US3572075A

US3572075A - Metal-rolling methods

Info

Publication number: US3572075A
Application number: US775266A
Authority: US
Inventors: Claus L Sporck
Original assignee: BP Chemicals Hitco Inc
Current assignee: BP Chemicals Hitco Inc
Priority date: 1957-06-19
Filing date: 1968-11-13
Publication date: 1971-03-23
Anticipated expiration: 1988-03-23

Abstract

The method of forming metal billets into desired cylindrical and circular shapes by imposing a squeezing force on the billet to cause the metal to flow in a direction for obtaining the desired shape while simultaneously imposing an additional force operating in a direction the same as the direction of metal flow caused by the squeezing force to assist the flow caused by the squeezing.

Description

United States Patent Inventor Claus L. Sporck Traverse City, Mich. Appl. No. 775,266 Filed Nov. 13, 1968 Patented Mar. 23, 1971 Assignee Hitco Gardena, Calif.

METAL-ROLLING METHODS 9 Claims, .8 Drawing Figs.

U.S. Cl. 72/86, 72/96 1nt.Cl B2lh 1/02, B2lh 19/14 Field 01' Search 72/86, 87,

[56] References Cited UNITED STATES PATENTS 704,316 7/1902 Gieshoidt 72/96 1,395,357 11/1921 Putnam 72/86 Primary Examiner-Lowell A. Larson Attorney-Frederick J. Olsson ABSTRACT: The method of forming metal billets into desired cylindrical and circular shapes by imposing a squeezing force on the billet to cause the metal to flow in a direction for obtaining the desired shape while simultaneously imposing an additional force operating in a direction the same as the direction of metal flow caused by the squeezing force to assist the flow caused by the squeezing.

' PAiENjIEDW :i SHEET 2 Hi gaadzvsmox. Q; 4 1

ATTORN EY.

METAL-ROLLING METHODS In general, the invention relates to methods for the roll formingof metal billets into parts of desired shape. The invention provides improved methods for the production of circular parts such as turbine discs, wheel discs, couplings and the like and for the production of cylindrical parts such as long, thin wall tubes.

Parts of the kind mentioned produced by the invention are characterized by qualities making a product far superior to similar products produced by conventional techniques. For example, parts produced by the methods described herein have enhanced mechanical strength properties, are dimensionally accurate and in most cases can be rolled to the exact desired shape or alternatively, to a shape which requires minimum machining. Moreover, the methods of the invention provide for the production of circular and tubular parts having the characteristics just noted and which are of a much larger diameter than is presently obtainable by known methods.

The methods of the invention will be described below in connection with the following drawings wherein:

FIG. 1 is a diagrammatic view illustrating a cylindrically shaped billet and a disclike part formed therefrom; v

FIG. 2 is an array of top and bottom sets of conically shaped rollers arranged for working a billet such as the one shown in FIG. 1;

FIG. Sis a plan view of the top set of rollers of FIG. I illustrating the offset condition of the axes of the rollers;

FIG. 4 is an elevational view of a'pair of rollers in position to work a cylindrical billet;

FIG. 5 is an elevational view showing the position of the rol lers P a i FIG. 6 is a prospective view of a conical roller having a helical groove on its working surface;

FIG. 7 is a diagrammatic view of a generally cylindrical billet and a long tubular part formed therefrom; and

FIG. 3 is a diagrammatic view illustrating a pair of generally cylindrically shaped rollers in position for working a cylindrical shaped blank to form the same into a long, thin-walled tube.

In FIG. 1 the cylindrically shaped billet B is formed by conventional techniques. The billet has the opening 2 and is symmetrical with respect to the axis A. By the methods of the invention forces are exerted on the opposite ends of the billet to cause the same to flow into the disclike part D which in the present instance is a turbine disc for use in jet aircraft engines. This is accomplished by the array of rollers such as those shown in FIG. 2 within which is nested the billet B.

Before proceeding, it is to be-observed that the term cylindrical as used herein refers to a billet of a shape as shown in FIG. 1 which is either solid or hollow.

The array of FIG. 2 comprises top and bottom sets of rollers. In the top set the

rollers

3, 4 and 5 are spaced about billet axis A approximately 120 apart. In the corresponding bottom set the

rollers

10, 11 and 12 are similarly spaced about the axis A. The billet B is rotatably mounted between the rollers. Thus, with reference to FIG. 4, the billet B is supported on pedestal 13 between the

rollers

5 and 12. Aninner spindle 14 extends through the opening 2 of the billet and into the pedestal 13. The spindle 14 prevents lateral movement of the billet during the working operation. The pedestal, billet and spindle are all rotatable in unison.

