US3884063A

US3884063A - Gear rolling

Info

Publication number: US3884063A
Application number: US446625A
Authority: US
Inventors: Richard W Tersch
Original assignee: Lear Siegler Inc
Current assignee: Nachi Machining Technology Co
Priority date: 1974-02-28
Filing date: 1974-02-28
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20
Also published as: CA1013181A; JPS579894B2; DE2508842C2; DE2508842A1; IT1026056B; JPS50120463A; FR2262569B3; FR2262569A1; GB1437680A

Abstract

Gears are finish rolled in mesh with one or more gear-like tools, the teeth of the gear and gear-like tool being in tight mesh and under very considerable pressure. Some of the surfaces of the teeth of the die which contact the tooth surfaces of the gear are smooth uninterrupted surfaces, while the surfaces of other teeth of the tool are relieved by providing grooves which extend generally in parallelism with the plane of rotation of the gear. This reduces the total area of contact and accordingly, assuming that the gear and die are urged towards each other with the same force, increases the pressure per unit area, providing greater penetration. Therefore, to maintain only the same pressure per unit area, the total force urging the die and gear together may be halved, thus permitting of lighter and less rigid machines. The unrelieved or smooth surfaces of the remaining teeth produce a smooth accurately formed surface on the teeth of the gear.

Description

United States Patent I l [111 3,884,063 Tersch May 20, 1975 GEAR ROLLING [75] Inventor: Richard W. Tersch, Grosse Pointe [57] ABSTRACT w d Mi h Gears are finish rolled in mesh with one or more gearlike tools, the teeth of the gear and gear-like tool [73] Asslgnee: r Santa Momma being in tight mesh and under very considerable pres- Cahf' sure. Some of the surfaces of the teeth of the die 22 2 ,1974 which contact the tooth surfaces of the gear are smooth uninterru ted surfaces, while the surfaces of [2]] Appl' 4464525 other teeth of the tool are relieved by providing grooves which extend generally in parallelism with the 52 US. Cl 72/102; 29/1592 plane of rotation of the g This reduces the total 51 Int. Cl. B21h 5/00 area of Contact and accordingly, assuming that the 58 Field of Search 72/102, 469; 29/1592 gear and die are urged towards each other with the same force, increases the pressure per unit area, pro- 5 R fe en e Ci d viding greater penetration. Therefore, to maintain UNITED STATES PATENTS only the same pressure per unit area, the total force urging the die and gear together may be halved, thus i282 permitting of lighter and less rigid machines. The unrelieved or smooth surfaces of the remaining teeth produce a smooth accurately formed surface on the Primary Examiner-Lowell A. Larson teeth of the gear Attorney, Agent, or FirmWhittemore, Hulbert & Belknap 26 Claims, 5 Drawing Figures GEAR ROLLING BRIEF SUMMARY OF THE INVENTION In recent years there has been a renewed interest in finishing of gears by rolling them in mesh with a gearlike die at parallel axes under pressure sufficient to cause a flow or displacement of metal in the surface of the gear teeth.

Inasmuch as the operation takes place with the gear and gear-like die in tight mesh and with a very considerable force acting between the die and gear in a radical direction parallel to a line joining the centers of the gear and die, it has been usual to provide two or more dies so as to balance the forces applied to the work gear. In a simple case, two dies are provided disposed diametrically opposite to each other with respect to the work gear. In a conventional operation, one of these dies is rotated in a fixed position and the other die while rotating, is moved toward and away from the stationary die so as to apply metal deforming pressure to the teeth of a work gear interposed between the dies.

Excellent results have also been obtained when the work gear is finished by rolling it in mesh with a single gear-like die. This of course requires the machine in which the gear is rolled to be of sturdy construction so as to withstand the forces developed by applying metal deforming pressure to the teeth of the work gear.

The gear rolling operation which forms the subject matter of the present invention is to be contrasted with a gear finishing operation known as gear shaving. In gear shaving a tool in the form of a gear-like body formed of tool steel is provided with a plurality of grooves or serrations which extend between the root and crest of the teeth and which intersect the side surfaces of the teeth in sharp corners capable of removing shavings of metal from the surface of the teeth. Gear shaving tools of this type are designed to operate in mesh with a work gear with the axes of the gear-and tool crossed in space. Accordingly, as the gear and tool rotate, the contacting portions of the teeth, even at the pitch line, have a component of relative motion extending from end to end of the gear teeth. This produces a sliding action which combined with the usual involute slide, produces a resultant relative sliding action which in turn causes the sharp corners provided at the surface of the cutter teeth to remove shavings of metal from the teeth of the gear.

