US 3108502 A
Description (OCR text may contain errors)
Oct. 29, 19 H. B. CHATFIELD 3,108,502
' PUNCH AND DIE ASSEMBLY Filed Oct. 22, 1959 2 Sheets-Sheet 1 INVENTOR Henry B. Chatf'ield m 91% f a m ATTORNEYS Oct. 29, 1963 I H. B. CHATFIELD 3,108,502
v PUNCH AND DIE. ASSEMBLY Filed 001;. 22, 1959 2 Sheets-Sheet 2 wg mfzzm ATTORNEYS United States Patent On ice Siddfidd Patented st. 29, 1963 3,108,502 PUNCH AND DIE ASSEMELY Henry B. Chatiield, Metaflo Research, R0. Box 2192, Santa Ana, Calif. Filed 0st. 22, 1959, Ser. No. 843,064 6 Claims. (U1. 78-9) This invention relates to punches for forging or compressing materials and more particularly to punches for compacting or cold forming material in a die.
The cost of some operations, particularly the compacting or cold forming of relatively small cup-shaped articles of steel, hard metal or other materials which require the application of extreme pressures has been prohibitive because of the high initial cost and short life of punches which must be made of a very hard steel or carbide and because of the necessity for slow operation.
Punches of the type referred to are commonly formed with a work engaging nose portion, a body portion of larger diameter than the nose portion and a head larger than the body portion which is mounted in a carrier which advances and retracts the punch. Such punches have been made by turning down a metal bar of the diameter of the head portion of the punch. The diffi'cult machining operations and especially the waste of expensive material makes such punches quite costly. These punches have also been very susceptible to fracture. Such failures have often been attributed to the tension stresses incident to stripping the punch from the work but it is believed that they have been primarily due to fatigue caused by vibrations concentrated at points of changing cross section even through the shoulders be smoothly rounded.
The present invention essentially obviates the problem of fatigue failure by providing a smooth surface punch that is of substantially uniform cross section from end to end and by providing a mounting head portion in the form of a press fitted collar or bushing composed of a material of considerably lower hardness than the material of the punch and of substantial thickness. The punch may have a compressive strength rating above 500,000 p.s.i. while the collar will have a compressive strength much lower than the punch and a tensile strength that is much higher.
The uniform cross section and smooth external surface of the punch insures that the vibrations of greatest intensity created by impact shocks wiil be adjacent the ends of the punch and the collar or bushing serves as an effective vibration dampener at the upper end of the punch. The mass acted upon by the punch, being softer than the .material of the punch, provides an effective vibration dampener during the forming operation. The-re is thus provided a punch of uniform cross section substantially free from shoulders or notches intermediate its ends so that the shock waves of greatest intensity will be adjacent its opposite ends where the energy of vibration is effectively absorbed by the mounting collar and by the material being formed.
A punch traveling at uniform velocity and coming into contact with a rigidly supported block of material to be formed would have instantly imposed thereon a load approximately its ultimate compressive strength. In order to protect the machinery from destructive shodks due to such impacts it has been found necessary either to so shape the slugs of material that are acted upon that they will gradually deform and act as shock absorbers, or to provide massive presses operating at extremely slow speeds. Either of these methods is economically burdensome. The shaping of the metal blank adds to the cost of production and is not always feasible. The slow press operation makes the cost of producing small fongings such as blind rivets, valve caps or the like prohibitive.
The present invention makes possible the use of relatively light weight rapidly moving machinery in the manufacture of small forgings by providing a thrust transmitting means between the punch and its actuator which effectively absorbs shocks of impact and insures gradual imposition of the axial thrust on the punch.
A stack of thin, relatively hard, flat, resilient disks are interposed between the thrust transmitting means and the punch. The disks should be of a hardness such that the compressive stresses to which they are subjected will not be beyond the elastic limit of the material of the disks and cause them to be plastically deformed.
