Abstract:
A punch assembly for a hydraulic punch press machine that affords the ability to compensate for punch point metal loss due to resharpening by facilitating accurate repositioning of the punch point relative to an aperture in a stripper plate through which the punch point member projects during a stamping stroke of the punch assembly.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to high-speed automated punch press for processing sheet metal and more particularly to the design of a punch assembly used in such machines. 
     II. Description of the Prior Art 
     In producing various products from sheet metal, it is often necessary to punch out predetermined hole patterns in the sheet metal workpiece. This is commonly done by a machine that positions a punch assembly and then employs a hydraulic ram to drive a sharpened punch point member of a predetermined shape through the workpiece and into a die underlaying the workpieces, thereby creating an aperture through the sheet metal of the predetermined shape. 
     After repeated strokes of the punch, the punch point can become dull and requires sharpening so that it is able to produce well-defined apertures. Repeated sharpening cycles remove metal from the punch point so that it can no longer punch through the workpiece and only serves to distort the workpiece, resulting in an undesired scrap rate. In most punch assemblies, when repeated sharpenings have been performed, it becomes necessary to replace the punch assembly&#39;s punch point with a new one, which is a relatively costly solution. Thus, a need exists for a way to increase the useful life of a punch assembly&#39;s punch point. The present invention satisfies this need. 
     Prior art metal punch assemblies have relied on the use of urethane strippers to allow punching close to non-flat workpiece surfaces. Such urethane strippers, however, suffer from the fact that they are incapable of accurately holding the workpiece during punching, resulting in misalignment of the punched hole. Also, urethane strippers are subject to rapid wear. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a punch assembly for a punch press machine where the punch assembly has a shank centrally located on an integrally formed circular alignment ring that serves to orient the punch assembly within the punch press in alignment with its ram. Coupled to the alignment ring by shoulder bolts is a stripper plate, the shoulder bolts permitting limited reciprocal displacement of the alignment ring toward and away from the stripper plate. Disposed between the alignment ring and the stripper plate are a plurality of compression springs that normally urge the stripper plate away from the alignment ring. Fastened to the undersurface of the alignment ring and movable therewith is a generally rectangular shim block that has a slot formed in its undersurface in which a punch point member is fastened. More particularly, the punch point member has an upper portion fastened within the slot of the shim block and a lower sharpened portion displaceable through a correspondingly-shaped aperture formed through a thickness dimension of the stripper plate. As the punch point member becomes shorter due to repeated sharpening, in accordance with a first embodiment, shim stock can be inserted between the undersurface of the adjustment ring and an upper surface of the shim block, thus placing the sharpened portion of the punch point member even with the bottom edge of the aperture formed through the thickness dimension of the stripper plate. In a second embodiment, a threaded connection between the shank and the alignment ring allows adjustment of the punch point member to accommodate for metal loss due to resharpening. 
     The use of a steel stripper plate along with mechanical springs offers distinct advantages over the use of urethane strippers as in the prior art. First, mechanical springs far outlast urethane and the use of a metal stripper plate is found to be superior in the ability to hold the workpiece in place during punching for more accurate hole placement. By proper design, as in the present invention, the stripper plate further serves as a guide for the punch point, a feature not achievable with urethane strippers. 
     The foregoing features and advantages of the invention will become apparent to persons skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which like numerals in the several views refer to corresponding parts. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first embodiment of the punch assembly of the present invention; 
         FIG. 2  is a cross-section view taken along line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a cross-section view taken along line  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a cross-section view taken along line  4 - 4  in  FIG. 1 ; 
         FIG. 5  is an enlarged, exploded perspective view of the punch assembly of  FIG. 1 ; 
         FIG. 6  is a plan view of an alternative stripper plate; 
         FIG. 7  is a perspective view of a second embodiment of the punch assembly of the present invention; 
         FIG. 8  is a top plan view of the embodiment of  FIG. 7 ; and 
         FIG. 9  is a cross-section taken along line  9 - 9  in  FIG. 8 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This description of the preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative teens such as “lower”, “upper”, “horizontal”, “vertical”, “above”, “below”, “up”, “down”, “top” and “bottom” as well as derivatives thereof (e.g., “horizontally”, “downwardly”, “upwardly”, etc.) should be construed to refer to the orientation as then described or as shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “connected”, “connecting”, “attached”, “attaching”, “join” and “joining” are used interchangeably and refer to one structure or surface being secured to another structure or surface or integrally fabricated in one piece, unless expressively described otherwise. 
