Abstract:
A composite stripper for use in conjunction with an apparatus for joining multiple pieces of sheet metal or other sheet material is disclosed. The composite stripper is an elongated cylindrical member having a compliant distal engagement portion which is compressible to clamp sheet materials tightly together and a relatively hard proximate portion to provide an adequate interface between the punch assembly and the material to be joined.

Description:
This application is a continuation-in-part of application Ser. No. 09/707,258, filed Nov. 6, 2000 now U.S. Pat. No. 6,430,795. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to joining sheet material, and more particularly to a molded stripper for use in the joining apparatus to form leak-proof or non-leak-proof joints. 
     The ability to permanently join multiple pieces of sheet material through punching or other manipulation and deforming them into an interlocking relationship has found widespread applications. In this regard, such method and apparatuses for permanently joining sheet metal or other sheet material items, with the capability of forming either leak-proof joints or conventional “lanced joints” has been the subject of numerous patents. Such apparatuses include die members that are movable laterally, transverse to the longitudinal movement of a punch against an anvil between the dies which are relatively simple but durable in construction, which utilize standard or specialized punches, which are compact and suitable in many different applications, which preserve the corrosive resistance of coated sheet material being joined, and which are suitable for use either in a small press or in a C-frame holder of a large press. In this regard, U.S. Pat. No. 5,581,860 and the related U.S. patents identified therein are exemplary of this technology. The disclosure of all such patent applications and issued U.S. patents are expressly incorporated by reference herein. 
     In pertinent part to the present invention, U.S. Pat. No. 5,581,860 discloses a punch assembly having a punch body mounting a circular punch. The circular punch includes a threaded portion which receives and threadingly supports a stripper retainer. Disposed within the stripper retainer is a stripper or sheet metal retainer biased to a stripping position by means of a coil spring. The stripper serves to clamp sheet material together before and during the formation of the joint. The stripper preferably has an outside diameter at the area of longitudinal engagement with the sheet metal to be formed that is greater than the inside diameter of the die opening when the dies are at their maximum laterally outward open position. This, coupled with biasing forces urging the stripper longitudinally against the sheet metal surrounding the joint in order to clamp the sheet metal pieces together, greatly reduces the tendency for the sheet material to flow longitudinally outward, away from the preferably fixed anvil inside of the die opening. 
     The coil spring used to generate the stripper biasing force has a tendency to degrade during repeated cycling. More specifically, the clamping forces generated by the stripper decrease as the stripper is repeatedly cycled, and may ultimately fail in a fatigue mode. Likewise, the metal tip of the stripper which contact the sheet material may scratch or mar the material being stripped. Although the prior art strippers perform satisfactorily, the present application is directed to additional improvements and refinements thereupon. 
     In accordance with the present invention, as defined in the appended claims, a composite urethane stripper is provided for use in conjunction with various punch assemblies to provide sufficient force to clamp the sheet materials tightly together before and during the forming of the joint. As presently preferred, the composite stripper utilizes two components of urethane having different hardness—an end portion utilizing a relatively hard durometer urethane and a middle portion utilizing a relatively soft durometer urethane. The configuration of the stripper is such that the clamping force may be accurately and repeatedly controlled. 
     These features, along with others discussed in more detail below provide a stripper for use in die assemblies of forming apparatuses that are more durable and reliable, more versatile, more stable, more widely applicable, and that typically require less maintenance to operate than previous strippers. 
     These and other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, taking in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to appreciate the manner in which the advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings only depict preferred embodiments of the present invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a perspective view of a punch assembly having a molded three-piece composite urethane stripper according to a first embodiment; 
     FIG. 2 is a cross-section of a punch assembly taken along line  2 — 2  shown in FIG. 1; and 
     FIG. 3 is a cross-sectional view of the molded composite urethane stripper of FIG.  2 . 
     FIG. 4 is a partial perspective view of the apparatus which is applicable to the formation of a leakproof joint according to the present invention. 
     FIG. 5 is a partial elevational view of the apparatus of FIG. 4, with a portion of the apparatus shown in longitudinal cross-section. 
     FIG. 6 is a partial sectional view of the apparatus of FIGS. 4 and 5, illustrating the apparatus substantially at the point of completion of a leakproof joint according to the present invention. 
     FIG. 7 is a perspective view of a punch assembly having a molded two-piece composite urethane stripper according to a second embodiment; 
     FIG. 8 is a cross-section of the punch assembly of FIG. 7 taken along line  8 — 8 ; and 
     FIG. 9 is a cross-sectional view of the molded composite urethane stripper of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the figures, punch assembly  10  includes punch holder  12  having punch  14  extending from body  16 . More specifically, as best seen in FIG. 2, blind bore  18  extends into body  16  from an upper surface  16   a  thereof and is adapted to receive punch  14 . Throughbore  20  is axially aligned with and extends into blind bore  18  from a lower surface  16   b  of body  16  and is adapted to receive locating pin  22 . A second throughbore  24  is formed in body  16  and is adapted to receive socket head cap screw  26 . In this manner, punch assembly  10  may be releasably secured to a movable die (not shown) of a joint forming apparatus. 