The stack of :disks is loosely supported so that between operations there is opportunity for air to enter the spaces between the disks. The space between the disks may be of an extremely small order of magnitude. In many instances a spacing of from .0002 to .0003 inch per disk has been found adequate although a space of from .0005 to .0007 inch is preferred. Normal surface irregularities of the flat rolled plate from which the disks are punched and surface irregularities due to hardening treatment are ordinarily sufiicient to provide the necessary spacing.
Because the disks are not absolutely flat, they will separate and allow a layer of air to enter between them Whenever the squeeze pressure is released. When the punch is advanced into engagement with the work, the fluid films admitted to the spaces between the disks upon the previous retraction of the punch, are subjected to pressure tending to create radial outward flow of fluid from between the plates. The pressure necessary to create the rapid how of fluid increases very rapidly as the gap between the disks becomes minute, so that the compressive thrust is gradually applied to the punch and excessive impact shocks are prevented even when the punch is rapidly moved into engagement with the work.
The cushioning action is dependent upon the number of disks rather than their thickness, but space requirements usually require relatively thin disks.
The sum of the spaces between the disks determines the lost motion between the punch and its actuator and the thickness of the disks determines the length of the space between the punch and the pressure applying portion of the punch carrier. Ordinarily hardened steel disks of from inch to /8 inch in thickness are satisfactory, but in some instances because of severe space restrictions, much thinner disks have been employed. The number of disks required will be dependent upon various factors such as the pressure to which the punch is subjected, the size of the disks and the average spacing between the disks. In some instances five disks may be suiiicient, in others as many as forty disks may be desirable.
Since the pressure applied to a cushioning disk varies inversely as the square of its diameter, it is desirable that the disks be of a diameter greater than that of the punch, so that disks of a metal that is of less hardness and of less compressive strength than the punch may be employed.
In order to accommodate disks of a diameter materially greater than that of the punch, an anvil of greater diameter than the punch may be interposed between the punch and cushioning disks. The anvil should be of a material of suflicient compressive strength to with-- stand without permanent deformation the maximum pressure to which the punch is subjected and may be of the same material as the punch.
Since the compression per unit of area onthe disks varies inverselyas the square of their diameter, it is obviously possible to employ much softer material for the disks if disks of large diameter are employed. If their diameter is sufliciently large, disks of resilient plastic material may be employed.
Failure of cold forming forging punches is often due to high bending or lateral nonaxial forces that act upon them while they are highly loaded because of the difficulty of maintaining perfect geometrical alinement of the punch and die. The present invention alleviates this difficulty by providing a die with a chamfer to facilitate entry of the punch and by mounting the punch to float radially in its carrier, the punch being centered by springlike radial forces that are sufiicient to overcome the gravitational and inertial forces to which the punch is subjected but which permit the punch to be shifted bodily and be guided by the die into axial alinement with the die. The punch is supported in its carrier for slight bodily lateral movements by a resilient centering means which may be in the form of an endless helical wire garter spring.
Objects of the invention are to provide means for protecting punches of the cold forging type by providing means for dampening vibration set up in the punch by impacts of the punch against the work, by providing means for absorbing the shocks due to such impacts and by providing means for eliminating severe non-axial stresses in the punch.
Reference should be had to the accompanying drawings forming part of this specification, in which:
FIGURE 1 is a vertical section showing an extrusion punch and die assembly with the punch retracted;
FIG. 2 is a vertical section showing the punch and die assembly with the punch at the end of its forming stroke;
FIG. 3 is a horizontal section through the die taken on the line indicated at 33 in FIG. 1;
FIG. 4 is a vertical section showing a compacting punch and die assembly embodying the invention with the punch retracted; and
FIG. 5 is a vertical section showing the punch at the end of its compacting stroke.
