Abstract:
A welding device includes a lower tip and an upper tip, as welding tips, and pressuring members. The pressuring members are supported by a support member disposed in the upper tip. The pressuring members are displaced by the action of pressuring member displacement mechanisms and, together with the upper tip, come into contact with a metal plate arranged on the outermost part of a laminated body.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a welding apparatus (device) for welding a stacked body of a plurality of workpieces. 
       BACKGROUND ART 
       [0002]      FIG. 74  is a schematic front view for illustrating a spot welding process for joining high resistance workpieces  1 ,  2 , which are made of a so-called high tensile strength steel and have a large thickness to exhibit a high electric resistance. The two high resistance workpieces  1 ,  2  are stacked to form a stacked body  3 . The stacked body  3  is gripped and pressed between a first welding tip  4  and a second welding tip  5 . When the first welding tip  4  and the second welding tip  5  are energized, a portion is heated to form a melted portion  6  in the vicinity of the contact surface between the high resistance workpieces  1 ,  2 . Then, the melted portion  6  is solidified to generate a solid phase, which is referred to as a nugget. 
         [0003]    Since the high resistance workpieces  1 ,  2  have the high electric resistance, a large amount of Joule heating is generated in the vicinity of the contact surface during the energization, so that the melted portion  6  grows larger as shown in  FIG. 75  in a relatively short time. Therefore, the melted portion  6  is liable to be scattered (spatter generation is liable to be caused). Thus, in the spot welding process for joining the high resistance workpieces  1 ,  2 , it is necessary to highly accurately control a welding current in view of preventing the spatter generation. However, such control cannot be achieved easily. This problem is caused even in the case of joining a thinner high tensile strength steel workpiece. 
         [0004]    In the case of joining three or more workpieces, the workpieces may contain different materials and may have different thicknesses. For example, as shown in  FIG. 76 , an outermost workpiece (a low resistance workpiece  7 ) may have the smallest thickness. Incidentally, in  FIG. 76 , the low resistance workpiece  7  is made of a mild steel, exhibits a low electric resistance, and is stacked on the high resistance workpieces  1 ,  2  shown in  FIGS. 74 and 75  to form a stacked body  8 . 
         [0005]    In the process of spot welding the stacked body  8 , a larger amount of Joule heating is generated in the vicinity of the contact surface between the high resistance workpieces  1 ,  2  than in the vicinity of the contact surface between the low resistance workpiece  7  and the high resistance workpiece  2 . This is because a higher contact resistance is generated in the vicinity of the contact surface between the high resistance workpieces  1 ,  2 . 
         [0006]    Therefore, in the stacked body  8 , a melted portion  9  is developed first in the vicinity of the contact surface between the high resistance workpieces  1 ,  2 . As shown in  FIG. 77 , the melted portion  9  may grow larger before another melted portion is developed in the vicinity of the contact surface between the low resistance workpiece  7  and the high resistance workpiece  2 . When the energization is continued to form the other melted portion in the vicinity of the contact surface between the low resistance workpiece  7  and the high resistance workpiece  2 , the spatter generation may be caused in the vicinity of the contact surface between the high resistance workpieces  1 ,  2 . 
         [0007]    However, if the energization is stopped, the melted portion and hence the nugget are not grown to a sufficiently large size in the vicinity of the contact surface between the low resistance workpiece  7  and the high resistance workpiece  2 . Accordingly, a desired bonding strength is hardly achieved between the low resistance workpiece  7  and the high resistance workpiece  2 . 
         [0008]    This problem may occur also with an indirect feeding type welding apparatus. 
         [0009]      FIG. 78  is a schematic side view of a stacked body  14  of three the metallic plates  11 ,  12 ,  13  gripped by an indirect feeding type welding apparatus  15 . The indirect feeding type welding apparatus  15  has a first welding gun (not shown) for supplying a welding current and a second welding gun  16  for welding the stacked body  14 . The welding current is transferred from the first welding gun through an external feed terminal  17  to the second welding gun  16 . Such a structure of the indirect feeding type welding apparatus  15  is known from Japanese Laid-Open Patent Publication No. 07-136771, Japanese Laid-Open Utility Model Publication No. 59-010984, etc. 
         [0010]    Specifically, the first welding gun has a positively (+) polarized upper electrode  18  and a negatively (−) polarized lower electrode  19 . The second welding gun  16  has an upper tip  20  corresponding to the first welding tip and a lower tip  21  corresponding to the second welding tip. The external feed terminal  17  is prepared by interposing an insulator  23  between conductive terminals  22   a,    22   b.  The upper electrode  18  and the upper tip  20  are electrically connected by the conductive terminal  22   a  and a lead  24 , and the lower electrode  19  and the lower tip  21  are electrically connected by the conductive terminal  22   b  and a lead  25 . 
         [0011]    In the welding process, the stacked body  14  is gripped between the upper tip  20  and the lower tip  21  of the second welding gun  16 . The welding current flows through the stacked body  14  from the upper tip  20  to the lower tip  21  in the thickness direction. A portion is heated to form a melted portion in the vicinity of each of the contact surface between the metallic plates  11 ,  12  and the contact surface between the metallic plates  12 ,  13 . Then, the melted portions are solidified to generate solid-phase nuggets, whereby the metallic plates  11 ,  12  are connected and the metallic plates  12 ,  13  are connected to each other. 
         [0012]    In a case where the metallic plates  11 ,  12  are the high resistance workpieces, which are made of a high tensile strength steel, have a large thickness, and exhibit a high electric resistance, and the metallic plate  13  is the low resistance workpiece, which is made of a mild steel and exhibits a low electric resistance, a larger amount of Joule heating is generated in the vicinity of the contact surface between the metallic plates  11 ,  12  (the high resistance workpieces) than in the vicinity of the contact surface between the metallic plates  12 ,  13  (the low resistance workpiece and the high resistance workpiece). This is because a higher contact resistance is generated in the vicinity of the contact surface between the metallic plates  11 ,  12 . 
         [0013]    Therefore, in the stacked body  14 , as shown in  FIG. 79 , a melted portion  26  is developed first in the vicinity of the contact surface between the metallic plates  11 ,  12 . The melted portion  26  may grow larger before another melted portion is developed in the vicinity of the contact surface between the metallic plates  12 ,  13 . When the energization is continued to form the other melted portion in the vicinity of the contact surface between the metallic plates  12 ,  13 , a part of the melted portion  26  may be scattered from a gap between the metallic plates  11 ,  12 , and thus the spatter generation may be caused around the gap. 
         [0014]    However, if the energization is stopped, the melted portion and hence the nugget are not grown to a sufficiently large size in the vicinity of the contact surface between the metallic plates  12 ,  13 . Accordingly, a desired bonding strength is hardly achieved between the metallic plates  12 ,  13 . 
         [0015]    In Japanese Patent No. 3894545, the applicant has proposed that, in the process of spot welding such a stacked body, the pressing force of the first welding tip, applied to the low resistance workpiece, is made smaller than that of the second welding tip. In this case, the contact pressure of the low resistance workpiece against the high resistance workpiece is reduced. Therefore, the contact resistance between the low resistance workpiece and the high resistance workpiece is increased, so that a sufficient amount of Joule heating is generated at the contact surface. Consequently, the nugget between the low resistance workpiece and the high resistance workpiece can be grown to approximately the same size as the nugget between the high resistance workpieces, whereby the resultant stacked body can exhibit an excellent bonding strength. 
       SUMMARY OF INVENTION  
       [0016]    A general object of the present invention is to provide a welding apparatus capable of forming a sufficiently large nugget in the vicinity of a contact surface between workpieces in a stacked body. 
         [0017]    A principal object of the present invention is to provide a welding apparatus capable of eliminating the possibility of spatter generation. 
         [0018]    According to an aspect of the present invention, there is provided a spot welding apparatus for spot welding a stacked body of a plurality of workpieces, comprising first and second welding tips, between which the stacked body is interposed, a pressing member for pressing an outermost workpiece of the stacked body, the first welding tip and the pressing member being brought into contact with different portions of the outermost workpiece, and a holder for holding the first welding tip and the pressing member, which is displaced by a holder displacement mechanism, wherein the holder has a pressing member displacement mechanism for displacing the pressing member, and the pressing member displacement mechanism is electrically isolated from the holder. 
         [0019]    According to another aspect of the present invention, there is provided a spot welding apparatus for spot welding a stacked body of a plurality of workpieces, comprising first and second welding tips, between which the stacked body is interposed, a first displacement mechanism for displacing at least one of the first and second welding tips, a pressing member for pressing an outermost workpiece of the stacked body, the first welding tip and the pressing member being brought into contact with different portions of the outermost workpiece, a second displacement mechanism for displacing the pressing member independently from the first or second welding tip, and a pressing mechanism for generating a pressing force of the pressing member. 
         [0020]    According to a further aspect of the present invention, there is provided an indirect feeding type welding apparatus comprising first and second welding guns, wherein a current is supplied from the first welding gun through an external feed terminal to the second welding gun, whereby the second welding gun is used for welding a stacked body of a plurality of workpieces, and the second welding gun contains first and second welding tips movable close to and away from each other, and further contains a displaceable pressing member for pressing an outermost workpiece of the stacked body. 
         [0021]    In any aspect, the pressing forces of the first welding tip and the pressing member are balanced with the pressing force of the second welding tip, so that the pressing force of the first welding tip is smaller than that of the second welding tip. Therefore, in the stacked body between the first welding tip and the substantially opposite second welding tip, the total of the pressing forces acts on a wider or larger area in a position closer to the second welding tip. Thus, the total force acting on the contact surface between the outermost workpiece (in contact with the first welding tip) and the adjacent workpiece is smaller than the total force acting on the other contact surface between the workpieces. 
         [0022]    Since the pressing forces are distributed in this manner, the contact area at the contact surface between the outermost workpiece and the adjacent workpiece is smaller than the contact area at the other contact surface between the workpieces. Therefore, the contact resistance can be made higher to increase the generation amount of Joule heating at the contact surface between the outermost workpiece and the adjacent workpiece. Consequently, the nugget can be grown larger on the contact surface, and thus the bonding strength can be improved, between the outermost workpiece and the adjacent workpiece. 
         [0023]    Since the metallic plates are pressed by the pressing member, the outermost workpiece can be prevented from separating from the adjacent workpiece. Consequently, spatter scattering of the softened melted portion from a gap between the outermost workpiece and the adjacent workpiece can be prevented. 
         [0024]    The first welding tip and the pressing member are preferably attached to one holder (support member). In this case, the welding apparatus can be prevented from having a complicated or large structure. Therefore, even in a case where an intricately-shaped stacked body is welded, the stacked body can be located in a desired welding position without interference from the first welding tip and the pressing member. 
         [0025]    It is preferred that the first displacement mechanism is used for displacing the first welding tip, the second displacement mechanism is used for displacing the pressing member, and the displacement mechanisms are independent from each other. In this case, the first welding tip and the pressing member can be easily contacted with and separated from the stacked body independently from each other. Thus, the pressing force of the pressing member acting on the stacked body can be easily controlled. 
         [0026]    The pressing member may be utilized as an auxiliary electrode having a polarity opposite to that of the first welding tip, and a branching current may flow from the first welding tip to the auxiliary electrode or from the auxiliary electrode to the first welding tip in an energization process. 
         [0027]    In this case, the current flows through the outermost workpiece in the direction from the first welding tip to the auxiliary electrode or the opposite direction. Therefore, the contact surface between the outermost workpiece and the adjacent workpiece is sufficiently heated by the current. Consequently, the nugget can be grown sufficiently larger at the contact surface, so that the resultant bonded product can be further excellent in bonding strength. 