Since the axes of the gear and tool in the case of shaving are crossed the instantaneous theorectical contact between the toothed portions is point contact. Due to metal displacement, and due further to the fact that the surface of the teeth of the shaving cutter are interrupted by serrations, the cutting action applied to the surface of the teeth of the gear covers an appreciable area. However, in order to cover the entire surface of the gear teeth uniformly from end to end, it is necessary to provide a relative traverse between the gear and cut ter, or to provide a longitudinally concave modification to the cutter teeth so as to cause them to envelop the mating teeth of the gear. Where traverse is provided, it may be in a direction parallel to the axis of the gear or transverse thereto, which latter direction of traverse brings about an operation known in the industry as diagonal traverse.

Reference is made to gear shaving at this time so as to make it clear that the present operation, while it employs a tool having superficial resemblance to a gear shaving cutter, is in fact quite different.

In order to carry out the gear finishing method of the present invention, the gear and gear-like die are adapted to mesh with their axes in parallelism. The teeth of the die are relieved by the provision of grooves which intersect the side surfaces of the teeth in lines which occupy planes parallel to the plane of rotation, or in other words, perpendicular to the axis of the die. While these lines of intersection may produce sharp corners which may be either acute right angular or 0btuse included angles, it will be understood that these edges do not perform a cutting operation. This is because the edges, even though sharp, cannot perform a cutting operation since they are moving always in parallelism with their own length. Accordingly, they are incapable of forming chips or at least are incapable of removing metal in significant amounts.

In practice, since the corners at the sides of the lands provided on the surfaces of the die teeth intermediate adjacent grooves are not cutting edges, it is usually preferred to render these surfaces dull as by aminor vapor blasting operation.

In some cases, in order to correct observed errors in rolled gears, it becomes desirable to provide a very minor angular displacement between the axes of the gear and tool or die. Such an adjustment is accomplished when the observed lead of the rolled gear varies slightly from the required lead. In such case an adjustment of a few minutes of arc will correct the lead error without introducing other significant errors. In such case, even this minor crossed axes relationship might produce chips, which however are completely avoided by rounding off or dulling the corners of the lands on the die teeth.

If a plurality of grooves are provided in the surface of a tooth or die, leaving unrelieved land surfaces between adjacent grooves, and if the lands and grooves have equal width, it will be appreciated that with the application of a predetermined radial load between the gear and tool, the pressure per unit area is double that which would be present with the same loading if the teeth of the die were smooth and uninterrupted. Conversely, to maintain the same pressure per unit area, it is possible to reduce the radial loading to about half that which would be necessary with smooth uninterrupted die teeth. Rolling under these conditions results in the land surfaces of the die teeth displacing metal from corresponding areas in the teeth of the work gear. In order that the entire surfaces of all teeth of the work gear shall be worked uniformly, it has heretofore been necessary to so design the pattern of grooves such that all areas of the teeth of the work gear are subjected to working pressure by the teeth of the die.

The production of dies having grooved teeth as described in the foregoing involves first the production of a gear-like die by a suitable gear cutting operation such as hobbing. Thereafter, the surfaces of the teeth are green ground to approximately final dimensions, leaving however surfficient stock to provide a gear grinding operation after heat treat, and the surfaces of the teeth of the die are provided with a plurality of parallel grooves extending so as to occupy planes perpendicular to the axis of the tool. This operation is normally performed by serrating cutters in the form of interrupted blades which move from the crest toward the root of the teeth.

The serrating blade has ribs which form the cutting edges at the end of the blade and these ribs may be shaped so as to provide grooves of different crosssectional shape. For example, the sides of the grooves may occupy planes perpendicular to the axis of the tool, in which case the included angle between each side of a groove and the adjacent uninterrupted tooth surface of the die tooth is 90 if the teeth of the die are spur teeth. If the teeth of the die are helical, the foregoing operation produces included angles at opposite sides of each rib intermediate a pair of adjacent grooves which are obtuse and acute.

Under the extreme pressure conditions prevailing in gear rolling, it is undesirable to have acute included angles at the side corners of the lands on the die teeth and accordingly, the side of the grooves which would otherwise intersect the side surface of the teeth to produce an acute included angle is inclined so as to form a 90 or an obtuse included angle.

In the production of the grooves on the teeth, each side of each tooth must separately be provided with the grooves and this operation is inherently time consuming and expensive.

