Abstract:
A fastener with a tapered head that frictionally mates with a tapered hole in a punched sheet. Frictional attachment is achieved with the fastener head remaining flush with the top of the panel. If a second panel of softer material is placed underneath against the backside of the first panel, in the same pressing step clinch features on the shank of the fastener attach to the second panel and attach the two sheets face-to-face. Attachment of the tapered head with the top panel exploits two different attachment phenomena: a locking taper and, depending on the choice of materials such as stainless steel, attachment by galling.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a non-provisional patent application of U.S. provisional patent application No. 62/197,850 entitled Tapered Head Tack Pin, filed Jul. 28, 2016, priority from which is hereby claimed. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to fasteners for connecting metal panels in face-to-face configuration. 
       BACKGROUND OF THE INVENTION 
       [0003]    To attach first and second panels with a fastener that extends through a hole in the first panel, the fastener usually must have a head that either abuts or otherwise attaches to the first panel. In some configurations, the head of the fastener lies flush or sub-flush with the first panel. In such configurations, the first panel is usually soft enough for the fastener head to embed into the panel to achieve the flush result. This option is not possible, however, if the first panel is composed of a very hard material. Therefore, it would be desirable to provide a fastener that joins top and bottom panels and is flush mounted to the top panel. It would also be desirable to provide a flush-mount fastener that can be flush mounted to a panel composed of a very hard material. 
         [0004]    Galling is a form of wear between sliding surfaces where attachment is the result of friction and adhesion. In the presence of a high force compressing the surfaces together, galling occurs as material from both surfaces is pulled with the contacting surface. Galling is caused by a combination of friction and adhesion between the surfaces, followed by a tearing of the crystalline structure of the materials involved. Therefore, it would also be desirable to provide an assembly of mating metallic panels wherein the panel material composition and attachment process are specifically selected to utilize galling as the primary or secondary attachment mechanism. 
       SUMMARY OF THE INVENTION 
       [0005]    During sheet metal punching, the sheet is supported on a die, which has a hole that is slightly larger than the punch to provide clearance for the punch and slug to pass through. When the hole is punched, a portion of the hole tears out to the larger diameter of the supporting die on the opposite side of the sheet. This condition is shown in  FIG. 2  and makes assembly with the fastener easy to make. The result is a hole with a portion that is divergent (expands radially) in the direction of the die. 
         [0006]    In a preferred embodiment, the invention provides a clinch fastener that is constructed and arranged to attach top and bottom panels and lies flush with the top panel once installed. In another preferred embodiment, the present invention comprises a clinch fastener for flush attachment of a first panel made from a hard material to a second panel made from a relatively soft material such as aluminum. The fastener has a tapered head that frictionally mates with the above-described tapered hole in a punched sheet. By turning the punched sheet die-side-up, and then installing the fastener into the die side of the tapered hole, a frictional attachment can be achieved with the fastener head remaining flush with the top of the panel. If, in the same pressing step, a second panel of softer material is placed underneath and against the backside of the first panel, clinch features on the shank of the fastener attach to the second panel and provide face-to-face attachment of the two sheets. Attachment of the tapered head with the top panel exploits two different attachment phenomena: a locking taper and, depending on the choice of materials, galling. 
         [0007]    In one preferred embodiment, the invention comprises a clinch fastener having from top to bottom: a frustoconical head having a bottom surface substantially perpendicular to a central vertical axis of the fastener; an undercut located immediately below the head; and a shank at the bottom of the fastener located immediately below the undercut. The fastener head preferably has a planar top surface substantially parallel to the bottom surface and the fastener&#39;s lateral cross section is circular and symmetrical about the vertical axis. The fastener shank can be barrel-shaped with a distal bottom end that is tapered. 
         [0008]    In another preferred embodiment, the invention comprises an assembly of a first top panel and a second bottom panel. The first top panel has a first compound circular hole with two concentric sections, namely, a tapered upper section downwardly convergent to a junction with a contiguous lower section of uniform diameter. A second bottom panel is positioned face-to-face with the first panel and has a second circular hole aligned with the lower section of the first hole. A clinch fastener with a frustoconical head joins the first and second panels. The fastener head preferably lies flush with a top surface of the first panel and is adhered to the first top panel by galling. The fastener further includes an undercut which receives the cold flow of metal from the bottom panel. In one embodiment, the fastener and the top panel can be composed of stainless steel. 
         [0009]    In a further embodiment, the invention comprises an assembly of mating metallic panels wherein the panel material composition and attachment process are specifically selected to utilize galling as the primary or secondary attachment mechanism. In this embodiment, the assembly may utilize the inventive connector described above to join metal panels face-to-face to utilize the attachment phenomena of a locking taper and/or galling. 
         [0010]    These and other objects and advantages will be apparent from the following drawings and description of the preferred embodiments. Before explaining numerous embodiments of the invention in detail, it is to be understood that the invention is not limited in its application or to the details of construction in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being carried out in various ways. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a bottom, front perspective view of a fastener in accordance with a preferred embodiment of the invention; 
           [0012]      FIG. 2  is a sectional view of a punch, die and metal panel, and shows the shape of a punched hole in the metal panel of an assembly in accordance with another embodiment of the invention; 
           [0013]      FIG. 3  is an enlarged sectional view of the fastener of  FIG. 1  connecting two metal panels; 
           [0014]      FIGS. 4A and 4B  are enlarged front elevation diagrams illustrating the locking taper forces of the fastener of  FIG. 1  during installation; and, 
           [0015]      FIGS. 5A-5D  are a series of enlarged sectional views showing a method of fastening two metal panels and a panel assembly in accordance with further embodiments of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    A fastener in accordance with a preferred embodiment of the invention is shown in  FIG. 1  and designated generally by reference numeral  10 . In this embodiment, the fastener  10  comprises a clinch fastener having a tapered frusto-conical head  11  with a bottom surface  15 , an undercut  13 , and a barrel-shaped shank  17 . The bottom surface  15  functions as a displacer of material in the receiving (lower) panel  34 . The undercut  13  extends from the bottom surface  15  and receives the displaced material of the lower panel  34 . The barrel-shaped shank  17  has a tapered, distal end for guiding the fastener into the receiving hole in a metal panel. The bottom displacer surface  15  is oriented substantially perpendicular to a central vertical axis “A”. 
         [0017]      FIG. 2  shows a punch  20  and die  22  for making a receiving hole in a metal panel  24  in which the fastener  10  is inserted. The punch  20  is typically ground to size and the die must have a clearance aperture  25  for the punch  20  and slug to pass through. The profile of the hole in the panel  24  after punching has an upper straight wall portion  23 , and a lower, tapered tear-out portion  26  having a larger diameter  26  than the upper portion. The “upper” and “lower” portions are described with reference to the orientation of the panel shown in  FIG. 2 ; however,  FIGS. 3 and 5 , the panel is shown inverted (compared to  FIG. 2 ) and the straight wall portion is located in the lower portion of the hole and the tapered portion is located in the upper portion of the hole. 
         [0018]      FIG. 3  shows an enlarged section of two panels  32 ,  34  that are connected using the fastener  10  described above and installed in accordance with an assembly method of the invention. In this embodiment, the head  11  of the fastener  10  has a shape that generally complements the shape of the hole in the upper panel  32 . The head  11  of the fastener  10  is dimensioned to be installed flush with the top surface  30  of the top panel  32 . The tapered head  11  maintains the mechanical ability to captivate the upper panel  32  to the fastener  10  in the upward direction. In the assembly shown in  FIG. 3 , the bottom panel  34  is composed of softer material than the fastener  10 , which permits the use of a clinch undercut  13 . The displacer surface  15  pushes metal from the bottom panel  34  into the undercut  13  located just above the shank, which holds the fastener  10  to the bottom panel  34 . In a preferred embodiment, the top panel  32  has nearly the same hardness as the fastener  10 . 
         [0019]    The locking taper feature of the fastener  10  is illustrated in  FIGS. 4A and 4B , which show both the installation and static condition of the fastener  10  as described in  FIGS. 1 and 3 .  FIGS. 4A and 4B  illustrate and describe the forces and scheme necessary to determine the minimal tapering required for self-locking. The orthogonal force of the uniformly distributed taper “Fn” is modeled at the midline of the conical section at “Dm”. Quite simply, the locking taper will retain the tapered portion of the fastener in the top panel when the vertical component of the friction force exceeds the vertical component of the normal force and the installation force is removed or is zero. The friction force acts in the direction opposite the direction the fastener is being pushed. When the installation force is removed, the vertical component of the normal force acts to push the fastener out of the top panel. The friction force holds the fastener in place in the opposite direction. 
         [0020]    Referring to the static diagrams of  FIGS. 4A and 4B , the theoretical force needed to extract the tapered connector “Fe” may be calculated as follows: 
         [0021]    Summation of the forces in the “Y” direction: 
         [0000]    
       
