Grinding Machine

Abstract

A grinding machine for forming a non-circular surface on a workpiece and means for preventing mis-formation of the surface due to lack of normalcy between the master cam and the cam follower.

Description

BACKGROUND OF THE INVENTION

It is common practice to form non-circular surfaces by use of the abrasive machining process. Generally speaking, the master cam is used to guide the relative motion between the workpiece and the grinding wheel in such a manner as to duplicate the master cam surface on the workpiece. In order to obtain an accuracy in this operation, it is desirable that a situation known as "normalcy" be maintained. This is defined as moving the master cam past the cam follower in such a way that the point of contact between the cam follower and the master cam surface and the center of the cam follower lie in the same plane at all times. This means that, similarly, the force between the workpiece and the grinding wheel remains in the same direction during the entire cycle of rotation of the master cam, so that the grinding wheel cuts always on the same line, namely where it is dressed and runs true.

Experience with grinding spindles shows that the grinding wheel runs true only in the line where it was dressed. Maintaining normalcy also allows wheel wear without part shape distortion due to "R-.alpha. error". This condition of normalcy is particularly useful in grinding the housing of the so-called "Wankel engine" in which a "figure eight" chamber (known as a "epitrochoid") is formed and there are some areas of the surface which have a high rate of angular change, both inwardly and outwardly of a basic circle. One way of accomplishing this is shown in the U.S. Pat. of Davies No. 2,421,548 where the master cam is made in the shape of an annulus with a drive roller on one side and a cam follower on the other side, the master cam being driven by one of the rollers. The master cam snakes its way between the two rollers, thus maintaining theoretical normalcy. A similar method of accomplishing this is shown in the Appleton et al U.S. Pat. No. 3,259,021 where the metal removing tool is a milling cutter but the master cam is an annulus which snakes its way through a group of rollers to maintain normalcy. Similarly, in the U.S. Pat. of Hoglund No. 3,663,188, the master cam is also in the form of an annulus which snakes it way between a drive roll and an idler roll or a pair or idler rolls to maintain normalcy. All of these constructions have the handicap that the driving of the master cam takes place through frictional contact with a roll. The wearing of the frictional roll causes inaccuracy in the grinding operation and the driving force (not being positive) is limited by the friction available. In the case of an engine housing, for instance, the normal tolerance would be 0.001 inch. In order to maintain this with the prior art cam grinders, it was necessary to operate at a very low speed with small amounts of metal removal. In a modern industrial plant, it is necessary not only to rotate the workpiece at a high rate of speed, but to remove a great deal of metal at the same time with low workspeed (creep feed for roughing). In the prior art the devices are simply not capable of transmitting sufficient horsepower to the rotation and movement of the cam and of the workpiece to produce such a high production operation.

Furthermore, the passage of sharp changes in radius of curvature through the guiding rolls (which are the principal operative mechanism of the prior art cam grinders) tends to injure the machinery, because these rolls must be relatively small in diameter and be mounted in bearings and the bearings are subjected to extreme shock as the rapid rate of change of radius of curvature takes place. The machines, therefore, are difficult to maintain in operation and require constant maintenance and replacement of bearings. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a grinding machine for generating a non-circular surface on a workpiece in which exact normalcy is maintained, while large amounts of power are available for the movement of the master cam and workpiece.

Another object of the present invention is the provision of a cam grinder in which the rolls which are subject to contact with the master cam surface are not subjected to wear.

A further object of the present invention is the provision of a cam grinder in which a no frictional driving action takes place against a master cam surface.

It is another object of the instant invention to provide a grinding machine for grinding cam-like workpieces in which the main drive of the workpiece and master cam takes place through conventional gearing and in which additional motion is provided to maintain precise normalcy.

A still further object of the invention is the provision of a cam grinder which is capable of high precision and heavy industrial production operation with a minimum of wear on the parts and requiring a minimum of maintenance and replacement of parts.

It is a further object of the invention to provide a grinding machine for producing cam-like articles which is capable of heavy duty industrial production usage, making use of relatively unskilled labor.

Another object of the invention is the provision of a cam grinder in which normalcy is produced by a direct drive, rather than a frictional drive, so that no wear or slippage takes place.

SUMMARY OF THE INVENTION

In general, the invention consists of a grinding machine for forming a non-circular surface on a workpiece having a base with a plane surface and having a table mounted on the said surface of the base for slidable movement in any direction in that plane. A workhead is rotatably located in the table for supporting and rotating the workpiece and a master cam is mounted on the workhead and has a surface representative of the surface to be formed. A wheelhead is mounted on the base for operative movement relative to the workhead and has an abrasive wheel for generating the said surface. A cam follower is provided for engaging the surface of the master cam and means is provided for joining the cam follower to the base to cause the cam follower and the abrasive wheel to move in the approximate path of the said surface of the master cam. A link joins the base to the table to correct variation in normalcy encountered during the rotation of the workhead.