In accordance with the present invention only approximately, or exactly, half of the tooth surfaces of the die are grooved, the remaining surface being left as smooth uninterrupted, usually involute tooth surfaces.

As a result of the elimination of one-half of the serration formation, the overall cost of production of the die is materially reduced. At the same time the use of effectively alternated grooved or relieved surfaces or smooth and unrelieved surfaces produces smooth completely finished tooth surfaces on the teeth of the rolled gears.

In a simplified form each tooth surface is acted on alternately by grooved or relieved surfaces and plain or unrelieved surfaces. The action of the grooved surface on the tooth surface of the gear is to produce shallow parallel indentations extending from root to crest thereof. The indentations are separated by areas which have not been depressed and which accordingly project minutely above the depressed or indented area acted on by the lands of the die tooth. When this surface again engages the die it is acted on by a smooth uninterrupted tooth surface which accordingly will initially contact only the undepressed or unindented areas which escaped the action of the lands of the die tooth during the previous passage of the gear tooth through the zone of mesh. If the width of the lands and grooves on the die tooth are equal it will be apparent that in general the work done by the grooved teeth is exactly equal to the work done by the smooth unrelieved teeth.

Moreover, the gear finishing operation includes a predetermined radial feed between the gear and the die to a predetermined depth, at which feed terminates and the gear and die rotate at a constant center distance. This dwell period results in all of the teeth of the gear being finished at full depth by a succession of passes of smooth uniterrupted die teeth, thus tending to produce a more uniform finish.

Accordingly, the invention has as two entirely dissimilar advantages, the savings in the processing time required to manufacture the dies and in the production of superior gears by an operation which nevertheless reduces the total force which must be applied between the gear and die.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary elevational view showing a portion of a die and gear in mesh with each other.

FIG. 2 is a sectional view on the pitch cylinder of the die, developed into a plane, showing lands on successive teeth extending at a helical lead.

FIG. 3 is a view similar to FIG. 2, showing staggered lands and grooves.

FIG. 4 is a view similar to FIGS. 2 and 3, showing a different arrangement of lands and grooves.

FIG. 5 is a diagrammatic view illustrating yet another arrangement of lands and grooves.

DETAILED DESCRIPTION In FIG. 1 there is shown a portion of a work gear 10 having teeth 12 in mesh with the teeth 14 of a gear-like die 16. In carrying out the gear-rolling operation either the gear or the die is rotated in mesh and the parts are fed towards each other, such for example as by upward movement of the die 16 to a position in which it has a predetermined center distance with respect to the gear and at which center distance the gear will be rolled to correct size and its teeth finished to predetermined dimensions.

In FIG. 2 there is shown a sequence of teeth designated T1, T2, T3, T4, T9. It will be observed that each of the teeth has a smooth uninterrupted surface at one side and has a grooved surface at the other side. It will further be observed that a gear tooth entering into the space between the teeth Tl T2 will be engaged on opposite sides by the grooved surfaces of die teeth. On the other hand, a gear tooth entering into mesh between the teeth T2 -T3 of the die is engaged on opposite sides by smooth surfaces.

It will be appreciated of course that a single tooth on the work gear does not successively occupy a space between the teeth Tl T2, T2 T3, T3 T4, etc. On the other hand, by providing successive teeth on the die which are relieved on opposite sides, and by providing the appropriate number of teeth so as to avoid prime numbers and the like, it may be assured that each tooth on the gear, during successive passes through the zone of mesh, is acted on alternately by a pair of smooth surfaces and a pair of grooved surfaces.

Referring to FIG. 2 it will be observed that the tooth Tl has smooth land surfaces designated L1 and groove surfaces G1. If it is assumed that the die is rotating so that successive teeth T1, T2, etc., move in the direction of the arrow the forwardly facing tooth surfaces having the land L1 and the groove G1 is preceded by tooth T3 which is the next tooth provided with forwardly facing land surface L3 and groove G3. Similarly, the tooth T5 is the next preceding die tooth having forwardly facing land surface L5 and groove G5. Teeth T7 and T9 are also provided with forwardly facing land surfaces L7 and L9 respectively, and grooves G7 and G9 respectively. It will further be observed that on this succession of teeth T1, T3, T5, T7 and T9 the lands are shifted laterally of the teeth by uniform amounts which bring a land L9 on the tooth T9 into circumferential alignment with a land Ll on the tooth TI. This constitutes 'what may be regarded as a lead or helix designated by the line 20 at which the land surfaces extend around the periphery of the die. Similar considerations establish a similar lead or helix designated by the line 22 for the corresponding rearwardly facing land surfaces on the teeth T2, T4, T6, T8, etc. With this specific arrangement, which is by no means critical, eight teeth constitute a series in which the lands and grooves are displaced lengthwise of the teeth by an amount equal to the combined width of land and groove.

