BACKGROUND OF THE INVENTION This invention relates to apparatus, attached to lathes, for machining a workpiece to provide the workpiece with a predetermined out-of-round shape. More particularly, this invention relates to such apparatus for machining pistons and cams.

Several types of apparatus are known for machining out-of-round shapes such as pistons, cams and the like. One such type of machine is of the cam-cam follower type, in which an out-of-round shape is transferred from a cam surface to the outer surface of the workpiece. A typical prior art device is depicted in Fig. 1.

In Fig. 1, the lathe has a spindle 10 and an adapter 12 interconnected with the lathe. A chuck 14 holds a workpiece 16 in place. In Fig. 1, workpiece 16 is a piston. As spindle 10 rotates, adapter 12, chuck 14 and workpiece 16 also rotate. A member 18 retains the piston in its correct axial position with respect to chuck 14.

A cam 20 is formed integral with spindle 18. Cam 20 has an out-of-round surface that is transferred to workpiece 16 via follower arm 22, cam follower 24, and a cutting tool 26. Follower arm 22 is pivotable about a pivot 28. Pivot 28 is affixed to a support 30, which in turn is interconnected with the lathe. A resilient compression spring 32 is interconnected between follower arm 22 and support 30. Compression spring 32 applies a force to follower arm 22 and cam follower 24 to keep the cam follower against cam surface 20 as spindle 10 and cam 20 rotate.

Follower arm 22 is typically about 16 inches long and has a substantial mass and inertia. Pivot 28 is disposed at an end of follower arm 22, so that cam follower 24 and cutting tool 26 are disposed on the same side of the follower arm. Also, compression spring 32 applies a force to follower arm 22 that is on

the same side of pivot 28 as cam follower 24 and cutting tool 26.

The arrangement depicted in Fig. 1 provides relatively accurate transference of the cam shape to the workpiece. However, the apparatus depicted in Fig. 1 has several significant disadvantages. One disadvantage is that the large mass of follower arm 22 causes a significant force to be applied to cam follower 24, thereby resulting in premature wear of the cam follower.

Yet another disadvantage of the apparatus depicted in Fig. 1 is that the high inertia of follower arm 22 requires a relatively strong force, applied by spring 32, to keep cam follower 24 against cam surface 20. The need for a relatively strong force further increases the wear of the cam follower.

Another significant disadvantage of the prior art apparatus depicted in Fig. 1 is that the high moment of inertia of the cam follower results in a relatively slow transference of the cam shape to the cutting tool and thus to the workpiece.

Another significant disadvantage of the prior art device depicted in Fig. 1 is that the movement of follower arm 22 results in unbalanced forces being applied to pivot bearing 34, which surrounds pivot 28, in the X, Y as well as Z directions. As a result, special preloaded bearings must be used for pivot bearing 34, and the bearings must be frequently replaced, resulting in machine downtime, loss of productivity, and increased expense.

Another disadvantage of the apparatus depicted in Fig. 1 is that the substantial size of follower arm 22 prohibits the apparatus depicted in Fig. 1 from being used in certain types of turret lathes, or prevents the use of certain tools and indexing around the lathe due to space limitations.

SUMMARY OF THE INVENTION Apparatus is disclosed for machining a workpiece to provide the workpiece with a predetermined out-of- round shape. The apparatus is used with a lathe and is of the cam-cam follower type, and includes a means for changing the relative position between the workpiece and the cutting tool, a means for transferring an out- of-round shape to the workpiece including a cam having an out-of-round shape, a follower arm, and a cam follower that engages the cam to pivot the follower arm, and a means for applying a force to the follower arm.

The follower arm is preferably made from a lightweight aluminum alloy, and has a centrally- disposed pivot such that the cam follower is on a first side of the pivot and the cutting tool is on an opposite second side of the pivot. In certain applications, the follower arm could be made from a magnesium or titanium alloy. The force applying means preferably includes an air cylinder or a resilient tension spring, which is interconnected between the lathe and the follower arm at a location on the second side of the pivot.

The pivot may be disposed at any location between the cam follower and the cutting tool. In a preferred embodiment, the distance between the pivot and the cam follower is between 1 to 3 times greater than the distance between the cutting tool and the pivot.

A key feature of the present invention is that all the X-forces applied to the pivot and the pivot bearing are in the same direction. Thus, standard self- lubricating pivot bearings or V-shaped bearing blocks may be used in place of the preloaded special bearings required in prior art machines. Another feature of the present invention is that the mass and thus, the inertia of the follower arm are greatly reduced, thereby substantially lessening the force applied to the cam follower. As a result, the

wear of the cam follower is substantially reduced, thereby decreasing machine downtime, increasing productivity, and decreasing the expense of operating the machine. Another important feature of the present invention is that the overall size of the apparatus is substantially reduced since the size of the follower arm itself is substantially reduced. As a result, the apparatus according to the present invention may be used with a wider variety of turret lathes, including those which have space limitations, and there is additional space available around the apparatus to permit indexing and the use of other tools.