         [0028]    The welding apparatus may further comprise, in the vicinity of the second welding tip, another auxiliary electrode having a polarity opposite to that of the second welding tip. In this case, after the branching current from the first welding tip to the auxiliary electrode (in the vicinity of the first welding tip) or from the auxiliary electrode to the first welding tip has vanished, another branching current may flow from the other auxiliary electrode (in the vicinity of the second welding tip) to the second welding tip or from the second welding tip to the other auxiliary electrode. 
         [0029]    In this case, the nugget can be grown sufficiently larger in the vicinity of the contact surface between the outermost workpiece against which the second welding tip is in abutment and the workpiece adjacent thereto. 
         [0030]    For example, in a case where the stacked body interferes with the first welding tip and the pressing member and thereby cannot be readily welded, it is preferred that a first support tip and a support pressing member are interposed between the first welding tip and the stacked body and between the pressing member and the stacked body respectively, and a second support tip is interposed between the second welding tip and the stacked body. 
         [0031]    In such a structure, pressing positions of the first and second welding tips and the pressing member can be away from the stacked body, while the first and second support tips and the support pressing member are brought into contact with the stacked body. Therefore, even when the stacked body has a complicated shape, the stacked body can be easily welded. 
         [0032]    In this structure, the pressing forces of the first support tip and the support pressing member are balanced with the pressing force of the second support tip. Thus, the total of the pressing forces acts on a wider area in a position closer to the second support tip than to the first support tip. 
         [0033]    It is to be understood that the support pressing member may act as an electrode in the same manner as the pressing member, so that a current may flow in the direction from the support tip to the support pressing member or the opposite direction. In this case, the current flows through the outermost workpiece in the stacked body. Therefore, the contact surface between the outermost workpiece and the adjacent workpiece is sufficiently heated by the current. Consequently, the nugget can be grown sufficiently larger at the contact surface, so that the resultant joined regions can be further excellent in bonding strength. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0034]      FIG. 1  is an enlarged view of essential features showing a welding apparatus (spot welding apparatus) according to a first embodiment of the present invention; 
           [0035]      FIG. 2  is an enlarged vertical cross-sectional view of essential features showing a holder in the spot welding apparatus of  FIG. 1 ; 
           [0036]      FIG. 3  is an enlarged vertical cross-sectional view of essential features showing a condition in which a downward movement of a pressing member shown in  FIG. 2  is moved downward; 
           [0037]      FIG. 4  is a schematic front view of essential features showing a stacked body to be welded, gripped by an upper tip (first welding tip), a lower tip (second welding tip), and pressing rods (pressing members); 
           [0038]      FIG. 5  is a schematic front view (with a graph) for illustrating an appropriate surface pressure distribution between an uppermost workpiece and a workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0039]      FIG. 6  is a schematic front view of the stacked body, gripped only by the lower and upper tips; 
           [0040]      FIG. 7  is a schematic vertical cross-sectional view of the stacked body at the start of energization for generating a current flow from the upper tip to the lower tip after the state of  FIG. 4 ; 
           [0041]      FIG. 8  is a schematic front view of essential features showing a stacked body different from that of  FIG. 4 , gripped by the lower tip, the upper tip, and the pressing rods (pressing members); 
           [0042]      FIG. 9  is a schematic front view of essential features showing a stacked body different from those of  FIGS. 4 and 8 , gripped by the lower tip, the upper tip, and the pressing rods (pressing members); 
           [0043]      FIG. 10  is a schematic front view of essential features showing the stacked body, gripped by the upper tip, the lower tip, and auxiliary electrodes in a welding apparatus (spot welding apparatus) according to a second embodiment of the present invention; 
           [0044]      FIG. 11  is a schematic vertical cross-sectional view of the stacked body at the start of energization for generating a current flow from the upper tip to the lower tip after the state of  FIG. 10 ; 
           [0045]      FIG. 12  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization continuously after the state of  FIG. 11 ; 
           [0046]      FIG. 13  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after only the auxiliary electrodes are separated from the stacked body; 
           [0047]      FIG. 14  is a schematic vertical cross-sectional view of the stacked body after completion of the energization (spot welding) by separating the upper tip from the stacked body after the state of  FIG. 13 ; 
           [0048]      FIG. 15  is a schematic front view of essential features showing a stacked body different from that of  FIG. 10 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0049]      FIG. 16  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the auxiliary electrodes are electrically disconnected from a negative terminal of a power source; 
           [0050]      FIG. 17  is a schematic vertical cross-sectional view of the stacked body at the end of the energization (spot welding); 
           [0051]      FIG. 18  is a schematic front view of essential features showing a stacked body different from those of  FIGS. 10 and 15 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0052]      FIG. 19  is a schematic vertical cross-sectional view of the stacked body after completion of the energization (spot welding); 
           [0053]      FIG. 20  is a schematic front view of essential features showing a stacked body different from those of  FIGS. 10 and 15 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0054]      FIG. 21  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the auxiliary electrodes in the vicinity of the upper tip are electrically disconnected from the negative terminal of the power source, and the auxiliary electrodes in the vicinity of the lower tip are brought into contact with a workpiece; 
           [0055]      FIG. 22  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the auxiliary electrodes in the vicinity of the lower tip are electrically disconnected from a positive terminal of the power source; 
           [0056]      FIG. 23  is a schematic vertical cross-sectional view of the stacked body in which a current flows from the lower tip and the auxiliary electrodes to the upper tip in the direction opposite to that of  FIG. 11 ; 
           [0057]      FIG. 24  is a schematic vertical cross-sectional view of a current flow from the upper tip to the auxiliary electrodes through the uppermost workpiece and the workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0058]      FIG. 25  is a schematic side view of essential features showing a welding apparatus (spot welding apparatus) according to a third embodiment of the present invention; 
           [0059]      FIG. 26  is an enlarged schematic front view of essential features showing the spot welding apparatus of  FIG. 25 ; 
           [0060]      FIG. 27  is a schematic front view of essential features showing a stacked body to be welded, gripped by a lower tip, an upper tip, and auxiliary electrodes; 
           [0061]      FIG. 28  is a schematic front view (with a graph) for illustrating an appropriate surface pressure distribution between an uppermost workpiece and a workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0062]      FIG. 29  is a schematic front view of the stacked body, gripped only by the lower and upper tips; 
           [0063]      FIG. 30  is a schematic vertical cross-sectional view of the stacked body at the start of energization for generating a current flow from the upper tip to the lower tip and the auxiliary electrodes after the state of  FIG. 27 ; 
           [0064]      FIG. 31  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization continuously after the state of  FIG. 30 ; 
           [0065]      FIG. 32  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after only the auxiliary electrodes are separated from the stacked body; 
           [0066]      FIG. 33  is a schematic vertical cross-sectional view of the stacked body after completion of the energization (spot welding) by separating the upper tip from the stacked body after the state of  FIG. 32 ; 
           [0067]      FIG. 34  is a schematic vertical cross-sectional view of a stacked body different from that of  FIG. 27 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0068]      FIG. 35  is a schematic vertical cross-sectional view of the stacked body in the process of generating a current flow from the upper tip to the lower tip after only the auxiliary electrodes are separated from the stacked body after the state of  FIG. 34 ; 
           [0069]      FIG. 36  is a schematic vertical cross-sectional view of the stacked body after completion of the energization (spot welding); 
           [0070]      FIG. 37  is a schematic vertical cross-sectional view of a stacked body different from those of  FIGS. 27 and 34 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0071]      FIG. 38  is a schematic vertical cross-sectional view of the stacked body at the end of the energization (spot welding); 
           [0072]      FIG. 39  is a schematic vertical cross-sectional view of a stacked body different from those of  FIGS. 27 ,  34 , and  37 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0073]      FIG. 40  is a side view of essential features showing a welding gun having auxiliary electrodes in the vicinity of the lower tip (second welding tip); 
           [0074]      FIG. 41  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the auxiliary electrodes in the vicinity of the upper tip are separated from the stacked body, and the auxiliary electrodes in the vicinity of the lower tip are brought into contact with the stacked body; 
           [0075]      FIG. 42  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the auxiliary electrodes in the vicinity of the lower tip are separated from the stacked body; 
           [0076]      FIG. 43  is a schematic vertical cross-sectional view of the stacked body in the process of flowing a current from the lower tip and the auxiliary electrodes to the upper tip in the direction opposite to that of  FIG. 27 ; 
           [0077]      FIG. 44  is a schematic vertical cross-sectional view of a current flow from the upper tip to the auxiliary electrodes through the uppermost workpiece and the workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0078]      FIG. 45  is a side view of essential features showing a welding gun having in a gun body a displacement mechanism for displacing auxiliary electrodes; 
           [0079]      FIG. 46  is a schematic side view of essential features showing a welding apparatus (indirect feeding type welding apparatus) according to a fourth embodiment of the present invention; 
           [0080]      FIG. 47  is an enlarged front view of essential features showing the indirect feeding type welding apparatus of  FIG. 46 ; 
           [0081]      FIG. 48  is a schematic front view of essential features showing a stacked body to be welded, gripped by a lower tip, an upper tip, and auxiliary electrodes; 
           [0082]      FIG. 49  is a schematic side view of essential features showing the stacked body to be welded, gripped by the lower tip, the upper tip, and the auxiliary electrodes; 
           [0083]      FIG. 50  is a schematic front view (with a graph) for illustrating an appropriate surface pressure distribution between an uppermost workpiece and a workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0084]      FIG. 51  is a schematic front view of the stacked body, gripped only by the lower and upper tips; 
           [0085]      FIG. 52  is a side view of essential features showing the stacked body at the start of energization for generating a current flow from the upper tip to the lower tip and the auxiliary electrodes after the state of  FIG. 48 ; 
           [0086]      FIG. 53  is a schematic vertical cross-sectional view of the stacked body in the state of  FIG. 52 ; 
           [0087]      FIG. 54  is a schematic vertical cross-sectional view of the stacked body in the process of further performing the energization continuously after the state of  FIG. 53 ; 
           [0088]      FIG. 55  is a side view of essential features showing the stacked body in the process of further performing the energization from the upper tip to the lower tip continuously after the current from the upper tip to the auxiliary electrodes is eliminated; 
           [0089]      FIG. 56  is a schematic vertical cross-sectional view of the stacked body in the state of  FIG. 55 ; 
           [0090]      FIG. 57  is a side view of essential features showing the stacked body after completion of the energization (spot welding); 
           [0091]      FIG. 58  is a schematic vertical cross-sectional view of the stacked body after the upper tip, the lower tip, and the auxiliary electrodes are separated from the stacked body after the state of  FIG. 57 ; 
           [0092]      FIG. 59  is a schematic vertical cross-sectional view of a stacked body different from that of  FIG. 48 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0093]      FIG. 60  is a schematic vertical cross-sectional view of the stacked body in the process of generating a current flow from the upper tip to the lower tip after the current flow from the upper tip to the auxiliary electrodes is eliminated after the state of  FIG. 59 ; 
           [0094]      FIG. 61  is a schematic vertical cross-sectional view of the stacked body after completion of the energization (spot welding); 
           [0095]      FIG. 62  is a schematic vertical cross-sectional view of a stacked body different from those of  FIGS. 48 and 58 , gripped by the lower tip, the upper tip, and the auxiliary electrodes, at the start of the energization; 
           [0096]      FIG. 63  is a schematic vertical cross-sectional view of the stacked body at the end of the energization (spot welding); 
           [0097]      FIG. 64  is a side view of essential features showing an indirect feeding type welding apparatus having an actuator for displacing the auxiliary electrodes; 
           [0098]      FIG. 65  is a side view of essential features showing an indirect feeding type welding apparatus using a changing-over switch instead of an ON/OFF switch; 
           [0099]      FIG. 66  is a side view of essential features showing the changing-over switch, turned from the state of  FIG. 65  to change a current pathway; 
           [0100]      FIG. 67  is a front view of essential features showing an indirect feeding type welding apparatus having support tips and support pressing members between the upper tip (the auxiliary electrodes) and the stacked body; 
           [0101]      FIG. 68  is a plan view for illustrating positional relations of the support tips and the support pressing members to the upper tip and the auxiliary electrodes around pressing parts; 
           [0102]      FIG. 69  is a side view of essential features showing a stacked body containing a workpiece having a vertical wall in a welding process; 
           [0103]      FIG. 70  is a plan view of the stacked body in the state of  FIG. 69 ; 
           [0104]      FIG. 71  is a front view of essential features showing current pathways in the state of  FIG. 67 ; 
           [0105]      FIG. 72  is a schematic vertical cross-sectional view of the stacked body where a current flows from the lower tip and the auxiliary electrodes to the upper tip in the direction opposite to that of  FIG. 52 ; 
           [0106]      FIG. 73  is a schematic vertical cross-sectional view of a current flow from the upper tip to the auxiliary electrodes through the uppermost workpiece and the workpiece located immediately beneath the uppermost workpiece in the stacked body; 
           [0107]      FIG. 74  is a schematic vertical cross-sectional view of a stacked body, gripped only by a lower tip and an upper tip, in the process of generating a current flow from the upper tip to the lower tip in a conventional spot welding method; 
           [0108]      FIG. 75  is a schematic vertical cross-sectional view of a melted portion grown larger after the state of  FIG. 74 ; 
           [0109]      FIG. 76  is a schematic vertical cross-sectional view of a stacked body different from that of  FIG. 74 , gripped only by the lower and upper tips, in the process of generating a current flow from the upper tip to the lower tip; 
           [0110]      FIG. 77  is a schematic vertical cross-sectional view of a melted portion grown larger after the state of  FIG. 76 ; 
           [0111]      FIG. 78  is a side view of essential features showing a conventional indirect feeding type welding apparatus; and 
           [0112]      FIG. 79  is a schematic vertical cross-sectional view of a stacked body, gripped only by a lower tip and an upper tip in the indirect feeding type welding apparatus of  FIG. 78 , in the process of generating a current flow from the upper tip to the lower tip. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0113]    Several preferred embodiments of the welding apparatuses of the present invention will be described in detail below with reference to the accompanying drawings. 
