Patent Publication Number: US-6220946-B1

Title: Active polishing of rotatable article surfaces

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/074,631, filed Feb. 13, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to precision polishing or superfinishing of surfaces of rotatable metallic articles, such as bearing journals, crankpins and cams of crankshafts and camshafts and other cylindrical and non-cylindrical articles. The material removal operations of the invention may be performed to improve surface finish, attain size or improve geometry of the finished surfaces. 
     BACKGROUND OF THE INVENTION 
     Current methods of finishing cylindrical surfaces are disclosed in numerous prior patents. For example: 
     U.S. Pat. No. 1,993,543 Egger shows polishing of crankshaft surfaces using abrasive tape held stationary by polishing shoes which provide force against the workpiece. Between polishing steps, the tape is advanced to provide a new abrasive surface for polishing. 
     U.S. Pat. No. 5,311,704 Barton II et al. shows microfinishing of bearing journals using either abrasive tape or hard abrasive inserts of various types. 
     U.S. Pat. No. 4,833,834 shows a traveling belt grinder for finishing cam surfaces of camshafts. An abrasive belt is driven by a drive pulley past the workpiece. A backup shoe provides force against the workpiece and guide and tension pulleys position and tension the belt. Coolant is provided to cool the workpiece and carry away removed material (sworf) from the grinding process. Subsequent polishing of the ground surfaces may be required to achieve the desired surface finish or to improve the part geometry or size tolerances. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved belt polishing machine having a moving endless abrasive coated polishing belt. The belt wraps around and travels over a substantial portion of a circumferential surface of a rotatable workpiece for finishing the surface. 
     Typical surfaces may comprise journals and crankpins of engine crankshafts, camshaft journals and cylindrical surfaces of other articles as well as, in some cases, camshaft cam surfaces or other non-circular surfaces. In general, the polishing machine includes drive means for driving the polishing belt in a continuous path, guide means for guiding the polishing belt around and into engagement with a substantial portion of the circumferential surface, tensioning means for applying tensioning force to the polishing belt during finishing operations, and coolant feeding means for applying coolant against the polishing belt for separating and carrying away removed material from the belt and cleaning the abrasive for a subsequent pass. 
     The guide means preferably include guide pulleys positioned on either side of and beyond the belt engaged surface of the workpiece to cause the belt to wrap around and engage a large portion of the surface for polishing it. The guide pulleys may also act as tensioning means or one or more separate tensioning pulleys may be employed on a polishing head carrying the main drive pulley and guide pulleys, as well as the workpiece. A backup shoe may be used to increase pressure of the belt against the workpiece, particularly where correction of the surface geometry is required. 
     The guide pulleys may be mounted on stationary or pivotable arms or on a pivoting carrier in order, for example, to allow oscillating motion for polishing the pins of a rotating crankshaft. Alternatively, a main and secondary polishing head may be shuttled horizontally and vertically to accomplish the same purpose. 
     The workpiece may be gaged in place by opening pivotal arms with the guide pulleys and retracting the main polishing head sufficiently to allow a gage means to move against and measure the workpiece while polishing continues on a lesser included angle of the circumferential surface. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic side view of a first embodiment of belt polishing machine according to the invention; 
     FIG. 2 is a fragmentary end view from the line  2 — 2  of FIG. 1 illustrating the relative positioning of a backup shoe; 
     FIG. 3 is a view similar to FIG. 1 but showing a second embodiment of belt polishing machine including an in-process gaging feature; 
     FIG. 4 is a pictorial view of a base mounted belt polishing machine illustrating a third embodiment of the invention adapted for simultaneously polishing multiple surfaces on a single shaft; 
     FIG. 5 is a view similar to FIGS. 1 and 3 showing a fourth embodiment of the invention capable of polishing crankpins; 
     FIG. 6 is a view similar to FIG. 5 showing an alternative embodiment of crankpin polishing machine; and 
     FIG. 7 is a view similar to FIG. 6 showing yet another embodiment of crankpin polishing machine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIGS. 1 and 2 of the drawings, numeral  10  generally indicates a belt polishing machine having a movable polishing head  12  carrying an endless abrasive coated polishing belt  14 . Belt  14  is driven in a recirculating path by a main drive pulley  16  and passes around leading and trailing guide pulleys  18  and  20 , respectively. Between the pulleys  18 ,  20  and offset toward the main drive pulley  16  is a cylindrical workpiece  22 . A separate work table, not shown, rotatably carries the workpiece  22  and may be arranged for lateral oscillation of the workpiece relative to the polishing belt  14 . 
