Abstract:
A hard machining process uses a cylindrical tool ( 21 ) of ceramic or other hard machining material of comparatively low cost that provides a circular cutting edge to be engaged in cutting relation with hardened workpiece ( 20 ) to be machined. As tool cutting edge wear occurs, the tool ( 21 ) is rotated through a small angle to bring into contact with the workpiece ( 20 ) a fresh portion of the cutting edge. It is not necessary to rotate that tool ( 21 ) sufficiently to effect replacement of the entire cutting edge portion in engagement with the tool. Sufficient angular movement of the tool is made to replace that of the cutting edge that has the greatest effect on the finish of the machined workpiece ( 20 ), which is the cutting location where the removed chip is thinnest.

Description:
The present application claims priority rights based on U.S. Provisional Application Serial No. 60/055,479 filed Aug. 12, 1997. 
    
    
     INTRODUCTION 
     This invention relates to a method and apparatus for hard machining a hardened workpiece, and more particularly to such methods and apparatus in which the cutting tool is moved to bring a fresh, unworn portion of a cutting edge into cutting engagement with the workpiece. 
     BACKGROUND OF THE INVENTION 
     There is growing interest in a machining operation known as “hard turning” or “hard machining.” This involves removing material from a workpiece in a machining operation with the workpiece in a hardened state. In the past, it has been customary to produce parts, such as ball and roller bearings, gears, cams, etc., that must be hardened to decrease wear, by the following sequence of steps: 
     rough machine a part in its soft state, 
     heat treat the part, and 
     rough and finish grind the part to provide desired accuracy and surface finish. 
     By contrast, in hard machining, the hardened part is machined to produce a part in a single operation instead of the more costly sequence indicated above. 
     Hard machining has become an option with the appearance of improved tool materials such as cubic boron nitride or polycrystalline cubic boron nitride (hereinafter “CBN” or “PCBN,” respectively) and ceramics. The CBN or PCBN is very expensive (comparable to diamond in this respect), while the latter, hard-cutting ceramics, have a much shorter tool life, but a much lower cost. It would be possible to use the less expensive ceramic tool material, a superior grade of carbide or other low cost tool material capable of hard machining, provided the nonproductive tool changing time could be reduced and the tool material could be used more efficiently. As used herein, then, a hard machining cutting tool is one having a cutting edge of one of the aforesaid materials capable of hard machining, to wit CBN, PCBN, hard-cutting, ceramics, superior grade carbide or other tool material capable of hard machining. 
     SUMMARY OF INVENTION 
     An improved method and apparatus for combining a comparatively coarse, roughing cut at a high removal rate (in a location of coarse cutting) and a finishing cut at a low removal rate (in a location of finishing cutting) are employed that are useful in hard turning operations. The new method and apparatus make it possible to use a much less expensive tool material such as ceramic in place of PCBN that is now used in hard turning operations. This is accomplished by offsetting the lower tool life of a lower cost tool relative to that of a PCBN tool by providing the tool with an extended cutting edge extending along a path of translation and moving the tool to move the cutting edge along the path of translation so that a fresh cutting edge portion is brought rapidly into cutting position. In particular, in a preferred embodiment of the invention, a fresh cutting edge portion is moved into position to replace the portion of a cutting edge being used for the finishing cut, which finishing portion of the cutting edge is less than the entire portion in engagement with the workpiece. 
     In a specific preferred embodiment, the method and apparatus of the invention achieves the above objectives by: 
     using a large diameter cylindrical tool of ceramic having a large number of new cutting edges (or cutting portions of the continuous circular cutting edge) along the periphery of the face of a single cylindrical tool, and reducing the nonproductive downtime to change tools to essentially zero by merely rotating the cylindrical tool through a small arc when a new cutting edge is required. 