         [0114]    Spot welding apparatuses will be described below. 
         [0115]      FIG. 1  is an enlarged view of a spot welding apparatus  110  according to a first embodiment. The spot welding apparatus  110  contains a robot having an arm (not shown) and a welding gun  114  supported on a wrist  112  of the arm. 
         [0116]    The welding gun  114  is a so-called C-type gun having an approximately C-shaped fixed arm  130  under a gun body  124 . A lower tip  132  is disposed as a second welding tip on the lower end of the fixed arm  130  in confronting relation to the gun body  124 , and extends toward the gun body  124 . 
         [0117]    The gun body  124  contains a ball screw mechanism (not shown) for displacing a holder (support)  140  in the vertical direction of  FIG. 1 . Thus, the ball screw mechanism is a holder (support) displacement mechanism for displacing the holder  140 . 
         [0118]    A displacement shaft  134  projects from the gun body  124  and extends toward the lower tip  132 , and is displaced by a ball screw in the ball screw mechanism in the vertical direction (arrow Y 2  or Y 1  direction) of  FIG. 1 . The ball screw is rotated by a servomotor (not shown) in the ball screw mechanism. 
         [0119]    The holder  140  is disposed on the end of the displacement shaft  134  to support an upper tip  136  used as a first welding tip and pressing members  138   a,    138   b.    
         [0120]    The pressing member  138   a  has an end  142   a  having a rod shape extending parallel to the upper tip  136 , and further has a base  144   a  having an approximately trapezoidal shape as viewed from the front. An air cylinder  146   a  is disposed as a pressing member displacement mechanism in the holder  140 , and the base  144   a  is connected with a piston rod  148   a  in the air cylinder  146   a.  The holder  140  is a conductor, and thereby can transfer a current to the upper tip  136 . 
         [0121]    As in the enlarged view of essential features showing in  FIG. 2 , the holder  140  has a bore  150   a,  into which the piston rod  148   a  is inserted. A sleeve  152   a  is inserted into the bore  150   a,  and a bearing  154   a  is inserted into the sleeve  152   a.  Furthermore, the piston rod  148   a  is inserted into the bearing  154   a,  and a piston  156   a  is slidably in contact with the sleeve  152   a.    
         [0122]    A round groove  158   a  is formed circumferentially on the side wall of the piston  156   a,  and a sealant  0 -ring  160   a  is placed in the round groove  158   a.  A stopper  162   a  is disposed on the head of the piston  156   a,  and extends toward the top of the bore  150   a.  The stopper  162   a  is composed of an insulator. 
         [0123]    The sleeve  152   a  is composed of an aluminum material or an aluminum alloy material, and its surface is subjected to a hard alumite treatment. Thus, an oxide film containing a hard alumite is formed on the outer and inner peripheral walls of the sleeve  152   a.  The oxide film has an insulating property, and also the sleeve  152   a  has an insulating property. In other words, the sleeve  152   a  is an insulator, whereby the piston  156   a  is electrically isolated from the holder  140 . 
         [0124]    Alternatively, the sleeve  152   a  may be composed of an insulator such as a bakelite material or the like. In a case where the sleeve  152   a  is composed of a conductive material, the sleeve  152   a  may be electrically isolated from the holder  140  by disposing an insulator therebetween. 
         [0125]    The piston rod  148   a  is inserted in a coil spring  164   a.  One end of the coil spring  164   a  is stopped by the top of the bearing  154   a,  and the other end is in contact with the bottom of the piston  156   a.  When the piston rod  148   a  is displaced (lowered) in the downward direction of  FIGS. 1 and 2 , the coil spring  164   a  is compressed. Meanwhile, the coil spring  164   a  acts to apply an elastic force for displacing (lifting) the piston rod  148   a  in the upward direction. 
         [0126]    A room  166   a  is formed between the bore  150   a  and the piston  156   a.  An air supply/discharge passage  168   a  is communicated with the room  166   a  as a through-hole in the holder  140 . The air supply/discharge passage  168   a  is connected with a tube in a compressed air supply/discharge mechanism (not shown). Thus, a compressed air is supplied to and discharged from the room  166   a  by the compressed air supply/discharge mechanism. 
         [0127]    The other pressing member  138   b  and the air cylinder  146   b  have the same structures as above. The components of the pressing member  138   b  and the air cylinder  146   b,  which are identical to those of the pressing member  138   a  and the air cylinder  146   a,  are denoted by identical reference numerals and are marked with an additional character “b” instead of “a”. Therefore, detailed explanations thereof are omitted. 
         [0128]    A stacked body  170   a  to be welded contains three metallic plates  172   a,    174   a,    176   a  arranged upwardly in this order. Each of the metallic plates  172   a,    174   a  has a thickness D 1  (e.g. about 1 to 2 mm), and the metallic plate  176   a  has a thickness D 2  smaller than the thickness D 1  (e.g. about 0.5 to 0.7 mm). Thus, the metallic plates  172   a,    174   a  have the same thickness, and the metallic plate  176   a  is thinner than the metallic plates  172   a,    174   a.  In other words, the metallic plate  176   a  has the smallest thickness among the three metallic plates  172   a,    174   a,    176   a  in the stacked body  170   a.    
         [0129]    For example, each of the metallic plates  172   a,    174   a  is a high resistance workpiece made of a so-called high tensile strength steel, such as a high-performance high tensile strength steel sheet JAC590, JAC780, or JAC980 (defined according to the Japan Iron and Steel Federation Standard). For example, the metallic plate  176   a  is a low resistance workpiece made of a so-called mild steel, such as a high-performance steel sheet JAC270 for press-forming (defined according to the Japan Iron and Steel Federation Standard). The metallic plates  172   a,    174   a  may be made of the same or different metal materials. 
         [0130]    The stacked body  170   a  to be welded is interposed between the lower tip  132  and the upper tip  136 , and is energized by the lower tip  132  and the upper tip  136 . The lower tip  132  is electrically connected to a negative terminal of a power source  178 , and the upper tip  136  is electrically connected to a positive terminal of the power source  178 . Therefore, in the first embodiment, a current flows from the upper tip  136  to the lower tip  132 . 
         [0131]    As described in detail hereinafter, the distances Z 1 , Z 2  between the upper tip  136  and the pressing members  138   a,    138   b  are controlled to achieve an appropriate pressure distribution in the metallic plate  176   a  and the metallic plate  174   a  located immediately beneath the metallic plate  176   a.    
         [0132]    In this structure, the servomotor in the ball screw mechanism, the compressed air supply/discharge mechanism with the air cylinders  146   a,    146   b,  and the power source  178  are electrically connected to a gun controller  179  serving as a control means. Thus, the operation, actuation, and deactuation of the servomotor, the compressed air supply/discharge mechanism, and the power source  178  are controlled by the gun controller  179 . 
         [0133]    The spot welding apparatus  110  of the first embodiment is basically constructed as described above. Operations and advantages of the spot welding apparatus  110  will be described below in relation to a spot welding method according to the first embodiment. 
         [0134]    In the spot welding method for welding the stacked body  170   a,  i.e. for joining the metallic plates  172   a,    174   a  to each other as well as joining the metallic plates  174   a,    176   a  to each other, first the robot moves the wrist  112  and thus the welding gun  114  to position the stacked body  170   a  between the lower tip  132  and the upper tip  136 . 
         [0135]    After the gun body  124  is lowered to a predetermined position, the servomotor in the ball screw mechanism is actuated to start the rotation of the ball screw under the control of the gun controller  179 . Then, the upper tip  136  and the pressing members  138   a,    138   b  are moved downward in the arrow Y 1  direction closer to the stacked body  170   a.  Consequently, the stacked body  170   a  is gripped between the lower tip  132  and the upper tip  136 . 
         [0136]    Meanwhile, the compressed air supply/discharge mechanism is actuated by the gun controller  179 , whereby the compressed air is supplied through the air supply/discharge passages  168   a,    168   b  to the rooms  166   a,    166   b.  The pistons  156   a,    156   b  are pressed by the compressed air in the rooms  166   a,    166   b,  so that the pistons  156   a,    156   b  and the piston rods  148   a,    148   b  are lowered down while compressing the coil springs  164   a,    164   b  as shown in  FIG. 3 . Leakage of the compressed air from the rooms  166   a,    166   b  is prevented by the O-rings  160   a,    160   b  attached to the pistons  156   a,    156   b.    
         [0137]    The piston rods  148   a,    148   b  are moved downward, and thus the pressing members  138   a,    138   b  disposed on the ends of the piston rods  148   a,    148   b  are lowered toward the stacked body  170   a  in the arrow Y 1  direction. Consequently, before, at the same time as, or after the gripping of the stacked body  170   a  between the lower tip  132  and the upper tip  136 , the pressing members  138   a,    138   b  are brought into contact with the metallic plate  176   a.    FIG. 4  is a schematic vertical cross-sectional view of the stacked body  170   a  in this step. 
         [0138]    The distances Z 1 , Z 2  between the upper tip  136  and the pressing members  138   a,    138   b  are controlled such that as shown in  FIG. 5 , a portion pressed by the upper tip  136  exhibits the highest surface pressure, and portions pressed by the pressing members  138   a,    138   b  exhibit the second highest surface pressure, at the contact surface between the metallic plates  176   a,    174   a.  The distance Z 1  is preferably equal to the distance Z 2 . 
         [0139]    In other words, at the contact surface, some portions exhibit surface pressures lower than the above high pressures obtained due to the upper tip  136  and the pressing members  138   a,    138   b.  Consequently, a pressing force distribution shown in  FIG. 4  is achieved. The distribution will be described in detail below. 