     Body  16  further includes throughbore  28  extending therethrough to receive retainer assembly  30  for releasably securing punch  14  within punch holder  12 . More specifically, retainer assembly  30  includes bolt  32  and retaining washer  34  received within throughbore  28 . Retainer washer  34  is threadedly secured within throughbore  28 . As retaining bolt  32  is tightened down, retaining washer  34  engages a shoulder portion  14   a  of punch  14 , thereby releasably securing punch  14  within punch holder  12 . 
     Composite stripper  36  is an elongated cylindrical member having a longitudinal bore  38  formed therethrough which is adapted to receive punch  14 . Composite stripper  36  includes base section  40 , middle spring section  42  and tip section  44 . As best seen in FIG. 3, the inside diameter of bore  38  varies through base section  40 , middle section  42  and top section  44 . More specifically, the inside diameter  38   b  of middle section  42  is slightly less than the outside diameter of punch  14 , and therefore is used to retain stripper  36  on the shank of punch  14  by a slight “press” or interference fit. The inside diameter  38   a  of base portion  40  is slightly larger than the outside diameter of punch  14  to provide a clearance for the shank of punch  14 . Similarly, the inside diameter  38   c  of tip portion  44  is slightly larger than the outside diameter of the tip  14   a  of punch  14  to provide a slight clearance around the tip  14   b  of punch  14 . The portion of throughbore  38   a ,  38   c  associated with base portion  40  and tip portion  44  may be molded or machined to suit the specific geometric shape of punch  14 . 
     As presently preferred, stripper  36  is of a composite construction in that it incorporates materials of varying hardness for achieving different functional features. As presently preferred, base portion  40  and tip portion  44  are made from a relatively hard durometer urethane, preferably of a hardness of approximately 75 D, to provide proper support and wear characteristics, while middle portion  42  is made from a relatively soft durometer urethane, preferably of a hardness of approximately 95 A, to control the overall clamping force generated when the composite stripper  36  is compressed. More specifically, the hard urethane on base portion  40  prevents extrusion of composite stripper  36  into any adjacent hole, such as throughbore  28  of punch holder  12 . This eliminates the need for a metal washer supporting composite stripper  36 . Utilizing hard urethane for tip portion  44  resists marring the material being stripped, a problem commonly encountered in the use of standard metal stripper tips. Furthermore, utilizing a relatively hard urethane tip minimizes the wear on composite stripper  36  associated with the repeated cycling during the adjoining operation. The use of a softer durometer urethane for middle section  42  provides sufficient compliance to generate the compression force necessary to adequately hold and clamp the material being joined. This gives a reliable stripping force with a consistent range of compression. Middle portion  42  and tip portion  44  of composite stripper  36  are preferably of a constant length regardless of the length of the punch used. In contrast, the length of base portion  40  may be modified to provide a desired overall length of composite stripper  36 . 
     Composite stripper  36  may be fabricated using a sequential process molding tip portion  44 , middle portion  42  and the base part  40 . Alternately, composite stripper may be fabricated by placing base portion  40  and tip portion  44  into a suitable mold and injection molding middle portion  42  in a manner sufficient to bond base and tip portions  40 ,  44  thereto. One skilled in the art will also recognize that other molding processes may be adapted for a given composite stripper configuration. Furthermore, colors may be utilized in the fabrication of composite stripper to specifically identify the stripper, e.g. heavy-duty versus medium-duty stripping forces, as well as to suit safety requirements, e.g. orange-colored tip portion. In addition, various logos and/or part numbers may be molded into the end face portion  44  prior to the insert molding process. 
     The composite stripper of the present invention allows for closer joining or piercing center distances than steel spring stripper assemblies with comparable forces. Moreover, the overall size of composite stripper  36  is smaller than conventional steel spring stripper which require larger encasements for constraining the coil spring. One skilled in the art will readily recognize that the overall length and effective compliance of composite stripper  36  is dictated by the particular apparatus, the joining process and the desired clamping forces. As such, the specific materials and the hardness of such materials associated with a composite stripper may be modified without deviating from the spirit and scope of the present invention. In this regard, a urethane polymer is the presently preferred material, however other suitable plastics which provide the desired durability, wear, compliance and hardness characteristics may be substitute, and thus are considered within the scope of the present invention. Furthermore, one skilled in the art will readily recognize that the specific spring force generated by composite stripper  36  is primarily dictated by the geometry, length and wall thickness, as well as the material selection of middle portion  42 . In this regard, one skilled in the art will readily appreciate that the present invention contemplates the use of strippers having various cylindrical configurations as well as cones and various polyhedral configurations may be incorporated into the present invention, and thus are considered within the scope of the present invention. 