     The substitution of a ceramic tool material for CBN or PCBN has an important thermal advantage in the combined roughing/finishing cut employed in hard turning. CBN has a much higher thermal diffusivity than does a ceramic. While this is advantageous relative to tool wear in conventional machining applications, this is not the case for the type of cut employed in hard turning as shown in FIG. 1, where is the depth of cut and f is the feed per revolution (fpr). In this case, a chip region of large thickness (t r ) responsible for most of the removal (and hence temperature rise) is contiguous with the finishing region of the chip having low thickness (t f ) (and hence low removal rate and low temperature rise). The finish of the surface produced is influenced primarily by the thickness of the chip t f , while the bulk of the material is removed in the region where the chip thickness is t r . The bulk of the heat generated and hence most of the tool wear will be in the t r  region. Relative to surface finish, tool wear in the t f  (finishing cut) region is more important than that in the t r  (comparatively coarse cut) roughing region. 
     Surface finish deterioration, the rate of which increases with temperature, determines the useable tool life in hard turning. It is therefore important that the higher temperature in the roughing region (region of large t r ) not flow into the finishing region (region of low t f ) to reduce tool life. The poorer thermal properties of ceramic relative to CBN is advantageous in this particular case. Also, a larger tool radius can mean a large arc of tool engagement and a longer heat travel distance from the hottest portion of the roughing region to the comparatively cooler finishing region. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood from the description of a preferred embodiment that follows and from the diagrammatic figures of the accompanying drawings. 
     In the drawings: 
     FIG. 1 is a fragmentary diagrammatic illustration showing the shape of a chip being formed when cutting is confined to the nose radius of a tool. 
     FIG. 2 is a fragmentary diagrammatic illustration showing a conventional hard turning operation with only a small portion of a tool face consisting of PCBN. 
     FIG. 3 is a fragmentary diagrammatic illustration showing the scallop left behind on the finished surface where the depth of the scallop (R m ) is the peak-to-valley roughness which depends upon the radius at the tool tip and the feed per revolution (f). 
     FIG. 4 is an enlarged diagrammatic illustration of a portion of the scallop of FIG. 3 showing the relation between peak-to-valley roughness (R m ) and centerline-average-roughness (R a ) where the centerline is located so that the areas above and below the centerline are equal, and R a  is the mean distance from the centerline to the surface of the scallop over one feed distance f. 
     FIG. 5 is a diagrammatic illustration showing the relation between a cylindrical tool of large radius and the work in accordance with the present invention. 
     FIG. 5A is a diagrammatic, fragmentary cross-sectional view along the line A—A of FIG.  5 . 
     FIG. 6 is a diagrammatic representation of an apparatus providing one means of rapidly indexing the cutting edge of the cylindrical tool of FIG. 5 each time a new surface is to be provided that eliminates the adverse effects of tool wear on the geometry of the surface left behind on the workpiece. 
     FIG. 6A is a diagrammatic top plan view, partially in section of a stepper motor and driven worm gear for effecting the rapid indexing of the apparatus of FIG.  6 . 
     FIG. 7 is a fragmentary diagrammatic view, partially block diagram, showing the tool of FIGS. 5 and 5A in the environment of a lathe. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Turning to FIG. 2, at present hard turning is performed using a tool  11  having an expensive polycrystalline CBN insert  12 . PCBN is a composite of small single crystal particles bonded together by sintering. A typical application is shown in FIGS. 1 and 2 where: 
     “f” is the feed rate, inches per rotation (hereinafter “ipr”), 
     “d” is the depth of cut, inches (hereinafter “in.”), 
     “N” are the rotations per minute (hereinafter “rpm” of a workpiece  15 , 
     “D” is the workpiece diameter, in., “V” is the cutting speed defined as πDN (in. per minute), perpendicular to the paper, and 
     “r” is the radius (in.) of the tool insert  12 . 
