Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This invention is related to commonly assigned and co-pending U.S. Ser. No. 10/094,128. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method and apparatus for clinching metal sheets together for assembling automotive vehicle structures. 
     BACKGROUND OF THE INVENTION 
     It is known that the manufacture of automotive vehicles often requires that metal sheets be attached to each other to form automotive vehicle structures. Clinching is one potential method of attaching such sheets. Clinching typically requires steps of stamping or otherwise cold forming corresponding indentations in at least two stacked metal sheets for frictionally or otherwise mechanically interlocking the sheets to each other. During conventional clinching processes, the metal sheets may require fairly substantial deformation of the sheets to form proper indentations. Such deformation can be particularly difficult to achieve in high strength metal sheets, which tend to be more brittle than certain lower strength metals, or require expensive heat treatment for relieving internal stresses. Therefore, there is a need for improved clinching techniques, apparatuses or both, for achieving high integrity attachment of metal sheets, particularly, sheets formed of advanced or high strength metals such as aluminum, magnesium, high strength steel and the like. 
     SUMMARY OF THE INVENTION 
     The present invention meets these needs by providing an improved method of clinching a first metal sheet to a second metal sheet, with particular utility in the formation of components for an automotive vehicle. The method involves clinching at least two sheets of metal with a punch and die assembly during or after contacting electrodes with the metal sheet for locally heating the metal sheet at the clinching locations. More specifically, the method includes a step of stacking a first metal sheet on a second metal sheet. Each of the sheets includes a first side and a second side and at least a portion of the second side of the first sheet is in overlapping contact with at least a portion of the first side of the second sheet for forming an overlapped region. Once the sheets are stacked, the first and second metal sheets are placed between a punch assembly and a die assembly. The punch assembly includes a punch surrounded by a first electrode, wherein the first electrode is adapted for contacting the first sheet. The die assembly includes a die surrounded by a second electrode, wherein the second electrode is adapted for contacting the second sheet. The first and second electrodes are each connected to an electrical energy source. Upon contacting the first and second electrodes with the metal sheets, the electrical energy source is capable of inducing an electrical current that flows between the first and second electrodes and the first and second metal sheets to elevate the temperature of the overlapped region of the first sheet and the second sheet. Mating indentations are punched within the overlapped region for additionally securing the first sheet to the second sheet. During formation of the indentations, an outer periphery of one of the indentations at least partially bonded to an inner periphery of another of the indentations. Additionally, the clinching die provides force to clinch the inner periphery onto the outer periphery. 
     The present invention also provides an apparatus for clinching a first metal sheet to a second metal sheet. The apparatus includes a punch assembly for stamping mating indentations in the first and second metal sheet while the first sheet is stacked upon the second sheet. The punch assembly includes a cylindrical punch moveable between at least a first position and a second position for forming the indentations. The punch assembly further includes a first electrode associated with the punch. A die assembly is also included in the apparatus for at least partially supporting the first and second sheets as the punch assembly stamps the indentations into the sheets. The die assembly includes a central cylindrical die defining a cup-shaped cavity for assisting in forming the indentations. The die assembly also includes an associated second electrode. The apparatus further includes an electrical energy source electrically connected to the first electrode and the second electrode for inducing a current between the first and second electrode and through the first and second sheets for elevating the temperature of portions of the first and second sheets prior to or during punching of the indentations into the portions. 
     The present invention thus provides an improved clinching apparatus and clinching technique for providing structurally improved indentations in stacked sheets thereby more securely fastening the sheets together. The ability to locally control the temperature of the sheets makes this invention particularly advantageous for the joining of high strength metals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects and advantages of the present invention will become apparent upon reading the following detailed description in combination with the accompanying drawings, in which: 
     FIG. 1 is a sectional view of a clinching apparatus prior to clinching a pair of stacked metal sheets to each other; 
     FIG. 2 is a sectional view of the clinching apparatus of FIG. 1 during clinching of the pair of stacked metal sheets to each other; 
     FIG. 3 illustrates the clinching apparatus of FIGS. 1 and 2 with a robot arm and an energy source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a first metal sheet  10  is clinched to a second metal sheet  12  by a clinching apparatus  14 . The clinching apparatus  14  includes a punch assembly  16  for stamping generally cup-shaped or generally cylindrical mating indentations  18 ,  20  into the metal sheets  10 ,  12  and a die assembly  22  for supporting the metal sheets  10 ,  12  and for assisting in the stamping or forming of the indentations  18 ,  20 . 