         [0140]    The gun controller  179  controls the rotating force of the servomotor for rotating the ball screw in the ball screw mechanism and the pressing forces of the compressed air against the pistons  156   a,    156   b  (the moving forces of the air cylinders  146   a,    146   b ) such that the total pressing force (F 1 +F 2 +F 3 ) of the upper tip  136  and the pressing members  138   a,    138   b  against the metallic plate  176   a  is well balanced with the pressing force (F 4 ) of the lower tip  132  against the metallic plate  172   a.  By this control, the total pressing force (F 1 +F 2 +F 3 ) applied to the stacked body  170   a  in the arrow Y 1  direction is made approximately equal to the pressing force (F 4 ) applied to the stacked body  170   a  in the arrow Y 2  direction. The pressing force F 2  is preferably equal to the pressing force F 3 . 
         [0141]    In this case, the relation of F 1 &lt;F 4  is satisfied. Therefore, as schematically shown in  FIG. 4 , in the stacked body  170   a,  the total pressing force of the lower tip  132  and the upper tip  136  acts on a wider (larger) area as the force proceeds from the upper tip  136  toward the lower tip  132 . Thus, the force acting on the contact surface between the metallic plates  174   a,    176   a  is smaller than the force acting on the contact surface between the metallic plates  172   a,    174   a.  In a case where the distances Z 1 , Z 2  are excessively small, the stacked body  170   a  does not have the above described portions, which exhibit surface pressures lower than the high pressures obtained due to the upper tip  136  and the pressing members  138   a,    138   b.  In this case, the appropriate distribution is hardly achieved. 
         [0142]    In a case where the pressing members  138   a,    138   b  are not used for satisfying the relation of F 1 =F 4 , a pressing force distribution schematically shown in  FIG. 6  is achieved in the stacked body  170   a  by the lower tip  132  and the upper tip  136 . As shown in  FIG. 6 , in this case, the total force acts uniformly over the stacked body  170   a  from the upper tip  136  to the lower tip  132 . In other words, the force acting on the contact surface between the metallic plates  174   a,    176   a  is equal to the force acting on the contact surface between the metallic plates  172   a,    174   a.    
         [0143]    In  FIGS. 4 and 6 , at the contact surface between the metallic plates  174   a,    176   a,  an area, on which the force acts, is represented by a thick solid line. As is clear from the comparison between  FIGS. 4 and 6 , the area, on which the force acts, is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Thus, the metallic plate  176   a  has an area pressed against the metallic plate  174   a,  and the area is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . In other words, the contact area between the metallic plates  174   a,    176   a  is smaller under the condition of F 1 &lt;F 4 . 
         [0144]    When the total pressing force is distributed from the upper tip  136  to the lower tip  132  in the above manner to achieve the smaller contact area between the metallic plates  174   a,    176   a,  a reaction force is generated in the direction from the stacked body  170   a  toward the upper tip  136 . In the first embodiment, the pressing members  138   a,    138   b  are subjected to the reaction force. 
         [0145]    As described above, the holder  140  having the pressing members  138   a,    138   b  and the air cylinders  146   a,    146   b  is supported by the displacement shaft  134  connected to the ball screw mechanism in the gun body  124 . Therefore, the reaction force acting on the pressing members  138   a,    138   b  is absorbed by the gun body  124  (the welding gun  114 ). 
         [0146]    Thus, the reaction force derived from the stacked body  170   a  can be prevented from acting on the robot. For this reason, the robot is not required to have a high rigidity. In other words, the robot can be reduced in size, resulting in low equipment investment. 
         [0147]    Next, the gun controller  179  sends, to the power source  178 , a control signal for starting energization. Then, as shown in  FIG. 4  (and  FIG. 6 ), a current i starts to flow in the direction from the upper tip  136  toward the lower tip  132 . This current flow is achieved because the upper tip  136  and the lower tip  132  are connected to the positive and negative terminals of the power source  178  respectively as described above. The contact surface between the metallic plates  172   a,    174   a  and the contact surface between the metallic plates  174   a,    176   a  are heated by Joule heating generated due to the current i. 
         [0148]    As described above, the contact area between the metallic plates  176   a,    174   a  is smaller in  FIG. 4  than in  FIG. 6 . Therefore, the contact resistance and the current density at the contact surface between the metallic plates  174   a,    176   a  are higher in  FIG. 4  than in  FIG. 6  (i.e. under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 ). Thus, the generated amount of Joule heating (i.e. the amount of generated heat) is larger under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Consequently, under the condition of F 1 &lt;F 4 , as shown in  FIG. 7 , a heated region  180  in the vicinity of the contact surface between the metallic plates  172   a,    174   a  and a heated region  181  in the vicinity of the contact surface between the metallic plates  174   a,    176   a  are grown to approximately the same size. 
         [0149]    The contact surface between the metallic plates  172   a,    174   a  and the contact surface between the metallic plates  174   a,    176   a  are heated to a sufficient temperature and melted by the heated regions  180 ,  181 . Thus obtained melted portions are cooled and solidified, whereby nuggets  182 ,  183  are formed between the metallic plates  172   a,    174   a  and between the metallic plates  174   a,    176   a  respectively. Though the nuggets  182 ,  183  are shown in  FIG. 7  to facilitate understanding, the nuggets  182 ,  183  are in the liquid-phase states of the melted portions during the energization. Such melted portions are shown in this manner also in the following drawings. 
         [0150]    As described above, the heated region  180  in the vicinity of the contact surface between the metallic plates  172   a,    174   a  and the heated region  181  in the vicinity of the contact surface between the metallic plates  174   a,    176   a  have approximately the same size. Therefore, also the nuggets  182 ,  183  have approximately the same size. 
         [0151]    In the process of forming the melted portion, the metallic plate  176   a  is pressed against the metallic plate  174   a  by the pressing members  138   a,    138   b.  The metallic plate  176   a  having a low rigidity can be prevented by such pressing from warping and thus from separating from the metallic plate  174   a  during the energization (heating). Thus, spatter scattering of the softened melted portion from a gap between the metallic plates  176   a,    174   a  can be prevented. 
         [0152]    The sleeves  152   a,    152   b  are interposed between the holder  140  and the pistons  156   a,    156   b  (or the bearings  154   a,    154   b ) respectively. As described above, the sleeves  152   a,    152   b  are an insulator, so that the current to be applied to the upper tip  136  is not transferred from the holder  140  to the pistons  156   a,    156   b  and hence the pressing members  138   a,    138   b.    
         [0153]    After the melted portions are sufficiently grown in a predetermined time, the energization is stopped, and the holder  140  is moved upward to separate the upper tip  136  from the metallic plate  176   a.  Alternatively, the upper tip  136  and the lower tip  132  may be electrically isolated only by lifting the holder  140  to separate the upper tip  136  from the metallic plate  176   a.    
         [0154]    At the same time as or after the stop of the energization, the compressed air is discharged from the rooms  166   a,    166   b  (see  FIG. 2 ) by the compressed air supply/discharge mechanism. Consequently, the elastic forces of the coil springs  164   a,    164   b  become higher than the pressing forces of the compressed air on the pistons  156   a,    156   b.  Thus, the pistons  156   a,    156   b  are moved upward by the elastic forces of the coil springs  164   a,    164   b,  and are returned to the original positions set before the compressed air supply. Also the pressing members  138   a,    138   b  are moved upward and returned to the original positions. 
         [0155]    By the upward movement, the stoppers  162   a,    162   b  disposed on the heads of the pistons  156   a,    156   b  are brought into contact with the tops of the bores  150   a,    150   b  (the rooms  166   a,    166   b ). The pistons  156   a,    156   b  are prevented from being further lifted by the contact. Since the stoppers  162   a,    162   b  are composed of an insulator as described above, even when the pistons  156   a,    156   b  are lifted and brought into contact with the tops of the rooms  166   a,    166   b  during the energization of the upper tip  136  and the lower tip  132 , the current is not transferred from the holder  140  to the pistons  156   a,    156   b.    
         [0156]    The operations from the start to the end of the spot welding method are performed under the control of the gun controller  179 . 
         [0157]    The energization is stopped in this manner, so that the heating of the metallic plates  172   a,    174   a,    176   a  is stopped. The melted portions are cooled and solidified with time to form the nuggets  182 ,  183  respectively. The metallic plates  172   a,    174   a  are joined to each other by the nugget  182 , and the metallic plates  174   a,    176   a  are joined to each other by the nugget  183 , to obtain a bonded product. 
         [0158]    The bonded product is excellent in the bonding strengths between the metallic plates  172   a,    174   a  and between the metallic plates  174   a,    176   a.  This is because a sufficient amount of Joule heating is generated and the nugget  183  is sufficiently grown at the contact surface between the metallic plates  174   a,    176   a  as described above. 
         [0159]    As described above, in the first embodiment, the nugget  183  between the metallic plates  174   a,    176   a  can be grown to a size approximately equal to that of the nugget  182  between the metallic plates  172   a,    174   a  while preventing the spatter generation, whereby the bonded product can be produced with the excellent bonding strength between the metallic plates  174   a,    176   a.    
         [0160]    Since only the air cylinders  146   a,    146   b  are attached to the common holder  140  for supporting the upper tip  136 , the welding apparatus can be prevented from having a complicated or large structure. Therefore, even in a case where an intricately-shaped stacked body is welded, the stacked body can be located in a desired welding position without interference from the pressing members  138   a,    138   b  and the upper tip  136 . 
         [0161]    Since the pressing members  138   a,    138   b  are closer to the air cylinders  146   a,    146   b,  the offset loads on the air cylinders  146   a,    146   b  can be easily reduced. 
         [0162]    In the first embodiment, the nugget  183  between the metallic plates  174   a,    176   a  can be grown further larger by increasing the pressing forces F 2 , F 3  of the pressing members  138   a,    138   b.  However, the size of the nugget  183  tends to become saturated at certain levels of the pressing forces F 2 , F 3 . In other words, the nugget  183  is hardly grown larger than a certain size by excessively increasing the pressing forces F 2 , F 3 . Furthermore, in the case of excessively increasing the pressing forces F 2 , F 3 , the pressing force F 1  has to be excessively lowered in order to balancing the total force of the pressing forces F 1 , F 2 , F 3  with the pressing force F 4 . As a result, the size of the nugget  182  between the metallic plates  172   a,    174   a  is reduced. 
         [0163]    Consequently, it is preferred that the difference between the pressing force F 1  of the upper tip  136  and the pressing forces F 2 , F 3  of the pressing members  138   a,    138   b  is determined in view of maximizing the sizes of the nuggets  182 ,  183 . 
         [0164]    In any case, various pressure application means such as spring coils, servomotors, and hydraulic cylinders may be used instead of the air cylinders  146   a,    146   b.    
         [0165]    The combination of the materials of the metallic plates  172   a,    174   a,    176   a  is not particularly limited to the above combination of the steel materials. The metallic plates  172   a,    174   a,    176   a  may be composed of any material as long as they can be spot-welded. For example, all the metallic plates  172   a,    174   a,    176   a  may be composed of a mild steel. Alternatively, the metallic plates  174   a,    176   a  may be composed of a mild steel while only the metallic plate  172   a  may be composed of a high tensile strength steel. 
         [0166]    Though the uppermost metallic plate  176   a  is thinner than the metallic plates  172   a,    174   a  to be welded in the above embodiment, the stacked body  170   a  is not limited thereto. A stacked body  170   b  shown in  FIG. 8  may be used instead of the stacked body  170   a.  In the stacked body  170   b,  a metallic plate  174   b  having the smallest thickness is interposed between metallic plates  172   b,    176   b.  In this case, for example, the metallic plate  172   b  is composed of a high tensile strength steel while the metallic plates  174   b,    176   b  are composed of a mild steel, but the combination of the materials is not particularly limited thereto. 
         [0167]    It is to be understood that the middle metallic plate may have the largest thickness, and the undermost metallic plate may be thinner than the other two metallic plates. 