More specifically, the workhead is provided with a gear concentric with an axis through the center of the workpiece and through the center of rotation thereof. The table is provided with a motor driving a one-half size pinion gear which, in turn, drives the workhead gear in a ratio of 2-to-1. A link is attached to the pinion gear at a point spaced from its center of rotation. The length of the link is adjustable by virtue of an extensible hydraulic cylinder whose length is determined by a secondary cam mounted on and rotatable with the pinion gear.

BRIEF DESCRIPTION OF THE DRAWINGS

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a perspective view of a grinding machine embodying the principles of the present invention,

FIG. 2 is a plan view of the grinding machine,

FIG. 3 is a vertical sectional view of the grinding machine taken on the line II--II of FIG. 2,

FIG. 4 is a vertical sectional view of the machine taken on the line III--III of FIG. 3,

FIG. 5 is a diagram showing the kinematic relationship of the parts of the machine,

FIG. 6 is a diagram of a grinding machine in which an approximation of normalcy is maintained by use of a plain crank,

FIG. 7 is a diagram of the present grinding machine showing how precise normalcy is obtained with a compensation,

FIG. 8 is a diagram showing normality error,

FIG. 9 is a diagram showing geometry error,

FIGS. 10-16 are electrical diagrams of circuitry used to obtain the grinding cycle, and

FIG. 17 is a diagrammatic representation showing the concept of normality and the way that the point of contact changes with wheel size.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, wherein are best shown the general features of the invention, the grinding machine indicated generally by the reference numberal 10, is shown as supporting a workpiece 11 on which a non-circular surface 12 is to be formed. In the illustration the workpiece is shown as the stator of a Wankel engine in which the surface 12 is a epitrochoid. The machine is provided with a base 13, having a plane upper horizontal surface 14 on which is mounted a table 15. A wheelhead 22 is mounted on a column 20 extending upwardly on the base. The wheelhead is provided with a spindle 23 on the outer end of which is mounted an abrasive wheel 24. Suitable guides and actuating mechanism are provided to mount the wheel on the base for operative movement relative to a workhead 17, extending upwardly from the table 15, and to the workpiece 11 so that the abrasive wheel 24 can generate the said surface 12. Generally speaking, movement parallel to the axis of the abrasive wheel 24, takes place by movement of the wheelhead 22 vertically, while feeding movements transverse to the axis take place by movement of the table 15 under the impetus of a motor 30. The kinematic diagram shown in FIG. 5 shows the various motions of the mechanism. Important points in the apparatus are indicated by their reference numerals, including the connecting rod anchor point 55, the axis 56 of the gear 31, the axis of the crank end 34, and the locus of the center of the crank worm gear 36. It can be seen that by laying out the locus 57 of the center of the epitrochoid as will be desirable to maintain normality, it is possible by conventional descriptive geometry methods to obtain the locus 58 of the center of the crank worm gear 36 necessary to obtain that angular motion of the table 15 necessary to produce the epitrochoid normality. One goes from the locus 58 to the locus 59 to the path the crank end 34 must take and thus you have a pattern for use in demonstrating the change in the length of the crank 29 produced by the hydraulic cylinder 38 in order to obtain normalcy. This pattern is transferred to the cam 39 which controls the cylinder 38.

Referring now to FIG. 2, wherein are also shown the general features of the invention, the grinding machine is shown as supporting the workpiece 11. It can be seen that the base 13 has the plane upper horizontal surface 14 on which is mounted the table 15. The underside of the table is provided with hydrostatic pockets 16 by which means the table is mounted on the said upper surface of the base for slidable movement in any direction in the plane of the surface. The workhead 17 is mounted in the table 15 in bearings 18 and it serves to support and rotate the workpiece 11.

Suitable clamping means is used for attaching the workpiece 11 to the workhead, but these are well known in the art and are not shown in the drawings in order to permit simplicity of explanation. At the lower end of the workhead 17 is mounted a master cam 19 having an internal surface 21 which is representative of the previously-mentioned surface 12 to be formed on the workpiece.

