EP0000762B1

EP0000762B1 - A punch stripper assembly and method of making same

Info

Publication number: EP0000762B1
Application number: EP19780100558
Authority: EP
Inventors: Joseph Sante Didonato; Timothy John Kwiecinski
Original assignee: Houdaille Industries Inc
Current assignee: Houdaille Industries Inc
Priority date: 1977-08-10
Filing date: 1978-07-31
Publication date: 1981-12-23
Also published as: DE2861451D1; US4166403A; CA1086214A; DK350778A; EP0000762A1

Description

The invention relates to the art of guiding punches to workpieces and stripping the punches from the workpieces to prevent damage or distortion of the workpieces. More specifically this invention deals with a plastics punch stripper and guide molded in situ around the active end of a punch within the exiting mouth of a rigid punch guide to a depth or thickness sufficient to support the punch throughout its stroke and with an end face having an inner periphery conforming exactly with the configuration of the punch to engage the workpiece in the area surrounding the punched out slug and lying flush with the periphery of the punched out hole.
From the art of guiding punches the GB-A-1 251 843 discloses a punch stripper assembly having a removable stripper body with a bore receiving the reduced head end portion of the punch and with a spigot of reduced cross section sitting within a counter bore in the lower end of the stripper body. The removable stripper body has its only effective bore completely unsupported peripherally by the lower part of the body. Therefore if the body is made from a plastics material it has no peripheral support when it engages a work piece on the die and it will be squeezed between the die supported work piece and the stripper causing the body to flatten and to deform. When the squeezing of the body also takes place while punching the bore into the plastics material with the head end portion of the punch with which it is to be used it is very difficult to punch a bore which provides an accurately fitting guide for the head end portion of the punch. Such a stripper body can not be provided economically with sufficient close tolerances to provide the desired lateral support for the reduced head end portion of the punch. This is particularly true since the stripper body is made as a separate unit and the possibility of misalignement between the stripper body and the stripper body is therefore apparent.
With respect to DE-C-1 778405 an appratus and a method is disclosed for providing a fitting guide between a guide plate or turret and a punch. The guide plate has a roughly processed bore lined in-situ with a plastics material. Since the plastics material within the bore of the guide plate or turret is obviously the only guide for the punch the plastics material is subjected to stresses which will cause it to flow and to bulge. Further the plastics material provided within the bore of the guide plate obviously does not function as a stripper to prevent bulging or dimpling of the marginal area surrounding the punched hole as the head end portion of the punch is stripped from the work piece.
It is an object of this invention to provide a combined punch guide and stripper formed of plastics material and reinforced by a surrounding rigid punch receiving member.
It is a further object to improve the quality and accuracy of punching operations by guiding the head end portion of the punch right up to its point of entry into a workpiece in an elastomeric plastics material stripper engaging the workpiece and reinforced against displacement or distortion by a surrounding rigid punch guide tube and further by pressing the stripper against the marginal area surrounding the punched hole while the punch is retracted and thus preventing bulging or dimpling of the marginal area as the punch is stripped from the workpiece.
According to the invention there is provided a punch stripper assembly comprising a punch having a cylindrical axial central portion and a reduced head end portion shaped to punch a desired configuration in a workpiece, a spring supported punch guide with the punch guide being a rigid guide tube adapted to slidably receive the cylindrical axial central portion of the punch, said guide tube comprising a molded plastics material stripper slidably supporting the reduced head end portion of the punch and continually engaging the punch head throughout its operating stroke, said stripper having an end face for bottoming on the workpiece to be punched. Said punch stripper assembly being improved in that the plastics material stripper has been moulded in-situ between the end portion and the guide tube and that the exposed end of the stripper has been subjected to a finishing operation rendering the exposed end of the stripper flush with the end face of the guide tube.
It is further provided that said punch guide tube has an internal groove and said stripper has an annular rib seated in said internal groove securing the stripper against longitudinal movement in the punch guide tube.
The method of making a punch stripper assembly with a setting flowable plastics material being cast into the recess between the punch guide and a punch in the guide to form a solid rigid stripper budy locked to the guide, comprises according to the invention the step of providing a punch body having a cylindrical axial central portion with a reduced active end portion; providing a rigid metal constant inner diameter tubular punch guide having an inner diameter substantially equal to the diameter of the central portion; inserting the punch in the tubular guide with the reduced active end portion positioned for projecting through a bottom of the guide, the punch being positioned to place a shoulder joining the reduced active end to the central portion internally of the guide thereby providing a recess internally of the guide between the shoulder and the bottom with the reduced active end of the punch extending at least to the bottom of the guide; in-situ casting a setting flowable plastics material into the recess in the guide at least to the level of the bottom end forming after curing a laterial guide for the active end of the punch during punching operation of the assembly, the cured plastics material being thus formed with a punch receiving opening therethrough substantially codimensional with said active end portion and subjecting the exposed end of the stripper to a finishing operation rendering the exposed end of the stripper flush with the end face of the guide tube.
It is a specific requirement of the method according to the invention to provide the steps of casting the plastics material in the exit end of the guide tube around the reduced head end portion of the punch, retracting the punch head end portion in the resulting casting and providing a smooth end face on the casting to provide a workpiece engaging surface.
The method of forming a punch stripper assembly further includes the step of coating the punch head end portion with a mold release agent before filling the recess.
The plastics material is selected for ease in . molding, toughness, bearing load and guide capacity, resistance to.deformation under stress and impact at the operating temperatures, and capability of being finished to a smooth end face which will not mar the workpiece. The plastics material should be elastomeric to avoid shattering under stress and since it is reinforced by the surrounding rigid metal punch guide, it will not shift. For ease in molding, it can be cast from a viscous hardenable fluid. Both thermoplastic and thermosetting resins are operative.
Urethane-type resins are preferred. The following resins of this type are useful examples:

