Piston turning machine

Abstract

A piston turning machine comprising an elongated shaft structure for rotating a pistong having a skirt which is to be machined to a non-circular shape wherein a cylindrical cam corresponding in cross-sectional shape to the shape desired for the piston skirt is interposed in the shaft structure at a position spaced sufficiently from the piston so that metal removed from the skirt does not encumber the cam. A cam follower and a turning tool are mounted on a reciprocable tool bar so that movement of the follower on the cam is translated into movement of the tool toward and away from the skirt in a manner such that for each increment of follower movement a reduced increment of tool movement is obtained to thereby minimize the effect of dimensional tolerance variations in the cam on the final shape of the piston skirt.

Description

This invention relates generally to piston turning machines and more particularly to an improved machine of this type which utilizes a cam in line with the piston being machined to control the machined shape of the piston thereby enabling machining of the piston to a wide variety of shapes.

Pistons for internal combustion engines are made of metal which is non-uniformly distributed about the axis of the piston thereby resulting in irregular thermal expansion of the piston when subjected to the high temperature conditions in a modern engine. It is necessary for efficient operation of the engine, therefore, that the cold piston have an irregular shape so that the hot piston will take a shape that will achieve the most favorable wear, sealing and friction conditions during engine operation. Pistons with elliptical cross sections are commonly used today to compensate for irregular expansion of the piston skirt which occurs, at least in part, because of the extra metal in the skirt at the wrist pin openings. U.S. Pat. No. 2,720,806 shows a machine for turning pistons so as to provide the skirts with an elliptical cross section. In addition to cutting the piston skirt in the form of an ellipse, it may also be desirable in the case of some pistons to taper the skirt in an axial direction or otherwise vary the cross sectional shape of the piston in an axial direction. The machine shown in U.S. Pat. No. 2,720,806 is capable of tapering the skirt. However, a machine such as disclosed in the aforementioned patent is limited in use to forming only certain piston shapes.

In accordance with the present invention, an elongated shaft structure is provided for rotating a piston which is releasably clamped on one end of the shaft structure so that the axis of the piston skirt is coincident with the axis of the shaft structure. A cam, corresponding in cross sectional shape to the shape desired for the piston skirt, is interposed in the shaft structure at a position remote from the piston so that the metal chips and the like removed from the skirt during machining cannot be deposited on the cam. A reciprocable tool bar is rotably mounted alongside the shaft structure and a cam follower and a turning tool are mounted on the tool bar. A spring associated with the tool bar maintains the cam follower in continuous engagement with the cam during movement of the tool bar to advance the turning tool along the length of the piston skirt. This results in machining of the piston skirt to a shape conforming to the shape of the cam which is generally cylindrical so that it is readily initially formed to whatever complex shape may be desired for the piston skirt.

The cam follower and the turning tool are arranged relative to each other and relative to the cam and the piston such that for each increment of movement of the cam follower, a much smaller increment of movement of the tool is obtained. Importantly, this arrangement enhances the precision machining of the piston because any variations from allowable tolerances in the cam are automatically reduced to smaller variations in the piston. This is accomplished by arranging the cam follower so that it engages the cam at an incident angle measured with respect to the radius of the cam, which is greater than the incident angle, measured with respect to the radius of the piston, at which the tool contacts the piston. This arrangement also involves arranging the cam follower so that the distance between the tool bar axis and the cam follower is greater than the distance from the tool bar axis to the point of engagement of the turning tool with the piston skirt. This arrangement enables the use of a cam which is not unduly large, namely, does not substantially exceed the diameter of the piston skirt while also obtaining the advantage of requiring large movement of the cam follower to obtain small movement of the turning tool. Thus, this invention provides improved piston turning apparatus which is capable of efficiently machining piston skirts to complex shapes within precision tolerance limits.

Further objects, features and advantages of this invention will become apparent from a consideration of the following description, the appended claims, and the accompanying drawing in which:

FIG. 1A is a side elevational view of the upper portion of the piston turning machine of this invention, with some parts broken away and other parts shown in section for the purpose of clarity, illustrating the turning tool in retracted position for commencing the machining of a piston skirt in solid lines, and in an advanced position in broken lines;

FIG. 1B is a side elevational view which is a continuation of FIG. 1A and shows the lower portion of the machine of this invention, the broken line at the top of FIG. 1B matching the broken line at the bottom of FIG. 1A;

FIG. 2 is a diagrammatic view showing the arrangement of the semi-finish and finish tool elements in the machine of this invention;

FIG. 3 is a transverse sectional view of a portion of the machine of this invention as seen from substantially the line 3--3 in FIG. 1A;

FIG. 4 is a diagrammatic view illustrating the relative positions of the turning tool and cam follower in the machine of this invention; and

FIG. 5 is a sectional view of the adjustable bearing structure in the machine of this invention, as seen from the line 5--5, in FIG. 1A.

