Patent Publication Number: US-RE29958-E

Title: Punch and stripper assembly

Description:
This is a reissue application of U.S. Pat. No. 4,031,787 which issued on June 28, 1977. .Iaddend. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a punch and stripper assembly for use in press-type of machine tools. 
     2. Prior Art 
     It has long been the practice, which still continues, to utilize a stripping spring in a punch and stripper assembly where the stripper spring is permitted to expand to its free length after a power stroke. Such practice causes the spring to be stressed from zero to a maximum, which minimizes spring life. Further, the length of the assembly cannot be absolutely controlled because of the inherent differences in the lengths of the springs of a production lot, and also because the free length of springs tends to change with flexing, particularly when a spring is new. When punch and stripper assemblies are utilized in contemporary high performance equipment, the tooling length or envelope must be controlled. An example of a punching device having a punch and stripper assembly of such prior type is shown in U.S. Pat. No. 2,953,051. 
     To provide a preload on a stripping spring is known, as shown in U.S. Pat. No. 3,079,824. However, the prior types of punch and stripper assemblies that do have a preload on the stripping spring are so constructed that special tooling is needed to order to put the assembly together or to disassemble it for service, such as needed to replace a broken spring. 
     SUMMARY OF THE INVENTION 
     The punch and stripper assembly of the invention includes a punch having an intermediate shoulder, an enlarged head threaded on an end of the punch, a compression spring which is preloaded by the adjustment of the head which engages the ram, the other end of the spring being engaged by a washer, a retainer acting between the punch shoulder and the washer, and a stripper sleeve for acting between the workpiece and the retainer. In a preferred form, the retainer is a split ring having separate sections retained together by being at least partially disposed in a counterbore in the washer. Further, the retainer preferably includes a cylindrical projection so that when the retainer is inverted, the effective position of the punch is altered. 
     Accordingly, it is an object of the present invention to provide an improved punch and stripper assembly. 
     Another object of the present invention is to provide a punch and stripper assembly having a preloaded stripping spring. 
     Another object of the present invention is to provide a punch and stripper assembly wherein its effective length or envelope size can be adjustably controlled. 
     A still further object of the present invention is to provide a punch and stripper assembly which can be assembled and disassembled without use of special tools even though the stripping spring is preloaded. 
     Yet another object of the present invention is to provide a punch and stripper assembly wherein effective length thereof can be controlled even though there has been considerable shortening of the punch as a consequence of regrinding to sharpen the punch. 
     Many other objects, features and additional advantages of the present invention will become apparent to those skilled in the art from the drawing and the detailed description relating thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of punch and stripper assembly provided in accordance with the present invention; 
     FIG. 2 is a similar view of the punch and stripper assembly of FIG. 1, but shown after having been readjusted as a consequence of punch sharpening; 
     FIG. 3 is a plan view of the device shown in FIGS. 1 and 2; and 
     FIGS. 4 and 5 are elevational views of modified forms of the part shown in FIG. 3. 
    