A urethane elastomer marketed under the trade name K-PRENE by DiAero, Division of Houdaille Industries, Inc., Lake City, Minnesota, the K-100 grade having the following properties is especially useful:

A poly ether polyurethane marketed under the trade name CONAP by Conap Inc. Olean, New York, the UC-22 grade is a liquid which will cure at room or elevated temperatures to provide the following very desired properties:
Thermosetting epoxy casting resins capable of hardening from viscous precurable liquids into hard tough solids either at room or elevated temperatures or with hardeners are also useful and a wide selection is available under trade names such as "EPI-BIS" from Dow Chemical Co., "NOVOLAC" from Union Carbide Corp., "EPI-REZ" from Celanese Co.
Other and further objects of this invention will become apparent to those skilled in the art from the following detailed description of the annexed sheets of drawings which, by way of a preferred example only, illustrate an embodiment of the invention.

Figure 1 is a fragmentary cross-sectional view of a turret punch equipped with a stripper and guide according to this invention and showing the relative positions of the parts in retracted position at the start of a punching step;
Figure 2 is a view similar to Figure 1 but showing the relative positions of the parts just before the punch enters the workpiece;
Figure 3 is a view similar to Figures 1 and 2 but showing the relative positions of the parts after the punch has passed through the workpiece;
Figure 4 is a view similar to Figures 1 to 3 but showing the relative positions of the parts upon retraction of the punch from the workpiece;
Figure 5 is a cross-sectional view along the line V-V of Figure 1;
Figure 6 is a bottom plan view taken along the line VI-VI of Figure 1;
Figure 7 is a somewhat diagrammatic cross-sectional view, with parts in elevation illustrating the casting or molding of the stripper body around the active end of the punch in the punch guide;
Figure 8 is a view similar to Figure 7 but illustrating the grinding of the molded stripper body flush with the bottom end edge of the punch guide;

