US2996848A - Grinding method and apparatus therefor - Google Patents

Description

Aug. 22', 1961 E. A. THOMPSON 2,995,848

GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 s Sheets-Sheet 1 Fla.

INVEN TOR. [4 EL 4. THOMPSON BY ATTORNEYS- Aug. 22, 1961 E. A. THOMPSON 2,996,848

GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 3 Sheets-Sheet 2 O Q *2 x ZR o "Q.

N 1 g g a t Z or i k INVENTOR. [4R4 A. THOMPSON AT QRNEYS.

Aug. 22, 1961 E. A. THOMPSON 2,996,843

' GRINDING METHOD AND APPARATUS THEREFOR Filed March 14, 1957 3 Sheets-Sheet 3 INVENTOR. H6 424. A. Tqompsorv A TTORNEYS 3 I f /WIfQWL;

2,996,848 GRINDING METHOD AND APPARATUS THEREFOR Earl A. Thompson, 1300 Hilton Road, Ferndale Station, Detroit 20, Mich. Filed Mar. 14, 1957, Ser. No. 645,987 22 Claims. (CI. 51-95) This invention relates to a grinding method and apparatus therefor. More specifically, the invention has to do with a method and apparatus for rough and finish grinding work pieces in a single operation.

Finish grinding operations as conventionally performed are relatively slow and tedious, and the time consumed in obtaining a desired finish by grinding increases as the fineness of the finish desired increases. Thus, in a conventional grinding operation where it is desired to obtain an extremely fine finish, say 2 or 3 microinches, such a finish is only obtainable practically by sparking ou that is, by causing the piece to make numerous passes across the face of the wheel without any relative radial feed between the work piece and the wheel, except that produced by the relief of deflections in the machine and work, so as to eventually remove all the high spots from the finished face of the work piece. This operation is slow for several reasons. In the first place, sparking out obviously consumes a lot of time. Furthermore, a finish grinding Wheel is a relatively hard wheel; and unless the work piece is fed across the face of the wheel very slowly, burning or burnishing will result.

It is an object of this invention to provide a method and apparatus for rough and finish grinding work pieces very rapidly in one operation.

A further object of the invention is to provide a method and apparatus for grinding work pieces in a very rapid manner and at the same time obtain a finish in the range of 1 /2 to 3 /2 microinches.

The invention contemplates a grinding machine wherein a rough grinding wheel and a finish grinding wheel are adjacently positioned on a rotating shaft so as to rotate about exactly the same axis. The machine includes a rotating spindle driving a work collet, the spindle and collet being fixedly mounted on a support that is reciprocated in an accurate predetermined path so that a work piece in the collet is caused to make a pass across the peripheral faces of the two grinding wheels. Means are provided on the machine for dressing the peripheral faces of the two grinding wheels so that as a work piece makes a pass across the faces of the two wheels, the depth of cut of the finish grinding wheel is a very amount, just suflicient to produce the desired surface finish with neglible stock removal. For this purpose the faces of the two wheels are dressed so that the finish wheel is only a very tiny amount higher than the roughing wheel and preferably the finish wheel is one of a higher degree of hardness than the roughing wheel.

In the drawings:

FIG. 1 is a side elevational view of a grinding machine in accordance with the present invention as viewed from the operators side of the machine.

FIG. 2 is a top view of the machine.

FIG. 3 is a sectional view taken generally along the lines 3-3 in FIG. 2 and showing the track arrangement for guiding the reciprocating movement of the table of the machine.

FIG. 4 is a sectional view generally along the lines 4-4 in FIG. 3 and showing the motor means for reciprocating the table of the grinding machine.

FIG. 5 is a generally diagrammatic view showing a manner in which the grinding wheels may be dressed in accordance with this invention.

2,996,848 Patented Aug. 22, 1961 FIG. 6 is a cross section on line 6-6 of FIG. 1 showing a portion of the wheel dressing mechanism.