     The polishing belt passes from the leading guide pulley  18  around the far side of the workpiece  22  and back to the trailing guide pulley  20  so that the abrasive coated side of the belt passes, or is wrapped, around nearly 180 degrees of the circumferential surface of the cylindrical workpiece  22 . If desired, the belt wrap could be much less, possibly as low as 25 or 30 degrees, as will subsequently be more fully discussed. The guide pulleys  18 ,  20  also act as belt tensioners, exerting forces on the belt in the direction of arrows  23  away from the drive pulley and thus holding the belt around the cylindrical workpiece so as to exert a polishing force on the outer cylindrical surface of the workpiece  22 . 
     A backup shoe  24 , also shown in FIG. 2 of the drawings, is optionally included in the polishing machine and may be used to exert additional force against the belt to selectively apply greater polishing pressure against selected areas of the cylindrical workpiece surface in order to preferentially shape or size the surface to a desired contour. For example, the shoe could be shifted laterally to various portions of the cylindrical surface being polished so as to reduce the diameter of portions which may be excessively large or to improve the roundness of portions which may have been machined in an eccentric or non-round condition or with an imperfect axial profile, such as tapered, convex or concave, etc. 
     Machine  10  is additionally provided with a coolant supply including a header  26  supported by a bracket  28  to the polishing head  12  and communicating through distribution tubes  30  with nozzles  32 . The nozzles are arranged to spray coolant onto the surfaces of the polishing belt  14  in a manner to separate and carry away removed material from the belt, thereby cleaning the abrasive for a subsequent polishing pass across the cylindrical workpiece surface  22 . 
     In operation, the workpiece  22  is rotated on its axis at a desired speed which may be relatively slow and should not be so fast as to cause runout of the cylindrical surface during the polishing process. At the same time, the polishing belt is driven by the main drive pulley at a linear speed which, together with the rotation of the workpiece, provides a relative speed of the belt over the cylindrical circumferential surface of the workpiece being polished that is at or near the ideal cutting speed for the polishing process being performed. 
     For example, if the workpiece is rotated in a clockwise direction as shown in FIG. 1, the main drive pulley will also be rotated in a clockwise direction. This causes the belt  14  to travel in the direction of the arrow  33  so that it passes around the leading guide pulley  18 , and then, in a counterclockwise direction, around the opposite side of the workpiece  22 . Thereafter, belt  14  travels around guide pulley  20  to the drive pulley  16 . Thus, the belt travels in a direction opposite to the surface of the workpiece  22  and the relative cutting speed of the belt is the sum of the linear travelling speeds of the two surfaces. This allows the polishing belt to be used in its most efficient manner, providing cutting at a speed best suited for the cutting or polishing operation. 
     The force of the belt against the workpiece is primarily determined by the force applied by the guide pulleys  18 ,  20  in tensioning the belt. This force, in turn, determines the rate of cutting or polishing action of the belt as it moves against the workpiece surface. If additional rounding or sizing of the workpiece surface is required, the backup shoe  24  may be applied against the outside, non-abrasive side, of the belt to apply additional cutting pressure against the workpiece. The backup shoe  24  may be laterally movable across the cylindrical surfacing of the workpiece in order to apply the additional force selectively at the locations where it is most needed. The polishing force may be increased by reducing the angle of belt wrap around the cylindrical workpiece. However, in general, an increased angle of belt wrap is better for correcting lobing and cylindricity of the polished surfaces. 
     Coolant is applied to the polishing belt at all times during the polishing operation through a header  26  and tubes  30 . Coolant washes the abrasive side of the belt, separating and carrying away material removed from the workpiece in the polishing process and thereby cleaning the abrasive for a subsequent polishing pass across the workpiece surface. The polishing belt  14  is therefore maintained in proper condition for the effective removal of metal from the polished surface which allows the use of the belt in polishing a large number of workpieces before the belt needs to be treated or replaced. 
     Referring now to FIG. 3, there is shown a second embodiment of belt polishing machine generally indicated by numeral  34 , and in which like numerals indicate like components. In this embodiment, the polishing head  12  is shown carrying an attached ball nut  36  which is engaged by a ball screw  38  for moving the head  12  in a fore and aft direction, toward and away from the workpiece  22 . The polishing head is additionally provided with optional tensioning pulleys  40  which exert outward forces on the belt. Thus, the guide pulleys  18 ,  20  may be relieved of the tensioning function provided in the first embodiment of FIGS. 1 and 2, although they may also be used as tensioning pulleys, if desired. 
