US2558035A - Method and apparatus for cold drawing - Google Patents

Description

June 26, 1951 P. w. BRIDGMAN 2,558,035

METHOD AND APPARATUS FOR COLD DRAWING Filed July 5. 1947 2 SheetS-Sheet 1 INVENTOR l3 1 Y Pliggv w. amosumu ATTORNEY June 26, 1951 Filed July 5. 1947 P. W. BRIDGMAN IIETHOD AND APPARATUS FOR COLD DRAIING 2 Sheets-Sheet 2 I an Y YI II'I'I rl l l J il hl l ll ll INVENTOR PERCY W. BRIDGMAN ATTORNEY Patented June 26, 1951 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR COLD DRAWING Percy W. Bridgman, Cambridge, Mass.

Application July 5, 1941, Serial No. 759,185

12 Claims. (01. 205-3) This invention relates to the technique of extrusion and more particularly to a novel method and apparatus for cold drawing metals, alloys and other plastic materials.

It has been well established by experiment that many metals and other plastic materials have greatly increased capacity for deformation without fracture if they are deformed while subjected to high hydrostatic pressure. For example, tension specimens of ordinary steel pulled while under a hydrostatic pressure of about 400,0000 p. s. i. elongated up to 300 fold (the reduction in area being 99.7%) before fracture, whereas similar specimens pulled at atmospheric pressure broke at elongations of less than two fold.

In view or the increased ductility imparted to metal by high pressure, it would appear that cold metal could be extruded through the die of a conventional extrusion press upon the application of suilicient pressure directly to the metal. However, such a procedure fails in practice, because the friction between the cold metal and the walls of the die is so great that the die will split before the pressure can be increased sufficiently to start extrusion.

In my copending application, Serial No. 668,654, a method was described wherein a solid plastic material such as a metal or alloy, is extruded by subjecting such material uniformly to a high hydrostatic pressure, thereby increasing its ductility and forcing the material, while in an increased state of ductility, through a die. In each of several modifications described therein, the body being extruded is subjected to high hydrostatic pressure on one side of the die and to atmospheric pressure on the other side. As a result, the body is under great axial compressive stress during its passage through the die and tends to deform laterally against the inner walls thereof, thus creating great frictional forces in opposition to the forces promoting the extrusion process. It was disclosed therein that the hydrostatic pressure required to produce a given reduction of area could be considerably reduced by combining the extrusion process with a drawing operation wherein a pulling force is exerted on the issuing metal. This has the disadvantage, however, that a large part of the material subjected to tension is no longer under the influence of the hydrostatic forces that confer improved ductility. Consequently, brittle materials, ordinarily capable of being extruded by hydrostatic pressure alone, frequently fail by tension in the combined process.

It is the primary object of the present invention to provide a method and apparatus for drawing a solid plastic material such as a metal or alloy through a die while under high hydrostatic pressure and in a greatly increased state of ductility. Another object of the invention is to provide a relief valve that is readily operable under high hydrostatic pressure. A still further object is to provide a simple method of firmly securing the reduced end portion of a body to be extruded to a guide member capable of exerting a high tensile force when hydrostatic pressure is applied to the surrounding fluid.

In general, the method of the present invention comprises passing a reduced portion of a body of solid plastic material through a die, subjecting the portions of said body on both sides of said die to high hydrostatic pressure and applying tension to said reduced portion of said body, for example by causing said pressure to force an extended portion of said reduced portion through an orifice.

The invention is further described with reference to the accompanying drawings, in which Fig. 1 is an elevation, in diagrammatic crosssection, of a hydrostatic drawing apparatus in accordance with the invention;

Fig. 2 is an enlarged perspective view, with a portion broken out, of the pull rod 28 of Fig. 1:

Fig. 3 is an elevation, in diagrammatic crosssection, of a modified form of the apparatus shown in Fig. 1;

Fig. 4 is a. sectional view taken along the line 4-4 of Fig. 3;

Fig. 5 is an elevation, in diagrammatic crosssection, of a preferred form of relief valve for use in the hydrostatic drawing apparatus of the invention.

