of fibers so oriented
then would be if a lesser quantity of the fibers were longitudinally
disposed. The circumferential fiber layers 34 disposed between the
layers of longitudinally oriented fibers provide a minimal transverse
strength in the composite to avoid any longitudinal cracks because
of the weak transverse strength of the graphite fiber composite.
In order to assure that the tool support contains at least about
50 to 60 volume percent fibers, the binder in excess of about 40
to 50 volume percent is driven from the composite prior to the curing
of the resin. This removal of the excess binder is achieved by pressing
the composite in a suitable die assembly at a pressure in the range
of about 400 to 500 psi. The application of the pressure loading
is preferably achieved at selected sequences during the fabrication
of the composite. For example, compaction of the composite to force
the excess resin from the system may be satisfactorily achieved
after each four or five layers of fibers are applied to the mandrel.
After applying the selected number of fiber layers, longitudinally
and circumferentially, on the fabrication mandrel to form the composite
of the desired wall thickness, the resin system is allowed to fully
cure so as to secure the fibers and the adjacent layers of fibers
tenaciously together. The resin system employed as the binder is
preferably an epoxy resin of the type curable at room temperatures
so to obviate the use of heat and other curing mechanisms which
could possibly detract from the integrity of the structure.
With the completion of the tubular graphite fiber-resin binder
composite, it is interfaced with the adapters 26 and 28. This interfacing
may be achieved by etching the surface of the metal adapters 26
and 28 and then adhesively bonding them into the tubular composite
10. The epoxy resin employed for binding together the fibers may
be used for attaching the adapters to the composite. Interfacing
the composite with the adapters 26 and 28 should be achieved with
considerable care since misalignment or a poor bond between the
mating surfaces will lower the stiffness of the tool support 10
as well as introduce inaccuracies in the performance of the grinding
head.
The stiffness of the tubular support 10 in the contour plane at
the grinding wheel is approximately 6.6 microinches per pound of
force applied against the grinding head for in-plane horizontal
loads. If these grinding forces are less than 25 pounds, the total
contour errors due to the deflection of the tool support 10 is only
about 160 microinches. In a demonstration of the invention, a graphite
cylinder was ground to a diameter of 27 inches and a height of 12
inches with the variation of the cylinder diameter being with .+-.
0.0003 inch.
The elastic modulus of the tool support 10 with 52 volume percent
fibers, an overall length of approximately 48 inches with a diameter
of about 11 inches at the large end and about 51/2 inches at the
small end is about 40 .times. 10.sup.6 psi which is substantially
greater than a steel tool support of a similar configuration which
has an elastic modulus of 30 .times. 10.sup.6 psi. The density in
grams per cubic centimeter of the tool support composite is 1.5
which compares favorably to that of a steel tool support which is
7.8 grams per cubic centimeter so as to provide a stiffness-to-weight
ratio 7 times greater than that of the steel support. The coefficient
of thermal expansion of the graphite-resin composite is essentially
zero whereas a steel tool support is about 8.5 .times. 10.sup.-6
/inch/1.degree. F. The damping factor which eliminates a substantial
amount of the detrimental chatter and vibration found in tool supports
of the nature described is such that the graphite fiber-resin composite
has a damping factor of about 20 times greater than that provided
by a steel support when relatively compared which provides a significant
improvement in the rigidity of tool supports. In fact, it is believed
that any additional improvements in the rigidity of the grinder
of the type described herein would have to be achieved in other
components since the elastic modulus of the tool support is significantly
greater than that of other components of the grinding machine.
It will be seen that the present invention provides a unique tool
support structure which is characterized by a high flexural stiffness
for minimizing errors resulting from machining forces, a high damping
capacity for minimizing the effect of vibration on the surface finish,
near zero coefficient of thermal expansion for reducing inaccuracies
caused by temperature variations in the environment and other machine
components, and a light weight for decreasing machine tool deflections
caused by the machine component weight.
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