The machine of the present invention comprises a punch assembly A and a die assembly B that are mounted for relative axial movement to bring the punch into and out of engagement with the die. A punch 1 that is of uniform cross sectional shape from end to end and of substantially uniform diameter is provided with a collar 2 at its upper end that is press fitted or shrunk upon the punch and that is preferably of a diameter at least 50% greater than that of the punch. The collar 2 is flush with the upper end of the punch 1 and is composed of a material softer than that of the punch 1. For example, the punch may be formed of extremely hard tool steel while the collar 2 is made of a much softer steel of high tensile strength. While the punch 1 is of substantially uniform diameter in cross section from end to end, it may have a cross sectional form other than circular. The punch is carried by a suitable support such as a press head 3 and is mounted in a tubular holder and guide 4 that is fixed to the head 3. The upper end of the tubular holder 4 has an axial bore 5 in which the collar 2 is received. The punch 1 and collar 2 are supported by parallel tie rods 6 which are parallel to the axis of the punch and that pass through the collar 2, an anvil 7 that rests upon the top of the punch 1 and a series of disks 8 that are interposed between the anvil and the head 3. The collar 2, anvil 7 and disks 8 are slidable on the tie rods 6 and have slight movement with respect thereto when the punch 1 is subjected to pressure. A thick flat disk 9 of a metal harder than that of the head 3 may be interposed between the stack of disks 8 and the head 3 to protect the metal of the head against deformation. The disks 8 may be formed by punching from a rolled steel sheet and afterwards hardened by heat treatment. Such disks are not perfectly flat and when stacked there are spaces between the disks which may be filled with air when the pressure on the punch is released.
At its lower end the holder 4 is provided with a counterbore 10 in which is fixed a tubular guide member 11 that is provided with an axial bore 12 through which the punch 1 extends. At its upper end the guide member 11 is provided with a counterbore 13 that provides an internal stripping shoulder 14. The lower end portion of the punch 1 is received within a follower sleeve 15 within which the punch 1 has a sliding fit, the sleeve 15 being received with a slight clearance in the bore 12 of the guide member and being provided with a collar 16 secured to its upper end that has a circumferential flange 17 that rests upon the shoulder 14. A coil spring 18 is interposed between the flange 17 and the underside of the collar 2 and serves to normally hold the follower sleeve 15 in its lowermost position and to press the punch 1 and the collar 2 upwardly against the anvil 7 and disks 8. The loose mounting of the follower 15 in the bore 12 permits the follower and punch to have limited bodily movements in a direction transverse to their axis Within the bore 12. The collar 2, anvil 7 and disks 8 also preferably have lateral play in the bore 5.
Below the shoulder 14 a centering ring 19 surrounds the tubular follower 15 and maintains yielding radial thrusts thereon that are distributed circumferentially so as to normally retain the follower and punch centrally within the bore 12. The ring 19, which may be in the form of a helically coiled wire garter spring, is mounted in an annular recess 20 opening to the bore 12, the recess 20 being of uniform depth throughout its circumference and of an internal diameter to hold the ring 19 in engagement with the follower 15 throughout the circumference thereof. The ring 19 is yieldable to permit slight bodily movements of the follower and punch and the loose fit of the collar 2 and anvil 7 facilitates such movements.
The die assembly B includes a tubular die 21 positioned to receive the punch and follower, a die case 22 that is mounted in a suitable support such as a press bed 23 and a longitudinally tapered collar 24 for applying an initial compression to the die 21. The die bottom may be formed by a knockout block 25 that may be actuated by a plunger 26 extending through the bed 23 to eject a formed article from the die. The tubular die 21 is of an internal diameter to slidably receive and guide the follower 15 and is provided with a chamfered upper edge 27 to guide the follower and punch into the die. By reason of the lateral yieldability of the centering means for the punch and follower, they are readily guided into the die 21 and brought into axial alinement with the die if initially misaligned. The punch and die are thus relieved of severe stresses which might otherewise be caused by high non-axial forces acting thereon due to misalignment.