As shown in Fig. l, the apparatus of the invention comprises a hydraulic cylinder B having three chambers adapted to hold a liquid or other suitable medium M under high pressure, and a relief valve l2. The upper chamber I3 is equipped with a plunger It for applying pressure to the medium M and is separated from the intermediate chamber l5 by the drawing die It which is supported by a shoulder IT in the cylinder B. The intermediate chamber ii, in turn, is separated from the lower chamber it by the extrusion plate l9, which has an orifice 20 and is supported on a shoulder 2|. The relief valve I! connected to the lower end of the hydraulic cylinder B may be of any suitable design, but in view of the high pressures used is preferably constructed in the novel manner described in connection with Fig. 5. In order to prevent leakage with the high pressures required in the medium M it is desirable that the forward end 22 of the plunger I8 is slideably fitted to the main portion thereof to provide space for compresslble packing rings 28 that are forced outwardly against the cylinder walls under the infiuence of endwise pressure. For the same reason, packing rings 28 should be used at the seat of the threaded joint connecting the relief valve l2 to the cylinder B. The packing rings may be of lead or other suitable material. Lead packing rings have been found to be particularly effective in maintaining pressures of 200,000 p. s. l. and higher.

It is not necessary that the medium M, employed in the drawing operation, be a liquid. In fact, at the pressures employed, it is most probable that oil, for example, is congealed into a soft solid. It is necessary, however, that a solid medium be notably softer than the material to be extruded and soft enough to transmit approximately hydrostatic pressure, i. c. it must support only a relatively small shearing stress. An advantage obtained by using a soft solid rather than a true liquid is that possible leakage of the medium around the die and piston may be more effectively prevented.

To assemble the apparatus for use, an end portlon 28 of the metal body 26 to be drawn is reduced in cross-section, as by machining or grinding, over a sufficient length to allow it to be passed through the die l5 and inserted in the closely fitting axial hole 21 in one end of the pull rod 28 which extends into the loosely fitting orifice 28 in the extrusion plate l8. With the novel design of the recessed end of the pull rod 28, the hydrostatic pressure exerted by the surrounding medium serves to join the pull rod securely to the reduced end portion of the body inserted in the recessed end thereof provided only that the edge of the joint is sealed air-tight by a non-porous sealing material such as soft solder 29. Referring to Fig. 2, a portion of the wall thickness of the axial hole 21 in the pull rod 28 is reduced to a few thousandths of an inch, for example by forming two diametrically opposite flattened portions 38 that extend along the outer surface of the pull rod for a length substantially equal to the depth of the hole. When the edge of the joint is sealed the application of hydrostatic pressure on the outer portion of the pull rod causes the thick portions of the walls to be forced inwardly with great pressure against the rod, while the flattened portions of the walls, being considerably thinner, readily give way by buckling. By making the axial hole 21 deep enough, an extremely large gripping force may be obtained. The forward end of the pull rod preferably is rounded to permit easy entrance thereof into passage 20 of Fig. 1.

With the apparatus assembled as shown in Fig. 1, pressure may be applied to the medium M within the cylinder B by exerting a downward force upon the plunger H. The equalization of pressure between the upper chamber l8 and the intermediate chamber l8 takes place almost instantaneousLv through suitable passages 8| preferably disposed concentrically about the orifice of the die I. By means of these passages the upper and intermediate chambers are so interconnected as to form a unitary drawing chamber subjecting both the entering and emerging portions of the body to a high hydrostatic pressure. The transmission of fluid pressure from the intermediate chamiber ii to the lower chamber l8,