With reference to the drawing, the piston turning machine of this invention, indicated generally at 10, is illustrated in FIGS. 1A and 1B as consisting of a main frame 12 on which an elongated shaft structure 14 is mounted for rotation about an axis 16 so as to rotate a piston 18 about its axis 20. The shaft structure 14 includes a chuck 22 which releasably holds the piston 18 on one end of the shaft structure 14. Any suitable drive mechanism can be utilized to rotate the shaft structure 14, a rotary drive motor 23 being illustrated as driving a belt 25 which in turn drives shaft pulley 26. Interposed in the shaft structure 14, between the ends thereof, is a hollow cylindrical cam 24 having an external surface 28. The surface 28, or some portion thereof, is configured to correspond to the final shape desired for the piston 18.

The shaft structure 14, at a position between the cam 24 and the drive pulley 26 is supported in an adjustable bearing assembly 29. Set screws 31, carried by the supporting frame 33 for the bearing assembly 29, are engageable with the bearing assembly 29 to shift it along a line 35 to in turn shift the shaft structure 14 along the same line and tilt the cam 24. To adjust the bearing assembly 29 within the frame 33 in order to tilt the cam 24, one set screw is first loosed, the other set screw is then tightened against the bearing assembly until it engages with the first set screw. As can readily be seen from FIG. 1A, a clearance between the bearing assembly 29 and the frame 33 exists thus enabling such adjustment.

Slidably mounted on the main frame 12 at a position parallel to the axis 16 is a reciprocable tool bar 30. A turning tool assembly 32 for machining the piston 18 is mounted on one end of the tool bar 30 and the opposite end of the tool bar 30 is secured to a bar 34 which is in turn mounted on the piston rod 36 for a fluid actuated cylinder assembly 38 that has its cylinder 40 mounted on the frame 12. When the piston and cylinder assembly 38 is actuated so as to retract the rod 36, the tool bar 30 is moved to the position shown in solid lines in FIG. 1B in which the turning tool 32 is retracted relative to the piston 18. When the assembly 38 is actuated to extend the piston rod 36, the tool 32 is advanced along the skirt portion 42 of the piston 18 to the final advanced position shown in broken lines. During such movement, a torsion spring 44 which extends between the mounting bar and an adjustable nut 46 on the tool bar 30 urges the tool bar 30 in a counterclockwise direction as viewed in FIG. 3 so as to maintain a cam follower 48 in engagement with the cam surface 28. The follower 48 is mounted on a support arm 50 carried by the tool bar 30, as shown in FIG. 3. The follower 48 is adjustable so that the diameter of the piston 18 can be varied or any wear of the turning tool 32 can be compensated for by adjusting the follower.

By virtue of the rotatable mounting of the tool bar 30 on the frame 12, movement of the cam follower 48 on the cam surface 28 causes movement of the turning tool 32 in a direction radially of the piston 18. The particular arrangement of the cam follower 48 and the tool 32 is best illustrated in FIG. 4 in which the axis of rotation of the tool bar 30 is shown at 52 and an imaginary line extending between the axis 52 and the axis 16 for the shaft structure 14 is indicated at 54.

It is clear from FIG. 4, that the distance, represented by the line 56, between the line 54 and the point of engagement between the cam surface 28 and the follower 48 is substantially greater than the distance, indicated by the line 58, between the line 54 and the point of engagement of the tool 32 with the piston skirt 42. This arrangement contributes to the desired relationship between tool 32 and follower 48 which provides for a reduction in each increment of radial movement of the follower 48 in radial movement of the tool 32.

It is also clear from FIG. 4 that by virtue of this arrangement, the distance from the tool bar axis 52 to the point of engagement between the cam follower 48 and the cam surface 28 is substantially greater than the distance between the axis 52 and the point of engagement between the tool 32 and the piston skirt 42. In addition, the incident angle A at which the tool 32 contacts the skirt 42, measured with respect to a radius of the piston 18 is substantially less than the incident angle B at which the cam follower 48 engages the cam surface 28 measured with respect to a radius of the cam 24, where a maximum angle is 90.degree., also contributes to the desired reduction in radial movement of tool 32 for each increment of movement of follower 48. As shown in FIG. 4, in a preferred embodiment of applicant's invention, the angle B is 90.degree..