Another feature of the present invention is that the reduced inertia of the cam follower according to the present invention enables pistons, cams and the like to be turned at much higher speeds when compared to prior art systems, thereby increasing the output of the machine. Another feature of the present invention is that the reduced inertia of the follower arm also allows a smaller force to be applied to the follower arm, thereby decreasing the force applied to the cam follower. Cam follower wear is thus substantially reduced.

These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiments, and the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a prior art apparatus for machining out-of-round shapes onto a workpiece.

Fig. 2 is a side view of an apparatus according to the present invention for machining out-of-round shapes onto a workpiece.

Fig. 3 is an end view of the apparatus depicted in Fig. 2.

Fig. 4 is a top view of the apparatus depicted in Fig. 2.

Fig. 5 is an end view of a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 2 through 4 depict a first embodiment of the present invention. Fig. 5 depicts an alternate, second embodiment. In each of Figs. 2 through 5, the directions of two of the major axes X, Y, and Z, are noted.

Referring to Figs. 2 through 4, the apparatus according to the present invention may be used with a computer numerical control (CNC) turret lathe that is used to turn a wide variety of workpieces. Suitable

CNC lathes are manufactured by Hardinge of Palmyra, New York under the trademark CONQUEST as well as by many other companies.

In Figs. 2 through 4, lathe 36 includes a rotatable spindle 37 and a turret or slide 38. Spindle 37 is rotated by one or more motors of lathe 36 in response to the control of a lathe computer. A support bracket 40 is interconnected with turret/slide 38. A spindle adapter 42 and a chuck 44 interconnect workpiece 46 with lathe 36 and spindle 37. Adapter 42 may have an integral out-of-round shaped cam 48; in the alternative, cam 48 may be a separate component from adapter 42.

The out-of-round shape of cam 48 is transferred to the outer surface of workpiece 46 by a cam follower 50 that is interconnected with a follower arm 52, a cutting tool 54 that engages the outer surface of workpiece 46, and a pivot 56. Follower arm 52 is preferably made of a lightweight material such as an aluminum alloy, and has a relief 58 therein to further reduce the mass and inertial of follower arm 52 as well as the moment of inertia of cam follower 50.

As best shown in Fig. 4, pivot 56 is at least partially surrounded by a pivot bearing 60. Pivot bearing 60 is a standard self-lubricating bearing or bushing such as a DU bearing made by Garlock and sold by a large number of suppliers. As an alternative, pivot bearing 60 may be a V-shaped bearing block since substantially all the forces on pivot 56 and thus on bearing 60 are in the positive X-direction as opposed to the alternating positive and negative directions as in the prior art apparatus depicted in Fig. 1. Ball or roller type bearings could also be used for bearing 60, but are not necessary for most applications.

Referring again to Fig. 4, forces in the Y- direction are borne by thrust bearings 62. These Y- direction forces include frictional forces created by the interaction between rotating cam 48 and cam follower 50, and include tangential cutting forces generated at the cutting tool.

Thrust bearings 62 are preferably thrust washers of the plastic impregnated type or are made of sintered metal. In the alternative, headed bushings or ball- type thrust bearings may be used.

The pivot shaft is preferably made from a hardened steel, and may be hollowed out to reduce its mass. As depicted in Fig. 4, follower arm 52 has a greater width at the point where the follower arm engages pivot 56 to increase follower arm stiffness and to lower the moment forces on the pivot bearings themselves.

Cam follower 50 is maintained in contact with cam 48 by a force applying means interconnected with follower arm 52. As depicted in Figs. 2 and 3, the force applying means may be a tension spring 64 interconnected between bracket 40 and follower arm 52. Spring 64 applies a tension force to follower arm 52, thereby keeping cam follower 50 against the cam surface. As best shown in Fig. 3, spring 64 is preferably disposed at a non-normal angle with respect to the axis of rotation of workpiece 46 and follower

airm 52. The purpose of this angle is to prevent any unstable condition from causing unwanted movement or vibration of spring 64.

An air cylinder may be used in place of spring 64. Indeed, it may be desirable to use an air cylinder to calibrate the spring force and then to substitute a less expensive, simpler tension spring after the required spring force has been determined.