In dies previously suggested for use as single rolling dies, it has been considered essential to provide the land surfaces in a particular pattern. The die of the present invention is not so critical since after each rolling operation performed on a particular tooth of the gear by grooved surfaces of die teeth, the work gear tooth is next acted on by a pair of smooth uninterrupted die teeth.

It will be apparent that this insures that all of the areas which were not acted on by the preceding grooved die teeth are acted on by the smooth uninterrupted surfaces of die teeth and this without requiring any particular registration between land areas and groove areas.

Thus, it is desirable that the unrelieved areas or the land area on each die tooth which operates on a gear tooth shall be equal to the total areas occupied by the grooves on such die teeth. This in general may be accomplished by forming lands and grooves of equal width. Where the combined land and groove width of the die teeth is evenly divisible into the length ofa gear tooth, this will insure that at all times the grooved die teeth act on one-half the area of the gear teeth, leaving the other half of the area of the gear teeth to be acted on by the smooth unrelieved surfaces.

From the foregoing it will be apparent that the die would operate satisfactorily if all of the land areas on all of the die teeth were in line circumferentially with each other. However, so that the action of the lands on the die teeth shall be distributed over the entire surface, it is preferred to provide the lands on successive teeth or on successively acting teeth (taking into account the number of teeth of the gear), so as to extend at a lead, or at least to be staggered.

Thus for example, successive teeth having lands facing in the same direction may have the lands on one tooth directly opposite the grooved surface on the adjacent similarly constituted tooth. Alternatively, the lands on a succession of teeth (taking into account those facing in the same direction or again successively acting teeth) may be offset from each other so that the lands on every third tooth are in circumferential alignment.

Such an arrangement is shown in FIG. 3 where a succession of teeth T10, T11, T12 T15 is shown. The tooth T has grooves G10 and lands L10 provided on the forwardly facing side of the tooth, assuming that rotation results in movement of the teeth in the direction of the arrow. The next preceding tooth having forwardly facing grooves and lands is the tooth T12 and it will be observed that the grooves G12 are in circumferential alignment with the lands L10 and that the lands L12 are in circumferential alignment with the grooves G10. The same consideration applies to the lands and grooves which face away from the direction of rotation. Thus, it will be observed that the rearwardly facing lands L13 are in circumferential alignment with rearwardly facing grooves G11 and G15 on the next adjacent teeth provided with rearwardly facing grooved surfaces. Thus, FIG. 3 represents an arrangement in which the lands and grooves are staggered on consecutive teeth having grooved surfaces facing in the same direction.

The most efficient practice of the present invention suggests that the total number of tooth surfaces 2N on a die shall be divided into N smooth or uninterrupted surfaces, (N/2) grooved surfaces facing in one direction, and (N/Z) grooved surfaces facing in the opposite direction. For this to be theoretically exactly possible of course, requires an even number of teeth in the die. If the number of teeth in the die is odd it will of course be necessary to provide one more grooved surfaces facing in one direction than the number of grooves facing in the opposite direction. This of course does not materially affect the results, since the number of teeth in the die ordinarily is relatively large, as for example 62.

In addition, for simplicity in manufacture, as well as for uniform finishing action, it is desirable to provide the grooved and plain surfaces in a regular pattern. Accordingly, the arrangements illustrated in FIGS. 2 and 3 are preferred in which alternate teeth are provided with plain and grooved surfaces on the same sides (with reference to the direction of rotation), and adjacent teeth are provided with plain and grooved surfaces on opposite sides. This arrangement, as clearly evident from FIGS. 2 and 3 means that a gear tooth entering into the space between adjacent die teeth is engaged on opposite sides either by a pair of grooved surfaces or a pair of plain surfaces.

A typical die designed for use with a gear having 27 teeth is provided with 62 teeth. If the die teeth are consecutively numbered, it will be apparent that a particu' lar tooth meshing to the rear of tooth No. l on the die will next pass through mesh between die teeth N0. 27 and No. 28, and thereafter between die teeth No. 54,No. 55; No. 19,No. 20; No. 46,No. 47; etc. After 62 passes, the gear tooth will again pass through mesh between die teeth No. 62,No. l.