FIG. 7 is a diagram showing the relative positions of the grinding wheel, the dresser bar, the work piece, and the work table guide arranged for grinding a spherical surface on the work piece.

FIG. 8 is a view corresponding to FIG. 7 showing the parts arranged to grind a flat surface on the work piece.

FIG. 9 is a cross sectional view on a very highly enlarged scale of a work piece partially ground in accordance with the present invention and illustrating in a schematic way the finish produced by the rough and finish grinding wheels in a grinding arrangement according to the present invention.

FIG. 10 is a diagram showing portions of the rough and finish grinding wheels.

Referring to the drawings, and particularly to FIGS. 1 and 2, a grinding machine is illustrated at 10 and includes an accurately journalled shaft 12 on which is mounted a rough grinding Wheel 14 and a finish grinding wheel 16. On the base of the machine, there is mounted a hydraulic control unit 18 of known construction for actuating the moving parts of the machine. On the bed 20 of the machine, there is mounted a table 22 which supports a housing 24 in which is journalled a spindle 26 that drives a work collet 28. Spindle 26 has a belt drive from a motor 29. The work piece is designated 30. Table 22 is designed to be reciprocated back and forth so that the work piece 30 is traversed once across the peripheral faces of grinding wheels 14 and 16 and returned out of contact therewith. The path of reciprocation of table 22 is determined by a guide bar 32 mounted on the bed of the machine as by screws 34 and having an arcuate cam face 36. The cam face 36 may be a circular are as shown in FIGS. 2 and 7 or the cam face may be straight as shown at 36' on a guide bar 32 in FIG. 8 On the underside of table 22, there is mounted a pair of rollers 38 that engage the arcuate face 36 of guide bar 32. Rollers 38 are urged against the arcuate cam face 36 of guide bar 32 by a bearing pad 40 that engages a shoulder 42 formed on the underside of table 22. Pad 40 is journalled as by a bearing 44 on a pin 46 extending transversely through the end portion of a plunger 48. Plunger 48 is slideably arranged in a bore 50 formed in the bed of the machine and the plunger is biased in a direction away from the grinding wheels 14 and 16 by a coil spring 52 acting between a snap ring 54 in bore 50 and a washer 56 mounted at the free end of plunger 48. With this arrangement, it will be observed that the table 2.2 is at all times rigidly backed in a hori- U zontal plane against the guide bar 32 so that there will be no tendency for the table to shift away from the grinding wheels as the work piece is caused to make a path across the grinding wheels.

The table 22 is arranged to be reciprocated through a predetermined cycle by the power unit 18. The particular means. employed for imparting a reciprocating movement to table 22 are shown in FIG. 4 and include a cylinder 5'8 pivotally supported on the bed of the ma: chine as by a pin 60 and a double-acting piston 62 within cylinder 58 and having a rod 64 that is pivotally connected to the underside of table 22 as by a pin 66. Passageways 68 and 70 are formed at the opposite ends of cylinder 5'8 to permit the admission of pressure fluid to the opposite working faces of piston 62 and thereby reciprocate table 22. Fluid for this purpose is delivered from the power unit 18 to the opposite ends of a cylinder 58 as required to produce the desired table movement. The extent of movement of table 22 in the feed direction is controlled by the engagement of an adjustable stop 72 on the table with a stop button 74 on the bed of the machine. The position of the table in the re tracted position is determined by an adjustable stop 76 on the bed of the machine that is engaged by a stop lug 78 mounted on the underside of the table 22.

The particular cycle of the table as well as the manner in which the table is actuated through its cycle by the power unit 18 preferably includes loading of the collet 28, unloading the collet, shifting the table 22 from the load and unload position to the feed position, feeding the work piece across the faces of the wheels and retracting the table to the load position. The complete cycle is performed very rapidly by the mechanism referred to and the actual feeding of a work piece, that is, for ex ample, about 1 inch in diameter, across the faces of the grinding wheels 14 and 16 consumes about 2 seconds.