     Turning now to FIGS. 4-6, an exemplary die assembly  110 , which is adapted to cooperate with punch  14  to form a leakproof joint is shown. Die assembly  110  includes die body  126  having an integral boss  130  and a circular annular shoulder  132  thereon. Integral boss  130  includes conical portion  131  interconnecting an anvil  136  with shoulder  132 . 
     Two or more arcuate die segments  138  are disposed around boss  130  and include die segment shoulders  144 , which are supported and engaged by shoulder  132  of die body  126 . Boss  130  is provided with a circular annular relieved portion  145  adjacent the shoulder  132 . This relieved portion  145  provides clearance so that the die segments  138  can primarily pivot outwardly about a lateral axis as the joint is being completed. 
     Die portions  138  are maintained in their normally-closed position, shown in FIG. 4, by means of a resilient, elastomeric band  146 , which surrounds the lateral sides of die segments  138  in order to resiliently bias die segments  138  in a laterally inward direction toward the longitudinal axis of the die assembly. Resilient band  146  is received within, and longitudinally restrained by, a laterally inwardly recessed groove  150  extending circumferentially around the die segments  138 . Although resilient band  146  can be composed of any of a wide variety of elastomeric materials suitable for particular applications of the present invention, it is preferred that resilient band  146  be composed of a urethane or urethane-containing material. 
     When die segments  138  are in their closed positions illustrated in FIG. 4, the upper surfaces  152  lie in a common plane and the abutting faces of adjacent die segments lie in planes indicated by reference numeral  154 . In the embodiment shown in FIG. 4, die portions  138  are provided with circular inside faces or recesses  156  that define a circular opening generally complementary with the lateral cross-sectional shape of punch  14 . Die portions  138  are also provided with radiused edge portions  158 , and are preferably uniformly spaced from punch  14 , in order to substantially avoid shearing or tearing sheet material items a and b during the deformation of sheet material. The downward force exerted on die portions  138  by punch  14  will tend (at least initially) to close, rather than open, the die assembly  110 . This is because the primarily pivotal and longitudinally supported engagement of die segment shoulders  144  with die body shoulder  132  is at a laterally-outward position relative to edge portions  158 . Furthermore, shoulder  132  defines a solid surface easily capable of handling the axial loads on die segments  138  during operation of the apparatus. 
     During operation, the action of punch  14  against anvil  136  causes lateral extrusion of the sheet material portions  160  to form the leakproof joint, in the manner illustrated in FIG.  6 . The resilient band  146 , in addition to contributing to a uniform and controlled formation of a joint, also provides substantial simplicity and economy in the manufacture, operation and maintenance of the die assembly. 
     Turning now to FIGS. 7-9, an alternate embodiment incorporating a two piece stripper  236  is provided according to a second embodiment wherein like reference numbers increased by 200 over those used in conjunction with composite stripper  36  will be used to designate like components. In addition, it will be appreciated that composite stripper  236  may be fabricated using the molding processes as described herein in relation to composite stripper  36 . 
     Composite stripper  236  is an elongated cylindrical member having a longitudinal bore  238 . Composite stripper  236  includes base section  240  and tip section  242 . As best seen in FIG. 9, the inside diameter of bore  238  varies between base section  240  and tip section  242 . Explained further, the inside diameter  238   a  of base section  240  is slightly larger than the outside diameter of punch  14  to provide a clearance for the shank of punch  14 . The inside diameter  238   b  of tip section  242  is slightly smaller than the outside diameter of punch  14  to provide a friction or interference fit. 
     As with composite stripper  36 , composite stripper  236  incorporates materials of varying hardness. In this way, base section  240  is preferably made of a relatively hard durometer urethane, preferably of a hardness of approximately 75 D, to provide sufficient support and wear characteristics. Tip section  242  is preferably made of a relatively soft durometer urethane, preferably of a hardness of approximately 95 A, to control the overall clamping force generated when the composite stripper  236  is compressed. The preferred materials as described being associated with composite stripper  236  provide the advantages associated with previously described stripper  36 . In this regard, utilizing a softer durometer urethane for tip section  242  provides sufficient compliance to generate the compression force necessary to adequately hold and clamp the material being joined. In addition, the hard urethane of base  240  prevents extrusion of composite stripper  236  into any adjacent hole, such as a throughbore incorporated in a punch holder. Tip section  242  is made of a distinct color from the base section  240 . Accordingly, tip section  242  is preferably made of a bright color, such as but not limited to, yellow to designate the area of visual interest to a user. Base section  240  is preferably made of a distinct color from said tip section  242  such as but not limited to safety orange. 
     The foregoing discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings, that various changes, modifications and variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims.