Abstract:
A clinching apparatus includes a clinching punch having a specifically dimensioned tapered surface which improves the strength of a clinch joint created therewith. In another aspect, a tapered surface adjacent a distal end of a clinching punch has an angle of 20-35° relative to a distal end. A further aspect of a clinching apparatus provides a tapered surface adjacent a distal end which is configured in accordance with a specific formula.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates generally to clinching apparatuses and more particularly to a clinching punch and joint formed therewith. 
         [0002]    It is known to use a punch and die to create a clinch joint between workpiece sheets. Such a device is disclosed in U.S. Pat. No. 5,177,861 entitled “Apparatus for Joining Sheet Material” which issued to Sawdon on Jan. 12, 1993, and is commonly owned with the present application. This patent is incorporated by reference herein. This prior device was a significant step in the industry but there is still room for further improvements. 
         [0003]    Another clinching construction is disclosed in U.S. Pat. No. 5,727,302 entitled “Die and Punch for Forming a Joint and Method of Making the Die” which issued to Sawdon on Mar. 17, 1998, and is commonly owned with the present application. This is also incorporated by reference herein. This prior punch includes a 5° frusto-conical taper which was intended to advantageously prevent high stress concentrations and to improve the tool life during high pressure tool flexure misalignments. 
         [0004]    In accordance with the present invention, a clinching apparatus includes a clinching punch having a specifically dimensioned tapered surface which improves the strength of a clinch joint created therewith. In another aspect, a tapered surface adjacent a distal end of a clinching punch has an angle of 20-35 degrees relative to a distal end. A further aspect of a clinching apparatus provides a tapered surface adjacent a distal end which is configured in accordance with a specific formula. Additional advantages and features of the present clinching apparatus can be ascertained from the following description and appended claims taken in conjunction with the present drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a side elevational view showing a clinching apparatus; 
           [0006]      FIG. 2  is a cross-sectional view, taking along line  2 - 2  from  FIG. 1 , showing the clinching apparatus; 
           [0007]      FIG. 3  is a perspective view showing a punch employed in the clinching apparatus; 
           [0008]      FIG. 4  is a side elevational view showing the punch; 
           [0009]      FIG. 5  is an enlarged side elevational view, taken within circle  5  from  FIG. 4 , showing the punch; 
           [0010]      FIG. 6  is an end elevational view showing the punch; and 
           [0011]      FIG. 7  is an enlarged diagrammatic view showing a clinch joint formed using the clinching apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring to  FIGS. 1 and 2 , a clinching apparatus  11  includes a punch assembly  13 , a die assembly  15  and material workpiece sheets  17  and  19 . Punch assembly  13  includes a punch holder  21 , a hollow stripper retainer  23 , a stripper  25 , a helically coiled spring  27  and a punch  29 . Retainer  23  is affixed to a periphery of holder  21 , and stripper  25  is a hollow member, a section of which is linearly moveable through an aperture  31  of retainer  23 . An axial length of stripper  25  is preferably greater than twice the diameter of an abutment flange  32  thereof. Spring  27  is located between flange  32  of stripper  25  and a facing surface of holder  21  so as to urge stripper  25  axially away from holder  21  but also allow stripper  25  to be compressed toward holder  21  during clinch joint forming when punch  29  is linearly advanced along its centerline and axial direction by a fluid powered actuator moving holder  21  and/or its attached frame. Exemplary actuators for moving either the punch or die are discussed in U.S. Pat. No. 6,115,898 entitled “Force Multiplying Apparatus for Clamping a Workpiece and Forming a Joint Therein” which issued to Sawdon on Sep. 12, 2000, and U.S. Pat. No. 8,650,730 entitled “Clinching Tool” which issued to Sawdon on Feb. 18, 2014, both of which are incorporated by reference herein. 
         [0013]    Die  15  includes a body  41 , and a fixed and stationary anvil  43  integrally upstanding from body  41 . Furthermore, die  15  additionally includes a circular-cylindrical shield  45  attached to body  41  with a set of three movable die blades  47  located between a generally cylindrical side of anvil  43  and shield  45 . A coiled spring  49 , elastomeric O-ring or other biasing member is located between shield  45  and blades  47  so as to urge distal ends of the die blades toward the side of anvil  43 . Punch holder  21  and die body  41  are fastened to a C-frame, fixture or other mechanism such that punch  29  and anvil  43  are always intended to be aligned with each other in all joint forming operating conditions in the preferred embodiment of the present punching apparatus. 
         [0014]    Reference should now be made to  FIGS. 2-6 . Punch  29  includes a generally circular-cylindrical body  61  which is elongated in an axial direction defined by a centerline CL of the punch. A recess  63  has an axially elongated shape machined in a peripheral surface of body  61  with a partially spherical depression  65  located at a trailing end thereof. A threaded fastener  67  and surrounding partially spherical retention ball or collar  69  engage with recess  65  to secure punch  29  to holder  21  in a removable manner such that a proximal or trailing end  71  of punch  29  firmly abuts against holder  21 . 
         [0015]    Punch  29  further includes a clinch joint-forming tip  81  which ends in a distal or leading end  83 . Tip  81  of punch  29  further includes a generally circular-cylindrical peripheral side surface  85  which is located between distal end  83  and an arcuately curved transition section  87 . Distal end  83  and a section with surface  85  are the portions of punch  29  that enter into and create a recessed cup for the clinch joint in a punching and deforming action. It is noteworthy that surface  85  has a generally constant diameter B without any discontinuities or steps therein. Alternately, a very small draft angle can be provided on surface  85 , but generally equal to 5° relative to the punch centerline. Moreover, arcuately curved section  87  may alternately have a frusto-conical shape thereto although various workpiece deformation or punch strength benefits may not be achieved. 
         [0016]    A frusto-conical taper  101 , annular in an end view, is located between distal end  83  and cylindrical side surface  85 . Taper  101  preferably has an angle F of 20-35 degrees relative to end  83 , more preferably 28-32 degrees, and most preferably 30 degrees for two workpieces  17  and  19  of differing thicknesses. Intersecting lines defined between sidewall surface  85 , taper  101  and distal end  83  all have angular corners in one version, however, it is alternately envisioned that one or both of the intersecting corners can employ a radius no greater than 0.02 inch. Furthermore, N is a sidewall neck thickness NL or NR of punch-side workpiece  17 , A is an axial length of body  61  of punch  29 , E is a minor radius measured between centerline CL of punch  29  and an intersection between tapered surface  101  and distal end  83 , t 1  is a minimum thickness of punch-side workpiece  17  at the bottom or button of the final clinch joint, and t 2  is a minimum thickness of die-side workpiece  19  at the bottom or button of the final clinch joint. 
         [0017]    Additionally, angle F for taper  101  of punch  29  is preferably determined according to the following formula regarding the desired final clinch joint  111  as illustrated in  FIG. 7 : 
         [0000]    
       