In the arrangement illustrated, it is desired to grind the end of work piece 30 with a radius, that is, the radius of the arcuate cam face 36 of guide bar 32. However, whether the work piece is ground fiat or with a radius, a means is provided for dressing the faces of grinding wheels 14 and 16 to the desired contour. In the arrangement shown, the dresseris shown at 80 and includes a diamond tool 82 carried by a holder 84 reciprocably mounted in a casing 86. Casing 86 is guided on a base 88 for traversing movement across the faces of wheels 14 and 16 and a cam follower member 90 on holder 84 engages a dressing bar 92 on base '88 to define the path of the diamond 82 as it traverses the peripheral faces of wheels 14 and 16. The dressing bar 92 is clamped in the vise jaws 93 in the customary manner. On the base 88 there is pivotally mounted a cross slide 95 (FIG. 6) having a pivot stub 97 received in a suitable bore in the cross slide 88. Suitable clamp screws 99 and a suitable scale and pointer 101 are provided for adjusting the angular position of the cross slide 95 with respect to the base 88. In the arrangement shown in FIGS. 2, 6 and 7, the diamond dresser is arranged to dress the peripheral faces of wheels 14 and 16 to a slight concave form, the wheel 16 being dressed with a shoulder 94 (FIG. so as to grind a chamfer around the edge of the work piece. Wheels 14 and 16 are spaced slightly apart so that there will be a clearance 96 therebetween, thus providing a very clear line of demarcation between the grinding faces of the two wheels, a line of demarcation that is readily perceptible to the operator when the wheels are being dressed. Such a spacing is not essential however, and the two wheels may, if desired, be in direct contact or even adjacent sections of a single composite wheel. The shoulder 94 may be formed as integral part of wheel 16 or it may be formed upon a separate wheel adjacent to it.

I have found that with an arrangement such as described herein, work pieces can be ground very rapidly and very smoothly if care is taken to dress the two wheels relative to one another so that the depth of cut of the finish wheel 16 is a minimum amount.

By way of explanation, referring to FIG. 9, the portion 98 of the surface of the work piece there illustrated is a highly magnified representation of a surface produced by a rough grinding wheel 14. For example, with a wheel of 60 grit, the finish of surface portion 98 might be normally on the order of microinches. Now to obtain the smoothest possible finish from the rough ground surface portion 98, the finish grinding wheel 16 should be dressed so as to merely remove the high spots on the surface portion 98, so that the ridges 100 are cut down approximately to the roots of the valleys 102. However, since the depth of these ridges on a 10 microinch finish is actually only 10 millionths of an inch, it may be very difiicult as a practical matter to dress the finish grinding wheel 16 so that it produces a step 104 on the work piece as it is being ground of only 10 millionths of an inch.

Even though the height or diameter to which the finish grinding wheel 16 is dressed is. largerthan the height or diameter of the rough grinding wheel 14 by an amount which initially may be somewhat greater than the optimum step 104 on the work piece, it is possible to obtain an excellent finish, however, without the step of sparking out. In other words, the cutting depth of the finish grinding wheel into the work piece is so small that the finish wheel does not burn the work. This condition is illustrated in FIG. 9 wherein the line 103 represents the surface of the rough grinding wheel 14 when the finish wheel 16 is freshly dressed to the proper height. The distance represented by the dimension 107 is the excess height of the finish wheel 16 over the rough wheel 14 when the former is freshly dressed.