The rollers of the sets are individually rotatably mounted and are arranged so that the two sets can be moved relative to each other in a direction along the axis A. For example, with reference to FIG. 4 the bottom set of rollers (as indicated for the roller 12) and the top set of rollers (as indicated by the roller 5) are rotatably mounted on carriage mechanism C. The carriages move the sets vertically so that the roller feed in a direction along axis A.

In the preferred embodiment, the top set of rollers are power driven and the billet is keyed to the spindle which is also power driven.

after having worked the billet of FIG. 1 into a disclike The relative motion of the rollers along the axis A imposes a squeezing force on the billet which causes the billet to expand radially outwardly between the rollers in a direction indicated by the arrows E. This reduces the height and increases the diameter of the billet to an extent as indicated in FIG. 5 where the roller 5 has moved downwardly to shape the billet into the disclike part D.

With reference to FIG. 4, it will be noted that the

rollers

5 and 12 extend respectively from the

points

15 and 16 adjacent the axis A outwardly across the face to the periphery. Further, with reference to FIG. 5, it will be apparent that the roller contact is maintained at all times during the rolling operation. This is particularly important and advantageous in that the metal in the billet is under compression at alltimes and this avoids cracking during the conversion of a cylindrical shape to a disclike shape.

The contact established between the rollers and the billet during the working operation is in the nature of a line contact and preferably the corresponding rollers of the sets are arranged with respect to one another so that the two lines of contact lie substantially in the same radial plane. In this manner the forces imposed on the billet by the top and bottom rollers act essentially in the same radial plane and therefore operate with maximum, loading efficiency.

The use of rollers on both sides of the billet for the rolling operation as above mentioned is advantageous in that practically all the available force is used in moving metal rather than overcoming external retarding forces, for example, such as would be the case if the bottom of the billet was supported on a plate rather than on reducing-rollers.

In connection with the flowing of disc D of FIG. 5 the invention contemplates alternative methods for providing the peripheral rim 17.

Preferably, there are three guide rollers spaced l20 apart and respectively located between the corresponding sets of the top and bottom rollers. One of the guide rollers is indicated at 18in FIG. 5. The guide rollers are controllably radially movable with respect to the axis A. For the working operation, the rollers are moved inwardly to engage the billet B and then as the blank starts to extrude are moved outwardly in a manner to impose a slight force on the extruding billet. This action guides and supports the billet. When the billet first begins to extrude away from the outer end of the roller, for example, the end 19, the outward motion of the guide rollers is stopped and the extruding billet will be forced against the rollers 18. As a consequence, the edge of the billet will extrude vertically to form the rim 17.

Alternatively, the rim 17 is formed by making a final squeeze with the top and bottom rollers. As the billet just begins to extrude away from the outer end 19 of the rollers, the top and bottom sets are then given a further movement toward each other. Since no reducing is then done on the portion outside of the end 19, a peripheral rim is formed.

The rollers are contoured so as to obtain desired shape on the disc. For example, in the present embodiment, the apex end of each roller is rounded (FIG. 5) to make a fillet connection 20 between the hub section 21 and the intermediate section 22 while the largeend of the roller is contoured to form the fillet 23 connecting the outer rim 17 with the intermediate section 22.

One of the important features of the invention is the providing of force additional to the squeezing force which functions to assist the squeezing force in moving the metal in the desired direction. The invention contemplates two ways of accomplishing this. In one arrangement the roller axes are offset with respect to the billet axis and in the otherarrangement the rollers are provided with helical grooves which can be used either in the offset or nonoffset condition. The offset arrangement will first be explained.

In FIG. 3 the lines 3a, 4a and 5a represent the respective roller axes and also the projection of the line-contact made by each roller with the end of the billet B. It will be readily apparent that the respective axes and lines of contact are offset from the billet rotational axis A. It is preferable that the amount of offset be within a range l5. The effect of this arrangement is to impose an additional force on the billet during the rolling operation. The additional force has a component which acts to move the metal radially outwardly or in the same direction as the metal is caused to move by the squeezing force. The other rollers impose similar force.

With reference to providing a helical groove to generate the additional force, reference is made to FIG. 6 where roller 24 is formed with the helical groove 25 of relatively shallow depth and minimum pitch. With roller rotation, the grooves grip the metal and pull the same radially outwardly and thereby impose a force which augments and amplifies the squeezing force and gets the metal moving radially outwardly.