The punch and die assembly shown in FIGS. 1, 2 and 3 is designed for backward extrusion and, as shown in FIG. 2, a block of metal in the die 21 is caused to flow backwardly between the punch and die to form the tubular wall of a cup-shaped forging, the follower 15 being forced upwardly with respect to the punch by the extruded metal against the yielding thrust of the spring 18.
The heavy endwise pressure exerted on the punch 1 during the forming operation is transmitted through the anvil 7 to the disks 8 which serve to cushion the impact stroke. The disks 8 are not perfectly flat and when stacked there are spaces between the disks which may be filled with air. When pressure is applied to the stacked disks compression of the stack is resisted not only by the resilience of the metal of the disks, but also by the pressure of the fluid films between the disks. When the punch contacts the work, pressure on the stack of disks tends to squeeze the films of air out from between the disks and, since the pressure is high, the initial flow of fluid from between the disks is extremely rapid.
The thickness of the disks 8 is dependent upon the Space requirements. It is desirable to provide a substantial numbers of disks in order to increase the cushioning action of the fluid films. The thickness of the disks may vary greatly, except that it is desirable that the thickness of the disks be accommodated to the space restrictions. It is desirable that the space between the disks be of extremely small magnitude. The average distance of the space between disks should be less than .001 and is preferably from .0005" to .0007", although in some instances a space as small as .0002" has been found to be adequate.
Usually the disks 8 are from /8" to A in thickness, but if the space restrictions require it, much thinner disks may be used. For example, a .003" thick precision rolled cold worked Swedish flapper valve shim stock which has a hardness of approximately 52 Rockwell C and a yield strength of approximately 280,000 p.s.i. has been successfully used.
Exact uniformity of spacing of the disks 8 is not essential. The space between the top of the anvil 7 and the bottom press head 3 may be sufiicient to accommodate a given number of disks of a given thickness with an aver-age spacing between them that is within the desired limits.
The anvil 7 and disks 8 should have a loose fit within the bore 5 and should be loosely held between the anvil 7 and the head 3. However, if the irregularity of the disk faces is such that an undesirable amount of lost motion between the anvil and piston is provided the disks may be placed under an initial axial compression to reduce the average spacing between them.
Since the pressure per uni-t of area exerted on the disks 8 is approximately equal to the pressure per unit of cross sectional area imposed on the punch 1 times the ratio of the cross sectional area of the punch to the area of the faces of the disks 8, it will be apparent that the hardness of the disks may be less if the diameter of the disks is increased. For example, if the diameter of the disks is twice the diameter of the punch 1, the pressure per unit of area exerted on the disk faces will be about onequarter the pressure per unit of area exerted upon the punch. If desired the diameter of the disks 16 may be enough to permit the use of relatively soft metal or even resilient plastic materials for the disks, the only requirement being that the material of the disks be resilient and of sufficient compressive strength to sustain the pressure to which they are subjected without plastic deformation.
The punch 1 and die 21 may be formed of materials of the high compressive strength necessary to form or cold forge articles of very hard metals or other materials. Both the punch and die may be formed of material having a compressive strength rating above 500,000 psi.
The mounting collar 2 may be formed of a steel softer than the mate-rial of the punch 1 and having a compressive yield point lower than 350,000 p.s.i.
By reason of the vibration dampening action of the collar 2 at the upper end of the punch and the vibration dampening action of the metal into which the punch is driven, the punch 1 is very effectively protected against fracture due to vibrations set up in the body of the punch. By reason of the effective shock absorbing action of the stack of disks 8, the severity of impact shocks is very greatly lessened. It has been found that punches constructed and mounted in accordance with the present invention can be operated at speeds much higher than heretofore practical and that the failure of punches due to breakage is practically eliminated, so that production can continue until a punch has worn down to a size less than that required to produce articles having dimensions within the tolerance permitted.