     For a sharp tool, and a reasonably rigid machine, the surface finish depends primarily upon a scallop  16  generated by the nose of the tool having a radius r. FIG. 3 shows this scallop  16  with feed (f) greatly magnified relative to r, and where R m  is the depth of the scallop. R m  is called the peak-to-valley roughness. To a good approximation: 
     
       
           R   m   =f   2 /8 r   (1) 
       
     
     Surface roughness is usually measured by the motion of the stylus of a measuring instrument perpendicular to the surface as it moves across the surface. In the USA, finish is expressed in terms of the arithmetic average deviation of a surface trace from its centerline (R a ). The centerline of the exaggerated scallop  16  is shown in FIG. 4, and the R a  value for this scallop is R m /4 to a good approximation. Therefor, 
     
       
           R   a   =f   2 /(32 r ), microinches.  (2) 
       
     
     This is the theoretical roughness. The actual measured roughness of a hard turned surface generated by the nose radius of a sharp tool will be very close to this value (less than a factor of 2 greater due to wear, tool chatter, etc.). Thus, the surface finish produced by a tool cutting completely on the insert radius will be independent of the depth of cut (d) and will depend only on the feed (f) and insert radius (r). 
     To obtain a suitable finish in hard turning f must be small. However, a small value of f requires a long time to make a cut of a required axial length and hence reduces productivity and increases cost. If r is increased from the normally used value of 0.03 in. to one in., the value of f may be increased by a factor of 5.8 to obtain the same R a . This is the first reason for greatly increasing r (increased feed rate to give the same finish, hence less production time and cost). 
     The second reason for increasing r is to use the tool material more efficiently. FIGS. 5 and 5A show a cylindrical tool  21  of large radius r. The tool  21  has a tool face  23  and a tool flank  24 , both as seen in FIG.  5 A. The tool  21  is mounted with its axis inclined to provide a negative rake angle α and a positive clearance angle β from a workpiece  20 . As seen in FIG. 5, only a small portion of the edge formed by the tool face and the tool flank will be in use. In order to remove a chip  25  of sufficient material to provide a finished surface of good integrity and finish the required depth of cut d will normally be several times the feed f. A conservative value would be ten (i.e. d=10f). 
     If an R a  of 10 μin. (0.25 um.) in hard turning is required (before superfinishing), and r=1 in. (2.54 cm.), then from equation 2, a feed of about 0.018 ipr (0.046 cm./r) would be required. For a conservative ratio of R a  actual to R a  theoretical of 2, the required feed would be 0.018/2 0.5 =0.013 ipr (0.033 cm./r.). If the depth of cut is ten times this value, then d=0.13 in. (0.033 cm.) The corresponding arc of contact between tool  21  and work  20  would be Φ=cos −1[ 1−0.13]=30°. 
     If the tool  21  is rotated 30° every time a tool change is indicated, then there would be 360°/30°=12 tool changes per cylindrical tool. It is only a ,mall portion θ of the arc of contact φ that determines the finish as compared to the portion of the arc of contract that provides the initial comparatively coarse cutting at the leading contact location. Therefore, it would not be necessary to have a new cutting edge extend over this total area of contract. FIG. 3 shows the scallop corresponding to a given R M  of 40 μin (1 um.) (=4R a ). The critical arc of tool-work contact in this case would be θ=cos −1[ 1−40×10 −6 ]=0.5. According to this, it would be possible to have 360/0.5=720 tool changes per cylindrical Sol. Theoretically, the number of new cutting edges per cylindrical tool  21  should be between 12 and 720. A practical value might be 200 for this example. 
     If the tool in the above example has a radius of 0.5 in. (1.27 cm.) instead of 1 in. (2.54 cm.), then the following results would obtain: 
     A required R a  of 10μin. (0.25 um.), 
     a required feed rate (f) equal to 0.0126 ipr (0.032 cm./r) (theoretically) or 0.009 ipr (0.023 cm./r) (practically, i.e. 0.0126/2 0.5  (0.032/2 0.5 )), 
     a depth of cut (d) equal to 10f or 0.09 in. (0.23 cm.), 
     a total arc of contact equal to cos −1 (1−0.09/0.5) or 35°, 
     a minimum tool change per cylindrical tool of 10, based on complete arc of contact Φ, 
     a critical arc Θ equal to cos −1 (1−40×10 −6 /0.5) or 0.72°, and 
     a max tool change per cylindrical tool of 360/0.72 or 500. 