         [0168]    The number of the metallic plates is not particularly limited to 3. For example, a stacked body  170   c  shown in  FIG. 9  may be used instead of the stacked body  170   a.  In the stacked body  170   c,  a metallic plate  174   c  is stacked on a metallic plate  172   c,  and the both are composed of a high tensile strength steel. 
         [0169]    In a second embodiment, the pressing members  138   a,    138   b  are used as auxiliary electrodes, to which a current is applied. The components of the second embodiment, which are identical to those of  FIGS. 1 to 9 , are denoted by identical reference characters. Therefore, detailed explanations thereof are omitted. 
         [0170]      FIG. 10  is a partially enlarged horizontal cross-sectional view of essential features showing a spot welding apparatus according to the second embodiment. The spot welding apparatus of the second embodiment is substantially equal to the apparatus of the first embodiment except that the pressing members  138   a,    138   b  are electrically connected to the negative terminal of the power source  178 . Also in the second embodiment, the current flows from the upper tip  136  to the lower tip  132 . In the second embodiment, the pressing members  138   a,    138   b  are hereinafter referred to as the auxiliary electrodes  190   a,    190   b  to facilitate the understanding of the differences from the first embodiment. 
         [0171]    As described above, the lower tip  132  and the auxiliary electrodes  190   a,    190   b  are electrically connected to the negative terminal of the power source  178 , and the upper tip  136  is electrically connected to the positive terminal of the power source  178 . Therefore, the auxiliary electrodes  190   a,    190   b  have polarities opposite to that of the upper tip  136  though all the components are brought into contact with the uppermost metallic plate  176   a  in the stacked body  170   a.  In the following drawings, when the upper tip  136  is electrically connected with the auxiliary electrodes  190   a,    190   b  and a branching current i 2  is generated, the polarities of the auxiliary electrodes  190   a,    190   b  are shown. On the other hand, when the branching current i 2  is not generated, the polarities of the auxiliary electrodes  190   a,    190   b  are not shown. 
         [0172]    The distances Z 3 , Z 4  between the upper tip  136  and the auxiliary electrodes  190   a,    190   b  are controlled such that some portions exhibit surface pressures lower than those from the upper tip  136  and the auxiliary electrodes  190   a,    190   b  to achieve an appropriate pressure distribution as in the first embodiment (see  FIG. 5 ). Therefore, the upper tip  136  is separated from the auxiliary electrodes  190   a,    190   b  at certain distances. However, when the distances Z 3 , Z 4  are excessively large between the upper tip  136  and the auxiliary electrodes  190   a,    190   b,  the resistances therebetween are increased, so that it is difficult to obtain a flow of the branching current i 2  to be hereinafter described (see  FIG. 12 ). 
         [0173]    Thus, the distances Z 3 , Z 4  are controlled such that the above appropriate surface pressure distribution is achieved in the metallic plates  176   a,    174   a,  and an appropriate branching current i 2  flows under the resistances between the upper tip  136  and the auxiliary electrodes  190   a,    190   b.    
         [0174]    The main part of the spot welding apparatus of the second embodiment is basically constructed as described above. Operations and advantages of the apparatus will be described below. 
         [0175]    In a spot welding method using the spot welding apparatus for spot welding the stacked body  170   a,  first the robot moves the welding gun to position the stacked body  170   a  between the upper tip  136  and the lower tip  132  in the same manner as the first embodiment. Thereafter, the upper tip  136  and the lower tip  132  are moved close to each other, whereby the stacked body  170   a  is gripped therebetween. 
         [0176]    Before, at the same time as, or after the gripping, the compressed air is supplied to the rooms  166   a,    166   b  through the air supply/discharge passages  168   a,    168   b  (see  FIGS. 1 and 2 ). The pistons  156   a,    156   b  and the piston rods  148   a,    148   b  are moved downward by the compressed air, so that the auxiliary electrodes  190   a,    190   b  are lowered toward the stacked body  170   a  in the arrow Y 1  direction. Consequently, the auxiliary electrodes  190   a,    190   b  are brought into contact with the metallic plate  176   a  as shown in the schematic vertical cross-sectional view of  FIG. 10 . Of course, the coil springs  164   a,    164   b  are compressed during the downward movement of the pistons  156   a,    156   b  and the piston rods  148   a,    148   b.    
         [0177]    Also in the second embodiment, the gun controller  179  controls the pressing forces F 2 , F 3  of the auxiliary electrodes  190   a,    190   b  against the metallic plate  176   a  such that the total pressing force (F 1 +F 2 +F 3 ) of the upper tip  136  and the auxiliary electrodes  190   a,    190   b  is well balanced with the pressing force F 4  of the lower tip  132 . 
         [0178]    Also in the second embodiment, it is preferred that the difference between the pressing force F 1  of the upper tip  136  and the pressing forces F 2 , F 3  of the auxiliary electrodes  190   a,    190   b  is determined in view of maximizing the sizes of the nugget between the metallic plates  172   a,    174   a  and the nugget between the metallic plates  174   a,    176   a  as in the first embodiment. 
         [0179]    Next, energization is started. In the second embodiment, since the upper tip  136  and the lower tip  132  are connected to the positive and negative terminals of the power source  178  respectively, as shown in  FIG. 11 , a current i 1  flows from the upper tip  136  toward the lower tip  132 . Heated regions  192 ,  194  are formed between the metallic plates  172   a,    174   a  and between the metallic plates  174   a,    176   a  respectively by Joule heating generated due to the current i 1 . 
         [0180]    Also the auxiliary electrodes  190   a,    190   b  having the negative polarities are in contact with the metallic plate  176   a.  Therefore, in addition to the current i 1 , the branching current i 2  flows from the upper tip  136  toward the auxiliary electrodes  190   a,    190   b.    
         [0181]    Thus, in the second embodiment, the branching current i 2  is generated not in the metallic plates  172   a,    174   a  but in the metallic plate  176   a.  As a result, the metallic plate  176   a  exhibits a larger current value in this method as compared to conventional spot welding methods using only the upper tip  136  and the lower tip  132 . 
         [0182]    In this method, another heated region  196  different from the heated region  194  is formed in the metallic plate  176   a.  As shown in  FIG. 12 , the heated region  196  is grown with time and then integrated with the heated region  194 . 
         [0183]    The contact surface between the metallic plates  174   a,    176   a  is subjected to heat from both of the integrated heated regions  194 ,  196 . Furthermore, in the second embodiment, the contact resistance at the contact surface between the metallic plates  174   a,    176   a  is higher than that at the contact surface between the metallic plates  172   a,    174   a  as in the first embodiment. Therefore, the contact surface is heated to a sufficient temperature and melted, so that a nugget  198  is formed between the metallic plates  174   a,    176   a.    
         [0184]    As the ratio of the branching current i 2  is increased, the heated region  196  can be made larger. However, when the ratio of the branching current i 2  is excessively high, the current value of the current i 1  is reduced, whereby the sizes of the heated regions  192 ,  194  are reduced. Thus, the size of the nugget  200  is liable to be reduced, while the size of the nugget  198  becomes saturated. The ratio of the branching current i 2  is preferably selected in view of growing the nugget  200  to a sufficient size under the current i 1 . 
         [0185]    For example, the ratio between the current i 1  and the branching current i 2  can be controlled by changing the distances Z 3 , Z 4  between the upper tip  136  and the auxiliary electrodes  190   a,    190   b  (see  FIG. 10 ) as described above. The ratio between the current i 1  and the branching current i 2  is preferably e.g. 70:30. 
         [0186]    A melted portion and hence the nugget  198  are grown with the passage of time as long as the energization is continued. Therefore, the nugget  198  can be sufficiently grown by performing the energization over an appropriate time. 
         [0187]    The current value of the current i 1  in the metallic plates  172   a,    174   a  is smaller than that in a conventional spot welding method. Therefore, the amount by which the metallic plates  172   a,    174   a  are heated can be prevented from excessively increasing in the process of growing the melted portion (the nugget  198 ) between the metallic plates  174   a,    176   a.  Consequently, the apparatus is capable of eliminating the possibility of the spatter generation. 
         [0188]    In this process, a melted portion to be solidified into the nugget  200  is formed by the current i 1  between the metallic plates  172   a,    174   a.  When the branching current i 2  is continuously applied, the total amount of the current i 1  is reduced, and the heated region  192  and hence the nugget  200  are liable to be reduced in size, as compared with the case without the branching current i 2 . 
         [0189]    Therefore, in the case of further increasing the size of the nugget  200 , it is preferred that only the auxiliary electrodes  190   a,    190   b  are separated from the metallic plate  176   a  as shown in  FIG. 13 , and even thereafter current continues to be conducted from the upper tip  136  to the lower tip  132 . When the auxiliary electrodes  190   a,    190   b  are separated from the metallic plate  176   a,  the current value of the current i 1  is increased, and the total amount of the current i 1  is increased in the energization. 
         [0190]    Only the auxiliary electrodes  190   a,    190   b  may be separated from the metallic plate  176   a  by using the compressed air supply/discharge mechanism for discharging the compressed air from the rooms  166   a,    166   b  (see  FIG. 2 ). The pistons  156   a,    156   b  are moved upward due to the elastic forces of the coil springs  164   a,    164   b  by discharging the compressed air. Thus, the pistons  156   a,    156   b,  the piston rods  148   a,    148   b,  and the auxiliary electrodes  190   a,    190   b  disposed on the ends of the piston rods  148   a,    148   b  are moved upward. Consequently, the auxiliary electrodes  190   a,    190   b  are separated from the metallic plate  176   a,  and are returned to the original positions. A negative pressure may be provided in the rooms  166   a,    166   b  to lift the piston rods  148   a,    148   b.    
         [0191]    As a result, the branching current i 2  vanishes, so that only the current i 1  flows in the metallic plate  176   a  from the upper tip  136  to the lower tip  132 , and the heated region  196  (see  FIG. 12 ) disappears. 
         [0192]    Thereafter, the metallic plates  172   a,    174   a  are under a common spot welding condition. Thus, the generated amount of Joule heating is increased in the thick metallic plates  172   a,    174   a,  whereby the heated region  192  is expanded and further heated to a higher temperature. The contact surface between the metallic plates  172   a,    174   a  is heated to a sufficient temperature and melted by the heated region  192  having the higher temperature, and the melted portion (the nugget  200 ) is grown larger. 
         [0193]    Thereafter, the energization may be continued until the melted portion (the nugget  200 ) grows sufficiently, e.g. until the melted portion for forming the nugget  200  is integrated with the melted portion for forming the nugget  198  as shown in  FIG. 14 . The relation between the energization time and the growth of the nugget  200  may be confirmed in advance by a spot welding test using test pieces. 
         [0194]    The contact surface between the metallic plates  172   a,    174   a  is preheated by the heated region  192  formed by passage of the current i 1  while the nugget  198  is grown between the metallic plates  174   a,    176   a.  Therefore, the affinity of the metallic plates  172   a,    174   a  with each other is improved before the melted portion to be converted to the nugget  200  is grown larger. Consequently, the spatter generation is hardly caused. 
         [0195]    As described above, in the second embodiment, the spatter generation can be prevented in both of the process of growing the nugget  198  between the metallic plates  174   a,    176   a  and the process of growing the nugget  200  between the metallic plates  172   a,    174   a.    
         [0196]    After the melted portion for forming the nugget  200  is sufficiently grown in a predetermined time, the energization is stopped, and the upper tip  136  is separated from the metallic plate  176   a  as shown in  FIG. 14 . Alternatively, the upper tip  136  and the lower tip  132  are electrically isolated by separating the upper tip  136  from the metallic plate  176   a.    
         [0197]    The operations from the start to the end of the spot welding method are performed under the control of the gun controller  179 . 