however, is purposely retarded by providing only a minimum amount of clearance between the pull rod 28 and the orifice 28. When the desired high hydrostatic pressure has become equalized in all three chambers, the relief valve I2 is quickly opened, allowing medium M to escape from the lower chamber l8 at a much greater rate than it can enter through the clearance between the pull rod 28 and the orifice 28. The resulting great diiference in pressure between the intermediate chamber l5 and the lower chamber I8 drives the pull rod 28 through the orifice 28 and thereby simultaneously draws the metal body 26 through the die I8. With proper coordination of the relief valve opening, the pull rod clearance, and the force applied to the plunger, the portions of the body entering and leaving the die are subjected to high hydrostatic pressure and are therefore in an improved state of ductility. Furthermore, under such conditions of coordination, the drawing process may be continued uninterruptedly until the lower end of the pull rod 28 reaches the end of its travel, for example as shown by the broken line in Fig. 1.

The use of a pull rod 28 is not essential to the operation of the apparatus, as the mechanically reduced portion of the body itself may be made of sufficient length to extend into the orifice 28, which need be only slightly larger in diameter than the throat of the die It. In this case the extrusion plate I9 may be positioned quite close to the die It, allowing only enough space between these members for the unrestricted flow of the oil or other medium from the passages 8| through the small clearance space between the reduced portion 25 of the metal body and the walls of the orifice 28.

It is sometimes diilicult to provide an amount of clearance between the pull rod 28 and the walls of the orifice 28 sufficient for easy insertion of the pull rod, yet not so large as to cause an undesirable loss of medium from the upper chambers l3 and I5 under the high pressures used. This difliculty may be avoided by providing a minimum of clearance between the extrusion plate orifice and the pull rod and by employing means to keep the end of the pull rod away from the extrusion plate orifice until the pressure in all three chambers has been equalized. A modification of the apparatus of Fig. l employing these features is shown in Fig. 3. In this embodiment, the hydraulic cylinder C has an upper chamber 8| separated from a lower chamber 82 by an extrusion bushing l8 supported at its flanged end 84 on the shoulder 45. The die 48 is coaxially mounted in the base of a cup-shaped die holder 41 which has an outer cylindrical wall 48 slideably disposed within the cylinder C, and a coaxial inner cylindrical wall 49 which protrudes beyond the outer cylindrical wall 48. Passages 58 extend from the space between walls 48 and M to orifices lying within the confines of the diametrical dimension of an axially located recess 58 in the upper portion of the extrusion bushing 43. A collar 5| rests upon the shoulder 52 in the inner wall of the cylinder C and at least portions of said collar flt slideablv around the protruding end of the inner wall 48. As shown in Fig. 4, passages for the flow of medium past the collar are provided by the spaces 58 between the slldeably fitting portions 88. However, if the entire inner surface of the collar is made to fit slideably around the protruding end of the inner wall, other passage means, such as holes extending through the thickness of the collar aseaoss s would be equally suitable. A liner ll adapted to screw into the upper portion of the hydraulic cylinder ilrmly clamps the collar Ii against the shoulder 52. A compression spring 56 acting between the collar Ii and the base ofthe die holder 41 is restrained from advancing the latter toward the extrusion bushing 44 by shear pins S'I extending from the slideably fitting portions ll of the collar I2 into the protruding end of the inner wall ll. These shear pins extend fromholesinthecollar ll totheholesinthe inner wall It, the holes being opposite each other when the spring 56 is sufficiently compressed to abut the edge of the outer wall 48 against the collar ii. The protruding end of the inner wall 49 extends suillciently beyond the outer wall II to enable the plunger II to establish contact therewith and shear the guide pins during the early part of its stroke. A relief valve is is connected to the lower chamber I! as in the previous modification.