A cam follower 25 extends upwardly from the base 13 to engage the surface 21 of the master cam. The cam follower is rotatably mounted on bearings 26 carried on a spindle 27 extending upwardly from the upper end of a pivot shaft 28, which itself extends downwardly into the base 13. The fact that the cam follower 25 and the abrasive wheel 24 are fixed relative to the base 13, causes the table 15, the workhead 17, and the workpiece 11 to move over the surface 14 relative to the base 13 in a manner dictated by the shape of the master cam surface 21.

A link 29 joins the base 13 to the table 15 to correct variations in normalcy occasionally during the rotation of the workhead 17. The workhead 17 is provided with a gear 31 which is concentric with an axis A-A through the center of rotation thereof. The table 15 is provided with a motor 32 which drives a pinion gear 33 which, in turn, drives the workhead gear 31. The link 29 is attached to the pinion gear 33 by means of a pin 34 at a point spaced from the center of rotation of the gear. The gear, incidentally, is mounted on a vertical shaft 35 having at its upper end a worm pinion 36 engaged by a worm 37 mounted on the shaft of the motor 32. The length of the link is made adjustable by virtue of an extensible hydraulic cylinder 38 whose length is determined by a secondary cam 39, bolted to the underside of the gear 33 and rotatable with it. The secondary cam 39 operates a hydraulic cylinder 41 which is connected by hydraulic lines 42 and 43 to operate the cylinder 38 in the link 29.

As has been mentioned, the base 13 is provided with a pivot shaft 28 that extends into the table 15 and provides for the rotation of the table about an axis B-B passing through the point of contact of the cam follower 25 with the surface 21 of the master cam 19. The table 15 includes an outer portion 44 on which the workhead 17 is mounted and an inner portion 45 in which the pivot shaft 28 is pivotally mounted. The outer and inner portions are joined for linear movement only therebetween parallel to the surface 14 of the base. For this purpose guide ways 46 are provided as is best evident in FIG. 4.

Means must be provided to advance and retract the abrasive wheel 24 from the workpiece 11 on occasion in accordance with normal grinding machine operation. For this purpose, the base 13 is provided with a sub-base 47, from which the pivot shaft 28 extends. The sub-base is linearly slidable in the main base 13 by means of guide ways 48, these ways being parallel in a horizontal plane to the ways 46 and located between the portion 47 and the base 13 of the machine. The relative position between the main base 13 and the sub-base 47 is selectable by means of a feed screw 49 suitably driven by the motor 30 mounted on the base 13. By the operation of the feed screw 49, it is possible to move the workpiece 11 relative to the abrasive wheel 24 to provide for the conventional feed retraction, compensation after dress, and like grinding machine movement of a conventional nature. Furthermore, the wheelhead 22 is provided with the traversing means which are well known in the art for introducing the abrasive wheel 24 longitudinally (vertically) into the interior of the workpiece and to provide for reciprocation on occasion. A dressing mechanism 52 is suitably mounted on the top of the table 15 for renewing the surface of the wheel in the usual way. Suitable flexible bellows 53 extend from the upper portions of the table 15 to the edges of the base 13 to prevent foreign matter from getting into the interior of the machine and particularly onto the surface 14 of the base. A pneumatic cylinder 54 is provided for biasing the table relative to the cam follower; in that respect it acts like a constant force spring.

The operation of the invention will now be readily understood in view of the above description. The workpiece 11 is mounted on the workhead 17 for the grinding operation. Presumably, the surface 12 has been rough machined and the grinding operation by means of the abrasive wheel 24 is a finishing operation. The wheelhead 22 is moved vertically to introduce the abrasive wheel into the opening in the workpiece in the usual way. This axial motion of the wheelhead 22 and the abrasive wheel takes place by means of a slide and a cylinder in the usual way. Lateral motion, however, is brought about by moving the entire workhead assemblage laterally by means of the motor 30 operating through the screw 49. This screw brings about various advances and retractions of the wheel, including motion necessary in order to dress by means of the dressing mechanism 52. The motor 32 is energized and operates through the gearing to rotate the workhead 17 in its bearing 18 relative to the table 15. The table 15, on the other hand, is biased by the pneumatic cylinder 54, so that the surface 21 of the master cam 19 presses continuously against the cam follower 25. Since the cam follower 25 on its spindle 27 and the shaft 28 are fixed to the base 13, the changes in distance of the surface 21 of the master cam relative to the axis A-A cause the entire workhead and table to move laterally relative to the abrasive wheel 24. This lateral motion takes place by means of the ways 46 shown in FIG. 4. If this were the only motion that took place in the cam grinder, the abrasive wheel 24 would be either "climbing" or "falling" in those areas of the epitrochoid surface that is to be formed. This would mean that more or less material would be removed than determined by the cam. For that purpose the suitably-formed cam 39 is provided to operate the link 29. This has the effect of swinging the table 15 relative to the base 13 in the manner shown in dotted lines in FIG. 2, which of course, is a gross exaggeration of the actual amount that such swinging takes place. The net result is to maintain the grinding wheel 24 always normal to the epitrochoid surface, so that the surface speed of cutting, the forces, and all the other grinding parameters remain the same. This brings about an exact duplication of the master cam surface in a manner which is independent of grinding wheel size variation from follower size. Furthermore, if it turns out that the master cam surface is not being exactly duplicated, a small change in the surface of the secondary cam 39 can be made very easily to compensate for the variations.