This invention also makes practical the cold working of metals of such hardness that'it has heretofore not been possible to extrude them commercially. For example, a stainless steel alloy having a hardness such that static pressure of approximately 550,000 p.s.i. was required to induce or initiate flow has been form-ed by means of a tungsten carbide punch with 6% cobalt having a compressive strength rating of 663,000 psi.
In FIGS. 4 and 5 an assembly for compacting material in a die is shown. In this modification the die assembly may be identical with that shown in FIGS. 1 to 3, but the punch assembly is somewhat simpler because of the omission of the follower. As shown in FIGS. 4 and 5, a punch 31 provided with a vibration dampening collar 32 is mounted in a holder 33 attached to a suitable support such as a press head 34. The holder 33 is provided with a bore 35 at its lower end in which the punch 31 has slight lateral play, the upper end of the tubular holder 33 being provided with a counterbore 36 that provides an internal shoulder 37 at its lower end upon which the collar 32 normally rests. An anvil 38 has a tapered lower end 39 and a bottom face 40 that is of the same diameter as the punch 31. A series of cushioning disks 41 are mounted in the counterbore 36 above the anvil 38 and engage with the under side of a block 42 which may be interposed between the upper end of the holder 33 and the press head 34. A centering ring 43 in the form of a garter spring is mounted in an annular recess 44 in the bore 35 and serves to yieldably center the punch 31 which is guided into the die 21 by means of the chamfer 27 in exactly the same way as a follower 15 in the modification above described.
It is to be understood that in accordance with the provisions of the patent statutes, variations and modifications of the specific devices herein shown and described may be made without departing from the spirit of the invention.
What I claim is:
1. In a forming press, the combination with an axially movable punch having a terminal impact receiving end face, of a carrier for said punch, said carrier having a thrust applying member spaced axially from said punch, a rigid thrust transmitting member engaging said impact face from the center to the margin thereof, and shock absorbing means interposed between said thrust transmitting and said thrust applying members consisting of a series of resilient metal disks stacked in face to face contact, said disks having slight irregularities which provide limited areas of contact between the disks and thin spaces for entry of fluid between opposed faces, said irregularities being reduced to lessen the volume of said spaces and expel fiuid from between said disks as the punch actuating pressure is applied, the material of said disks having sufiicient compressive strength to sustain the punch actuating pressure without plastic deformation, said thrust applying and thrust transmitting members having disk engaging faces that conform to the faces of the disks which they engage, the average spacing of the disks of said stack being at least two ten thousandths of an inch and not greater than one thousandth of an inch.
2. The combination as set forth in claim 1, in which the disks are of a diameter greater than that of the punch and in which the thrust transmitting member has a disk engaging face of a diameter substantially the same as that of the disks.
3. The combination as set forth in claim 2, in which the diameter of said disks is at least twice the diameter of said punch.
4. A forming press as set forth in claim 1 in which said punch has a cylindrical body which is flat at each end face and is substantially uniform in cross section from one end face to the other, and in which a vibration dampening collar is tightly fitted to the periphery of said body adjacent its impact-receiving face, said punch being formed of a material having a compression yield point above 500,000 p.s.i., said collar being composed of a steel having a compression yield point less than 350,000 psi. and having an end face flush with said impact receiving face.
5. A forming press as set forth in claim 1 including a die for receiving the work engaging end of said punch opposite said impact receiving end face, said punch being movable axially with respect to the die and into said die, a holder for said punch in which the punch is loosely mounted for axial movements and for limited bodily movements transverse to the axis of the punch, and resilient means mounted in said holder and interposed between said holder and the periphery of said punch for yieldably centering the punch in the holder, said die having a tubular punch guiding wall chanifered at its punch receiving end for guiding the punch into the die and for axially aligning the punch with the die.
6. A forming press as set forth in claim 5 in which an elongated follower sleeve has a sliding fit on the punch, a spring pressing said sleeve toward the die entering end of the punch, and means for limiting the axial movement of the sleeve toward said end of the punch to position the sleeve for engagement with the chamfered end of the punch guiding wall of the die.
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