The proper relation of the dressed heights of the two wheels can be readily ascertained by running a colletheld work piece across the rough grinding wheel 14 and partially across the finish grinding wheel 16 slowly and without rotating the collet. When a work piece is ground in this manner, the line of demarcation between the rough ground surface portion 98 and the finish ground surface portion 106 is defined by the shoulder 104 which can be measured with a very high degree of accuracy with present day high precision measuring instruments. When the step produced on the work piece is equal to approximately from one to fifteen times the microinches of finish produced by the rough grinding wheel 14, then the finish grinding wheel 16 is suitably dressed to remove only the high spots or ridges produced by the rough grinding wheel in the short time during which finish grinding occurs. The result is that an extremely smooth surface is produced; and since the finish grinding wheel is cutting down a minimum of material, it is doing a minimum of work. Thus, the work piece can be fed across the face of the finish grinding wheel at a relatively high rate without burning the work piece and the leading corner of the wheel wears away at a relatively slow rate.

A preferred way to obtain the desired relationship in the finish and rough wheels is to utilize a substantially harder wheel for a finish wheel and to dress both wheels from the same dresser bar surface 92 with the same diamond and follower. The extra hardness of the finish wheel results in a higher depressed surface than that of the rough wheel even though the dresser bar 92 is not provided with a step at the point corresponding to the gap 96 between the two wheels. The additional deflections of the machine parts and the oil films in the journals is sufficient to cause the finish wheel to dress out higher than the rough wheel due solely to its additional hardness. Thus, one satisfactory method is to dress both wheels by a single final pass of the diamond dresser 82 across the full face width of both wheels. Alternatively, if the finish wheel is left too high by this procedure, as will occur where there is a large differential in hardness between the two wheels, the diamond may be given a second pass across the finish wheel only, without, however, feeding the diamond inwardly for this purpose.

The height to which the finish wheel is dressed will vary for each application and the optimum conditions can best be found by trial of variations in dressing method and in relative hardness of the two wheels.

Steel work pieces about an inch in diameter have been ground in this manner using a 60 grit rough grinding wheel and a 320 grit finish grinding wheel to obtain a finishof 1 /2 to 2 microinches in less than 2 seconds.

With the wheels dressed to produce a shoulder '107 as shown in FIG. 9, about 100 millionths of an inch, a finish of 3 /2 microinches is readily obtained. Preferably, as a practical matter, this step 107 is held at about 50 millionths of an inch or less where a finish of 2% microinches can be consistently obtained.

As illustrated in FIGS. 5, 6, 7, and 8, the finish wheel 16 is preferably dressed with a shoulder 94. This is obassesses fi tained by a corresponding shoulder 118 formed on the cam bar 92. The purpose of the shoulder 94 is to provide an automatic breaking or slight charnfering of the corner on the finished face of the work piece without a separate operation. In order to obtain this, the stroke of the table 22 is adjusted by means of stop 72, FIG. 4, to bring the edge of the work piece into brief and limited contact with the shoulder 94 as the retraction of the spindle begins. This results in a small chamfer being formed at the periphery of the finish-ground surface.

'It will thus be seen that the present invention provides an improved apparatus for grinding work pieces in both a rough and finishing operation in a single pass in the minimum amount of time. The provision of both a rough and finish Wheel mounted on a common axis for simultaneous or immediate successive engagement with the Work piece, without rechucking, results not only in a saving of time but in a higher degree of accuracy than conventional methods. Furthermore, the particular relationship of the working surfaces of the two grinding wheels results in an ideal functioning of the finish wheel, namely, to remove only the stock of the high portions left by the rough Wheel without removing significant additional quantities of metal not necessary for the production of the desired high quality surface finish. In this way, the work pieces may be produced in a far shorter interval than heretofore accomplished with as high or higher quality of surface finish.

Thus other combinations of relative height and width of the rough and finish Wheels may be selected and other dressing sequences be utilized in order to maintain the desired small increment of height in the finish wheel above the rough wheel. If the wheels be selected of equal hardness, the rough wheel may be dressed down below the height of the finish wheel by one or more extra passes of the diamond across the rough wheel only Without touching the finish wheel after the initial pass of the diamond across both wheels. The same procedure may be utilized where it is desired to use a harder rough wheel than the finish wheel.