The foregoing may be considered as describing the ideal situation. However, advantages are obtained by the elimination of any number of grooved surfaces, in that the cost of forming such surfaces is likewise eliminated.

Other arrangements of plain and grooved teeth are of course possible.

In FIG. 4 there is shown a development of a portion of a die in which alternate teeth have plain or uninterrupted side surfaces, and the remaining teeth are pro vided with grooves at both sides. Thus, each gear tooth is engaged at opposite sides by a plain die tooth surface and a grooved die tooth surface. By appropriate selec tion of the number of teeth, each gear tooth may be engaged on the same side alternately by a plain and grooved die tooth surface.

If the number of die teeth is odd, two consecutive die teeth may be plain.

Another arrangement is suggested in the diagrammatic view of FIG. 5, where the die 30 may be divided into segments designated 31, 32, 33 and 34. All of the teeth in alternate segments may be plain, and in the remaining segments all die teeth may be grooved. In general, it is desirable to provide an even number of segments so that at least approximately half of the side surfaces of the die teeth will be plain and the other half grooved. It is not strictly necessary for the number of teeth to equal in each segment, and where a segment contains one or more extra teeth, such segment is preferably provided with plain teeth.

The extreme condition is where the number of segments is about equal to one-half the number of die teeth, in which case a sequence of two plain teeth is followed by two grooved teeth, etc.

It will be understood of course that the forces involved are such as to apply pressure to the engaged surfaces of the gear teeth sufficient to produce a plastic deformation capable of reforming the gear teeth into desired form.

While reference has been made to a flow or displacement of metal resulting in depressed or indented areas, it is of course to be understood that the total displacement resulting from a great may passes of a particular gear tooth through the zone of mesh results in only a very few thousandths of an inch at most so that the actual flow of metal on a single pass by a grooved tooth is so small that even prior to correction by engagement with the smooth unrelieved die tooth surfaces which follow each engagement with a grooved tooth, the deviation of the gear tooth surface from a perfectly smooth uniform surface from end to end would be determinable only by extremely close inspection. After the finishing operation is complete, terminating as it does in an appreciable dwell at which rotation is continued without further infeed, the surfaces of the gear teeth are well within commercial tolerance for precision gears.

Reference is made herein to the side surfaces of teeth. It is to be understood that by this term is meant the tooth surfaces extending from end to end of the teeth (from side to side of the gear or die) and from root to crest thereof, these being the surfaces which in mating gears are adapted to contact each other.

While his convenient to provide an arrangement in which each gear tooth enters into every tooth space of the die, which is a condition which can usually be attained or at least closely approached, this is not necessary, and any arrangement in which in general each side surface of each gear tooth is engaged consecutively by plain and grooved die tooth surfaces is advantageous.

What I claim as my invention is:

1. A gear rolling die for roll finishing gears comprising a gear-like body having teeth generally conjugate to the form required on a work gear, a substantial number of the side surfaces of the die teeth facing in each direction with respect to a given direction of rotation being provided with grooves extending in planes perpendicular to the axis of the die and defining therebetween lands also extending in planes perpendicular to the axis of the die, a further substantial number of the side surfaces of the die teeth facing in each direction with respect to the given direction of rotation being smooth continuous surfaces, all of the die tooth surfaces being either grooved or being smooth continuous surfaces.

2. A die as defined in claim 1 in which the number of grooved die tooth surfaces facing in one direction is substantially equal to the number of grooved die tooth surfaces facing in the opposite direction.

3. A die as defined in claim 1 in which the number of grooved die tooth surfaces is substantially equal to the number of smooth die tooth surfaces.

4. A die as defined in claim 1 in which the width of lands and grooves are substantially equal.

5. A die as defined in claim 1 in which the sides of said grooves define included angles with the side surfaces of said die teeth which are at least 90.

6. A die as defined in claim 5 in which the land areas of successively acting die teeth are out of circumferential alignment.

7. A die as defined in claim 5 in which the land areas of successively acting die teeth extend at a lead around said die.

8. A die as defined in claim 5 in which the land areas of successively acting die teeth are staggered on consecutive similarly facing grooved die tooth surfaces.

9. A die as defined in claim 1 in which substantially all of said die teethare smooth on one side and grooved on the other.

10. A die as defined in claim 9 in which adjacent die teeth have confronting grooved surfaces, or confronting smooth surfaces.

11. A die as defined in claim 1 in which approximately one-half of the die teeth are grooved on both sides, the remainder being plain on both sides.