Fig. 2 depicts other important features of the present invention. As depicted in Fig. 2, cam follower 50 and cutting tool 54 are disposed on opposite sides of pivot 56. Spring 64 is interconnected with follower arm 52 at a location that is on the same side of pivot 56 as cutting tool 54. In a preferred embodiment, spring 64 is interconnected with follower arm 52 at a location that is actually between pivot 56 and cutting tool 54.

The arrangement depicted in Fig. 2 is substantially different from prior art arrangements as depicted in Fig. 1 in that the follower arm pivot in the present invention is disposed between the cam follower and the cutting tool.

This arrangement allows the follower arm to be of substantially smaller size and mass when compared to the follower arm 22 of Fig. 1. For example, follower arm 52 in the present invention may have a mass on the order of 1 to 1.5 pounds and a length of about 6 to 8 inches, whereas the prior art follower arm 22 typically has a mass on the order of about 10 pounds and a length of about 13 inches. The reduced length of the follower arm according to the present invention allows the apparatus of the present invention to be used with lathes having significant space limitations.

Also, the reduced size of the present invention allows indexing equipment and other tools to be more easily used where space considerations are important when compared to prior art devices like that depicted in Fig. 1.

Pivot 56 in Figs. 2 through 4 may be disposed at any point between cam follower 50 and cutting tool 54. The positioning of the pivot between the cam follower and the cutting tool is dependent upon a number of factors, including the desired speed of the machine and the desired accuracy of the cut. To achieve maximum rotational speed of workpiece turning, pivot point 56 should be disposed near the midpoint between cam follower 50 and cutting tool 54. However, this positioning may diminish the accuracy of the cut to a certain degree. For maximum accuracy, cutting tool 54 should be positioned as close to the pivot point 56 as possible. However, this arrangement may reduce rotational speed since the moment of inertia of cam follower 50 may be increased.

The inventor has found that a preferred position of the pivot point is such that the distance between the cam follower and the pivot point is approximately two-thirds of the total length of the follower arm, with the distance between the pivot point and the cutting tool being approximately one-third of the overall follower arm length. These ratios provide a desirable tradeoff between cutting tool accuracy and production speed. At these ratios, it is believed that the production speed will be approximately double the speed of prior art devices as depicted in Fig.l. Also, it is believed that cam follower 50 will last approximately 20 times or more longer than cam follower 24 in the Fig. 1 prior art apparatus, primarily due to the reduced inertia and reduced force being applied to the cam follower. This substantial increase in longevity of the cam follower reduces machine downtime and expense caused by the need to frequently replace the cam follower in prior art machines. Another advantage of the present invention is that the effect of wear on the pivot bearing is substantially reduced compared to prior art devices since all of the X-forces are in the same direction.

In prior art devices such as that depicted in Fig. 1, the X-forces have components in both the positive and negative directions, thereby necessitating special preloaded bearings for the pivot bearing. The pivot bearing in the prior art apparatus must be constantly adjusted or replaced to insure proper operation. On the other hand, the moment on the pivot bearing due to Y- direction forces is substantially reduced in the present invention, thereby substantially lessening the forces on the pivot bearing. A standard pivot bearing or V-block may be used, and the standard pivot bearing lasts substantially longer when compared to the prior art bearings.

Cam follower 50 is preferably made from a bearing quality plastic material which has a low mass and low friction so that the cam surface is not damaged. In the alternative, the cam follower could be a roller bearing or may be made from a sintered metal.

Cutting tool 54 may be made from a diamond or carbide material, or may be an indexable insert-type tool if follower arm 52 includes a holder for the stick-type cutting tool.

Fig. 5 depicts an alternate embodiment of the present invention. In Fig. 5, pivot 56 and its respective pivot axis are disposed relatively lower in the X-direction than cam follower 50 and cutting tool 54 in Figs. 2 through 4.

That is, in the embodiment of Figs. 2 through 4, cutting tool 54, pivot 56, and cam follower 50 are disposed substantially in the same plane with each other. In the second embodiment depicted in Fig. 5, however, the pivot axis is slightly lower in the X- direction than the line connecting the cam follower and the cutting tool. The arrangement in Fig. 5 has the advantage of reducing the frictional forces on the cam follower and on the pivot bearing when compared to the embodiment depicted in Figs. 2 through 4. The embodiment depicted in Fig. 5 may also permit

relatively deeper cuts to be made by the cutting tool when compared to the embodiment depicted in Figs. 2 through 4.

The apparatus according to the present invention is preferably used to create out-of-round workpieces like 5 those used on the skirts of pistons for internal combustion engines. The apparatus may also be used to create out-of-round cams or antirotation shafts.

While several embodiments of the present invention have been shown and described, alternate embodiments will be apparent to those skilled in the art and are within the intended scope of the present invention. Therefore, the invention is to be limited only by the following claims.