A portion of the body I! to be extruded reduced in cross-section, as hereinbefore de scribed, in order that it may be passed through the die oriflce, but the reduced portion 82 is made of only such length that it or the end of an attached pull rod, is adjacent to but not within the orifice H of the extrusion bushing ll. with the relief valve it closed, suflicient force is applied to the plunger Bl to produce the desired hydrostatic pressure in the oil or other medium M contained within the cylinder C. By means of passages 50 and GI this pressure immediately equalizes itself throughout the entire interior of the cylinder. While maintaining the force on the plunger, the relief valve II is opened to permit some of the medium M to escape from the cylinder as the plunger descends under constant application of force. In its descent, the plunger OI makes contact with the protruding edge of the inner cylinder wall 49 of the die holder 41' and severs the shear pins 51. This allows the compression spring 56 to force the reduced end I! ofthemetalbody6!,ortheendofapullrodat-' tached thereto, into the snugly fitting oriflce I, and brings the die holder 41 to rest against a blocking means comprising the upper surface oi the flange ll of the extrusion bushing 43. The end portion 62 of the body within the oriilce I completely blocks the flow of oil from the upper portion of the apparatus into the lower chamber 42. A high hydrostatic pressure is thus maintained in the upper chamber II and in the recess 58 whereas the pressure in the lower chamber 02 continues to drop through escape of oil through the open relief valve 58. When the pressure difference becomes sufllciently great, the end portion 62 of the body is forced through the oriflee Si by hydrostatic extrusion. It is thus apparent that the body being drawn is subjected to high hydraulic pressure both before entering and upon emerging from the die, similarly as in the apparatus of Fig. 1.

With the high fluid pressures used. often ranging up to 250,000 p. s. i. and higher, ordinaw high pressure valves have been found unsuitable, particularly where rapid opening of the valve is of primary importance. Thus, in the apparatus of Fig. 1 it is essential that relief valve ll be adjusted from fully shut to fully open almost instantaneously, as otherwise a considerable amount of oil or other medium would pass through the clearance between the pull rod and the walls of the extrusion orifice before the pres- Q suneintheiowerchamber would drop sufflciently fastfortherodtobcginextruding.

Apreferredformofreliefvalveshowninflg- 5 comps-hes a cylindrical housing H having an externallythreaded reduced portion ll adapted. for example. to cooperate with a threaded recess intheendofcylindersBorCinFigs. lor 3. respectively.- An axial inlet passage 12 of small diameter extends from the outer surface of the reduced portion II to the apex of a conical valve seat I3, forming the base of a recess 14 axially disposed in the end wall of the pressure cylinder II. An outlet passage It extends from the side wall of the recess ll. in the vicinity of the conical seat II. to an external connection ll on the housingH. Withintherecess'llisaloosely fitting closure member such as a hardened steel ball ll adapted to eifect a fluid-tight seal of the inlet passage I! when the said ball is forced against the conical seat 13. Pressure against the ball is provided by contact with a boss II on the forward end ll of a plunger acting within cylinder Ii. In view of the high pressures used this plunger preferably consists of two slideably fltted parts 8| and ll separated by a compressible packing 81 capable of being deformed outwardly against the cylinder walls under the influence of endwise pressure. The mouth of the pressure cylinder II is sealed by the cylinder head II, which is seated against the shoulder 84. Fluid-tight seating of the cylinder head is assured by providing compressible packing 85 between the said cylinder head and the reduced portion I of the flrmly tightened gland nut 81. The cylinder head 83 has a long stem 88 passing through an axial opening in the gland nut 81 and terminating in 'a threaded end adapted for connectingtheaxialpassagelsinthestem 88 with an auxiliary hydraulic pmnp.

In operating the relief valve for use in the drawing apparatus of the invention. fluid medium F under pressure supplied by an auxiliary pump drives the plunger within the cylinder 15 against the hardened ball ll. causing the latter to be forced against the conical seat 13. This effectively seals the inlet passage I2 and prevents the escape of fluid medium from the cylinder of the drawing apparatus. Since the crosssectional area of the relief valve plunger is very much greater than the effective area of the hardened ball. a relatively small fluid pressure from an auxiliary pump is sufllcient to seal the valve against the high pressure existing in the main cylinder of the drawing apparatus. To open the relief valve, pressure is removed from the plunger by releasing fluid medium F in any suitable manner. This permits the plunger and the hardened ball to descend under the influence of fluid presure from the main cylinder. thus connecting passage I! with the outlet passage II and thereby providing a path of escape for medium from the main apparatus cylinder.