It will be understood further that the invention may be adapted to a variety of grinding machine configurations other than rotary face grinding, such, for example, as internal and external cylindrical grinding, centerless grinding, and in fact wherever a surface to be ground is presented successively to the abrading surfaces of a rough and a finish grinding wheel, both rigidly mounted upon a common arbor and having the dressed wheel heights and the duration of finish grinding selected and limited in accordance With the principles hereinbefore described.

What is claimed is as follows:

1. The method of rough and finish grinding hard metal work pieces in one operation which comprises rotating a pair of adjacently and fixedly positioned rough and finish grinding wheels about the same axis, moving a work piece in an accurately predetermined path across the peripheral face of first the rough grinding wheel and then the finish grinding wheel and maintaining the faces of the two wheels dressed so that as the work piece moves along said path of travel, the depth of cut of the finish grinding wheel is about to 100 millionths of an inch.

.2. The method of rough and finish grinding hard metal work pieces in one operation which comprises rotating a pair of adjacently and fixedly positioned rough and finish grinding Wheels about the same axis, moving a work piece in an accurately predetermined path across the peripheral face of first the rough grinding wheel and then the finish grinding Wheel and maintaining the faces of the two wheels dressed so that as the work piece moves along said path of travel, the depth of cut of the finish grinding Wheel is about equal to the microfinish in inches produced on the work piece by the rough grinding wheel.

3. The method of rough and finish grinding hard metal work pieces in one operation which comprises rotating :1 pair of adjacently and fixedly positioned rough and finish grinding wheels about the same axis, moving a work piece in an accurately predetermined path across the peripheral tace of first the rough grinding wheel and then the finish grinding wheel and maintaining the faces of the two wheels dressed so that as the work piece moves along said path of travel, the finish grinding wheel is higher than the rough grinding wheel by a distance in the range of about 50 millionths of an inch down to the microfinish in inches produced on the work piece by the rough grinding wheel.

4. The method called for in claim 3 wherein the work piece is rotated about an axis generally perpendicular to the axis of rotation of the grinding wheels.

5. The method called for in claim 3 wherein the work piece is moved in an arcuate path across the face of said grinding wheels.

6. The method of rough and finish grinding hard metal Work pieces in one operation which comprises rotating a pair of adj acently and fixedly positioned rough and finish grinding Wheels about the same axis, mounting the work piece to be ground in a work holder, moving the work holder in an accurately predetermined path so as to cause the face of the work piece to be ground to be fed successively across the peripheral faces of the rough and finish grinding Wheels and maintaining the faces of the two wheels dressed so that as the work piece moves along said path of travel, the finish grinding wheel is higher than the rough grinding wheel by a distance in the range of about 50 millionths of an inch down to the microfinish in inches produced on the Work piece by the rough grinding wheel.

7. Apparatus for rough and finish grinding hard metal work pieces in a single operation comprising a solidly journalled rotatable shaft, a rough grinding Wheel and a finish grinding wheel fixedly mounted side by side on said shaft, a work support, means for moving said work support in an accurately predetermined path such that a work piece on said Work support is fed across the peripheral face of first the rough grinding wheel and then the finish grinding wheel, the peripheral face of the finish grinding wheel being radially disposed outwardly relative to the peripheral face of said rough grinding wheel by a distance within the range of about 10 to millionths of an inch.

8. Apparatus for rough and finish grinding hard metal work pieces in a single operation comprising a solidly journalled rotatable shaft, a rough grinding wheel and a finish grinding wheel fixedly mounted side by side on said shaft, a work support, means for moving said work support in an accurately predetermined path such that a work piece on said work support is fed across the peripheral face of first the rough grinding wheel and then the finish grinding wheel, the peripheral face of the finish grinding wheel being radially disposed outwardly relative to the peripheral face of said rough grinding Wheel by a distance at least equal to the microfinish in inches produced on the work piece by the rough grinding wheel.

