Abstract:
A resistance welding apparatus having a pair of programmable ball electrodes carried on universally movable positioners. The positioners are programmed to move the ball electrodes simultaneously along a seam line so that the ball electrodes clamp and support opposite sides of a pair of stacked workpieces and are electrically charged to form resistance seam welds along the seam lines to connect the workpieces.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to resistance seam welding and, more particularly, to a resistance seam welding apparatus and method.  
       BACKGROUND OF THE INVENTION  
       [0002]     Resistance seam welding is known in the art for joining metal workpieces. The workpieces may be stacked or otherwise held in temporary assembly. The workpieces are then clamped between a pair of roller electrodes, which locally compress the workpieces. The electrodes are energized, causing electrical current flow through the workpieces to locally heat the workpieces between the electrodes and thereby form a weld. During this time the workpieces and the rollers are moved relative to one another to elongate the weld and thereby form a seam weld between the workpieces.  
         [0003]     Roller electrodes commonly have a larger contact area than conventional electrodes, which distributes welding current over a larger workpiece area. Thus a greater flow of electric current is required than with conventional electrodes to heat the larger area and form a weld. The larger contact area of a roller electrode also creates welding limitations when the workpieces to be joined are contoured or non-planar. In addition, roller wheels must be turned as they are moved relative to the workpieces to create a curved or angled seam weld.  
         [0004]     Thus, it is desirable to provide a resistance seam welding electrode which provides a smaller contact area than conventional rollers to reduce electric current requirements. It is also desirable to provide an apparatus for resistance seam welding having electrodes adapted for universal directional motion to conform to any desired weld pattern or part shape.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a resistance seam welding apparatus having a pair of universally programmable movable positioners each preferably carrying a ball electrode. The positioners are programmed to move the ball electrodes simultaneously along a seam line so that the electrodes clamp on opposite sides of a pair of workpieces as current is passed through the workpieces, between the electrodes, to form a resistance seam weld.  
         [0006]     In an exemplary embodiment, the resistance seam welding apparatus may include a suitable holding fixture or support adapted to carry a structural assembly formed of stacked workpieces.  
         [0007]     The resistance seam welding apparatus may further include a first programmable positioner in the form of a programmable robot. The positioner includes an end effector or holder carrying a coupler shank with a part-spherical socket for carrying a ball electrode therein. A retainer ring is attached to the shank, to retain the ball electrode in the socket. If desired, a cooling passage may extend within the jointed arm and into the coupler shank to provide liquid coolant flow to remove heat from the coupler shank and the ball electrode.  
         [0008]     The resistance seam welding apparatus may also include a second programmable positioner located beneath the support. The second positioner includes a holder mounting a coupler shank with a part-spherical socket carrying a ball electrode therein. A retainer ring is attached to the end of the shank to retain the ball electrode within the socket. If desired, a cooling passage may extend within the lower coupler shank to provide liquid coolant flow to remove heat from the coupler shank and the electrode.  
         [0009]     An electric current source, such as a transformer is attached to the base of the robot and supplies welding current to the upper and lower electrodes through the positioners.  
         [0010]     The first and second positioners of the invention may be used with workpieces having differing configurations, which may be accommodated by merely programming the positioners. The ball electrodes provide smaller welding contact points than do conventional roller wheels. These reduce the amount of contacting surface area between the electrodes and the surfaces of the workpieces, which reduces the amount of welding current required to form a weld. The ball electrodes allow the positioners to move the ball electrodes over curves and bends in the workpieces without loosing contact. The ball electrodes also allow the positioners to freely move the electrodes in any direction without having to steer the electrodes.  
         [0011]     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic pictorial view of a resistance seam welding apparatus according to the invention;  
         [0013]      FIG. 2  is a schematic cross-sectional view through a ball electrode assembly for the resistance seam welding apparatus of  FIG. 1 ; and  
         [0014]      FIG. 3  is a schematic cross-sectional view through an alternative ball electrode assembly for the resistance seam welding apparatus of  FIG. 1 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring first to  FIG. 1  of the drawings, numeral  10  generally indicates a workstation containing an apparatus  11  configured for resistance seam welding of workpieces. A temporary assembly of workpieces, such as a tunneled floor pan  12  and a tunnel undershield  14  are supported in the apparatus for welding into a structural assembly  15  in the form of a floor pan assembly for a vehicle. The structural assembly  15  includes an upper side  16  of the floor pan  12  and a lower side  17  of the undershield  14 .  
         [0016]     The resistance seam welding apparatus  11  includes a suitable holding fixture or support  18  adapted to carry the temporary assembly of the structural floor pan  12  and undershield prior to and during welding of these workpieces into the structural assembly  15 .  