The assembly 10 of Figures 1 to 4 illustrates one station of a turret punch machine having the molded plastics punch stripper and guide of this invention in position in the assembly. The assembly 10 includes a punch holder or turret 11 having a vertical punch receiving hole 12" therethrough. A tubular punch guide 13 is sus-' pended in the hole 12 by an outturned top flange 14 resting on an apertured platform 15 which is supported by springs 16 on the top of the turret 11. Pins 17 on the turret 11 center the springs 16 and slide through holes in the platform 15.
A punch 18 is slidably mounted in the tubular punch guide 13 and has a reduced diameter upstanding shank 19 slidable through a collar 20 resting on the flange 14 and projecting into the top of the tubular guide 13
A coil spring 21 surrounds the shank 19 and is bottomed on the collar 20 at one end and is compressed by a nut 22 threaded on the other end of the shank 19. The punch 18 is thus spring-suspended in the punch guide 13 from the spring-suspended platform on the turret 11.
A ram 23 engages the nut 22 to activate the punch guide and punch as hereinafter more fully described.
To prevent rotation of the punch 18 in the guide 13 and to prevent rotation of the punch guide 13 in the hole 12, the punch has a laterally extending pin 24. This pin 24 extends through a vertical slot 25 in the punch guide 13 to project into a groove 26 in the hole 12 of the turret 11. The slot 25 opens through the top flange 14 of the guide 13 but has a closed bottom end 27 preventing the punch 18 from dropping through the guide 13.
The punch 18 has a reduced configured active end 28 with a flat bottom end face 29 and a flared top shoulder 30 extending to the full diameter of the punch. This active end 28 of the punch may have any desired configuration and size to form the desired hole in a workpiece W supported on a die 31 of the punch assembly 10. The die 31 has a hole 32 shaped and sized to receive the active end 28 of the punch.
The active end 28 of the punch is slidably guided in a molded plastics member 33 which also serves to strip the punch from the workpiece without damaging the workpiece. This member 33 is integrally bonded in the bottom end of the guide tube 13 with its outer periphery 34 surrounded and supported by the tube which is formed of non-yielding material such as steel.
The guide and stripper member 33 has an inner periphery 35 surrounding the active end of the punch 26 in intimate bearing relationship. This inner periphery 35 has a height greater than the length of the stroke of the punch 18 so that the active end 28 of the punch is always slidably guided by this inner periphery.
The member 33 has a finished smooth end face or edge 36 which is flush with the bottom end 37 of the punch guide 13.
The upper end 38 of the member 33 is flared or beveled from the inner periphery 35 to the outer periphery 34. An annular rib 39 is provided on the member 33 to seat in a groove 40 in the inner periphery of the punch guide 13 to anchor the member against sliding in the punch guide.
In operation of the assembly 10, the nut 22 is tightened to compress the spring 21 thereby raising the punch 18 so that its end face 29 is either flush with or retracted into the stripper 33 at the start of the punching operation and before the ram 23 engages the nut 22 as illustrated in Figure 1. Then, as the ram 23 engages the nut 22, as shown in Figure 2, the thrust of: the ram is applied to the platform 15 through the spring 21 thereby lowering the platform 15 and compressing the springs 16. This lowering of the platform projects the punch guide 13 beyond the turret 11 to seat the end face 36 of the member 33 on the workpiece W around the die hole 32. If this end face 36 is flush with the bottom end edge 37 of the punch guide 13, this end edge 37 will also seat on the workpiece. Thus, as the assembly moves from the starting position of Figure 1 to the position of Figure 2, the workpiece W is finally clamped between the die 31 and the stripper and guide member 33 in the area surrounding the active end 28 of the punch.
As the ram 23 descends to the position of Figure 3, the spring 21 is further compressed to accomodate ejection of the active end 28 of the punch 18 beyond the stripper and guide member 33 and the guide tube 13 with the end 29 of the punch entering the workpiece and cutting a slug S from the workpiece which drops through the die hole 32. This forms a hole H in the workpiece of the exact size and configuration as the leading end 28 of the punch. Since the workpiece W is tightlv clamped against the die 31 in the marginal area thereof surrounding the hole H and since the inner periphery 35 of the member 33 is in intimate conforming bearing contact with the punch and thereby extends right up to the periphery of the hole H, very accurate and clean-cut punchings are obtained.
After the slug S has been punched out of the hole H, the ram 23 is retracted as shown in Figure 4 whereupon the spring 21 raises the punch 18 to withdraw its active end 28 within the punch guide 13 and stripper member 33. The spring 21, however, holds the platform 15 at its lower position to continue to press the stripper member 33 against the workpiece so that the active end of the punch 28 will be withdrawn from the punched hole H while the area of the workpiece surrounding the hole is still clamped tightly against the die. This prevents bulging or dimpling of the workpiece as the punch is stripped from the workpiece.
Following the retraction of the active end 28 of the punch into the punch guide and stripper 33, the ram 23 is raised back to the position of Figure 1 allowing the springs 16 to expand for raising the platform to lift the punch guide 13 back into the turret hole 12 and away from the workpiece.
In accordance with this invention, the guide and stripper member 33 is molded in situ in the punch guide 13 around the active end of the punch 28. Thus, as shown in Figure 7, the punch 18 is positioned in the guide tube 13 so that the active end 28 projects beyond the end edge 37 of the guide tube but with the flared shoulder 30 of the punch below the groove 40 in the guide tube. This provides an open top molding cavity with an outer wall formed by the inner periphery and groove of the guide tube 13, an inner wall formed by the periphery of the active end of the punch 28, and a bottom wall formed by the shoulder 30. A pour tube or funnel 41 may conveniently be mounted on the guide tube 33 with a skirt 42 embracing the end of the tube and a shoulder 43 bottomed on and covering the end edge 37 of the tube. The inner wall of the guide tube or funnel 41 is of the same diameter as the inner wall of the punch guide 13 and with the skirt 42 centering the tube 41 on this punch guide, the open top of the molding cavity is extended beyond the end 37 of the guide tube 13. The end 28 and the shoulder 30 of the punch 18 together with the interior wall of the funnel tube 41 are coated with a release coating 44 and a viscous hardenable resin is then poured through the tube 41 to fill the mold cavity to a level above the end edge 37 of the guide tube 13. As shown in Figure 7, the open top mold cavity C is filled with the viscous flowable resin R to a level just above the top end edge 37 of the guide tube 13.
After the resin R filling the mold cavity C has hardened, the tube 41 is removed and the punch 18 retracted in the guide 13. The top end face of the molded resin is ground flush with the end edge 37 of the guide tube 13 as shown in Figure 8.
A grinding wheel illustrated at 45 can conveniently form the flat active end face 36 of the guide and stripper member 33.
Since the guide and stripper 33 is formed in situ in the punch guide and around the active end of the punch, it becomes an integral fixed part of the punch guide tube 13 and conforms exactly with the configuration and size of the . active end of the punch. The rigid punch guide enveloping the molded plastics member 33 prevents lateral shifting or deformation of the plastics material but since the preferred material is an elastomer, it can flow under excessive stresses to avoid shattering under high impact loads. The active end face 36 of the plastics member 33 is polished smooth so as not to mar or scratch the workpiece W. The end edge 37 of the guide tube 13 may be polished.
From the above descriptions, it should be understood that this invention provides a punch assembly with a molded plastics material stripper that also acts as a guide for the active end of the punch and is reinforced against lateral displacement by a surrounding rigid punch receiving member.