         [0198]    When the energization is stopped in the above manner, the heating of the metallic plates  172   a,    174   a  is stopped. The obtained melted portion is cooled and solidified with the passage of time, whereby the metallic plates  172   a,    174   a  are joined to each other by the nugget  200 . 
         [0199]    Consequently, in the stacked body  170   a,  the metallic plates  172   a,    174   a  are joined to each other, and the metallic plates  174   a,    176   a  are joined to each other, to obtain a bonded article as a final product. 
         [0200]    The bonded product is excellent in the bonding strengths between the metallic plates  172   a,    174   a  and between the metallic plates  174   a,    176   a.  This is because the nugget  198  between the metallic plates  174   a,    176   a  is sufficiently grown under the flow of the branching current i 2  in the metallic plate  176   a  as described above. 
         [0201]    As described above, in the spot welding apparatus of the second embodiment, the auxiliary electrodes  190   a,    190   b  can be formed only by electrically connecting the pressing members  138   a,    138   b  to the negative terminal of the power source  178 . Therefore, the structure of the spot welding apparatus is not complicated due to the auxiliary electrodes  190   a,    190   b.    
         [0202]    Also in the second embodiment, the offset loads on the air cylinders  146   a,    146   b  can be easily reduced as in the first embodiment. 
         [0203]    Also in the second embodiment, the object to be welded is not limited to the stacked body  170   a.  The number of the metallic plates, the materials, and the thicknesses may be variously changed in the stacked body. Several specific examples will be described below. 
         [0204]    In the stacked body  170   b  shown in  FIG. 15 , the metallic plate  174   b  having the smallest thickness is interposed between the metallic plates  172   b,    176   b  as described above. For example, the metallic plate  172   b  is a high resistance workpiece composed of a high tensile strength steel, and the metallic plates  174   b,    176   b  are low resistance workpieces composed of a mild steel. 
         [0205]    In a case where the stacked body  170   b  is spot-welded only by the upper tip  136  and the lower tip  132 , the contact surface between the metallic plates  172   b,    174   b  is melted first. This is because the metallic plate  172   b  is the high resistance workpiece, whereby the contact resistance between the metallic plates  172   b,    174   b  is higher than that between the metallic plates  174   b,    176   b.  Therefore, when the energization of the upper tip  136  and the lower tip  132  is continued to sufficiently grow the nugget at the contact surface between the metallic plates  174   b,    176   b,  the spatter generation may be caused at the contact surface between the metallic plates  172   b,    174   b.    
         [0206]    In contrast, as shown in  FIG. 15 , since the auxiliary electrodes  190   a,    190   b  are used in the second embodiment, both the heated regions  192 ,  194  are formed at the contact surface between the metallic plates  172   b,    174   b  and the contact surface between the metallic plates  174   b,    176   b  respectively. This is because the contact surface between the metallic plates  174   b,    176   b  is sufficiently heated by the branching current i 2  in the metallic plate  176   b  in the same manner as the above stacked body  170   a.    
         [0207]    Consequently, nuggets  202 ,  204  are formed as shown in  FIG. 16 . After the branching current i 2  has vanished, the current i 1  may be continuously applied. In this case, for example, as shown in  FIG. 17 , a sufficiently larger nugget  206  can be developed over the contact surface between the metallic plates  172   b,    174   b  and the contact surface between the metallic plates  174   b,    176   b.    
         [0208]    As is clear from the above explanations of the spot welding of the stacked assemblies  170   a,    170   b,  by using the auxiliary electrodes  190   a,    190   b,  the heated regions and hence the nuggets can be shifted closer to the auxiliary electrodes  190   a,    190   b.    
         [0209]    Though the metallic plate  172   b  is composed of the high tensile strength steel and the metallic plates  174   b,    176   b  are composed of the mild steel in the above example, of course, the combination of the materials are not particularly limited thereto. 
         [0210]    The stacked body  170   c  shown in  FIG. 18  is provided by stacking the metallic plate  174   c  on the metallic plate  172   c  and may be spot-welded by using the auxiliary electrodes  190   a,    190   b,  the both metallic plates being composed of a high tensile strength steel. As shown in  FIGS. 75 and 77 , in the case of not using the auxiliary electrodes  190   a,    190   b,  the melted portions  6 ,  9  grow larger at the contact surface between the metallic plates  172   c,    174   c  (the high resistance workpieces  1 ,  2 ) in a relatively short time. Therefore, the spatter generation is liable to be caused. 
         [0211]    In contrast, as shown in  FIG. 18 , since the auxiliary electrodes  190   a,    190   b  are used in the second embodiment, a heated region  210  is formed at the contact surface between the metallic plates  172   c,    174   c,  and a heated region  212  is formed above the contact surface (i.e. in the vicinity of the auxiliary electrodes  190   a,    190   b  in the metallic plate  174   c ). This is because the metallic plate  174   c  is sufficiently heated by the flow of the branching current i 2  in the metallic plate  174   c.  Thus, also in this case, the heated regions and hence the nuggets (see  FIG. 18 ) can be shifted closer to the auxiliary electrodes  190   a,    190   b.    
         [0212]    Consequently, the contact surface between the metallic plates  172   c,    174   c  is softened, thereby improving the sealing property. Thus, even when the current i 1  is continuously applied to form a sufficiently large nugget  214  as shown in  FIG. 19 , the spatter generation is hardly caused. 
         [0213]    Spot welding of a stacked body  170   d  shown in  FIG. 20  will be described below. The stacked body  170   d  is obtained by stacking a low resistance metallic plate  172   d  composed of a mild steel, high resistance metallic plates  174   d,    176   d  composed of a high tensile strength steel, and a low resistance metallic plate  215   d  composed of a mild steel in this order from below. The metallic plates  172   d,    215   d  has thicknesses smaller than those of the metallic plates  174   d,    176   d.    
         [0214]    The auxiliary electrodes  190   a,    190   b  are disposed in the vicinity of the upper tip  136 , and furthermore auxiliary electrodes  190   c,    190   d  are disposed in the vicinity of the lower tip  132 . The auxiliary electrodes  190   c,    190   d  are electrically connected to the positive terminal of the power source  178 , and thereby have a polarity opposite to that of the lower tip  132 . The auxiliary electrodes  190   c,    190   d  can be located in this manner by disposing the holder  140  and the air cylinders  146   a,    146   b  in the vicinity of the lower tip  132  as in the vicinity of the upper tip  136 . 
         [0215]    As shown in  FIG. 20 , the stacked body  170   d  is gripped between the upper tip  136  and the lower tip  132 . Before, at the same time as, or after the gripping, only the auxiliary electrodes  190   a,    190   b  are brought into contact with the metallic plate  215   d.  When the energization is started, the current i 1  flows from the upper tip  136  to the lower tip  132 , and the branching current i 2  flows from the upper tip  136  to the auxiliary electrodes  190   a,    190   b.  Then, nuggets  116 ,  118  are formed at the contact surfaces between the metallic plates  174   d,    176   d  and between the metallic plates  176   d,    215   d  respectively. 
         [0216]    Then, as shown in  FIG. 21 , the auxiliary electrodes  190   a,    190   b  are electrically disconnected from the negative terminal of the power source  178  to eliminate the branching current i 2 . Before, at the same time as, or after the disconnection, the auxiliary electrodes  190   c,    190   d  are brought into contact with the metallic plate  172   d.  As a result, a branching current i 3  flows through the undermost metallic plate  172   d  from the auxiliary electrodes  190   c,    190   d  to the lower tip  132 . 
         [0217]    When the branching current i 2  vanishes, the growth of the nugget  218  is stopped. Meanwhile, the current i 1  continuously flows from the upper tip  136  to the lower tip  132 , and therefore the nugget  216  is grown larger at the contact surface between the metallic plates  174   d,    176   d.  Furthermore, another nugget  220  is formed at the contact surface between the metallic plates  172   d,    174   d  by the branching current i 3 . 
         [0218]    Then, as shown in  FIG. 22 , the auxiliary electrodes  190   c,    190   d  are separated from the metallic plate  172   d  to eliminate the branching current i 3 , whereby the growth of the nugget  220  is stopped. Thereafter, by continuously applying the current i 1 , only the nugget  216  at the contact surface between the metallic plates  174   d,    176   d  may be further grown larger and may be integrated with the nuggets  218 ,  220 . 
         [0219]    The object to be welded may have a complicated shape. As described above, even in this case, the object to be welded can be located in a desired welding position without interference from the upper tip  136  and the auxiliary electrodes  190   a,    190   b.    
         [0220]    Though the auxiliary electrodes  190   a,    190   b  are separated from the metallic plate  176   a  prior to the upper tip  136  in the second embodiment, the auxiliary electrodes  190   a,    190   b  and the upper tip  136  may be separated from the metallic plate  176   a  at the same time. 
         [0221]    As shown in  FIG. 23 , a current may flow from the lower tip  132  on the metallic plate  172   a  to the upper tip  136  on the metallic plate  176   a.  Also in this case, the auxiliary electrodes  190   a,    190   b  on the metallic plate  176   a  have polarities opposite to that of the upper tip  136 . Thus, the lower tip  132  and the auxiliary electrodes  190   a,    190   b  are electrically connected to the positive terminal of the power source  178 , and the upper tip  136  is electrically connected to the negative terminal of the power source  178 . Consequently, the current i 1  flows from the lower tip  132  to the upper tip  136 , and the branching current i 2  flows from the auxiliary electrodes  190   a,    190   b  to the upper tip  136 . 
         [0222]    As shown in  FIG. 24 , the branching current i 2  may flow not only in the metallic plate  176   a  on the upper tip  136  but also in the metallic plate  174   a  located immediately beneath the metallic plate  176   a.    
         [0223]    The auxiliary electrodes  190   a,    190   b  are separated from the metallic plate  176   a  in the above manner. Alternatively, a switch may be disposed between the auxiliary electrodes  190   a,    190   b  and the power source  178 , and only the branching current, which flows in the direction from the upper tip  136  to the auxiliary electrodes  190   a,    190   b  or the opposite direction, may be stopped by turning the switch to the disconnected (off) state. In this case, of course, the switch is turned to the connected (on) state to form the heated region  196 . 
         [0224]    In any case, the auxiliary electrode is not particularly limited to the above-described two auxiliary electrodes  190   a,    190   b  having the long rod shape. For example, one, three, or more long rods may be used as the auxiliary electrodes. In the case of using three or more auxiliary electrodes, a plurality of the auxiliary electrodes  190   a,    190   b  may be contacted with and separated from the outermost metallic plate at the same time in the same manner as the two auxiliary electrodes  190   a,    190   b.  Each auxiliary electrode may have a ring shape surrounding the lower tip  132  or the upper tip  136 . 
         [0225]    The auxiliary electrodes  190   a,    190   b  in the spot welding apparatus of the second embodiment may be electrically isolated from the power source  178  to perform the spot welding method of the first embodiment. Thus, in the spot welding apparatus of the second embodiment, the auxiliary electrodes  190   a,    190   b  can be energized or not energized, and thereby can be used only as the pressing members or used also as the electrodes for generating the branching current i 2 . 
         [0226]    Furthermore, though the C-type welding gun is used in the first and second embodiments, the welding gun may be a so-called X-type gun. In this case, the lower tip  132  and the upper tip  136  may be mounted on a pair of openable and closable chucks respectively. When the chucks are opened or closed, the lower tip  132  and the upper tip  136  are moved away from or close to each other. 
         [0227]    It is to be understood that the stacked body may contain five or more metallic plates. 
         [0228]    A welding apparatus (spot welding apparatus) according to a third embodiment will be described below. 
         [0229]      FIG. 25  is a schematic side view of a spot welding apparatus  310  according to a third embodiment, and  FIG. 26  is an enlarged front view of a main part thereof. The spot welding apparatus  310  contains a robot having an arm (not shown) and a welding gun  314  supported on a wrist  312  of the arm. 
