Abstract:
A stripper unit incorporates a precision threaded plug that preloads a mechanical die spring that, in turn, preloads the movable or traveling stripper adjacent the punch tip. The precision threaded plug simultaneously accurately locates the stripper unit on a commercially available ball-lock punch retainer. The stripper unit is entirely self-contained on the face of the ball-lock punch retainer, thus minimizing the space required on the press platen.

Description:
This application claims the benefit of provisional patent application No. 60/706,310, filed Aug. 8, 2005. 

   BACKGROUND OF THE INVENTION 
   The field of the invention pertains to punches and dies for forming holes and depressions in metals. In particular, the invention pertains to devices and accessories for preventing work pieces from adhering to punches during high-speed, high production manufacturing. 
   Examples of modern punch retainers are shown in applicant&#39;s previous U.S. Pat. Nos. 5,337,835, 5,410,932 and Des. 351,395. Views of these punch retainers are also shown in  FIGS. 1 and 2 . Modern punches for certain applications utilize either metal or polyurethane strippers to prevent work piece drag on the punch. The metal stripper units typically extend over the sides of the punch retainers or otherwise interfere with the close placement of punch retainers. Polyurethane strippers require separate metal retainers to accurately position the strippers on the punch retainers. With a view to eliminating some of the disadvantages attendant to previous stripper units, the following more compact stripper unit has been developed. 
   SUMMARY OF THE INVENTION 
   The new stripper unit incorporates a precision threaded plug that preloads a mechanical die spring and simultaneously locates the stripper unit on a commercially available ball-lock punch retainer. The ball-lock punch retainers are commonly used in the tool, die and metal stamping industry, and the new stripper unit is an improvement on various punch stripping units commercially available today. Use of the preloaded mechanical die spring provides for time-tested long life. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top plan view of a punch retainer; 
       FIG. 2  is a bottom plan view of a punch retainer; 
       FIG. 3  is a top plan view of the new stripper unit; 
       FIG. 4  is a side exploded view of the new stripper unit; 
       FIG. 5  is a side cross-section of the new stripper unit; 
       FIG. 6  is a bottom plan view of the new stripper unit; and 
       FIG. 7  is a side view of the new stripper unit on a punch retainer. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Illustrated in  FIGS. 1 and 2  are the top and bottom views of a typical commercially available ball-lock punch retainer  10 . The punch retainer  10  includes bolt holes  12  for attaching the punch retainer to a press platen (not shown), threaded holes  14  with dowel locating holes at bottom, and punch retention hole  16 . 
   In  FIG. 3 , the top plan view of the new stripper unit  18  illustrates the very robust stripper body  20  and the movable stripper  22 . The movable stripper  22  is formed with a hole  24  shaped to match the shape of the punch (not shown). 
   In  FIG. 4 , the components of the stripper unit  18  are shown in order of assembly. The stripper  22  is first inserted into the stripper body  20  followed by the mechanical die spring  26  and the precision threaded plug  28 . The precision threaded plug  28  screws into a precision threaded hole  30  in the bottom  32  of the stripper body  20  and seats on an accurately machined counterbore ledge  34 . 
   The upper bore  36  in the stripper body  20  includes a flat  38 , such that the upper bore  36  has a “D” shape. The stripper  22  is also “D” shaped to match the D-shape of the upper bore  36  and thereby provide assembly only in its correct position.  FIG. 5  illustrates in cross-section the completely assembled stripper unit  18 . As shown, the precision threaded plug  28  is tightly seated on the counterbored ledge  34 . It is appreciated that  FIGS. 4 and 5  teach bottom  32  is generally flat and that threaded plug  28  screws into the precision threaded hole  30  leaving a flush surface across the bottom  32 . 
   In  FIG. 6 , the bottom view of the stripper unit  18  shows the precision threaded plug  28  formed with a hexagonal hole  42  therethrough. The flats  44  of the hexagonal hole  42  are sized to accurately contact the punch (not shown) and thereby accurately align the stripper unit  18  over the punch retention hole  16 . Optionally, the hexagonal hole  42  may be made slightly smaller, whereby each flat  44  is formed with a slight concavity matching the curvature of the punch retention hole  16 . A hex wrench can be used to tighten the precision threaded plug  28  in place. 
   Thus, the stripper unit  18  can be perfectly aligned on the punch retainer  10 , as shown in  FIG. 7  with a punch (not shown) through the hexagonal hole  42 , before a machine screw (not shown) is inserted through hole  46  in the stripper unit  18  and threaded into hole  14  of the punch retainer  10  to fasten the stripper unit to the retainer. 
   The construction above provides a number of improved features. The precision threaded plug  28  locates the stripper unit  18  while simultaneously pre-loading the die spring  26  for improved die spring life. Because the plug  28  has a precision machined hexagonal hole  42 , the stripper unit  18  is accurately positioned and retained on its mating retainer  10 . The hexagonal hole  42  provides an alien wrench hex over the precision hole to provide for easy assembly and disassembly. The new design provides that all die spring  26  and stripping forces are completely isolated in the very robust stripper body  20  with no force being transferred to any outside component. Fully assembled, the stripper unit  18  is completely “self-contained,” eliminating the risk of components being lost, or, even upon catastrophic failure, any of the units&#39; components being jettisoned into the stamping die. In addition, it is appreciated that  FIGS. 5 and 7  teach the stripper unit  18  can be attached to the punch retainer  10  as a single unit and the bottom  32  can fit flush against the punch retainer  10 . 
   The precision threaded plug  28  is designed to bottom out in its mating pocket in the stripper body  20 , which automatically sets the die spring  26  pre-load and is fail safe, so the spring cannot be overloaded. 
   Unlike some stripper units currently available, the new stripper unit  18  universally fits all standard ball-lock punch retainers, and most importantly requires no retainer alterations. 
   The movable or traveling stripper  22  is preferably of a hardened tool steel construction with a lubricating coating to promote smooth stripping movement. The stripper  22  is guided not only by the “D” body shape, but also by the stripper&#39;s head diameter or hole size and shape  24 . This provides a much improved condition that will ensure the stripper  22  cannot twist or cock and jam during production and will better facilitate side load to the stripper when stripping non-flat surfaces. 
   The round portion of the stripper  22  and all the round features of the stripper unit  18  are co-located along the centerline of the punch and pierced hole, which simplifies design and manufacture of the stripper unit. The unique “D” shape  40  of the stripper  22  provides a failsafe assembly, allowing the stripper  22  to be assembled in the stripper body  20  only in the correct position. 
   The pre-loading of the die spring  26  and stripper  22  provides instant stripping force upon contact with the work piece. The instant stripping force on contact facilitates simplification of the calculations and timing questions, which are issues, especially with respect to cam activated die applications. The pre-loading also makes the overall stripper unit  18  more compact. The stripper body  20  is a one-piece construction designed with a robust heel for the attachment hole  46 , making the stripper unit  18  more stable and stronger than other stripper units.