9. Apparatus for rough and finish grinding hard metal work pieces in a single operation comprising a solidly journalled rotatable shaft, a rough grinding wheel and a finish grin-ding Wheel fixedly mounted side by side on said shaft, a work support, means for moving said work support in an accurately predetermined path such that a Work piece on said work support is fed across the peripheral faces of first the rough grinding wheel and then the finish grinding wheel, the peripheral face of the finish grinding wheel being radially disposed outwardly relative to the peripheral face of said rough grinding Wheel by -a distance Within the range of about 50 millionths of an inch down to the microfinish in inches produced on the Work piece by the rough grinding Wheel.

10. The apparatus called for in claim 9 wherein said Work support is adapted to move said work piece in a plane parallel to the axis of rotation of said shaft.

11. The apparatus called for in claim 9 including means for rotating said work support about an axis generally perpendicular to the axis of rotation of said shaft.

12. The apparatus called for in claim 9 wherein said means for moving the work support in an accurately predetermined path comprise guide means fixedly mounted relative to said shaft and spaced generally parallel therefrom, guide means on said work support engaging said other guide means on the side thereof adjacent said shaft, at least one of said guide means defining said predetermined path, means urging said guide means on the work support against the fixedly mounted guide means and means for reciprocating said work support in the path defined by the interengaged guide means.

13. A combined rough and finish grinding apparatus comprising a machine frame, a grinding wheel arbor carried by the frame, means for driving the arbor, a work piece support also carried by the frame, means for causing relative traversing movement between the work and the anbor, a rough grinding wheel and a finish grinding wheel both rigidly attached to the arbor and both presenting cutting surfaces to the work lying in the path of traverse with the cutting surface of the finish wheel closer to the work by an amount which is of the same order of magnitude as the roughness of the work but not great enough to cause burnishing of the work piece and a common dressing means for both wheels in adjustably fixed relation to the arbor.

14. A combined rough and finish grinding apparatus comprising a machine frame, a grinding wheel arbor carried by the frame, means for driving the arbor, a work piece support also carried by the frame, means for causing relative traversing movement between the work and the arbor, a rough grinding wheel and a finish grinding wheel, both rigidly attached to the arbor and both presenting cutting surfaces to the work lying in the path of traverse with the cutting surface of the finish wheel closer to the work by an amount which is of the same order of magnitude as the roughness of the work but not great enough to cause burnishing of the work piece the Width of the cutting surfaces along the path of traverse and the speed of the traversing means being correlated to limit the grinding by the finish wheel to an interval sufiicient to remove substantially only the ridges produced by the rough grinding wheel without removing a significant amount of the unground body of the work piece and a common dressing means for both wheels in adjustably fixed relation to the arbor.

15. A combined rough and finish grinding apparatus comprising a machine frame, a grinding wheel arbor carried by the frame, means for driving the arbor, a work piece support also carried by the frame, means for causing relative traversing movement between the work and the arbor, a rough grinding wheel and a finish grinding wheel, both rigidly attached to the arbor and both preseting cutting surfaces to the work lying in the path of traverse with the cutting surface of the finish wheel closer to the work by an amount which is of the same order of magnitude as the roughness of the work and not great enough to cause burnishings of the work piece the width of the cutting surfaces along the path of traverse and the speed of the traversing means being correlated to limit the grinding by the finish wheel to an interval sufficient to remove substantially only the ridges produced by the rough grinding wheel without removing a significant amount of the unground body of the work piece and a single dressing tool having a traversing means for passing the tool across the cutting surfaces of both wheels successively in a single pass.