The advantages of the invention are quite clear. The chief advantage is that the power enters the system to drive the workpiece through smooth, continuous-mesh gearing, which means that there is no wear of a driving roll and no slippage, as was encountered with prior art devices. Even the modification of movement provided by the link 29 are positive displacement movements. There is no wearing of a drive roll which will change the speed of the workpiece. Furthermore, extremely large amounts of power can be transmitted to the workpiece, so that the grinding operation takes place very rapidly. In the past, with friction type drives, the amount of power that could be transmitted through the small cam follower wheel was extremely limited; it was necessary to use long grinding cycles which made the grinding operation economically non-feasible.

FIG. 6 shows what happens when an ordinary crank is used in place of the crank 29. It shows the increment of angular error that results. In FIG. 7 by use of the cam 39 a satisfactory correction is made by use of the cylinder 38 and the pilot valve 41 to give normalcy with no angular error.

FIG. 8 shows how normality error occurs in the graph 60 when a plane crank is used, while the normality error with no crank at all, is shown in the graph 61. These are the errors which the present invention seeks to overcome. FIG. 9 shows how the normality error results in an error in geometry in the formation of the epitrochoid. Graph 62 being the geometry error when no crank is used at all and graph 63 showing the error when plain crank without the correction cylinder 38 is used. FIGS. 10 through 16 are electrical schematic, showing in general, the electrical apparatus necessary to produce the grinding cycle. Many reference numerals, such as the numeral 100, are used to indicate the free ends of various wires or leads which are connected together to obtain the overall electrical schematic. It was necessary, because of the limitations of drawing space, to divide the electrical diagram into the various parts.

FIG. 10, for instance, has a lead 100 for index to new wheel. A lead 102 for index to roughing wheel and a lead 103 for index to finishing wheel consists of a number of integrated circuits and logic circuits also includes a lead 104 for new wheel position. A lead 105 for roughing wheel position and a lead 106 for finishing wheel position. From wheel switches 107 provided for determining the new wheel position and a similar set of thumb-wheel switches 108 are provided for determining roughing wheel position. A four decade storage register 109 is provided for determining finishing wheel position. A four decade comparator 110 is also provided with three lines 111, 112, and 113 which receive signals when A is greater than B, A is equal to B and A is less than B, respectively.

The portion of the control shown in FIG. 11 have a second size override lead 122, a feed reset lead 123, and a first size override lead 124. Various other lines extending from this portion of the control bear the numbers 125 through 152.

FIG. 12 shows a compensate lead 153, as well as leads 154 and 155 associated with the feed slide. A portion of the circuitry contains a two decade BCD down-counter 156. A safety timer 157, and a feed system clock 158. A feed-mode line 159 extends from the circuitry.

FIG. 13 has a second size retract lead 182, a first size retract lead 183, a feed-mode lead 184 and a clock feed memory 185. The circuitry includes a pick-feed timer 186 and a three decade comparator 187. A number of thumb wheel switches are provided including the second size retraction switch 188, a first size retraction switch 189, a standard compensation switch 190, a course compensation switch 191, a course pick 192, a fine pick 193 all connected to a cable 181 leading to the two decade BCD down-counter 156 in FIG. 12. The comparator 187 is similarly provided with a cable 194.

FIG. 14 contains a number of important elements including a decade BCD to seven segment decoder/driver 198 and a cross-slide position digital readout 199; includes a number of thumb wheel switches including a wheel-wear switch 200, a wheel size switch 201, a course compensation switch 202. Extending from the circuitry is a wheel-wear output lead 203, a wheel size output lead 204, a course compensation output lead 205, and a cross-slide zero output lead 206.

Referring now to FIG. 15, it can be seen that the controls connecting with the cross-slide stepping motor 30, there being four drivers 214, 215, 216, and 217 connected to it. In the circuitry of FIG. 15 is a down-counter 218 in the form of a four decade BCD up/down counter with retentive memory. Also, provided is a pulse-two-step converter 219 and a three decade BCD up/down counter 220. Included in the circuitry also, is an auto reset timer 221, there being from the counter 220 a cable 222.