         [0017]     The resistance seam welding apparatus  11  further includes a first positioner in the form of a robot  20 . If appropriate, any other suitable form of programmable positioner may be substituted for the robot  20  within the scope of the invention.  
         [0018]     The robot  20  includes a base  22  supporting a jointed arm  24  with an end effector or holder  26  carrying an electrode assembly  28 , assembly  28  best shown in  FIG. 2 . Assembly  28  includes a coupler shank  30  with an end  32  defining a part-spherical socket  34  receiving a ball electrode  35 , which acts as an upper electrode. A retainer ring  36  is attached to the end  32  to retain the ball electrode  35  within the socket  34 . The retainer ring  36  may be formed of any suitable material, for example it could be formed of graphite to provide heat resistant lubrication for the ball electrode  35  within the socket  34 .  
         [0019]     When the ball electrode  35  is retained in the socket  34 , the ball should be able to roll in any direction within the socket to allow the positioner  20  to move the upper electrode in any direction along the surface of the structural assembly  15 . A coolant passage  37  for circulating liquid coolant extends through the jointed arm  24  and into the coupler shank  30  to allow welding heat to be transferred from the upper electrode to the liquid coolant.  
         [0020]     The resistance seam welding apparatus  11  also includes a second programmable positioner  39  located beneath the support  18 . The positioner  39  includes a base  40  that is linearly movable along rails  41  extending about the length of the support  18 . A plurality of control arms  42  carry a positionable holder  43  mounting a coupler shank  44  which may be similar to coupler shank  30 . The positionable coupler shank  44  has an end  45  defining a part-spherical socket  46  adapted for receiving a ball electrode  47 , which acts as a lower electrode. A retainer ring  48  attaches to the end  45  to retain the ball electrode  47  within the socket  46 . The retainer ring  48  may be formed of graphite to provide lubrication for the ball electrode  47  within the socket  46 . The ring  48  could be made of other suitable materials, and lubrication, if needed, could be provided by other means.  
         [0021]     When the ball electrode  47  is retained in the socket  46 , the ball should be able to roll in any direction within the socket to allow the positioner  39  to move the lower electrode in any direction along the surface of the structural assembly  15 . A cooling passage  49  adapted for circulating liquid coolant extends through the holder  43  and into the coupler shank  44  to allow welding heat to be transferred from the lower electrode to the liquid coolant.  
         [0022]     A transformer  50  provides welding current for the ball electrodes  35 ,  47 .  
         [0023]     Preferably the electrodes  35 ,  47  are spherical and universally rotatable within their respective sockets  34 ,  46  to create ball contact points for multidirectional seam welding. The ball electrodes should be of adequate size to move across the surfaces of the structural assembly while maintaining adequate contact with the structural assembly to form a continuous seam weld.  
         [0024]     In operation, the spatial coordinates of the structural assembly  15  are programmed into the positioners  20 ,  39 . A structural assembly  15  comprising workpieces  12 ,  14  in temporary assembly with opposing surfaces in contact along a seam line are placed onto the support  18  of the welding apparatus  11 . The first and second positioners  20 ,  39  subsequently position the upper and lower ball electrodes  35 ,  47  at a first selected location  52  along the seam line so that the electrodes engage opposite sides  16 ,  17  of the structural assembly  15 .  
         [0025]     The transformer  50  then energizes the ball electrodes  35 ,  47  to cause welding current to travel between the electrodes through the first selected location  52  to form a weld  54  between the electrodes. As the weld  54  forms between the ball electrodes  35 ,  47 , the positioners  20 ,  39  move the electrodes along the opposite surfaces of the structural assembly  15  to form a seam weld. As the positioners  20 ,  39  move the ball electrodes along the surfaces  16 ,  17 , the electrodes  35 ,  47  roll within their respective sockets  34 ,  46  similar to a ball point pen to maintain contact with the surfaces  16 ,  17  of the workpieces  12 ,  14 . During this time, the positioners  20 ,  39  adjust positioning of the ball electrodes  35 ,  47 , as needed, to maintain optimal electrode contact with the workpieces  12 ,  14  for optimal weld quality.  
         [0026]     The ball electrodes  35 ,  47  and their sockets should be made of suitable heat resistant high current (low resistance) materials, such as copper zirconium to maximize electric current through the workpieces and limit temperatures of the electrodes and excessive heat loss to the coolant in the coupler shanks  30 ,  44 . Other suitable materials may also be used if desired.  