16. Apparatus for rough and finish grinding a hard metal work piece in a single pass which comprises a solidly journalled unitary rotating shaft, an abrading device fixedly carried thereby and having side-by-side sections of coarse and fine grit, respectively, with the fine grit section slightly higher than the coarse grit section, a wheel dressing device mounted in adjustably fixed relation to the shaft and arranged to dress the working faces of both sections in predetermined relation to each other, means for causing relative motion between said device and a work piece along a fixed path parallel to the abrading surfaces and with no component of motion in a direction perpendicular to such surfaces whereby the work piece is subjected to a major stock removing action in passing onto and across the coarse grit section and to only a surface improvement action requiring no sparkout in passing onto and across the fine grit section.

17. The method of simultaneous rough and finish grinding a hard metal work piece which comprises rotating an abrading device with side-by-side sections of coarse and fine grit, dressing the sections to slightly different relative heights with respect to the work piece, and causing a relative motion between the work piece and the abrading device to remove a major part of the grinding stock in passing onto and across the coarse grit section and to merely improve the surface in passing onto and across the fine grit section.

18. The method of rough and finish grinding a hard metal work piece which comprises dressing a multi-sectioned abrading device to a predetermined surface contour of the sections with respect to each other, relatively moving the work piece and the abrading device along a path parallel to the surface of the abrading device first into a stock-removing engagement with a coarse abrading surface and immediately thereafter into a mere surface-improving engagement with a fine abrading surface without in-feed motion in a direction perpendicular to the abrading surface.

19. The method of rough and finish grinding a hard metal work piece which comprises dressing a multi-sectioned abrading device to a predetermined surface contour of the sections with respect to each other, relatively moving the work piece and the abrading device along a path parallel to the surface of the abrading device first into a stock-removing engagement with a coarse abrading surface under substantial perpendicular force and immediately thereafter into a mere surface-improving engagement with a fine abrading surface while some perpendicular force is still maintained and without in-feed motion in a direction perpendicular to the abrading surface.

20. The method of grinding a hard metal work piece to size and finish in a single pass which comprises rotating an abrasive device having side-by-side a rough grinding surface and a finsh grinding surface dressed to a height a few millionths of an inch above the rough grinding surface and causing relative movement between the work and the device in a direction parallel to said surfaces to remove stock by an interfering engagement with and a traverse across the rough grinding surface and during the latter part of said relative motion improving the surface finish by traversing across the finish grinding surface without in-feed motion in a direction perpendicular to the grinding surface.

21. Apparatus for rough and finish grinding a hard metal work piece in a single pass which comprises a solidly journalled unitary rotating shaft, an abrading device fixedly carried thereby and having side-by-side sections of coarse and fine grit and of different hardness, respectively, with the fine grit section slightly higher than the coarse grit section, a wheel dressing device mounted in adjustably fixed relation to the shaft and arranged to dress the working faces of both sections in predetermined relation to each other, means for causing relative motion between said device and a work piece along a fixed path parallel to the abrading surfaces and with no component of motion in a direction perpendicular to such surfaces whereby the work piece is subjected to a mag'or stock removing action in passing onto and across the coarse grit section and to only a surface improvement action requiring no spark-out in passing onto and across the fine grit section.

22. The method of grinding a hard metal work piece to a finish equal to that obtained by a conventional spark-out operation, but in a time not substantially greater than that required for a rough grinding operation comprising subjecting the Work piece to the immediately successive action of a coarse and a fine abrading wheel section both rigidly related and rotating on a common axis with the surface of the fine section projecting beyond the surface of the coarse section by a small distance approximating the surface roughness left by the coarse section.

References Cited in the file of this patent UNITED STATES PATENTS Winn Nov. 28, 1911 Sheehan Jan. 7, 1913 Busler May 18, 1915 Ames Oct. 16, 1917 Hanson Mar. 6, 1923 Norton July 30, 1929 Raule Dec. 27, 1932 St. John July 7, 1936 Dunbar et a1. Apr. 23, 1940 Binns Dec. 10, 1940 Gumper Mar. 12, 1946 Holman et a1 June 6, 1950 Holman Feb. 13, 1951 FOREIGN PATENTS Germany Sept. 15, 1911

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