     In machine  34 , the guide pulleys  18 ,  20  are mounted on pivotable arms  42  which are shown in solid lines in their optional outwardly pivoted positions. Dashed lines show the positions of the guide pulleys  18 ,  20  during the normal polishing process performed by the machine. Machine  34  is additionally provided with a gage slide  44  which carries a gage head  46  that has measuring fingers shown in position on opposite sides of the workpiece surface so as to measure or indicate the diameter of the surface during the polishing operation. 
     In operation, the polishing process begins with the gage head  46  and its slide  44  shifted to the right in the drawings, so that it is withdrawn from its measuring position. The polishing head  12  is also shifted to the right so that the drive pulley  16  is moved to the location shown in dashed lines indicated at  16 . This also causes the polishing belt to surround about one-half of the circumference of the cylindrical surface of the workpiece  22  as the pivotable arms  42  swing inwardly and the guide pulleys  18 ,  20  are positioned as indicated by the dashed lines labelled  18 ,  20 . 
     When the polishing process is near completion, the polishing head  12  may be retracted to the position shown. This allows the arms  42  to pivot outwardly so that the polishing belt  14  now engages only a relatively small portion of the surface of the workpiece covering an included angle of perhaps 30 degrees. This allows the gage head slide  44  to be moved inward and the fingers of the gage head to be positioned on opposite sides of the polished surface in order to measure its diameter. This measurement occurs while the polishing operation continues on the reduced portion of the cylindrical surface of the workpiece  22 . Thus, gaging in process is allowed during the polishing process so that the polished surface can be accurately sized and the polishing process terminated when the desired diameter or geometrical configuration is reached. 
     Referring now to FIG. 4, there is shown the general layout of a complete foundation mounted belt polishing machine  50 . Machine  50  includes a mounting base  52  carrying a polishing head or slide  54 . Head  54  mounts a belt drive motor  56  rotatably driving a shaft  58  carrying a plurality of, in this case two, drive pulleys  16 . Each of the drive pulleys  16  drives a polishing belt  14  which passes over a tensioning pulley  40  carried by a pulley support mounted on the polishing head  54 . Each of the polishing belts  14  then passes around leading and trailing guide pulleys  18 ,  20  which are carried by means, not shown, on the polishing head  40  for movement in fore and aft fashion on the base  52 . 
     The polishing belts  14 , as in the embodiment of FIG. 1, pass around about half of a circumferential surface of a cylindrical workpiece, such as a bearing journal  62 . The bearing journals  62  are carried on a workpiece  64  that is rotatably supported and driven by a work drive motor  66 . The drive motor and an associated tailstock  68  are, in turn, supported by a worktable  70  on which the workpiece shaft may be moved laterally or oscillated to varying positions, as desired, for polishing various other surfaces of the workpiece shaft as required. 
     In operation, the polishing process is accomplished as previously described with regard to the embodiment of FIG.  1 . When each polishing step is completed, the polishing head  54  is retracted, moving the polishing belts away from the polished surfaces and allowing the workpiece shaft  64  to be laterally moved to a position for polishing additional journals or other cylindrical surfaces, if desired, or to be removed completely from the machine for replacement by a new workpiece. 
     FIGS. 5-7 illustrate various embodiments of belt polishing machines adapted especially for polishing the crankpins of crankshafts while the crankshafts are rotated in the machine. The machines can also be used to polish the main journals of such crankshafts or other shafts as shown in previous embodiments, but this process is not illustrated in FIGS. 5-7. 
     FIG. 5 illustrates a polishing machine  72  which is, in many respects, similar to machine  34  of FIG. 3, and wherein like numerals indicate like parts. Thus, the polishing head  12 , polishing belt  14 , drive pulley  16 , ball nut  36 , ball screw  38 , and tensioning pulleys  40  are all similar to the FIG. 3 embodiment, and function in the manner there described. In addition, the leading and trailing guide pulleys  18 ,  20  are mounted at the ends of pivoting arms  74  which are freely movable, as will be subsequently described. 
     Machine  72  is arranged for polishing one or more crankpins  76  of a crankshaft, not shown. In the polishing operation, each crankpin  76  rotates around the axis, not shown, of the crankshaft from an upper zero degree position shown in solid lines to 90 degree, 180 degree, and 270 degree positions shown in dashed lines. In FIG. 5, the crankpin  76  is shown in the initial solid position with the polishing belt  14  curved about and engaging nearly half of its periphery and the pivoting arms  74  pivoted upwardly to position the crankpin  76  generally between and inward of the guide pulleys  18 ,  20 . 