     Shown in  FIG. 1  and identified generally by numeral  10  is a preferred embodiment of a punch assembly for a punch press machine, such as the TruPunch 1000 available from Trumph Inc. of Farmington, Conn. It is seen to comprise a generally cylindrical shank  12  having a central bore  14 , the shank also having a tapered notch  16  for mating with a hydraulic ram forming a part of the overall punch press. The shank  12  is centrally disposed and projects upwardly from an alignment ring  18  that is used to orient the punch assembly with respect to the punch press in which the punch assembly is designed to be used. Disposed in a U-shaped slot  20  formed inward from the upper surface of the alignment ring  18  is a key  22  that is held in place by a cap screw and that is also designed to fit into a key way formed in the hydraulic ram (not shown) used to drive the punch assembly  10 . 
     A stripper plate  24  is coupled to the alignment ring  18  by a plurality of shoulder bolts, best seen in the cross-sectional view of  FIG. 4  and identified by numeral  26 . The heads  28  of the shoulder bolts  26  are recessed in counterbores, as at  30 , formed inward of the upper surface of the alignment ring  18 . The threaded portions  32  of the shoulder bolts  26  mate with internal threads in bores drilled through the thickness dimension of the stripper plate  24 , all as best seen in the cross-sectional view of  FIG. 4 . The use of shoulder bolts for coupling the stripper plate  24  to the alignment ring  18  allows a reciprocal displacement of the alignment ring toward and away from the stripper plate. Alternatively, a bushing to set the height and a standard screw may be employed. 
     A plurality of compression springs, as at  34 , in  FIGS. 1 and 2  are disposed between the alignment ring  18  and the stripper plate  24  for normally urging the stripper plate away from the alignment ring. Both the alignment ring and the stripper plate include cylindrical pockets, as at  35 , formed in facing surfaces thereof for capturing the opposed end portions of the compression springs  34 . 
     As best seen in the view of  FIG. 5 , an aperture in the form of a rectangular slit  36  is formed through the thickness dimension of the stripper plate and which is centered within a downwardly projecting ridge  38  having beveled sides as at  40 . By having this ridge, a workpiece to be punched need not be totally flat because only the bottom of the ridge surrounding the aperture contacts the workpiece. 
     In  FIG. 2 , it can be seen that a generally rectangular recess  41  is formed inward in the bottom surface of the alignment ring  18 , that recess being aligned with and centered relative to a central axis of the shank  14  and fitted into this recess is a shim block  42  that is held in place at cap head screws, as at  44  ( FIG. 4 ). The shim block has a centrally disposed slot  46  as best seen in  FIG. 5  formed in its undersurface. As seen in  FIG. 2 , an alignment pin  47  fitted down the bore  14  drilled upward along a center line of the shank  12  also fits through a bore  49  in the shim block  42  to ensure accurate registration. 
     A punch point member  48  has an upper portion thereof fitted into the slot  46  of the shim block  42  and is fastened to the shim block by retaining setscrews  50  screwed into threaded bores  52  that extend transversely through the sidewalls of the shim block  42  to intersect with and retain the punch point member  48 . The alignment pin  47  also extends into a notch  51  ( FIG. 3 ) centered in the upper portion of the punch point member  48  to locate it with respect to the shim block  42  and the shank  12 . 
     Formed in the upper surface of the stripper plate  24  in registration with a lower portion of the shim block  42  is an accurately machined inset  54  having upwardly extending bosses  56  for guiding the shim block  42  and therefore the punch point member  48  during its descent stroke and subsequent rise in a punching operation. 
     The punch point member illustrated in the drawings is designed to create a long narrow rectangular slit in a workpiece as at  55  positioned between the stripper plate  24  and a die member  56 . A die member is built into the overall punch press and is separate from the punch assembly comprising the present invention. To facilitate punching such a slit in the workpiece  55 , the punch point member  48  is provided with a sharpened, somewhat V-shaped working edge as best seen in  FIG. 3 . Those skilled in the art can appreciate that if a different shaped hole is to be punched, the punch point member would be machined to have the desired shape and the shim block  42  would be machined to accommodate receiving the particular punch point member. Further, the shim block and punch point member may be formed as a unitary piece. 
     In operation, the punch assembly shown in  FIG. 1  would be inserted into a punch press, such as a Trumph TruPunch 1000 and appropriately oriented by fitting the key  22  into a keyway of the punch press. The machine&#39;s hydraulic ram is arranged to cooperate with the upper surface of the alignment ring  18  and the shank  12  to move the stripper plate  24  in contact with a workpiece  55  and pressing workpiece  55  onto the die  56 . The force of the ram also compresses the compression springs  34  as the alignment ring  18  slides downward guided by the shoulder bolts  26 . Because of the manner in which the shim block  42  and the punch point member  48  are fastened to the undersurface of the alignment ring  18 , a point will be reached at which the sharpened lower edge of the punch point member will press through the workpiece  55  and force a slug through the die  56  leaving an open slit. 