         [0230]    The welding gun  314  is a so-called C-type gun having an approximately C-shaped fixed arm  318  under a gun body  316 . A lower tip  320  is disposed as a second welding tip on the lower end of the fixed arm  318 , and extends toward the gun body  316 . 
         [0231]    The gun body  316  contains a ball screw mechanism (not shown) for displacing, in the vertical direction (the arrow Y 2  or Y 1  direction) of  FIGS. 25 and 26 , a holder  324  having an upper tip  322  as a first welding tip. Specifically, the holder  324  is disposed on the end of a displacement shaft  326 , which projects from the gun body  316  and extends toward the lower tip  320 . The displacement shaft  326  is displaced by a ball screw in the ball screw mechanism in the vertical direction of  FIG. 25 , and thus the upper tip  322  is displaced by the holder  324 . 
         [0232]    Thus, the ball screw mechanism is a first displacement mechanism for displacing the upper tip  322 . The ball screw is rotated by a servomotor (not shown) in the ball screw mechanism. 
         [0233]    A substantially plate-shaped bracket  328  (support member) is attached to the body of the upper tip  322 . The bracket  328  has a through-hole  329 , which has a diameter approximately equal to the body diameter of the upper tip  322 . The body of the upper tip  322  is inserted and fitted into the through-hole  329 . 
         [0234]    As shown in detail in  FIG. 26 , two actuators  330   a,    330   b  are disposed in the bracket  328 . Auxiliary electrodes  334   a,    334   b,  which act as pressing members, project from tubes  332   a,    332   b  in the actuators  330   a,    330   b  and extend parallel to the upper tip  322 . The auxiliary electrodes  334   a,    334   b  are displaced by the actuators  330   a,    330   b  close to and away from the lower tip  320  (in the arrow Y 1  and Y 2  directions). Thus, the actuators  330   a,    330   b  act as second displacement mechanisms for displacing the auxiliary electrodes  334   a,    334   b  and as pressing force generation/control mechanisms for generating and controlling pressing forces of the auxiliary electrodes  334   a,    334   b.    
         [0235]    A stacked body  340   a  to be welded contains three metallic plates  342   a,    344   a,    346   a  arranged upwardly in this order. Each of the metallic plates  342   a,    344   a  has a thickness D 3  (e.g. about 1 to 2 mm), and the metallic plate  346   a  has a thickness D 4  smaller than the thickness D 3  (e.g. about 0.5 to 0.7 mm). Thus, the metallic plates  342   a,    344   a  have the same thickness, and the metallic plate  346   a  is thinner than the metallic plates  342   a,    344   a.  In other words, the metallic plate  346   a  has the smallest thickness among the three metallic plates  342   a,    344   a,    346   a  in the stacked body  340   a.    
         [0236]    For example, each of the metallic plates  342   a,    344   a  is a high resistance workpiece made of a so-called high tensile strength steel, such as a high-performance high tensile strength steel sheet JAC590, JAC780, or JAC980 (defined according to the Japan Iron and Steel Federation Standard). For example, the metallic plate  346   a  is a low resistance workpiece made of a so-called mild steel, such as a high-performance steel sheet JAC270 for press-forming (defined according to the Japan Iron and Steel Federation Standard). The metallic plates  342   a,    344   a  may be made of the same or different metal materials. 
         [0237]    The stacked body  340   a  to be welded is interposed between the lower tip  320  and the upper tip  322 , and is energized by the lower tip  132  and the upper tip  136 . The lower tip  320  and the auxiliary electrodes  334   a,    334   b  are electrically connected to a negative terminal of a power source  350 , and the upper tip  322  is electrically connected to a positive terminal of the power source  350 . Therefore, in the third embodiment, a current flows from the upper tip  322  to the lower tip  320  and the auxiliary electrodes  334   a,    334   b.  Thus, the auxiliary electrodes  334   a,    334   b  have polarities opposite to that of the upper tip  322  though all the components are brought into contact with the uppermost metallic plate  346   a  in the stacked body  340   a.    
         [0238]    As described in detail hereinafter, the distances Z 3 , Z 4  (see  FIG. 27 ) between the upper tip  322  and the auxiliary electrodes  334   a,    334   b  are controlled to achieve an appropriate pressure distribution in the metallic plate  346   a  and the metallic plate  344   a  located immediately beneath the metallic plate  346   a.    
         [0239]    In this structure, the servomotor in the ball screw mechanism and the power source  350  are electrically connected to a gun controller  352  serving as a control means. Thus, the operation, actuation, and deactuation of the servomotor and the power source  350  are controlled by the gun controller  352 . 
         [0240]    The spot welding apparatus  310  of the third embodiment is basically constructed as described above. Operations and advantages of the spot welding apparatus  310  will be described below in relation to a spot welding method according to the third embodiment. 
         [0241]    In the spot welding method for welding the stacked body  340   a,  i.e. for joining the metallic plates  342   a,    344   a  to each other as well as joining the metallic plates  344   a,    346   a  to each other, first the robot moves the wrist  312  and thus the welding gun  314  to locate the stacked body  340   a  between the lower tip  320  and the upper tip  322 . 
         [0242]    After the gun body  316  is lowered to a predetermined position, the servomotor in the ball screw mechanism is actuated to start the rotation of the ball screw under the control of the gun controller  352 . Then, the displacement shaft  326  is lowered in the arrow Y 1  direction, whereby the upper tip  322  and the auxiliary electrodes  334   a,    334   b  are moved downward closer to the stacked body  340   a.  Consequently, the stacked body  340   a  is gripped between the lower tip  320  and the upper tip  322 . 
         [0243]    Meanwhile, the gun controller  352  sends a control signal to the actuators  330   a,    330   b,  so that the actuators  330   a,    330   b  acts to perform the downward movement. Consequently, the auxiliary electrodes  334   a,    334   b  are lowered toward the stacked body  340   a  in the arrow Y 1  direction. 
         [0244]    Thus, before, at the same time as, or after the gripping of the stacked body  340   a  between the lower tip  320  and the upper tip  322 , the auxiliary electrodes  334   a,    334   b  are brought into contact with the metallic plate  346   a.    FIG. 27  is a schematic vertical cross-sectional view of the stacked body  340   a  in this step. 
         [0245]    The distances Z 3 , Z 4  between the upper tip  322  and the auxiliary electrodes  334   a,    334   b  are controlled such that as shown in  FIG. 28 , a portion pressed by the upper tip  322  exhibits the highest surface pressure, and portions pressed by the auxiliary electrodes  334   a,    334   b  exhibit the second highest surface pressure, at the contact surface between the metallic plates  346   a,    344   a.  The distance Z 3  is preferably equal to the distance Z 4 . 
         [0246]    In other words, at the contact surface, some portions exhibit surface pressures lower than the above high pressures obtained due to the upper tip  322  and the auxiliary electrodes  334   a,    334   b.  Consequently, a pressing force distribution shown in  FIG. 28  is achieved. The distribution will be described in detail below. 
         [0247]    The gun controller  352  controls the rotating force of the servomotor for rotating the ball screw in the ball screw mechanism and the moving forces of the actuators  330   a,    330   b  such that the total pressing force (F 1 +F 2 +F 3 ) of the upper tip  322  and the auxiliary electrodes  334   a,    334   b  against the metallic plate  346   a  is well balanced with the pressing force (F 4 ) of the lower tip  320  against the metallic plate  342   a.  By this control, the total pressing force (F 1 +F 2 +F 3 ) applied to the stacked body  340   a  in the arrow Y 1  direction is made approximately equal to the pressing force (F 4 ) applied to the stacked body  340   a  in the arrow Y 2  direction. The pressing force F 2  is preferably equal to the pressing force F 3 . 
         [0248]    In this case, the relation of F 1 &lt;F 4  is satisfied. Therefore, as schematically shown in  FIG. 27 , in the stacked body  340   a,  the total pressing force of the lower tip  320  and the upper tip  322  acts on a wider (larger) area in a position closer to the lower tip  320  than the upper tip  322 . Thus, the force acting on the contact surface between the metallic plates  344   a,    346   a  is smaller than the force acting on the contact surface between the metallic plates  342   a,    344   a.  In a case where the distances Z 3 , Z 4  are excessively small, the stacked body  340   a  does not have the above described portions, which exhibit surface pressures lower than the high pressures obtained due to the upper tip  322  and the auxiliary electrodes  334   a,    334   b.  In this case, the appropriate distribution is hardly achieved. 
         [0249]    In a case where the relation of F 1 =F 4  is satisfied without using the auxiliary electrodes  334   a,    334   b,  a force distribution shown in  FIG. 29  is achieved in the stacked body  340   a  by the lower tip  320  and the upper tip  322 . As shown in  FIG. 29 , in this case, the total force acts uniformly over the stacked body  340   a  from the upper tip  322  to the lower tip  320 . In other words, the force acting on the contact surface between the metallic plates  344   a,    346   a  is equal to the force acting on the contact surface between the metallic plates  342   a,    344   a.    
         [0250]    In  FIGS. 27 and 29 , at the contact surface between the metallic plates  344   a,    346   a,  an area, on which the force acts, is represented by a thick solid line. As is clear from the comparison between  FIGS. 27 and 29 , the area, on which the force acts, is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Thus, the metallic plate  346   a  has an area pressed against the metallic plate  344   a,  and the area is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . In other words, the contact area between the metallic plates  344   a,    346   a  is smaller under the condition of F 1 &lt;F 4 . 
         [0251]    When the total pressing force is distributed from the upper tip  322  to the lower tip  320  in the above manner to achieve the smaller contact area between the metallic plates  344   a,    346   a,  a reaction force is generated in the direction from the stacked body  340   a  toward the upper tip  322 . In the third embodiment, the auxiliary electrodes  334   a,    334   b  are subjected to the reaction force. 
         [0252]    As described above, the bracket  328  having the auxiliary electrodes  334   a,    334   b  is supported by the displacement shaft  326  connected to the ball screw mechanism in the gun body  316 . Therefore, the reaction force acting on the auxiliary electrodes  334   a,    334   b  is absorbed by the gun body  316  (the welding gun  314 ). 
         [0253]    Thus, the reaction force derived from the stacked body  340   a  can be prevented from acting on the robot. For this reason, the robot is not required to have a high rigidity. In other words, the robot can be reduced in size, resulting in low equipment investment. 
         [0254]    Next, the gun controller  352  sends, to the power source  350 , a control signal for starting energization. Then, as shown in  FIG. 30 , a current i 1  flows in the direction from the upper tip  322  toward the lower tip  320 . This current i 1  flow is achieved because the upper tip  322  and the lower tip  320  are connected to the positive and negative terminals of the power source  350  respectively as described above. The contact surface between the metallic plates  342   a,    344   a  and the contact surface between the metallic plates  344   a,    346   a  are heated by Joule heating generated due to the current i 1 , whereby heated regions  360 ,  362  are formed respectively. 
         [0255]    As described above, the contact area between the metallic plates  346   a,    344   a  is smaller in  FIG. 27  than in  FIG. 29 . Therefore, the contact resistance and the current density at the contact surface between the metallic plates  344   a,    346   a  are higher in  FIG. 27  than in  FIG. 29  (i.e. under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 ). Thus, the generated amount of Joule heating (i.e. the amount of generated heat) is larger under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Consequently, under the condition of F 1 &lt;F 4 , as shown in  FIG. 30 , the heated region  360  in the vicinity of the contact surface between the metallic plates  342   a,    344   a  and the heated region  362  in the vicinity of the contact surface between the metallic plates  344   a,    346   a  are grown to approximately the same size. 
         [0256]    The auxiliary electrodes  334   a,    334   b  having the negative polarities are in contact with the metallic plate  346   a.  Therefore, in addition to the current i 1 , a branching current i 2  flows from the upper tip  322  toward the auxiliary electrodes  334   a,    334   b.    