    
     AS SHOWN ON THE DRAWINGS 
     The principles of the present invention are particularly useful when embodied in a punch and stripper assembly such as shown in FIG. 1, generally indicated by the numeral 10. A device or assembly of this type can be used in numerous types of punch presses such as turret presses, single station presses, and sub-press tooling. In such type of punch press tooling, a punch ram (not shown) acts on the upper end of the punch and stripper assembly 10 to force it against a workpiece (not shown), a reactive force being provided by a suitable die (not shown) disposed on the opposite side of the workpiece, all as is well known. Further, with tooling of this type, some type of lifting means is employed, schematically indicated by a springloaded adapter II which holds the punch and stripper assembly 10 in an elevated position after the ram has retracted therefrom in its return stroke. 
     The punch and stripper assembly 10 includes a punch 12 having one end 13 adapted for cutting or forming, and thus for coacting with the die. The punch 12 has an enlarged intermediate body portion 14 by which the punch is guided during reciprocation, there being a shoulder 15 which faces away from the end 13 and from which a reduced diameter portion 16 extends. The upper end of the punch 12 is a threaded end portion 17, there being a second shoulder 18 facing away from the cutting end 13. An enlarged head 19 is threaded and is carried on the threaded end portion 17, it having an upper surface 20 which is directed to be engaged by the ram. A set screw 21 is carried by the head 19 for acting against the punch 12 to hold the head 19 in a selected position. The head 19 has a pair of oppositely facing flat surfaces 22 which facilitate the selective positioning of the head 19 on the punch 12. 
     A compression spring 23 has an upper end which engages the lower surface of the head 19, the lower end of the spring 23 engaging against a washer 24 which has a bore 25 and a counterbore 26. A retainer 27 acts between the punch shoulder 15 and the washer 24, and such engagement is maintained at all times by the force from the spring 23. The retainer 27 comprises a split ring that has a pair of separate sections 28, 29, best seen in FIG. 3, which, as shown in FIG. 1 extend at least partially into the counterbore 26 in the washer 24. As shown in FIGS. 1 and 3, the retainer 27 has a cylindrical projection 30 which extends into the bore 25. Thus the cylindrical projection 30 is of lesser diameter than that of the counterbore 26 and projects from the retainer 27 in only one direction, such projection being axially beyond the counterbore into the bore 25 in the washer 24. The assembly 10 further includes a stripper sleeve 31 which has a lower end 32 for engaging the workpiece and which surrounds the end portion 13 of the punch 12. At its upper end 33, the sleeve 31 engages the retainer 27. Adjacent to the end 33, the sleeve 31 has a groove in which a second retainer 34 is disposed. The second retainer 34 is in the form of a wire spring which substantially encircles the sleeve 31, the retainer 34 having one end 35 that extends through an opening in the sleeve 31. When the punch and stripper assembly 10 is being handled separately from the punch press, the end 35 of the retainer 34 acts against the shoulder 18 to hold the assembly in assembled relationship. The retainer 34 thus firmly engages and encircles the sleeve 31 adjacent to the first retainer 27 and provides a means against which a lifting force can be applied to the assembly 10. 
     The assembly is put together in the following manner: The head 19 is threaded onto the end portion 17 of the punch 12 with only approximately two threads engaging. The spring 23 is then slipped on as is the washer 24. The retainer 27 is then put into position about the reduced diameter portion 16, the washer 24 then being disposed nearer the upper end of the punch. The head 19 is then tightened which can be done by hand or with the assistance of a non-special hand tool, and the tightening is continued until the upper end of the punch 12 almost reaches the upper surface 20 of the head 19. The is the position illustrated in FIG. 1. The stripper sleeve 31 is then slipped over the other end of the punch, and then the second or spring retainer 34 is snapped into position. The assembly 10 is then ready for use in a punch press. 
     The assembly is assembled with ease and it has a predetermined length, thus facilitating its use in presses where the length must be accurately established. In that the spring 23 is preloaded, maximum spring-life is obtained. 
     In place of the retainer 27, a split retainer 36 (FIG. 4) may be utilized. Prior to assembly, the retainer 36 has an open gap that permits it to be passed over the reduced diameter portion 16 of the punch 12. Thereafter, the retainer 36 is deformed so that it has a sliding fit with the portion 16. Normally the retainer 36 is not thereafter removed from the punch 12. 
     Another form of retainer is shown at 37 in FIG. 5. Here the retainer is in the form of a solid ring which had an initial diameter such that it could be slipped over one of the ends of the punch 12. After it is in a position of registration with the reduced diameter portion 16, it too is deformed, as by swagging, to be permanently carried on the punch 12. 
     In a typical installation, the punch 12, when new, will enter a new die by 0.062 inches. As a consequence of usage, the tip 13 will need to be sharpened, and thus shortened by whatever length is ground off. If 0.010 inch is ground off, then the head 19 can be backed off by a corresponding amount, thus maintaining the effective length of the punch. However, in a typical installation, the head can be backed off only so much from the initial illustrated position, 0.053 inch being a typical maximum amount. Assuming that 0.053 inch has been ground off, the head 19 is then backed off a corresponding amount. However, this amount, the maximum permitted by the ram location, still provides 0.072 inch preload on the spring. If the retainer 36 or 37 is used, after there has been 0.053 inch removed from the punch tip, the punch would have reached the end of its usable life. Even with a grind increment of 0.053 inches, die penetration of at least 0.009 inch will be assured because such grind increment is less than the initial die penetration. 
     However, according to the preferred form of the invention, when the retainer 27 is used, it can be inverted as shown in FIG. 2 to provide a second grind increment of 0.053 inches, thus doubling the effective life of the punch. While FIG. 1 illustrates the assembly adjusted for a new punch, FIG. 2 illustrates the same assembly adjusted for a punch that has had its first maximum grinding increment removed. With the retainer 27 inverted, the cylindrical projection 30 then exceeds into the sleeve 31 so that the shoulder 15 is arrested that much sooner. However, the upper end 33 of the sleeve 31 still engages the retainer 27 at a point which is substantially flush with the bottom of the washer 24. Thus FIG. 2 shows the punch tip 13 that has been shortened and shows the head 19 backed off the threaded end portion 17 by a corresponding amount. 
     Although various minor modifications might be suggested by those versed in the art, it should be understood that I which to embody within the scope of the patent warranted hereon, all such embodiments as reasonably and properly come within the scope of my contribution to the art.