         
           
             E 
             = 
             
               
                 B 
                 2 
               
               - 
               
                 
                   
                     P 
                     thk 
                   
                   2 
                 
                  
                 
                   ( 
                   
                     1 
                     
                       tan 
                        
                       
                           
                       
                        
                       F 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where 2E is a minor diameter of side surface  85  where the tapered surface intersects distal end  83 , B is a major diameter of side surface  85  where tapered surface  101  intersects side surface  85 , P thk  is a nominal (unclinched) punch-side workpiece  17  material thickness, and F is the angle of tapered surface  101 . 
         [0018]    The angular dimensions for the present taper  101  have been found to be significantly superior in joint performance as compared to conventional punch tips which have no taper or a mere 5° transitional taper, especially for a workpiece arrangement where punch-side workpiece  17  is less than half the nominal thickness of die-side workpiece  19  as measured prior to joint formation. One such arrangement is where workpieces  17  and  19  are mating components or housings in a refrigerator appliance, or alternately an oven appliance, a dish washer appliance, a clothes washer appliance, a clothes dryer appliance or the like. The specific tip dimensional ranges disclosed herein have been found to better control the workpiece material flow throughout the duration of the material joining process, especially creating a more constant and thicker punch-side workpiece thickness NL and NR at the most inwardly constricted diameter  113  of die-side workpiece  19  where the workpieces interlock together after full clinch joint formation. The further interlocking deformation areas are at LL and LR. In contrast, a traditional punch with no taper at the end-to-sidewall transition is prone to causing undesired tearing or piercing of the punch-side workpiece adjacent constricted diameter  113 . 
         [0019]    By way of further comparison for one example of the present apparatus, workpieces  17  (HRB  55 , textured pre-painted) and  19  (HRB  60 ) are both steel, where the punch-side workpiece has a nominal thickness of 0.19 inch and the die-side workpiece has a nominal thickness of 0.40 inch, clinched together by a punch with taper angle F of 30°. This construction was found to have a finished clinch joint peel strength of at least 25 pounds of force and with a shear strength of at least 75 pounds, while the conventional and untapered punch version had a peel strength of less than 15 pounds of force. In another example, 0.026 inch nominally thick stainless steel (HRB  71 ) was used for punch-side workpiece  17  with HRB  55  steel of 0.40 inch nominal thickness for die-side workpiece  19 ; this generated a peel strength of at least 60 pounds and a shear strength of at least 185 pounds for a 30° taper angle F on punch  29 . In these examples, a diameter C B  of a clinch joint outer button circular periphery is 0.188 inch, a button cap height Z is 0.038 inch, and a total button bottom thickness X is 0.021 inch. Furthermore, the preceding examples employed punch  29  with dimensions of A=3.937 inch, B=0.120 inch, C p =0.480, E=0.044, F=30° and G=0.015 inch. It should be appreciated that these dimensions are simply exemplary and the dimensions may vary (although the joint performance may also then vary). 
         [0020]    While the preferred embodiment of the present clinching apparatus has been disclosed, it should be appreciated that other variations are possible. For example, while three movable die blades have been disclosed, it should be appreciated that two, four or other quantities of die blades surrounding or partially surrounding an anvil of the die assembly may be employed although certain advantages of the present die may not be realized. Furthermore, a punch of a polygonal body periphery can alternately be used although certain manufacturing and cost advantages may not be obtained. The shapes of the punch holder, stripper, spring and stripper retainer can be varied, however, various advantages may not be realized. A moving anvil, fixed die blades, and the absence of the disclosed shield may alternately be used, although such an arrangement would forego many advantageous features of the present device. Additionally, aluminum and other workpiece materials may be employed with the present clinching apparatus although the exact dimensions may need to be slightly varied to account for the material flow differences. It should be appreciated that other modifications and variations may be made to the preferred apparatus without departing from the spirit and scope of the present invention.