From the above description, it is seen that this invention provides a piston turning machine 10 in which the cam 24 is on the same center line as the piston 18 that is being machined. This arrangement eliminates eccentricity problems, problems due to changes in the rate of speed at which the shaft structure 14 is rotated, surface feed variation problems and tolerance variation problems. Furthermore, the remote location of the cam 24 relative to the piston 18 positively precludes any danger of chips resulting from machining of the piston 18 encumbering the surface of the cam 24. The cam 24 can be of any complex cross-sectional shape desired for the piston skirt 42 and the cross-sectional shape of the cam 24 can be varied along the length of the cam to achieve a particular skirt shape such as a barrel shape. Thus, a wide variety of piston shapes can be achieved with the in-line cam and piston arrangement of this invention.

Furthermore, by virtue of the mounting and relative positions of the tool 32 and follower 48, each increment of radial movement of the follower 48 results in a lesser increment, one third in a preferreed embodiment of the invention, of radial movement of the tool 32 with respect to skirt 42. This enables the achievement of very accurately machined skirts 42. As shown in FIG. 4, the final shape of the work does not correspond exactly to the shape of the cam and the cam does not have to be larger in diameter than the work. In fact, in the illustrated embodiment, the cam is smaller. This is due to the non-radial movement of the tool 32 on the work. The shape of the cam is calculated initially to produce the desired shape of the piston skirt.

In the operation of the apparatus 10, the chuck 22 is operated to grip a piston 18 to be machined while the tool 32 is in its retracted position shown in FIG. 1B. In the event the cam 24 is to be changed, the tool bar 30 can be rotated in a clockwise direction as shown in FIG. 3 to move the follower 48 to a clearance position shown in broken lines. After the cam change, the tool bar is rotated in a reverse direction to engage the follower 48 with the cam surface 28, the shaft structure 14 is rotated at high speed, and the tool bar 30 is advanced to advance the tool 32 along the piston skirt 42 to the final position shown in broken lines in FIG. 1B. During such advancing movement of the turning tool 32, the piston skirt 42 is machined to a shape controlled by the predetermined shape of the cam surface 28. The tool bar 30 is then returned to its retracted position shown in FIG. 1B. This cycle can then be repeated as many times as necessary. If it is desired that the longitudinal profile of the piston 18 be changed, it can be accomplished by adjusting the bearing assembly 29 and thereby deflecting the shaft structure 14 along the line 16 in FIG. 4 which corresponds to the line 35 in FIG. 5. This adjustment can also compensate for misalignment of the cam 24 and piston 18 without necessitating reworking of the cam 24. In the illustrated embodiment of the invention, the tool assembly 32 has a semi-finish tool element 63 (FIG. 2) and a finish tool element 64 angularly spaced on the assembly from the element 62. The work is rotated in the direction of the arrow 66 and, in FIG. 2, the element 62 is in machining position. When it is desired to finish machine, the assembly 32 is rotated to put the element 64 in machining position. When machining has been completed, the chuck 22 is operated to release the machined piston 18, and a spring pressed ejector mechanism 60 operates to eject the machined piston 18. A second piston 18 to be machined is then positioned in the chuck 22, as shown in FIG. 1B, and the above cycle is repeated.

Claims

What is claimed is:

1. Apparatus for machining piston skirts to a predetermined cross-sectional shape comprising:

a. an elongated rotatable shaft having an axis,

b. means for supporting a piston having a skirt to be machined on one end of said shaft so that the axis of said skirt is substantially coincident with said shaft axis,

c. cam means intermediate the ends of said shaft corresponding in cross-sectional shape to said predetermined cross-sectional shape, said cam means having a diameter not substantially greater than the diameter of the piston, said cam means having an external surface and an axis substantially coincident with said shaft axis,

d. a support bar having a longitudinal axis spaced from said shaft axis, said support bar being mounted for movement substantially parallel to said shaft axis,

e. a turning tool secured to said bar and extending toward said shaft axis for engagement with the skirt of a piston on said support means, and

f. cam follower means secured to said bar and engaged with said cam means, said cam follower means engaging said cam means at a point which is a certain distance from a line extending between said support bar axis and said shaft axis, and said turning tool engaging said piston at a point which is a distance from said line less than said certain distance, and bearing means supporting said shaft at a position spaced from said cam means, said bearing means being movable transversely of said shaft for deflecting said cam means.

2. Apparatus according to claim 1 further including spring means urging said bar in a direction to maintain said cam follower means in engagement with said cam means.