Finally, the portion of the circuitry shown in FIG. 16 includes a decoder/driver 223 in the form of a three decade BCD two seven segment decoder/driver, as well as a feed position digital readout 224. There are several thumb wheel switches including a first size switch 225, an intermediate size switch 226, and a second size switch 227. Output leads include a feed reset output lead 228, a first size output lead 229, an intermediate size output lead 230, and a second size output lead 231.

The control system shown in FIGS. 10 through 16 are connected in the usual way to the conventional grinding machine controls to produce the motion indicated.

In the preferred embodiment grinding machines having the dimensions indicated in FIG. 5, was used for grinding a Wankel housing, whose surface to be ground had a silicon carbide/nickel plate which was to be finished to exact size. The machine was loaded manually with a power clamping of the workpiece onto the table. The vertical slide moved the abrasive wheel down to the rough position and started the work rotation at one rpm and indexed the diamond wheel into grinding position. Creep feed was used for this portion of the cycle. The next step was to feed the diamond wheel 0.005 inch to 0.006 inch for a rough grind with creep feed at one rpm occupying about 25 horsepower for the cut for about 1.2 revolutions of the workpiece. Next a semi-finish took place with the diamond wheel with the work speed at 30 rpm and feeding approximately 0.001 inch. Eight revolutions of the workpiece took place. Next an alternate abrasive wheel on the same spindle in the form of a cylindrical finishing wheel was indexed into grinding position by moving the wheelhead vertically. The work clamp was released to relieve force on thermal distortions and reclamped again. Finish grinding took place for about 0.002 inch on diameter with a multi-dress. About one-half reciprocation of the wheel took place and there was a change of feed-rate from a first feed-rate to a second feed-rate by use of an in-process gage. Finally, the finishing wheel was retracted and the vertical slide was raised to the loading position and the workhead rotation was stopped. The in-process gage was retracted and loading took place of the new workpiece.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come with the scope claimed.

Claims

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A grinding machine for forming a non-circular surface on a workpiece, comprising

a. a base having a plane surface,

b. a table mounted on the said surface of the base for sliding movement in any direction in that plane,

c. a workhead rotatably located in the table for supporting and rotating the workpiece,

d. a master cam mounted on the workhead and having a surface representative of the said surface to be formed,

e. a wheelhead mounted on the base for operative movement relative to the workhead and having an abrasive wheel for generating the said surface,

f. a cam follower for engaging the said surface of the master cam,

g. means joining the cam follower to the base to cause the cam follower and the abrasive wheel to move in the approximate path of the said surface of the master cam, and

h. a link joining the table to the base to correct variations in normalcy during the rotation of the workhead.

2. A grinding machine as recited in claim 1, wherein the workhead is provided with a gear concentric with an axis through the center of rotation thereof, and wherein the table is provided with a motor driving a pinion gear which in turn drives the workhead gear in a ratio of a whole integer.

3. A grinding machine as recited in claim 2, wherein the link is attached to the pinion gear at a point spaced from its center of rotation.

4. A grinding machine as recited in claim 3, wherein the length of the link is adjustable by virtue of an extensible hydraulic cylinder whose length is determined by a secondary cam mounted on and rotatable with the pinion gear.

5. A grinding machine as recited in claim 4, wherein the secondary cam operates a hydraulic cylinder which is connected by hydraulic lines to operate the cylinder in the link.

6. A grinding machine as recited in claim 1, wherein the base is provided with a pivot shaft that extends into the table and provides for the rotation of the table about an axis passing through the point of contact of the cam follower with the master cam.

7. A grinding machine as recited in claim 6, wherein the table includes an outer portion on which the workhead is mounted and an inner portion in which the pivot shaft is pivotally mounted, the outer and inner portions being joined for linear movement only therebetween parallel to the said surface of the base.

8. A grinding machine as recited in claim 6, wherein the base is provided with a sub-base from which the pivot shaft extends, the sub-base being linearly slidable mounted in the base, the relative position between the base and sub-base is selectable by means of a motor-driven feed screw.

9. A grinding machine as recited in claim 1, wherein the abrasive wheel consists of two non-coextensive portions mounted on the same spindle, one portion being intended for rough grinding and the other for finish grinding.

10. A grinding machine as recited in claim 9, wherein means is provided for retaining the difference in size of the two portions for the purpose of regulating feeding and compensation.