Gear Rolling

Abstract

Method and apparatus for producing gears from circular blanks by rolling them in metal deforming pressure contact with one or more toothed dies. The configuration of the space between the teeth of the die or dies is carefully controlled so as to provide surfaces which prevent the formation of "rabbit ears" and which accordingly eliminates the "cold shut" previously located at the crests of the teeth of gears rolled from circular blanks.

Description

BRIEF SUMMARY OF THE INVENTION

Several different methods of rolling gears from circular blanks have been employed, but in general these have resulted in causing the metal displaced from a circular blank to be formed upwardly into separate projections which are referred to as "rabbit ears." These projections eventually reach the bottom of the space between the teeth of the rolling die or dies and are forced together, leaving opposed abutting surfaces extending into the tooth from the crest thereof, which are commonly referred to as "cold shuts."

The methods of rolling gears which have produced this phenomenon include the rolling of gears between a pair of opposed gearlike dies in which the dies are relatively moved toward each other into the material of the circular blank. It also includes rolling operations in which the circular blank is acted on by opposed oppositely moving tools in the form of modified racks. It also includes operations in which two or more rolling dies are generally tapered either with their axes parallel or crossed, and in which the circular blank is advanced axially through the space between the rolling dies.

In accordance with the present invention it is preferred to employ a pair of opposed specially modified rolling gearlike dies. With this arrangement it is of course apparent that each of the dies opposes the lateral thrust of the other so that it is unnecessary to provide extremely rigid support for the work blank as it is rotated during the tooth generating operation. However, in its broadest aspect the present invention may be practiced with a single die providing the die and gear blank are adequately supported against lateral displacement.

In the operation the gear blank is moved axially relative to one or both of the rolling dies. Where the dies are provided in an opposed pair, the axial movement of the blank is through the space between the dies, and more specifically, the feeding movement of the gear takes place with its axis occupying a plane parallel to the axes of both dies.

In order to continuously prevent the local displacement of metal at the sides of the tooth spaces as they are being formed in the gear, which results in the so-called "rabbit ears," the spaces between the teeth of the die or dies is given a configuration so that surfaces thereof prevent the formation of the "rabbit ears" and cause the metal displaced from the tooth spaces of the blank to produce a flow of metal toward the center of the teeth.

In general, this may of course be accomplished by providing dies which have teeth and tooth spaces appropriately tapered to accomplish the foregoing result. However, since the shape of the tooth as it is being generated by a flow of metal displaced from the tooth spaces of the blank is not critical during intermediate stages, it is sufficient to provide root surfaces on the die sufficient to cause the metal displaced by the die teeth to flow laterally into the median portion of the teeth of the blank. Only in the portion of the die or dies which produce the final shape of the teeth is it critically important to have the tooth spaces including the flanks adjacent the root and usually also the root surfaces, to be configured to produce the required tooth configuration of the work gear.

The foregoing results are accomplished in general by providing a die which at one end has a smooth circular cross section whose diameter is the generating diameter of the die or substantially equal thereto. From this circular end of the die teeth extend to the other side of the die which increase in height to give a generally conical outside diameter. Also, the spaces between the teeth increase generally uniformly in depth. At the ends of these teeth and tooth spaces where they run out into the cylindrical die form, the tooth and tooth spaces may be of equal circumferential extent. Moreover, measured along the same diameter throughout the length of the teeth from end to end, the teeth and tooth spaces may remain of equal circumferential extent. Alternatively, this condition need not prevail, so long as in any radial plane, the dies provide adequate space to receive the displaced metal from tooth spaces of the gear blank, and have surfaces which shape the flow of displaced metal to prevent formation of "rabbit ears."