Where the helical groove arrangement is used, it is preferable that all of the rollers have the grooves of the same dimension and that both the top and bottom sets of rollers be power driven. Also, it is to be observed that the hand of the groove on each roller be the same so that the resultant effect is to exert outwardly directed forces on both. ends of the billet. Further, it will be noted that when the rollers (top and bottom) have made working contact with the respective ends of the billet, portions of the groove on each roller and portions between the groove make line contact with the billet.

When the helical-type rollers are used, it is preferred that the same be arranged in the offset condition in order to augment the force generated by offset condition per se. However, it is pointed out that inasmuch as the helical groove provides a means for obtaining the additional force, it is contemplated that helical type rollers can be set up in a nonoffset condition, that is to say, with their axes and lines of contact going directly through the rotational axis of the billet.

I have described the rollers of FIG. 2, etc. as being conical in shape. This shape is essential in order to avoid rubbing between the billet and roller. With the conical shape substantially true, rolling action is obtained and this minimizes roller wear and frictional drag. The angle of the rollers of FIGS. 2- is shown as 90. The angle can be less if desired.

The offset and helical groove arrangements as described above produce dual forces acting on the metal substantially at right angles to one another and move the metal in the same direction. This force orientation is advantageous in that the metal moves at a faster rate and the amount of force needed is minimized.

As a further means for improving the extrusion rate and for minimizing the required force, the invention contemplates oscillating the roller during the working operation.

In one aspect the oscillation is in a direction back and forth along the line of contact between the roller and the billet and in another aspect the roller is oscillated with the small end of the roller as the pivot point. In both instances the frequency of oscillation is small preferably in the order of about 30 c.p.s. and the amplitude is also small being in the order of a fraction of an inch. 1

In oscillating the roller in a direction along the line of contact, it is preferable to use rollers-having a helical groove as described heretofore. The effect of this motion is to permit the roller to burrow into the billet so to speak and thereby permit faster roller feed which causes the metal to move radially outwardly at a faster rate. When oscillating the roller with the small end as a pivot, the line of contact partakes of a sweeping action on the billet surface and this also has a burrowing effect and is conducive to faster roller feed and billet extrusion rate.

The dual-force concept as mentioned above is especially useful in the rolling of long cylindrical tubes. With reference to FIG. 7 the cylindrical billet 26 is rolled into the long tubular member 27 by the technique which will be described in connection with FIG. 8.

In FIG. 8 the billet 26 is rotatably mounted and means are provided for imposing a force on the billet to move the same to the right in the direction of the arrows 28. A pair of

rollers

30 and 31 are adapted to respectively work opposite sides of the wall of the billet 26. The rollers are power driven and are identical in construction. The rollers have tapered

sections

32 and 33 and

cylindrical sections

34 and 35. The tapered section 32 is provided with a helical groove 36. The section 33 has helical groove 37. Preferably, each helical groove is of relatively small depth and of minimum pitch.

In working a billet, the

rollers

30 and 31 are moved apart so that the end 40 of the blank can be moved slightly into the mouth or space 41. The rollers are then driven and the roller 31 moved inwardly to engage the blank between the rollers. Alternatively, the rollers may be set up in the position shown and the billet 26 moved to the right so that the end 40 is forced into the space 41.

Depending upon the hand of the helical grooves, either the rollers per se or external force causes the billet to move through the rollers during the working operation.

Where the grooves of the respective rollers are of different hand, the grooves will have no pulling effect when the rollers are rotated and so the billet stands still. In this instance an external force is exerted. on the billet to move the same through the rollers.

Where the external grooves on the respective rollers are on the same hand, the rotatibn of the roller will cause the grooves to grip the billet and pull the same to the right between the rolleis. With the tapered condition a squeezing force is exerted on the wall of the billet so that the same is gradually reduced in thickness until such time as the billet enters the space 42 between the

cylindrical sections

34 and 35. The cylindrical sections iron out the wall as it extrudes away from the rollers.

As will be apparent from the foregoing, movement of the billet through the rollers for the working operation is accomplished by means of two forces acting substantially at right angles to each other. One is the squeezing force developed by the tapered condition. This force acts radially on the billet. The other force is developed by either the helical groove exerting a pulling force or an external force moving the billet through the rollers. In either of the latter instances, the force is axial.

The arrangement described above has particular advantage in that the amount of reduction taken can be relatively large and the final wall thickness precisely controlled by the

cylindrical sections

33 and 34. Furthermore, the arrangement is conducive to the extrusion of relatively thin wall tubes of high dimensional accuracy and this is accomplished without the necessity of large, highly expensive mandrels.