One advantage of the relief valve described is that it possesses ease and rapidity of operation. Furthermore. there is no tendency for any of the moving parts to stick; and the fact that the closure member is forced straight against the seat without rotation is of particular advantage when using high pressures.

Metals and other materials. which cannot be successfully drawn at room temperature, may be hot drawn at much lower than normal temperamres by means of the method and apparatus of the invention. This modification requires only the application of heat to the specimen to be drawn.

The increased ductility of a material under high hydrostatic pressure makes it possible in some instances to draw coiled rod through the die. Such rod straightens itself out in passing through the die and emerges in the same condition as if it had been straight initially. This feature is of considerable practical significance, since it permits an increased length of material to be drawn in a given size hydraulic chamber.

Due to the great increase in ductility, metals drawn under high hydrostatic pressure acquire a similarly great increase in strain hardening. The degree of strain hardening may be determined from the flow stress," which is an increasing linear function of the natural strain (natural logarithm of the ratio of initial area to final area). Because of the great strains induced by drawing under hydrostatic pressure, it is possible to strain harden to flow stresses much greater than the maximum which can be attained by drawing the metal through reducing dies at atmospheric pressure without fracture.

While in this description specific examples of methods and means for carrying out the invention have been described in detail, these examples are given by way of illustration merely, and it is to be understood that the invention is not limited thereby but encompasses all methods and means within the scope oi the appended claims.

What is claimed is:

1. An apparatus for drawing a body of solid plastic material through a die while under the influence of hydraulic pressure, comprising a first hydraulic chamber having at least one fixed end wall; a die coaxially disposed within said first chamber and having the emergent end thereof facing said end wall; passage means connecting portions of said first chamber adjacent said die; a coaxial passage in said end wall at least equal to the diameter of the throat of said die; a second hydraulic chamber communicating with said coaxial passage; medium within both chambers capable of transmitting hydraulic pressure; a relief valve connected to said second chamber; and means for applying pressure to said medium.

2. An apparatus according to claim 1, comprising a spring for urging said die toward said end wall; restraining means for said motion operable by said means for applying pressure to said medium; and blocking means for maintaining a space between said die and said end wall.

3. An apparatus according to claim 2, comprising a die coaxially mounted in the base of a cupshaped member having an outer wall slidably disposed within said first hydraulic chamber, a

a coaxial inner wall having an end protruding beyond said outer wall, and passages interconnecting opposite faces of said base; a collar affixed to the inner wall said hydraulic chamber; said collar having portions slidably surrounding the protruding end of said inner wall of the cupshaped member and passages connecting portions of said cylinder on opposite sides of said collar; a compression spring between said collar and said base restrained by shear pins extending from said collar into said inner wall of said cup-shaped member; and a plunger in said hydraulic chamber capable oi establishing contact with and moving said inner wall of the cup-shaped member to sever said shear pins.

4. An apparatus as claimed in claim 3, comprising a pull rod adapted to be secured to a reduced end portion of a body capable of passing through 8 said die, said pull rod having a larger diameter than the throat of said die and adapted to flt snugly within the coaxial passage in said end wall.

5. An apparatus according to claim 3, comprising a shoulder within said hydraulic cylinder to act as a support for said collar. and a liner adap"- ed to fit within said cylinder, said liner having an externally threaded portion adapted to cooperate with an internally threaded portion of said cylinder to permit clamping said collar rigidly in place.

6. In an apparatus for drawing a body of solid plastic material through a die while the lateral surfaces of said body on each side of said die are subjected to hydrostatic pressure, a pull rod having an axial recess in one end thereof adapted to receive a reduced end portion of said body, said recess having a reduced wall thickness at one or more portions of the circumference capable of transmitting radial force to the body portion within said recess when the wall of said recess is subjected externally to said hydrostatic pressure.