It is important to provide a carefully matched die or pair of dies to a cylindrical work piece blank to produce the required results. It is of course obvious that all of the metal displaced from the tooth space of the blank goes into the portion of the teeth extending radially outwardly beyond the original diameter of the blank. This statement of course is subject to recognition of the fact that in rolling a gear of finite axial dimension, some material may be displaced axially at one side or the other or both, of the blank. However, the intermediate portion of the blank, or intermediate blanks where a number are rolled in a continuous sequence, do not exhibit this axial flow and hence, the die should be designed to accommodate all of the metal displaced from the tooth spaces and to form it into the required upper portion of the teeth.

It is accordingly an object of the present invention to provide a method and apparatus for producing gears by a rolling operation in which the displacement of metal into the tooth form is controlled to eliminate or substantially eliminate the formation of a "cold shut."

Other objects and features of the invention will be more apparent when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view showing the formation of "rabbit ears" in a rolled gear.

FIG. 2 is a diagrammatic view illustrating the dies and their arrangement for the rolling of gears.

FIG. 3 is a fragmentary sectional view on the line 3-3 of FIG. 2.

FIG. 4 is a diagrammatic view illustrating a sequence of sectional views perpendicular to the axis of a die showing the shape of the teeth.

FIG. 5 is a diagrammatic view illustrating initial, intermediate, and final tooth form of the blank.

FIG. 6 is a fragmentary elevational view of the die.

FIG. 7 is a fragmentary edge view of the die.

FIG. 8 is a fragmentary sectional view on the line 8-8, FIG. 6.

FIGS. 9 and 10 are fragmentary sectional views showing modifications of dies.

DETAILED DESCRIPTION OF THE INVENTION

In the production of gears or other toothed members by means which result in the flow of metal from tooth spaces of the gear into the teeth, there is a tendency for the metal to follow the path of least resistance and initially, to be displaced locally into projections next adjacent to the sides of the tooth spaces as they are formed. As more and more material is displaced from the tooth space these local projections reach substantial dimension. They are referred to as "rabbit ears."

Referring first to FIG. 1 there is shown an intermediate stage in the production of a gear by a rolling operation in which opposed rolls are fed radially into a rotating blank. Here it will be observed that metal has been displaced from an initial outside diameter indicated at 10 to form recesses 12 which are the beginnings of the tooth spaces between the teeth of the gear. The teeth during this stage of the process are indicated generally at 14, and teeth in different stages of formation are specifically designated 14a, 14b, 14c and 14d. It will of course be understood that as more and more metal is displaced from the tooth spaces 12, this metal will flow into the teeth and will be shaped by the coacting flanks of the teeth of the rolling die into the desired configuration such for example as an involute form.

The tendency of the metal displaced from the grooves or recesses 12 to follow the path of least resistance results in the formation of localized projections 16a which are referred to as "rabbit ears." As the rolling operation continues and more and more material is displaced from the spaces 12, the teeth grow in height as indicated at 14b, 14c and 14d. As indicated at 14b the "rabbit ears" here designated 16b, are substantially higher than as indicated at 16a and the tooth 14b is beginning to assume a required shape, as for example an involute shape. The tooth designed at 14c is the result of still further displacement of metal from the groove 12 into the tooth form and here it will be observed that the "rabbit ears" 16c are being forced toward each other and into contact. In the final tooth form these "rabbit ears" have been forced together into contact with each other but without establishing a homogeneous material at the crests of the teeth. This produces what is known as a "cold shut," here designated at 18, and this represents a weakening of the tooth which in the past has required removal of the crests of the teeth by grinding or cutting to, for example, an outside diameter designated at 20, which simply eliminates the portion of the tooth in which the "cold shut" is present.