I claim: 1. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet, comprising the steps of:

rotatably mounting a pair of conically shaped rollers and respectively engaging the opposite ends of said billet with the rollers each engagement being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line of contact being offset from the billet axis and the lines of contact of the respective rollers lying in the same radial plane; causing roller rotation and also causing relative rotation as between the rollers and the billet in a direction around the billet axis; and

while maintaining said roller contacts, moving the rollers relatively toward one another in a direction along the billet axis to impose force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation said offset condition imposing a force on the billet causing the metal to move radially outwardly.

2. A method according to claim 1 further including the step of oscillating each said roller about the small end thereof so that the line of contact swings back and forth on the end surface of the billet.

3. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet, comprising the steps of:

causing roller rotation and also causing relative rotation as between the rollers and the billet in a direction around the billet axis; and

while maintaining said roller contacts moving the rollers relatively toward one another in a direction along the billet axis to impose force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation said offset condition imposing a force on the billet causing the metal to move radially outwardly. V

4. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the steps of:

rotatably mounting a pair of conically shaped rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet;

causing roller rotation and also causing relativerotation between the rollers and the billet in a direction about the billet axis;

moving the rollers relatively I toward one another in a direction along the billet axis to respectively engage the opposite ends with portions of' the groove and portions between the groove, the contact of said portions, in plan, being substantially a line contact extending from a point adjacent the axis'of the billet radially outwardly across the surface to the periphery; and

' while maintaining said roller contacts'continuing said relative movement to impose force of the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation each helical groove imposing a force on the billet to cause the metal to move radially outwardly.-

5. The method according to claim 4'further including the step of oscillating each said roller in a direction along the line of contact with the billet.

6. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the steps of:

rotatably mounting a pair of conically shaped-rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet; causing roller rotation and also causing relative rotation of the rollers and the billet in a direction about the billet axis;

moving the roller relatively toward one another in a direction along the billet axis to respectively engage opposite ends of the billet with portions of said groove and portions between the grooves, the contact of said portions, in plan, being substantially aline contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line contact being offset from the billet axis and the lines-of contact of the respective rollers lying in the same radial plane; and

said roller motion maintaining said contacts and imposing force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the working operation each helical groove imposing force on the billet to cause the metal to move radially outwardly and said ofi'set condition imposing force on the billet to cause the metal to move radially outwardly. r 7. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the Ste 5 of:

rotatably mounting a pair of conically s aped rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet; causing roller rotation and also causing relative rotation of the rollers and the billet in a direction about the billet axis; v 1 moving the rollers relatively toward one another in a direction along the billet axis to respectively engage opposite ends of the billet with portions of said groove and portions between the grooves-the contact of said portions, in plan, being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line contact being offset of the billet axis; and said roller motion maintaining said contacts and imposing force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the working operation each helical groove imposing force on the billet to cause the metal to move radially outwardly and said offset condition imposing force on the billet to cause the metal to move radially outwardly. I v 8. The method of reducing the wall thickness and increasing the length of a cylindrical billet comprising the steps of:

rotatably mounting a pair of rollers in spaced-apart relationship, the axes of the rollers extending generally in the same direction and each roller having a helical groove extending along its axis; rotatably supporting a cylindrical billet; causing relative movement between the rollers and the billet to cause one roller to engage the outside surface of the billet and the other roller to engage the inside surface of the billet whereby the wall of the billet occupies the space between the rollers; and

causing the rotation of the rollers and also causing relative.

draw the same through the space in a direction along. the roller axes, and the rollers working the wall of the billet to reduce the wall thickness.

9. The method of reducing the wall thickness and increasing the length of a cylindrical billet comprising the steps of:

rotatably mounting a pair of tapered rollers in spaced apart relationship, the axes of the rollers extending generally in the same direction and the tapered section of each roller having a helical groove extending along its axis;

rotatably supporting a cylindricalbillet;

causing relative movement between the rollers and the billet to cause one roller to engage the outside surface of the billet and the, other roller to engage the inside surface of the billet whereby the wall of the billet occupies the space between the rollers; and

causing the rotation of the rollers and also causing relative rotation between the rollers and the billet, the relative rotation between the rollers and the billet causing each helical groove to exert force on the wall of the billet and draw the same through the space in a direction along the roller axes, and the tapered sections of the rollers working the wall of the billet to reduce the wall thickness.

helical groove to exert force on the wall of the billet and

Claims

1. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet, comprising the steps of: rotatably mounting a pair of conically shaped rollers and respectively engaging the opposite ends of said billet with the rollers each engagement being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line of contact being offset from the billet axis and the lines of contact of the respective rollers lying in the same radial plane; causing roller rotation and also causing relative rotation as between the rollers and the billet in a direction around the billet axis; and while maintaining said roller contacts, moving the rollers relatively toward one another in a direction along the billet axis to impose force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation said offset condition imposing a force on the billet causing the metal to move radially outwardly.

3. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet, comprising the steps of: rotatably mounting a pair of conically shaped rollers and respectively engaging the opposite ends of said billet with the rollers each engagement being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line of contact being offset from the billet axis; causing roller rotation and also causing relative rotation as between the rollers and the billet in a direction around the billet axis; and while maintaining said roller contacts moving the rollers relatively toward one another in a direction along the billet axis to impose force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation said offset condition imposing a force on the billet causing the metal to move radially outwardly.

4. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the steps of: rotatably mounting a pair of conically shaped rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet; causing roller rotation and also causing relative rotation between the rollers and the billet in a direction about the billet axis; moving the rollers relatively toward one another in a direction along the billet axis to respectively engage the opposite ends with portions of the groove and portions between the groove, the contact of said portions, in plan, being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery; and while maintaining said roller contacts continuing said relative movement to impose force of the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the rolling operation each helical groove imposing a force on the billet to cause the metal to move radially outwardly.

5. The method according to claim 4 further including the step of oscillating each said roller in a direction along the line of contact with the billet.

6. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the steps of: rotatably mounting a pair of conically shaped rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet; causing roller rotation and also causing relative rotation of the rollers and the billet in a direction about the billet axis; moving the roller relatively toward one another in a direction along the billet axis to respectively engage opposite ends of the billet with portions of said groove and portions between the grooves, the contact of said portions, in plan, being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line contact being offset from the billet axis and the lines of contact of the respective rollers lying in the same radial plane; and said roller motion maintaining said contacts and imposing force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the working operation each helical groove imposing force on the billet to cause the metal to move radially outwardly and said offset condition imposing force on the billet to cause the metal to move radially outwardly.

7. The method of converting a cylindrically shaped metal billet into a disclike part by reducing the height and increasing the diameter of the billet comprising the steps of: rotatably mounting a pair of conically shaped rollers each roller having a helical groove and positioning the rollers respectively adjacent opposite ends of the billet; causing roller rotation and also causing relative rotation of the rollers and the billet in a direction about the billet axis; moving the rollers relatively toward one another in a direction along the billet axis to respectively engage opposite ends of the billet with portions of said groove and portions between the grooves, the contact of said portions, in plan, being substantially a line contact extending from a point adjacent the axis of the billet radially outwardly across the surface to the periphery and the projection of the line contact being offset of the billet axis; and said roller motion maintaining said contacts and imposing force on the billet to cause the same to expand radially outwardly between the rollers whereby to reduce the height and increase the diameter and form said disclike part and during the working operation each helical groove imposing force on the billet to cause the metal to move radially outwardly and said offset condition imposing force on the billet to cause the metal to move radially outwardly.

8. The method of reducing the wall thickness and increasing the length of a cylindrical billet comprising the steps of: rotatably mounting a pair of rollers in spaced-apart relationship, the axes of the rollers extending generally in the same direction and each roller having a helical groove extending along its axis; rotatably supporting a cylindrical billet; causing relative movement between the rollers and the billet to cause one roller to engage the outside surface of the billet and the other roller to engage the inside surface of the billet whereby the wall of the billet occupies the space between the rollers; and causing the rotation of the rollers and also causing relative rotation between the rollers and the billet, the relative rotation between the rollers and the billet causing each helical groove to exert force on the wall of the billet and draw the same through the space in a direction along the roller axes, and the rollers working the wall of the billet to reduce the wall thickness.

9. The methOd of reducing the wall thickness and increasing the length of a cylindrical billet comprising the steps of: rotatably mounting a pair of tapered rollers in spaced apart relationship, the axes of the rollers extending generally in the same direction and the tapered section of each roller having a helical groove extending along its axis; rotatably supporting a cylindrical billet; causing relative movement between the rollers and the billet to cause one roller to engage the outside surface of the billet and the other roller to engage the inside surface of the billet whereby the wall of the billet occupies the space between the rollers; and causing the rotation of the rollers and also causing relative rotation between the rollers and the billet, the relative rotation between the rollers and the billet causing each helical groove to exert force on the wall of the billet and draw the same through the space in a direction along the roller axes, and the tapered sections of the rollers working the wall of the billet to reduce the wall thickness.