     Even at smaller tool radii (⅛ inch (0.32 cm.) for example), the advantages of the invention can begin to be seen with a feed rate in excess of 0.003 or 0.004 in. per revolution (0.0076 or 0.010 cm. per revolution (cm./r)). Likewise, when the arc of engagement of the tool edge and workpiece exceeds as small an angle as even 10° and the incremental replacement of cutting edge is accomplished by a movement of less than 5°, the appreciable increase in tool usage between replacements or regrinding can be appreciated. Moreover, when the angular movement effected by the tool drive for movement of a fresh unworn portion of cutting edge into position is 1° or less, very significant increases in tool usage times are observed. 
     Preferably, in order to take full advantage of the multiple cutting edge aspect of a relatively large cylindrical tool, the cutting edge should be as perfectly circular as possible and the tool rotation should be performed rapidly and without shift of the center of rotation. There are many ways in which this might be accomplished. From the foregoing, it can be seen that even engagement of the tool cutting edge over a total arc length corresponding to Φ=10° and rotation of the tool through an angle Θ of no more than 5° will result in considerably extended use of the ceramic tool by 72 or more incremental movements of fresh cutting edge into the finish cut portion of the arc of tool engagement. 
     One application of the above principle is illustrated in FIG.  6 . Here the tool  21  is a disc-shaped ceramic material (or other comparatively low cost tool material suitable for hard machining). The tool  21  is attached to the top of a metal piston  32  restrained by a retaining member  33  with a lapped bearing surface  34 , with very small clearance. After the ceramic tool  21  is attached, the tool face  23  and tool flank  24  surfaces are diamond ground with the piston  32  in place in the lapped surface  34  of the retaining member  33 . The tool face  23  is generally perpendicular to the axis of the cylindrical piston  32 . The tool flank  24  is substantially perpendicular to the tool face, but may be varied to present the correct cutting edge with respect to the workpiece. 
     The tool  21  is contacted with the workpiece (not shown is FIG.  6 ). The tool  21  is rotated as needed to maintain a cutting surface capable of producing the required finish. After the entire edge of the cylindrical tool has been utilized, it may be possible to regrind the tool face and tool flank surfaces with the piston in place in the lapped retaining member  33  to obtain a whole new set of cutting edges. 
     A means for rapid and precise rotary indexing and prevention of rotation of the cylinder during cutting can also be provided. This is achieved by a gear  40  attached to the bottom of the piston  32 . An adjustable stepping motor  41  actuates a worm gear  42  engaging the gear  40 . A thrust bearing  44  supports the gear  40  and piston  32  for rotation. 
     As illustrated in FIG. 7, the tool  21  and its movable support provisions  35 , in accordance with the present invention, may be used in the environment of a conventional machine tool, such as a lathe having a rotational drive  51  for rotating the workpiece  20 . A longitudinal feed  52  effects relative movement (feed) between the tool  21  and the workpiece  20 . A transverse drive  53  forces the tool  21  into cutting engagement with workpiece  20  and thus establishes the arc of engagement between the cutting surface and the workpiece as illustrated in FIG.  5 . Of course, whether in the setting of a lathe or other machine tool suitable for hard machining or hard turning, drives like those drives  51 ,  52  and  53  diagrammatically indicated in FIG. 7 can be conventional drives as known to those skilled in the art and suitable for turning, lapping, milling or other machining operations. 
     The foregoing, preferred embodiment of the invention described above should not be understood to limit the spirit and scope of the invention as set forth in the appended claims. For example, those skilled in the art will recognize that other means for supporting and moving the tool can be employed to bring a fresh, unworn portion of cutting edge in cutting relation with the workpiece.