According to the present invention, a gear blank 22 is provided which as illustrated is provided with a central hole 24 for the reception of a support arbor. A pair of dies 26, 28 are provided, the axis of which, as indicated at 30, are parallel and establish a plane containing these axes which also contains the axis of the blank 22 during the rolling operation. Subsequently, the exact tooth form of the dies 26, 28 will be described in some detail, but for the moment it will be observed that the teeth 32 of the dies, which for the time being may be considered as identical, are longitudinally tapered. Specifically, the teeth 32 have outwardly tapered crests 34 and inwardly tapered roots 36. This taper or inclination is of course with respect to the axis of the die. In general, it may be said at this point that the shape of the spaces between the teeth 32 and particularly at and adjacent the roots thereof, have a configuration so that any "rabbit ears" tending to be formed by the entry of the crests of the teeth of the die into the blanks engages these surfaces and hence, the formation of the "rabbit ears" is prevented. Instead, the displaced material is caused to flow toward the centerline of the teeth.

The portion of the dies 26, 28 at the left hand edge thereof, as seen in FIG. 2, is cylindrical and is adapted to engage or closely approach the smooth cylindrical outer surface of the blank 22.

In order to carry out the operation the dies 26 and 28 are rotated in the same direction through positive acting gearing which maintains the dies 26 and 28 in exactly properly timed relationship. This relationship is such that if the gear has an even number of teeth, centerlines of teeth on the dies 26 and 28 simultaneously pass through the plane containing the axes of the dies. If the blank contains an odd number of teeth, the centerline of a tooth on one of the dies 26 or 28 passes through the plane containing the axes of the dies while a plane bisecting the tooth space of the other die is passing through the same plane.

A loading chute indicated diagrammatically at 40 is provided receiving a number of blanks 22. The foremost blank is advanced from the chute by suitable means into a locator in alignment with a blank supporting arbor 42 having a reduced portion 44 adapted to fit snugly within the hole 24 in the blank and defining a shoulder 46 engageable with the end or side of the blank. Suitable power means are provided for advancing the arbor 42 axially through the space between the dies. As soon as the blank 22 is engaged by the smooth cylindrical adjacent ends of the dies, or alternatively, when its advancing edge is engaged by the tapered crests 34 of the die teeth, rotation will be imparted to a blank 22 in properly timed relation to the rotation of the dies. As the blank is advanced material is displaced by the tapered crests of the teeth of the dies into the appropriately tapered root spaces thereof while the formation of "rabbit ears" is prevented as previously suggested.

When the completed gear has passed beyond the dies 26 and 28 a stripper 48 mounted on a support 50 is inserted behind the finished gear, here designated 22a and the arbor 42 is retracted.

The actual operation is carried out with the dies 26 and 28 rotated at substantial speeds, preferably the speed of rotation being such as to produce a surface speed approximately 130 feet per minute. The arbor 42 is advanced at a rate which depends upon the material of the blanks, the diameter thereof, and other considerations. However, it has been found that satisfactory results have been obtained when the rate of advance is approximately 17 inches per minute.

The teeth of the dies 26 and 28 may be modified to have the teeth of a final short section untapered to produce a finishing action on the teeth of the gear. Alternatively, it may be desirable to provide separate finishing dies having unmodified teeth for finishing the gear as it emerges from the tapered toothed dies 26 and 28. These finishing dies may be abutted against the sides of the rolling dies or separated therefrom. With this arrangement the axial advance of the blank may be speeded up.