12. A die as defined in claim 1 in which the die is divided into sections, all of the teeth in each sector being either plain or grooved on both sides.

13. A die as defined in claim 12 in which the number of sectors is even, and half of the sectors contain plain teeth and the remainder of the sectors contain grooved teeth.

14.A die as defined in claim 13 in which all of the sectors are of substantially equal angular extent.

15. A die for finish rolling work gears in the form of a hardened gear having teeth generally conjugate to the desired form of teeth on the work gears, the teeth on said die having side surfaces engageable with the side surfaces of the teeth of work gears rotated at parallel axes and in tight mesh therewith, the side surfaces of the teeth of said die being substantially equally divided between smooth uninterrupted tooth surfaces and relieved surfaces having a plurality of parallel elongated grooves therein extendirig between the root and crest thereof in directions perpendicular to the axis of said die and defining spaced parallel lands therebetween.

16. A die as defined in claim 15 in which the width of said lands is substantially equal to the width of adjacent grooves.

17. A die as defined in claim 15 in which said lands and grooves are all of substantially the same width.

18. A die as defined in claim 17 in which the sum of the width of a land and groove is substantially evenly divisible into the width of a gear tooth. surface, measured from end to end thereof.

19. A die as defined in claim 15 in which each die tooth is provided on one side with grooves and is smooth and uninterrupted on the other side.

20. A die as defined in claim 18 in which the confronting side surfaces of each adjacent pair of die teeth are either grooved or smooth and uninterrupted.

21. A die as defined in'claim 18 in which alternate die teeth are provided with grooves on corresponding sides.

22. The method of roll finishing gears which comprises rotating the gears in tight mesh at parallel axes with a hardened toothed gear-like die under pressure sufficient to cause plastic deformation of the metal of the gear teeth at the surfaces thereof, causing each gear tooth to pass through meshing engagement with the die successively in pressure engagement with die tooth surfaces relieved so as to apply pressure only to selected areas of the surfaces of the gear teeth, and then in pressure engagement with smooth unrelieved die tooth surfaces to apply pressure initially only to those areas of the gear teeth to which no pressure was applied during the preceding passage through meshing engagement with the die.

23. The method as defined in claim 22 which comprises rotating the gears as aforesaid with only a single die.

24. The method as defined in claim 22 which comprises causing a gradual relative infeed between the die and gears during rotation, and providing a dwell at the position of minimum center distance to provide a finishing action in which each gear tooth surface is engaged a plurality of times by smooth unrelieved surfaces to provide smooth surfaces on the finished gear to crest thereof and perpendicular to the axis of the die.

Claims

5. A die as defined in claim 1 in which the sides of said grooves define included angles with the side surfaces of said die teeth which are at least 90*.

9. A die as defined in claim 1 in which substantially all of said die teeth are smooth on one side and grooved on the other.

14. A die as defined in claim 13 in which all of the sectors are of substantially equal angular extent.

15. A die for finish rolling work gears in the form of a hardened gear having teeth generally conjugate to the desired form of teeth on the work gears, the teeth on said die having side surfaces engageable with the side surfaces of the teeth of work gears rotated at parallel axes and in tight mesh therewith, the side surfaces of the teeth of said die being substantially equally divided between smooth uninterrupted tooth surfaces and relieved surfaces having a plurality of parallel elongated grooves therein extending between the root and crest thereof in directions perpendicular to the axis of said die and defining spaced parallel lands therebetween.

18. A die as defined in claim 17 in which the sum of the width of a land and groove is substantially evenly divisible into the width of a gear tooth surface, measured from end to end thereof.

21. A die as defined in claim 18 in which alternate die teeth are provided with grooves on corresponding sides.

24. The method as defined in claim 22 which comprises causing a gradual relative infeed between the die and gears during rotation, and providing a dwell at the position of minimum center distance to provide a finishing action in which each gear tooth surface is engaged a plurality of times by smooth unrelieved surfaces to provide smooth surfaces on the finished gear teeth.

25. The method as defined in claim 23 which comprises causing a gradual relative infeed between the die and gears during rotation, and providing a dwell at the position of minimum center distance to provide a finishing action in which each gear tooth surface is engaged a plurality of times by smooth unrelieved surfaces to provide smooth surfaces on the finished gear teeth.

26. The method as defined in claim 22 in which the relief in the die tooth surfaces is constituted by spaced parallel narrow elongated grooves extending from root to crest thereof and perpendicular to the axis of the die.