Claims

1. A punch stripper assembly comprising a punch (18) having a cylindrical axial central portion and a reduced head end portion (28) shaped to punch a desired configuration in a workpiece, a spring supported punch guide with the punch guide being a rigid guide tube (13) adapted to slidably receive the cylindrical axial central portion of the punch, said guide tube (13) comprising a molded plastics material stripper (33) slidably supporting the reduced head end portion (28) of the punch (18) and continually engaging the punch head throughout its operating stroke, said stripper (33) having an end face (36) for bottoming on the workpiece to be punched, characterized in that - the plastics material stripper (33) has been molded in-situ between the end portion (28) and the guide tube (13) and that the exposed end of the stripper (33) has been subjected to a finishing operation rendering the exposed end of the stripper flush with the end face of the guide tube (13).

2. The assembly of claim 1, characterized in that, - said punch guide tube (13) has an internal groove (40) and said stripper (33) has an annular rib (39) seated in said internal groove (40) securing the stripper against longitudinal movement in the punch guide tube.

3. The method of making a punch stripper assembly with a setting flowable plastics material being cast into the recess between a punch guide and a punch in the guide to form a solid rigid stripper body locked to the guide, characterized by the steps of - providing a punch body having a cylindrical axial central portion (18) with a reduced active end portion (28); - providing a rigid metal constant inner diameter tubular punch guide (13) having an inner diameter substantially equal to the diameter of the central portion; - inserting the punch in the tubular guide (13) with the reduced active end portion (28) positioned for projecting through a bottom (37) of the guide, the punch being positioned to place a shoulder (30) joining the reduced active end (28) to the central portion (18) internally of the guide (13) thereby providing a recess (C) internally of the guide (13) between the shoulder (30) and the bottom (37) with the reduced active end of the punch extending at least to the bottom (37) of the guide - in-situ casting a setting flowable plastics material (R) into the recess in the guide at least to the level of the bottom end (37) forming after curing a lateral guide for the active end (28) of the punch during punching operation of the assembly, the cured plastics material being thus formed with a punch receiving opening therethrough substantially codimensional with said active end portion (28) and then subjecting the exposed end of the stripper (33) to a finishing operation rendering the exposed end of the stripper flush with the end face of the guide tube (13).

4. The method of claim 3, characterized by - casting the plastics material in the exit end of the guide tube around the reduced head end portion of the punch, retracting the punch head end portion in the resulting casting and providing a smooth end face on the casting to provide a workpiece engaging surface.

5. The method of claim 3, characterized by, including the step of coating the punch head end portion with a mold release agent before filling the recess.

6. The method of claim 3, characterized by - casting the plastics material to a level beyond the end face of the guide tube (13).