     When the punching force of the ram is removed, the compression springs  34  will again expand to increase the separation between the stripper plate  24  and the alignment ring  18 , thus stripping the workpiece  55  free from the punch point member  48 . 
     As explained earlier, after repeated punch strokes, the working edge of the punch point member  48  tends to dull and requires sharpening. This, of course, decreases the height dimension of the punch point member and a point will be reached where the punch point member cannot be further sharpened and expected to continue to function properly. In prior art systems, this required replacement of the punch point member. In the case of the present invention, however, the life of the punch point member can be extended by simply unscrewing the four shoulder bolts holding the stripper plate to the alignment ring and removing the four bolts securing the shim block to the undersurface of the alignment ring and inserting a piece of shim stock as at  58  in  FIG. 5  between the shim block  42  and the alignment ring  18 , as best seen in  FIG. 5 . The shoulder bolts and screws would then be replaced. This has the effect of adjusting the extent that the lower portion of the punch point member extends through the aperture  36  in the bottom of the stripper plate and compensates for the amount of material removed from the punch point member in previous sharpening operations. 
       FIG. 6  is included merely to show that the stripper plate  24  can comprise separate halves  60  and  62  with a precision gap established between the facing edges  64  and  66  rather than a unitary piece as best seen in  FIG. 5 . 
     Alternative Embodiment 
       FIG. 7  illustrates an alternative embodiment for adjusting the effective length of a punch point member following repeated sharpening of its working edge. Here, only the shank, alignment ring and the compression springs are shown, but those skilled in the art can readily perceive from these views how the stripper plate is attached by shoulder bolts as in the embodiment of  FIG. 1 . 
     The alternative embodiment is indicated generally by numeral  100  in  FIG. 7  and is seen to include an alignment ring  102  with a key  104  that functions like key  22  in the embodiment of  FIG. 1  to accurately align the punch assembly in a punch press. Rather than having the shank  106  integral with the alignment ring  102  as in the earlier embodiment, as seen in  FIG. 9 , the shank member  106  has an externally threaded base portion  108 . 
     The punch point holder block  110  has a generally rectangular lower portion  112  of a lesser width dimension than its upper portion  114  and formed inwardly from a top surface of the upper portion  114  is a cylindrical bore  115  having internal threads. The upper portion  114  of the punch point holder block  110  is machined to have a sliding fit within a rectangular pocket  116  formed in the undersurface of the alignment ring  102 . 
     The shank  106  has its upper portion projecting upward through a central bore in the alignment ring  102  and a cylindrical base portion  118  that is externally threaded allowing it to be screwed into the threaded bore  115  in the upper portion  114  of the punch point holder block  112 . A retaining ring  120  holds the shank  106  in place relative to the alignment ring  102 . 
     As in the earlier described embodiment, punch point member  122  is accurately aligned with the shank  106  by the inclusion of a centering pin  124  that extends through a bore in the punch pint holder block  110  and into a notch  126  formed in a top portion of the punch point member. 
     In  FIG. 8 , there can be seen a series of regularly spaced notches, as at  128 , formed in the alignment ring and surrounding the shank  106 . 
       FIG. 9  shows a spring-loaded slidable latch  132  disposed in a slot  134  formed vertically in the periphery of the shank. The latch  132  has a finger  136  that can be made to fit into any of the slots  128  to prevent rotation of the shank  106  relative to the alignment ring  102 . However, by lifting the latch  132  against the force of the spring, the finger  136  is removed from a slot  128 , allowing the shank to be rotated and screwed up or down, depending on the direction of rotation and thereby accurately adjusts the position of the working end of the punch point member  122  relative to the exit opening  36  of a stripper plate  24  ( FIG. 5 ) without changing the length of the projecting upper portion of the shank relative to the top surface of the alignment ring. The magnitude of the displacement is then determined by the pitch of the threads on the base  108  of the shank and the degrees of rotation and between the slots  128  selected as a stop point. 
     After a number of sharpening operations on the punch point member  122 , the downward adjustment is made by lifting the latch  132  so that its finger  136  no longer resides in a slot  128  while at the same time rotating the shank clockwise when viewed in  FIG. 8 . This displaces the punch point holder block  110  downward. When a desired setting is reached, the spring-loaded latch  132  is released into a selected one of the slots  130 . 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.