7. In an apparatus for drawing a body of solid plastic material through a die while the lateral suriace portions of said body on each side of said die are subjected to hydrostatic pressure, a cylindrical pull rod having an axial recess in one end adapted to receive a diametrically reduced end portion of said body, the wall thickness of said pull rod being reduced in one or more places over a length substantially equal to the depth of said recess, whereby radial pressure is transmitted to the body portion within said recess when the wall of said recess is subjected to external hydrostatic pressure, and a rounded portion at the other end.

8. An apparatus for passing a body of solid plastic material through a die while under the influence of hydrostatic pressure, comprising a first hydraulic chamber having at least one fixed end wall; a die disposed within said first chamber and having the emergent end thereof facing said end wall; passage means connecting portions 01' said first chamber adjacent said die; a coaxial passage in said end wall at least equal to the diameter of the throat of said die; a second hydraulic chamber communicating with said coaxial passage; medium within both chambers capable of transmitting hydraulic pressure; means for applying pressure to said medium; and a relief valve comprising a housing having a valve chamber communicating with said second hydraulic chamber, said valve chamber having a valve seat and a lateral outlet passage and opening into a pressure chamber; a closure member adapted to cooperate with said valve seat to seal said inlet passage; a plunger comprising two slidably fitted coaxial members of substantially equal diameters separated by compressible packing material capable of being deformed outwardly against the inner walls of said pressure chamber, said plunger having a coaxial boss capable of enterim said valve chamber and forcing said closure member against said seat; and fluid-tight passage means associated with said pressure chamber to actuate said plunger.

9. A method of drawing a body of solid plastic material through a die, which comprises reducing a portion of said body to a cross-section adapted to pass through the throat of said die, passing said reduced portion through the throat of said die. subjecting the lateral surfaces of said body extending on both sides of said die to a hllh hrassaoss 9 drostatic pressure, and applying an axially directed force to said body capable of urging the unreduced portion of said body through said die.

10. A method of drawing a body of solid plastic material through a die, which comprises reducins a portion 01' said body to a cross-section adapted to pass through the throat of said die, passing said reduced portion through the throat of said die, and subjecting the lateral surfaces oi said body extending on both sides of said die and a transverse surface adapted to transmit axial force to said body to a high hydrostatic pressure, said hydrostatic pressure acting against said transverse surface imparting a resultant axial force upon said body capable of urging the unreduced portion of said body through said die.

1'1. A method of drawing a body of solid plastic material through a die. which comprises reducing a portion of said body to a cross-section adapted to pass through the throat of said die, passing said reduced portion through the throat of said die. and subjecting the lateral surfaces of said body on both sides of said die and a transverse surface 01 the unreduced portion of said body to a high hydrostatic pressure, said hydrostatic pressure acting against said transverse surface imposing a resultant axial force upon said body capable of extruding the unreduced portion of said body through said die.

12. A method or drawing a body of solid plastic 10 material through a die, which comprises reducing a portion of said body to a cross-section adapted to pass through the throat of said die, passing said reduced portion through the throat of said die, affixing to said reduced portion a tension member of larger cross-section than the cross-section of said reduced portion, thereby providing a transverse surface adapted to transmit axial force to said body, and subjecting the lateral surfaces of said body extending on both sides of said die, and said transverse surface to a high hydrostatic pressure, said hydrostatic. pressure acting against said transverse surface producing a resultant axial force upon said body capable of pulling the unreduced portion of said body through said die.

PERCY W. BRIDGMAN.

REFERENCES cr'rnn The following references are of record in the file of this patent:

Um'I'ED STATES PATENTS Number Name Date 524,504 Robertson Aug. 14, 1894 524.505 Robertson Aug. 14, 1894 524,506 Robertson Aug. 14, 1894 524,508 Robertson Aug. 14, 1894 1,767,038 Anderson June 24, 1930 1,777,611 Grohek Oct. 7, 1930

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