The design of the roll dies and the initial diameter of the work blank are critical factors determining the success or failure of this operation. Inasmuch as the blank is driven in rotation solely by its engagement with the rolling dies, it is essential to provide an initial diameter on the blank as determined by the circumferential pitch of the teeth of the dies measured at the minimum tooth crest diameter such that the initial impression made by a tooth of one die will be timed so as to register accurately with a tooth of the opposite die after the initial rotation of 180.degree.. On the other hand, it is elementary that, disregarding material displaced axially beyond the ends of the original blank, all of the material displaced from the tooth spaces appears in material which in the finished gear, lies outwardly of the initial outside diameter. It is of course appreciated that the exact material displaced from a tooth space does not necessarily appear in the outer portion of a tooth. Instead, material may be displaced toward the midplane or centerline of a tooth resulting in generally radially outward flow of other material beyond the original outside diameter of the gear. The original outside diameter of the gear thus in general constitutes the rolling or generating diameter which may or may not coincide with the nominal pitch diameter of the gear. Accordingly, the rolling die is designed by first determining the required full tooth configuration of the gear, thereafter determining a blank diameter such that the cross-sectional area of the tooth spaces within the diameter shall be approximately equal to the cross-sectional area of the tooth form lying radially outwardly of the diameter. With this determined, the next step is to design a roll with an appropriate number of teeth of proper configuration and with the tooth space configuration designed as to the root surfaces and flank surfaces adjacent the root so as to substantially completely prevent formation of "rabbit ears" and to cause material displaced by the teeth of the rolling dies to cause a flow of material toward the centerlines of the teeth, thus preventing formation of "rabbit ears".

The design of the die rolls is critical. If insufficient space is left for the controlled flow of material into the tooth formation of the gear by reason of insufficient tooth space formed in the die, excessive pressures result. On the other hand, if excess space, and particularly space adjacent the edges of the tooth spaces of the gear during the generating operation is present, then the "rabbit ears" form and having once formed cannot thereafter be eliminated.

As stated before, the exact shape of the tooth space between adjacent teeth of the die particularly, the exact shape or configuration of the root portion thereof, is not critical except at the ends of the teeth where the final formation of the teeth of the work gear occurs. Accordingly the configuration of the root portions of the teeth of the die, in sections adjacent the entry end of the blank, may include inclined portions engageable initially by material tending to flow into the formation of "rabbit ears" and terminating in a channel formed by the intersection of these inclined surfaces or in a proper case, by a separate channel intermediate the inclined surfaces into which flow of displaced material occurs as it is displaced along the inclined surfaces toward the midplane of the gear teeth.

While the foregoing describes an operation employing roll dies of particular form which, when designed with maximum perfection completely eliminates the "cold shut" heretofore present in roll gears, it is to be understood that the present method may be practiced with die rolls which merely reduce the formation of "rabbit ears" and the consequent "cold shut" so that the finished rolled gear may exhibit a superiority over present rolled gears only in the fact that a "cold shut" at the crests of the teeth thereof is of a reduced amount. In this case, as heretofore, a certain amount of material at the crests of the teeth may be removed to remove the "cold shut" although with the practice of the present invention this removal will be of less material. With this occurring, it is of course necessary to select the initial roll diameter in accordance with the final desired tooth configuration, taking into account the fact that the final operation will remove a predetermined amount of material at the crests of the teeth.

Referring now to FIGS. 5--7 there are shown detailed views illustrating the shape of the teeth of the gear rolling dies. In FIG. 6 the die proper is designated 60 and it is provided with a multiplicity of teeth 62 separated by tooth spaces 64. The teeth 62 taper and more specifically, have a tapered crest 66 inclined outwardly from the smaller side of the die and thus giving the die an overall generally conical configuration. In these FIGS. the taper imparted to the gear teeth, both to the crests and roots thereof, as well as the chordal thickness of the teeth, is exaggerated for clarity. The bottom of the tooth spaces or roots 68 are inclined oppositely from the inclination of the crests 66 of the teeth. It will be observed that at the small end of the cutter the periphery is essentially an uninterrupted circle 70. This circle constitutes the rolling diameter of the die and hence, of the gear produced thereby. This diameter on the gear will be adjacent but not necessarily coincident with the pitch diameter of the finished gear.

The taper of the teeth is preferably arranged so that any lead check measured along a side of a tooth and along a line parallel to the axis of the tooth or measured at constant radial distance from the axis of the die, will show the same lead at both sides of the teeth. In the illustrated die shown in FIGS. 5--7 the teeth, for simplicity, are shown as spur teeth. However, helical teeth will conform to the same requirements and the lead or helix angle at opposite sides of the teeth, measured along lines equidistant from the axis will be the same on both sides of the teeth.

For convenience and clarity, reference characters are applied to like portions of the teeth in accordance with the following description: The crests of the teeth are constituted by the areas 72. The sides or flanks of the teeth are constituted by the areas 74. The areas at the roots of the teeth are designated 76. The chordal thickness of the teeth measured on the rolling diameter 70 is uniform from end to end of the teeth and is equal to the uniform chordal spacing between adjacent tooth surfaces measured on this diameter.

Referring now to FIG. 4 there is diagrammatically shown in a single view the progression in the cross-sectional shape of the gear rolling die and particularly, the toothed portions thereof. In this FIG. the shape of a tooth and tooth space as viewed in a diametral plane adjacent the inlet side of the die, as for example at the plane designated A in FIG. 2, is illustrated in FIG. 4 where the tooth is designated 80a and the tooth space is designated 82a. The circular arc 84 represents the smooth continuous circular outside diameter of the dies at the inlet end where the teeth 80 terminate. It will be observed in FIG. 4 that the angular dimension of the tooth 80a is substantially equal to the angular extent of the recesses 82a. Moreover, the height of the tooth portion 80a is approximately equal to the depth of the tooth space 82a. With this arrangement the material displaced by the tooth section 80a is adapted to be received in and substantially fill the tooth spaces 82a. Moreover, it will be observed that the root surfaces 86a of the tooth spaces 82a are in position to be engaged by "rabbit ears" as they tend to be formed adjacent the sides of the tooth space produced by the tooth 80a. As illustrated in FIG. 4, the root surfaces 86a and the adjacent side surfaces of the tooth space prevent formation of "rabbit ears" and guide the flow of displaced material toward the middle portion of the tooth space intercepted by the radial line 88a.

Instead of providing a substantially circumferential root surface 82 a, it is within the contemplation of the present invention to provide a root surface such as illustrated at 89a which causes all of the metal displaced by the tooth to be guided toward the metal portion of the tooth space. In this case not only is there an elimination of "rabbit ears," but the metal is not permitted to be displaced at the extreme side of a tooth space. Instead, all of the material is displaced centrally. The only requirement at this time is that the root surfaces between the teeth of the rolling die do not permit the growth of "rabbit ears" and it is quite satisfactory to displace all of the metal toward the central portion of the gear teeth at this stage of formation.

At a radial section further across the width of the die, the height of the die teeth have increased, as indicated at 80b, and correspondingly the depth of the space between adjacent teeth has increased as indicated at 82b. Again, the requirement is observed that the depth of the tooth space below the rolling diameter 84 and its configuration shall be such as to receive all of the metal displaced by the die tooth 80b. Further, it is a requirement that the root surface 82b shall be located in position to cause a flow of metal displaced by the tooth 80b toward the middle portion of the gear teeth.

Similarly, a further section through the die 26, as for example at C, indicates a tooth 80c whose crest is substantially higher than the crest of the same tooth at the section B. Similarly, the depth of the tooth space has increased so that the root surface 82c is as shown. Again, the arrangement is such that the cross-sectional area of the tooth space below the rolling diameter 84 shall be equal to the cross-sectional area of the tooth 80c above the rolling diameter within suitable limits. It will be observed that the tooth 80c has begun to assume recognizable involute form.

Finally, adjacent the outlet side of the die 26 the die tooth assumes the shape illustrated at 80d and the root surface is located at 82d. This represents substantially a tooth form conjugate to the desired tooth to be produced on the gear.

In the foregoing the fact that some metal may be displaced axially of the blank has been ignored. In practice where axial flow of metal occurs and the amount of such flow can be predicted, the die design will be modified so that less cross-sectional area will be required below the rolling diameter because of the fact that not all of the metal displaced by the tooth portion above the rolling diameter is displaced into the tooth space.

In this illustration it will be observed that in cross sections A,B,C and D through the die 26, the chordal thickness of the tooth at the rolling diameter remains constant throughout, as does the chordal spacing between adjacent teeth on the rolling diameter.

Referring now to FIG. 5 there is diagrammatically illustrated the generation or formation of teeth on a solid blank 90. It will of course be understood that where the blank 90 is referred to as solid, this does not eliminate the possibility of a central hole such as the hole 24 illustrated in FIG. 2. It does require the formation of teeth in their complete form from material provided in a blank having initially a smooth cylindrical surface. In this FIG. the width of the die is designated by the arrow 92 and the solid blank 90 as it engages the left-hand end of the die has a smooth circular or cylindrical outside diameter 94. It is understood that the work gear 90 as it progresses is moved in a direction parallel to its axis and the diagrammatic showing of FIG. 5 is merely to indicate the growth of the circle containing the crests of the teeth and the diminution of the circle containing the roots of the teeth as the work gear is advanced axially from one side to the other of the die. The work gear at the position 90a has had its peripheral portion formed so that the circle 94a contains the crests of the partially formed teeth and the circle 96a contains or is tangent to the roots of the teeth as partially formed at such time.

When the blank has advanced to the position indicated at 90b the crests of the partially formed teeth are contained in the circle 94b, and the circle 96b contains or is tangent to the roots of the teeth.

When the gear emerges beyond the outlet end of the die or dies as illustrated at 90c, the teeth have been formed such that the circle 94c contains the crests of the teeth and the circle 96c contains or is tangent to the root surfaces thereof.

It will be observed by comparing construction lines 97 and 98 with the reference line 99 that the increase in height of the gear teeth above the rolling diameter which corresponds to the line 99, is equal or substantially equal to the increase in the depth of the tooth space below the rolling diameter. Moreover, as suggested on this FIG., the increase in height of the crests of the teeth progresses substantially uniformly and equally with the increase in root depth.

The foregoing describes a rolling die which has been satisfactorily used in pairs to produce gears from solid material as described in the foregoing. However, it is appreciated that the precise tooth form of the actual dies which have been tested in practice is not critical. The single essential requirement is that the teeth shall be tapered from end to end both to have an increasing tooth height as well as an increasing depth of space between the teeth, and further, that the configuration of the surfaces defining the space between the teeth of the die shall include surfaces in position to intercept the material of the blank which would otherwise be displaced locally into "rabbit ears," and effective to cause this material to flow inwardly toward the center of the gear tooth as it is being formed by metal displaced from portions of the blank which are to become tooth spaces in the finished gear.

While the foregoing specific disclosure has been of an arrangement in which the axes of the dies are parallel to each other and are parallel to the axis of the blank, it will be apparent that the invention is applicable also to an arrangement in which the dies are designed to operate with their axes crossed with respect to the axis of the gear. In this case it will of course be understood that the axes of the dies are crossed with respect to each other at an angle double the angle at which the axes of the individual dies cross with respect to the axis of the gear.

In the embodiment of the invention so far described the increase in crest height of the die teeth and the decrease in root diameter of the die teeth are substantially equal from end to end and are at a constant rate.

Referring now to FIG. 9 there is illustrated a different embodiment of the invention in which the die 100 has teeth 102, the crests 104 of which are of curved configuration from the small end toward the large end of the teeth. In this embodiment, the root diameter 106 is illustrated as of uniformly decreasing diameter.

The embodiment of the invention illustrated in FIG. 10 is generally similar. In this case the die 110 has the teeth 112 the crests 114 of which increase at a diminishing rate from the small end toward the large end, thus producing the curved configuration illustrated in the FIG. In this embodiment of the invention the root diameter 116 of the teeth increases at a diminishing rate generally corresponding to the increase in rate of the crest diameter.

These embodiments of the invention permit a variation in controlling the flow of material and in general, produce a more substantial rate of flow during initial formation of the teeth with a diminishing rate of flow as the teeth of the blank are formed into their final configuration.

The dies disclosed herein and the method of their use in rolling gears from cylindrical blanks (as opposed to finish rolling) represents a significant advantage over the dies and type of rolling in which opposed dies are moved radially into a blank having an initial outside diameter approximately equal to the pitch diameter, because in this case the blank diameter is initially contacted by the die roll or die teeth at approximately the pitch diameter thereof, and therefore the operating conditions for this method are virtually similar to those of present finish rolling of substantially fully cut teeth. In other words, because starting diameter of rolls is the pitch diameter, the circular pitch of the roll and gear are nearly identical.

Claims

We claim:

1. A gear rolling die in the form of a circular gear having a rolling diameter and having teeth which are tapered longitudinally to have crests of increasing height and tooth spaces of increasing depth, and in which the surfaces at the bottoms of the tooth spaces at and adjacent the roots of the die teeth are disposed to prevent formation of "rabbit ears" on a work gear blank as the die displaces metal therefrom by intercepting "rabbit ears" as they tend to form directly adjacent the tooth spaces being formed in a work gear and guiding the flow of metal toward the middle portions of the gear teeth as they are formed.

2. A die as defined in claim 1, the die teeth being of increasing crest diameter and of diminishing root diameter from end to end, the cross-sectional area of the tooth portions within the rolling diameter in any radial plane being generally equal to the area of the tooth space portions outside such rolling diameter, and the surfaces of the roots and tooth flanks of the portions of the tooth spaces being shaped to guide metal displaced from a gear blank by penetration thereinto of teeth of the die toward the middle portion of the gear teeth.

3. A die as defined in claim 2, the die teeth being of increasing crest diameter and of diminishing root diameter from end to end, the cross-sectional area of the tooth portions within the rolling diameter in any radial plane being generally equal to the area of the tooth space portions outside such rolling diameter, and the root surfaces of the tooth spaces of the die being of increasing depth from the flanks of the die teeth toward the middle portion thereof.

4. A die as defined in claim 3 in which the die teeth increase in crest diameter and decrease in root diameter generally equally from one end to the other.

5. A die as defined in claim 4 in which the rate of increase in crest diameter from one end to the other of the die teeth is substantially uniform.

6. The method of forming gears which comprises rolling a gear blank in contact with a gearlike die, causing progressive penetration of the teeth of the die into the periphery of the blank to form tooth spaces in the blank and to displace material radially outwardly into teeth intermediate the tooth spaces, and confining and guiding the material of the blank as it is displaced radially outwardly to prevent local displacement of material directly adjacent the sides of the tooth spaces as they are formed.

7. The method of claim 6 which comprises providing a pair of diametrically opposed dies having tapered teeth, and feeding the blank axially through the space between the dies.

8. The method of claim 7 in which the die teeth increase in crest diameter and decrease in root diameter generally equally from one end to the other.

9. The method of claim 8 in which the axes of the dies and the axis of the blank are parallel.

10. A die as defined in claim 1 in which in diametral planes adjacent the zone at which the die teeth merge into the rolling diameter, the bottom surfaces of the tooth spaces incline from approximately the rolling diameter toward the center of the tooth spaces and form a generally V-shaped surface effective to displace metal displaced by tooth portions of the die from the blank toward the central portions of the spaces between the die teeth.

11. A die as defined in claim 1 in which the die teeth have substantially constant leads as measured at constant radial distance from the die axis.

12. A die as defined in claim 1 in which the die teeth have a substantially constant chordal thickness as measured at the rolling diameter.

13. A die as defined in claim 12 in which the chordal width of the space between the die teeth as measured at the rolling diameter is substantially constant and substantially equal to the chordal thickness of die teeth as measured at the rolling diameter.

14. A die as defined in claim 1 in which the width of the die teeth at the starting end thereof is substantially equal to the tooth spacing.