     The punch assembly  16  includes a generally elongated metal stripper  24  having an opening  26  extending down a length of the stripper  24 . An elongated cylindrical steel punch  28  of the assembly  16  is received in the opening  26  and the punch  28  is moveable along a length of the opening  26  between at least a first position, as shown in FIG. 1, and a second position, as shown in FIG.  2 . The punch  28  may be moved hydraulically, mechanically, electrically, pneumatically or otherwise. Preferably, the punch assembly  16  also includes a spring  30  attached to the stripper  24 , the punch  28  or both that is biased against the motion of the punch  28  from its first to its second position for assisting in retracting the punch  28  after clinching as further described below. 
     A copper electrode  34  of the punch assembly  16  is generally annular and surrounds at least a portion of the stripper  24  and the hole  26  through which the punch  28  moves. A generally annular insulator  36  of the punch assembly  16  is disposed between the stripper  24  and the electrode  34  to electrically separate the electrode  34  from the stripper  24  and the punch  28 . The insulator  36  may be formed of an insulative material such as a plastic, polymer, ceramic, or the like. In one preferred embodiment, the insulator  36  is a laminate formed with a fabric or paper molded with a synthetic resin. 
     In FIGS. 1 and 2, the punch  28 , the hole  26 , the spring  30 , the insulator  36  and the electrode  34  are generally cylindrical, coaxial or both about an axis (not shown) extending centrally along their lengths. Preferably, a housing (not shown) can be used to fasten the electrode  34 , the insulator  36 , and the stripper  24  together. Alternatively, other conventional fasteners or fastening techniques may be used. 
     The die assembly  22  includes a generally cylindrical die  44  having a central cylindrical opening or cavity  46 . Preferably, the cylindrical die  44  includes three clinching blades  48  that are positioned in an annular arrangement to substantially surround a central cylindrical member  50 . Also preferable, an elastic band  52  surrounds the clinching blades  48  to maintain the blades  48  around the central member  50 . As seen, the blades  48  form a generally annular and cylindrical wall  54  for defining the cavity  46 . Alternatively, however, other dies may replace the die  44  shown. For example, the die  44  may be formed as a single part providing a cavity defined by a sloping annular wall for forming the cavity in a frusto-conical shape. 
     The die assembly  22  further includes a generally cup shaped electrode  60  with an annular portion  62  and a base portion  64  that cooperatively define a cavity for receiving the die  44 . Preferably, the die assembly  22  also includes a generally cup-shaped insulator  68  with an annular portion  70  and a base portion  74  defining a cavity wherein the insulator  68  is formed of a material similar to the material of the insulator  36  of the punch assembly  16 . As shown, the insulator  68  fits flush within the cavity of the electrode  60  and the die  44  is received in the cavity of the insulator  68  for electrically separating the die  44  from the electrode  60 . By changing the dimensions of the insulator  68 , the die  44  or both, a variety of different dies having a variety of different sized or shaped cavities may be interchanged within the cavity of the electrode  60  if desired. The components of the punch assembly  16  and the die assembly  22  may be fastened together as desired by conventional fasteners, adhesives, a housing and the like. 
     The punch assembly  16 , the die assembly  22  or both may be mounted to various apparatus for moving the punch assembly  16  or the die assembly  22  relative to each other, such as robots, C-frames and hard tooling such as a die set. In the exemplary embodiment shown in FIG. 3, the punch assembly  16  is attached to a robot arm  84  that can move the punch assembly  16  as needed or desired. The die assembly  22  is stably positioned adjacent the robot arm  84 . 
     An energy source  86  such as a transformer or other energy source is electrically coupled to the electrodes  34 ,  60  of the punch assembly  16  and the die assembly  22  for providing electrical current to those electrodes  34 ,  60 . 
     Referring to FIGS. 1 and 2, the first metal sheet  10  and second metal sheet  12  each include a first side  90  and a second side  92 . The first sheet  10  is stacked upon the second sheet  12  such that at least a portion of the second side  92  of the first sheet  10  is in substantially continuous contact with at least a portion of the first side  90  of the second sheet  12  at a location for forming the indentations  18 ,  20 . The sheets  10 ,  12  may be formed of several metals. Preferably, the sheets  10 ,  12  are formed of a high strength or advanced metal such as aluminum, magnesium, high strength steel or the like with thicknesses ranging between 0.6 mm and 3.0 mm although thicker of thinner sheets may also be used. 