         
           
             
               ∑ 
               Fy 
             
             = 
             
               0 
               = 
               
                 Fe 
                 - 
                 
                   ( 
                   
                     2 
                     * 
                     
                       1 
                       2 
                     
                     * 
                     μ 
                     * 
                     Fn 
                     * 
                     
                       cos 
                        
                       
                         ( 
                         
                           α 
                           2 
                         
                         ) 
                       
                     
                   
                   ) 
                 
                 + 
                 
                   ( 
                   
                     2 
                     * 
                     
                       1 
                       2 
                     
                     * 
                     Fn 
                     * 
                     
                       sin 
                        
                       
                         ( 
                         
                           α 
                           2 
                         
                         ) 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             Fe 
             = 
             
               Fn 
                
               
                 ( 
                 
                   
                     μ 
                     * 
                     
                       cos 
                        
                       
                         ( 
                         
                           α 
                           2 
                         
                         ) 
                       
                     
                   
                   - 
                   
                     sin 
                      
                     
                       ( 
                       
                         α 
                         2 
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0022]    Understanding that the coefficient of friction “μ”=tan(ψ) where ψ is an implicit sliding angle 
         [0000]    
       
         
           
             ϕ 
             = 
             
               arctan 
                
               
                 ( 
                 μ 
                 ) 
               