     In operation, the crankshaft is rotated in a clockwise direction, as shown by the arrows  77 . As the crankshaft moves one quarter turn clockwise from the zero degree position shown to the 90 degree position, the pivoting arms  74  swing downward to a horizontal position through the forces exerted by the polishing belt. At the same time, the polishing head  12  is shuttled to the right in order to maintain the relative position of the guide pulleys  18 ,  20  with respect to the crankpin  76 . Continuing clockwise motion of the crankshaft to the 180 degree position, pivots the arms  74  downward while the polishing head  12  is shuttled leftward back to the original position shown. Then the arms  74  extend at a downward angle with the guide pulleys  18 ,  20  on either side of and outward of the crankpin  76 . 
     In similar fashion, continued rotation of the crankpin to the 270 degree position, pivots the arms upward to their horizontal positions while the polishing head is shuttled to the left to maintain the relative positions of the crankpin and guide pulleys. Return of the crankpin to the upper zero degree position shown in solid lines, returns the pivoting arms to the upward positions illustrated in the drawing. In this manner, the complete surface of each guide pin may be polished while the crankshaft is rotated. The polishing belt  14  travels over the surface of the rotating crankpin while the pivot arms oscillate and the polishing head  12  reciprocates so as to maintain the polishing belt  14  in the proper relation and maintain the polishing force under all positions of the crankpin. 
     Referring now to FIG. 6, there is shown a modified polishing machine  78  similar to that of FIG. 5, wherein like numerals are used for like parts. FIG. 6 differs in that the guide pulleys  18 ,  20  are carried on parallel arms  80  which are supported by a pivoting carrier  82  so that the arms are pivoted up and down together in order to cause the polishing belt  14  to polish a crankpin  76  while the crankshaft is rotating in the same manner as described with respect to FIG.  5 . 
     Finally, FIG. 7 illustrates another modification of a polishing machine  84  in which the polishing head is separated into a main polishing head  86  which is movable in a fore and aft direction, and a secondary polishing head  88 , which is movable in an up and down direction and is carried on the main polishing head. The secondary polishing head carries fixed parallel arms  90  which support the leading and trailing guide pulleys  18 ,  20  which guide the polishing belt  14  around a crankpin workpiece  76  in the same manner as previously described. Again, the crankpin is orbited in a circular pattern while the crankshaft rotates in a clockwise direction, and the main polishing head  80  is shuttled forward and backward as necessary to accommodate the fore and aft portions of the rotating motion, while the secondary polishing head  88  is shuttled down and up to accommodate the up and down portions of the rotating motion of the crankpin  76 . In this manner, the crankpin surface is polished in fundamentally the same way as was the case for the previously described embodiments  72 ,  78  of FIGS. 5 and 6. 
     The belt polishing machine of the present invention, as exemplified by the various embodiments previously described, provides numerous advantages over previously known polishing machines. Since the abrasive coated polishing belt is continuously moved past the workpiece, it presents a constant flow of new abrasive that has the removed material or sworf constantly washed off by the cooling fluid that is always being applied. The workpiece material is removed rapidly due to the continuous active velocity of the fresh abrasive which is moved, in relation to the workpiece surface, at the most efficient cutting speed and without a buildup of sworf which is common among other polishing machine arrangements. 
     Optimum cutting speeds for typical abrasive grains are commonly accepted to be between 4,000 and 10,000 surface feet per minute (SPFM). In contrast, in polishing machines where the polishing action is dependent entirely on rotation of the work surface past a stationary abrasive, the relative speed is generally less than ten percent of the optimum cutting speed. The present invention also is superior in the fact that the workpiece may be rotated slowly so that vibration is avoided while the polishing belt may be moved rapidly in order to obtain the desired relative cutting speed of the belt over the polished surface. 
     Since the belt is constantly presenting new abrasive grits, it is not necessary to oscillate the belt or the workpiece in most circumstances, although this can be done if it is found to be necessary in a particular instance. Also, the nearly 180 degree wrap of the abrasive belt causes a natural cylindrical shaping of the workpiece. However, lesser angles of belt wrap can provide greater surface pressures and thus result in faster metal removal if desired. 
     Previously ground surfaces are often not truly cylindrical so that polishing with a machine of the type described here may be used to correct the cylindricity as well as to round off lobed imperfections and create a truly circular cylindrical journal or other surface. Thus, polishing with a machine according to the invention can significantly reduce cycle times for polishing crank and camshaft journals and crankpins, as well as bearing and seal surfaces and other rotatable surfaces of all shafts. This new machine provides a superior product, which is not subject to the buildup of sworf in the polishing belt that is common among other methods. The ability to back off the polishing head so as to provide for in-process gaging is an additional advantage, since the size of the polished surface may be checked and adjusted during the polishing process itself without stopping the process for gaging. 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.