     The stacked sheets  10 ,  12  are placed between the punch assembly  16  and the die assembly  22  of the clinching apparatus  14 . Preferably, the sheets  10 ,  12  are placed upon the die assembly  22  such that the second side  92  of the second sheet  12  contacts the die assembly  22 . Thereafter, the punch assembly  16  is contacted with first side  90  of the first sheet  10  (e.g., using the robot arm  84  or another apparatus) to clamp the sheets  10 ,  12  between the punch assembly  16  and the die assembly  22 . 
     When the sheets  10 ,  12  are clamped between the assemblies  16 ,  22 , the electrode  34  of the punch assembly  16  is in contact with the first side  90  of the first sheet  10  and the electrode  60  of the die assembly  22  is in contact with the second side  92  of the second sheet  12 . The energy source  86  induces an electric current that flows between the two electrodes  34 ,  60  through each of the sheets  10 ,  12 . Advantageously, the current may be applied for as short as about {fraction (1/30)} of a second using about 20 kiloamps of electricity for aluminum, however, different levels of energy may be used for different amounts of time depending on the application. The current provides energy to the sheets  10 ,  12  thereby elevating the temperature of (i.e., resistive heating) at least a portion of each of the sheets  10 ,  12  (i.e., the overlapped region) to a desired temperature. Preferably, the heated portions are the portions in which the indentations  18 ,  20  are to be formed. 
     Thereafter, the punch  28  is moved from its first position to its second position as shown in FIG. 2 to form the indentations  18 ,  20  in mating relation to each other (i.e., the indentation  18  in the first sheet  10  is securely fit within the indentation  20  in the second sheet  12 ) in the heated portions. As the indentations  18 ,  20  are stamped into the sheets  10 ,  12 , the wall  54  of the clinching die  44  provides force against the outer periphery of the indentation  20  in the second sheet  12  to clinch the inner periphery of the indentation  20  in the second sheet  12  about the outer periphery of the indentation  18  in the first sheet  10  thereby forming a joint. In the embodiment wherein a plurality of clinching blades  48  are surrounded by the elastic band  52 , the blades  48  may flex slightly outward to assist in forming and clinching the indentions  18 ,  20 . After formation of the indentations  18 ,  20 , the spring  30  retracts the punch  28  from the indentations  18 ,  20  such that the sheets  10 ,  12  may be removed from the die assembly  22  together. 
     Advantageously, clinching the sheets  10 ,  12  after heating the portions of the sheets  10 ,  12  to be clinched allows the indentations  18 ,  20  to be more easily formed without causing the structural defects that can be caused by cold forming techniques. Additionally, the heated inner periphery of the indentation  20  in the second sheet  12  tends to bond or weld to the heated outer periphery of the indentation  18  in the first sheet  10  thereby further securing the first sheet  10  to the second sheet  12 . 
     Although, the assemblies shown use electrodes coupled to an electrical energy source, it is contemplated that other energy sources suitable for locally treating the indented sheets, such as lasers (e.g., carbon dioxide or N:Yag lasers) may be attached to or form part of the punch assembly  16 , the die assembly  22  or both. It is further contemplated that the electrodes  34 ,  60  may not surround the punch  28  or die  44 , but may be otherwise associated with or adjacent the punch  28  or die  44  or that the electrodes  34 ,  60  may be integrally formed as the punch  28  or die  44 . 
     The method and apparatus described above may be used for attaching several different automotive components that have sheet metal or sheet metal portions. Examples include peel joints, lap joints, various vehicle panels such as door panels, decklids, hoods, sunroof applications and the like. Furthermore, the overlapped regions of the sheets may be continuously bonded or intermittently bonded over some or all of its area. 
     Advantageously, clinching according to the present invention is inexpensive, can improve joint consistency, and can extend the life of tooling used to make the clinched joints. 
     It should be understood that the invention is not limited to the exact embodiment or construction which has been illustrated and described but that various changes may be made without departing from the spirit and the scope of the invention.

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