             
           
         
       
       
         
           
             Fe 
             = 
             
               Fn 
               * 
               
                 ( 
                 
                   
                     
                       
                         sin 
                          
                         
                           ( 
                           ϕ 
                           ) 
                         
                       
                       
                         cos 
                          
                         
                           ( 
                           ϕ 
                           ) 
                         
                       
                     
                     * 
                     
                       cos 
                        
                       
                         ( 
                         
                           α 
                           2 
                         
                         ) 
                       
                     
                   
                   - 
                   
                     sin 
                      
                     
                       ( 
                       
                         α 
                         2 
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             Fe 
             = 
             
               
                 Fn 
                 
                   cos 
                    
                   
                     ( 
                     ϕ 
                     ) 
                   
                 
               
                
               
                 ( 
                 
                   
                     
                       sin 
                        
                       
                         ( 
                         ϕ 
                         ) 
                       
                     
                     * 
                     
                       cos 
                        
                       
                         ( 
                         
                           α 
                           2 
                         
                         ) 
                       
                     
                   
                   - 
                   
                     sin 
                      
                     
                       ( 
                       
                         α 
                         2 
                       
                       ) 
                     
                   
                 
                 ) 
               
               * 
               
                 cos 
                  
                 
                   ( 
                   ϕ 
                   ) 
                 
               
             
           
         
       
       
         
           
             Fe 
             = 
             
               
                 Fn 
                 
                   cos 
                    
                   
                     ( 
                     ϕ 
                     ) 
                   
                 
               
               * 
               
                 sin 
                  
                 
                   ( 
                   
                     ϕ 
                     - 
                     
                       α 
                       2 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0023]    For a locking condition: Fe=0 
         [0024]    Therefore: 
         [0000]    
       
         
           
             
               ϕ 
               - 
               
                 α 
                 2 
               
             
             = 
             0 
           
         
       
     
         [0025]    α=2ψ 
         [0000]      α=2arctan(μ)
 
         [0026]    The angle for locking can be defined in terms of the coefficient of friction as 
         [0000]      α≦2arctan(μ)
 
         [0000]      α=2arctan(0.06) Therefore α=6.87 0  
 
         [0000]    This locking taper force (Fe) was calculated using a conservative coefficient of friction for lubricated metal on metal of 0.06. 
         [0027]    In another preferred embodiment, the locking taper force (Fe), dimensions of the tapered hole, and dimensions of the tapered head are calculated by taking into account “galling”, which is another contributing locking mechanism between the mating sheets. Galling is a form of wear between sliding surfaces. For the fastener and assembly shown in  FIGS. 1-5 , the sliding surface is the interface between the tapered head  11  of the fastener  10  and the punched hole in the harder panel  24 . In the presence of a high force compressing the surfaces together, galling occurs as material from both surfaces is pulled with the contacting surface. Galling is caused by a combination of friction and adhesion between the surfaces, followed by a tearing of the crystalline structure of the materials involved. The galling surfaces deposit material on the mating surface, effectively creating a friction or cold weld. Common materials that are prone to galling are titanium, stainless steel, and aluminum. 
         [0028]    An assembly of two mating panels and a method of assembling the panels in accordance with a preferred embodiment of the invention are illustrated in  FIGS. 5A-D . Referring to  FIG. 5A , the fastener  10  is initially positioned in the hole in the top panel  32  in the orientation shown therein with the tapered surfaces properly aligned. Next, as seen in  FIG. 2 , a press tool  53  forces the fastener  10  downwardly into the top panel  32  from the tapered side of the hole while the bottom panel  34  is supported by an anvil  54 . Then, as seen in  FIGS. 5C and 5D , when the fastener  10  is pressed by the tool  53  against the anvil  54  with sufficient force, material from both panels flow slightly to form a uniform boundary between the two panels, which creates a very tight fit at high pressure between the fastener  10  and panels  32 ,  34 . In this configuration, the fastener  10  can only be removed in the reverse direction of its installation. Furthermore, the tapered fastener head  11  is locked in the top panel  32  by the above-described locking taper force or galling or both. 
         [0029]      FIG. 5A  shows a profile of a punched hole, which has a first portion with straight walls, and a second portion which is conical and faces upward. As installation of the fastener progresses,  FIGS. 5B and 5C  show how the interface between the fastener and the hard top panel become unified in a common geometry. Pressure has made the top panel flow slightly to the fully conical shape, perfectly mated to the fastener  10 . The tapered head locking fastener exhibits high forces between the mating tapered surface, as well as high friction during installation. In one exemplary embodiment of the invention, when a stainless steel fastener is pressed into a hard stainless steel top panel, galling occurs and aids in the retention of the fastener. The same can be said of any other combination of metals prone to galling.  FIG. 5D  shows the fully-installed fastener clinched into the softer bottom panel  54  resulting in the attachment of the two panels. 
         [0030]    The foregoing is to be considered illustrative only of the principles and possible embodiments of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, suitable modifications and equivalents may be resorted to, all falling within the scope of the invention which shall be determined only by the following claims and their legal equivalents.