         [0027]     During the welding process, coolant is circulated through the cooling passages  37 ,  49  to remove excess heat from the coupler shanks  30 ,  44  and the ball electrodes  35 ,  47 .  
         [0028]     The electrodes  35 ,  47 , may be sequentially repositioned at subsequent selected locations  52  to allow the electrodes to form multiple seam welds  54  along multiple seam lines. Once all of the seam lines  52  are welded the structural assembly  15  is completed and removed from the support  18 .  
         [0029]     The ball shape of the electrodes  35 ,  47  improves the versatility of the seam welding apparatus  11  by allowing the positioners  20 ,  39  to move the electrodes over various contours on the surfaces of the workpieces to form non-planar or non-linear seam welds. In addition, the ball electrodes  35 ,  47  allow the positioners  20 ,  39  to move the electrodes in a 360 degree pattern along the surface of the structural assembly  15  to form continuous closed pattern seam welds.  
         [0030]     When the retaining rings are formed of a lubricating material, such as graphite, the retaining rings  36 ,  48  provide lubrication for the ball electrodes  35 ,  47  within the sockets  34 ,  46  to allow the ball electrodes to roll freely over the surfaces  16 ,  17  of the workpieces  12 ,  14 .  
         [0031]     The ball shape of the electrodes  35 ,  47  provide small surface area contacting the surfaces  16 ,  17  of the structural assembly  15 . As a result, the amount of current required to form a weld between the ball electrodes  35 ,  47  can be reduced, thereby increasing the efficiency of the welding apparatus  11 .  
         [0032]      FIG. 3  shows an alternative electrode assembly  60  similar to electrode assembly  28  where like numbers indicate like parts. Assembly  60  includes a coupler shank  62  with an end  64  defining a part-spherical socket, not shown, adapted to retain a ball electrode  35 . The socket is provided with a groove  68  adapted to retain a conductive leaf type spring  70  within the socket and engaging the ball electrode  35 . The spring  70  is preferably formed of a carbon material that provides lubrication between the ball electrode and the coupler shank. A retainer ring  72  is attached to the end  64  to retain the ball electrode  35  and the spring  70  within the socket.  
         [0033]     The spring  70  provides a large conductive contact with the ball electrode  35  to provide welding current to the ball electrode. The spring  70  also reduces friction by providing lubrication between the ball electrode  35  and the coupler shank  62 . In addition, the spring  70  may movably suspend the ball electrode  35  within the socket of the coupler shank  62  to allow the ball electrode to move axially within the socket.  
         [0034]     When the ball electrode  35  is retained in the socket, the ball electrode should be able to roll in any direction within the socket to allow a positioner to move the ball electrode in any direction along the surface of the structural assembly  15 . A coolant passage  74  for circulating liquid coolant extends into the coupler shank  62  to allow welding heat to be transferred from the ball electrode  35  to the liquid coolant.  
         [0035]     In operation, electrode assembly  60  operates similarly to electrode assembly  28  in that the ball electrode  35  rotates within the socket to allow a positioner to move the electrode assembly over the surfaces of a structural assembly  15 .  
         [0036]     It should be understood, that either one of the ball electrodes  35 ,  47  may be replaced with any suitable electrode that will conduct current from the remaining ball electrode through the assembly opposite the locations of the remaining ball electrode.  
         [0037]     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.  
       GP-304381 NUMBER SHEET 07002.266  
       [0000]    
       
           10 . workstation  
           11 . apparatus (welding)  
           12 . workpiece—floor pan  
           14 . workpiece—undershield  
           15 . structural assembly (floor pan)  
           16 . upper side  
           17 . lower side  
           18 . support  
           20 . robot (first positioner)  
           22 . base  
           24 . jointed arm  
           26 . end effector/holder  
           28 . electrode assembly  
           30 . coupler shank  
           32 . end  
           34 . socket  
           35 . ball electrode  
           36 . retainer ring  
           37 . coolant passage  
           38 .  
           39 . second positioner  
           40 . base  
           41 . rail  
           42 . control arms  
           43 . holder  
           44 . coupler shank  
           45 . end  
           46 . socket  
           47 . ball electrode  
           48 . retainer ring  
           49 . cooling passage  
           50 . transformer  
           52 . selected location  
           54 . weld  
           56 .  
           58 .  
           60 . electrode assembly  
           62 . coupler shank  
           64 . end  
           66 .  
           68 . groove  
           70 . spring  
           72 . retainer ring  
           74 . coolant passage  
           76 .  
           78 .  
           80 .  
           82 .  
           84 .  
           86 .  
           88 .  
           90 .  
           92 .  
           94 .