         [0257]    Thus, in the third embodiment, the branching current i 2  is generated not in the metallic plates  342   a,    344   a  but in the metallic plate  346   a.  As a result, the metallic plate  346   a  exhibits a larger current value in this method as compared to conventional spot welding methods using only the upper tip  322  and the lower tip  320 . 
         [0258]    In this method, as shown in  FIG. 31 , another heated region  364  different from the heated region  362  is formed in the metallic plate  346   a.  As shown in  FIG. 36 , the heated region  364  is grown with time and then integrated with the heated region  362 . The contact surface between the metallic plates  344   a,    346   a  is subjected to heat from both of the integrated heated regions  362 ,  364 . In the following drawings, when the upper tip  322  is electrically connected with the auxiliary electrodes  334   a,    334   b  and the branching current i 2  is generated, the polarities of the auxiliary electrodes  334   a,    334   b  are shown. On the other hand, when the upper tip  322  is electrically isolated from the auxiliary electrodes  334   a,    334   b  and the branching current i 2  is not generated, the polarities of the auxiliary electrodes  334   a,    334   b  are not shown. 
         [0259]    The contact surface between the metallic plates  342   a,    344   a  and the contact surface between the metallic plates  344   a,    346   a  are heated to a sufficient temperature and melted by the heated regions  360 ,  362 ,  364 . Thus obtained melted portions are cooled and solidified, whereby nuggets  370 ,  372  are formed between the metallic plates  342   a,    344   a  and between the metallic plates  344   a,    346   a  respectively. Though the nuggets  370 ,  372  are shown in  FIG. 31  to facilitate understanding, the nuggets  370 ,  372  are in the liquid-phase states of the melted portions during the energization. Such melted portions are shown in this manner also in the following drawings. 
         [0260]    The nugget  372  between the metallic plates  344   a,    346   a  can be grown further larger by increasing the pressing forces F 2 , F 3  of the auxiliary electrodes  334   a,    334   b.  However, the size of the nugget  372  tends to become saturated at certain levels of the pressing forces F 2 , F 3 . In other words, the nugget  372  is hardly grown larger than a certain size by excessively increasing the pressing forces F 2 , F 3 . Furthermore, in the case of excessively increasing the pressing forces F 2 , F 3 , the pressing force F 1  has to be excessively lowered in order for balancing the total force of the pressing forces F 1 , F 2 , F 3  with the pressing force F 4 . As a result, the size of the nugget  370  between the metallic plates  342   a,    344   a  is reduced. 
         [0261]    Consequently, it is preferred that the difference between the pressing force F 1  of the upper tip  322  and the pressing forces F 2 , F 3  of the auxiliary electrodes  334   a,    334   b  is determined in view of maximizing the sizes of the nuggets  370 ,  372 . 
         [0262]    As the ratio of the branching current i 2  is increased, the heated region  364  can be made larger. However, when the ratio of the branching current i 2  is excessively high, the current value of the current i 1  is reduced, whereby the sizes of the heated regions  360 ,  362  are reduced. Thus, the size of the nugget  370  is liable to be reduced, while the size of the nugget  372  becomes saturated. The ratio of the branching current i 2  is preferably selected in view of growing the nugget  370  to a sufficient size under the current i 1 . 
         [0263]    For example, the ratio between the current i 1  and the branching current i 2  can be controlled by changing the distances Z 3 , Z 4  between the upper tip  322  and the auxiliary electrodes  334   a,    334   b  (see  FIG. 27 ) as described above. The ratio between the current i 1  and the branching current i 2  is preferably e.g. 70:30. 
         [0264]    In the process of forming the melted portion, the metallic plate  346   a  is pressed against the metallic plate  344   a  by the auxiliary electrodes  334   a,    334   b.  The metallic plate  346   a  having a low rigidity can be prevented by such pressing from warping and thus from separating from the metallic plate  344   a  during the energization (heating). Thus, spatter scattering of the softened melted portion from a gap between the metallic plates  346   a,    344   a  can be prevented. 
         [0265]    The melted portion and hence the nugget  372  are grown with the passage of time as long as the energization is continued. Therefore, the nugget  372  can be sufficiently grown by performing the energization over an appropriate time. 
         [0266]    The current value of the current i 1  in the metallic plates  342   a,    344   a  is smaller than that in a conventional spot welding method. Therefore, the amount of generated heats of the metallic plates  342   a,    344   a  can be prevented from excessively increasing in the process of growing the melted portion (the nugget  372 ) between the metallic plates  344   a,    346   a.  Consequently, the apparatus is capable of eliminating the possibility of the spatter generation. 
         [0267]    In this process, a melted portion to be solidified into the nugget  370  is formed by the current i 1  between the metallic plates  342   a,    344   a.  When the branching current i 2  is continuously applied, the total amount of the current i 1  is reduced, and the heated region  360  and hence the nugget  370  are liable to be reduced in size, as compared to the case without the branching current i 2 . 
         [0268]    Therefore, in the case of further increasing the size of the nugget  370 , it is preferred that only the auxiliary electrodes  334   a,    334   b  are separated from the metallic plate  346   a  as shown in  FIG. 32 , while the energization of the upper tip  322  and the lower tip  320  is continued. When the auxiliary electrodes  334   a,    334   b  are separated from the metallic plate  346   a,  the current value of the current i 1  is increased, and the total amount of the current i 1  is increased until stopping conduction of the electric current. 
         [0269]    Only the auxiliary electrodes  334   a,    334   b  may be separated from the metallic plate  346   a  by using the actuators  330   a,    330   b  for moving the auxiliary electrodes  334   a,    334   b  upward in the direction from the lower tip  320  (in the direction of the arrow Y 2 ). 
         [0270]    As a result, the branching current i 2  vanishes, so that only the current i 1  flows in the metallic plate  346   a  from the upper tip  322  to the lower tip  320 , and the heated region  364  (see  FIG. 31 ) disappears. 
         [0271]    Thereafter, the metallic plates  342   a,    344   a  are under a common spot welding condition. Thus, the Joule heating value is increased in the thick metallic plates  342   a,    344   a,  whereby the heated region  360  is expanded and further heated to a higher temperature. The contact surface between the metallic plates  342   a,    344   a  is heated to a sufficient temperature and melted by the heated region  360  having the higher temperature, and the melted portion (the nugget  370 ) is grown larger. 
         [0272]    Thereafter, the energization may be continued until the melted portion (the nugget  370 ) grows sufficiently, e.g. until the melted portion for forming the nugget  370  is integrated with the melted portion for forming the nugget  372  as shown in  FIG. 33 . The relation between the energization time and the growth of the nugget  370  may be confirmed in advance by a spot welding test using test pieces. 
         [0273]    The contact surface between the metallic plates  342   a,    344   a  is preheated by the heated region  360  formed by the current i 1  flow while the nugget  372  is grown between the metallic plates  344   a,    346   a.  Therefore, the affinity of the metallic plates  342   a,    344   a  with each other is improved before the melted portion to be converted to the nugget  370  is grown larger. Consequently, the spatter generation is hardly caused. 
         [0274]    As described above, in the third embodiment, the spatter generation can be prevented in both of the process of growing the nugget  372  between the metallic plates  344   a,    346   a  and the process of growing the nugget  370  between the metallic plates  342   a,    344   a.    
         [0275]    After the melted portion is sufficiently grown in a predetermined time, the energization is stopped, and the displacement shaft  326  is moved upward to separate the upper tip  322  from the metallic plate  346   a  as shown in  FIG. 33 . Alternatively, the upper tip  322  and the lower tip  320  are electrically isolated by moving the displacement shaft  326  upward to separate the upper tip  322  from the metallic plate  346   a.    
         [0276]    The operations from the start to the end of the spot welding method are performed under the control of the gun controller  352 . 
         [0277]    When the energization is stopped in the above manner, the heating of the metallic plates  342   a,    344   a  is stopped. The obtained melted portion is cooled and solidified with the passage of time, whereby the metallic plates  342   a,    344   a  are joined to each other by the nugget  370 . 
         [0278]    Consequently, in the stacked body  340   a,  the metallic plates  342   a,    344   a  are joined to each other, and the metallic plates  344   a,    346   a  are joined to each other, to obtain a bonded article as a final product. 
         [0279]    The bonded product is excellent in the bonding strengths between the metallic plates  342   a,    344   a  and between the metallic plates  344   a,    346   a.  This is because the nugget  370  between the metallic plates  344   a,    346   a  is sufficiently grown under the branching current i 2  flowing in the metallic plate  346   a  as described above. 
         [0280]    As described above, in the third embodiment, the nugget  372  between the metallic plates  344   a,    346   a  can be grown to a size approximately equal to that of the nugget  370  between the metallic plates  342   a,    344   a  while preventing the spatter generation, whereby the bonded product can be produced with the excellent bonding strength between the metallic plates  344   a,    346   a.    
         [0281]    The spot welding apparatus  310  can be prepared only by attaching the bracket  328  having the actuators  330   a,    330   b  to the displacement shaft  326  in a known spot welding apparatus. Thus, the spot welding apparatus can be prevented from having a complicated or large structure due to the auxiliary electrodes  334   a,    334   b.  Therefore, even in a case where an object to be welding has an intricate shape, the stacked body can be located in a desired welding position without interference from the auxiliary electrodes  334   a,    334   b  and the upper tip  322 . 
         [0282]    The object to be welded is not limited to the stacked body  340   a.  The number, the materials, and the thicknesses of the metallic plates may be variously changed in the stacked body. Several specific examples will be described below. 
         [0283]    In a stacked body  340   b  shown in  FIG. 34 , a metallic plate  344   b  having the smallest thickness is interposed between metallic plates  342   b,    346   b.  For example, the metallic plate  342   b  is a high resistance workpiece composed of a high tensile strength steel, and the metallic plates  344   b,    346   b  are low resistance workpieces composed of a mild steel. 
         [0284]    In a case where the stacked body  340   b  is spot-welded only by the upper tip  322  and the lower tip  320 , the contact surface between the metallic plates  342   b,    344   b  is melted first. This is because the metallic plate  342   b  is the high resistance workpiece, whereby the contact resistance between the metallic plates  342   b,    344   b  is higher than that between the metallic plates  344   b,    346   b.  Therefore, when the energization of the upper tip  322  and the lower tip  320  is continued to sufficiently grow the nugget at the contact surface between the metallic plates  344   b,    346   b,  the spatter generation may be caused at the contact surface between the metallic plates  342   b,    344   b.    
         [0285]    In contrast, as shown in  FIG. 34 , since the auxiliary electrodes  334   a,    334   b  are used in the third embodiment, heated regions  374 ,  376  are formed at the contact surface between the metallic plates  342   b,    344   b  and the contact surface between the metallic plates  344   b,    346   b  respectively. This is because the contact surface between the metallic plates  344   b,    346   b  is sufficiently heated by the branching current i 2  in the metallic plate  346   b  in the same manner as the above stacked body  340   a.    
         [0286]    Consequently, nuggets  378 ,  380  are formed as shown in  FIG. 35 . After the branching current i 2  has vanished, the current i 1  may be continuously applied. In this case, for example, as shown in  FIG. 36 , a sufficiently larger nugget  382  can be developed over the contact surface between the metallic plates  342   b,    344   b  and the contact surface between the metallic plates  344   b,    346   b.    
         [0287]    As is clear from the above explanations of the spot welding of the stacked assemblies  340   a,    40   b,  by using the auxiliary electrodes  334   a,    334   b,  the heated regions and hence the nuggets can be shifted closer to the auxiliary electrodes  334   a,    334   b.    
         [0288]    Though the metallic plate  342   b  is composed of the high tensile strength steel and the metallic plates  344   b,    346   b  are composed of the mild steel in the above example, of course, the combination of the materials are not particularly limited thereto. 
         [0289]    In  FIG. 37 , a stacked body  340   c,  which is provided by stacking a metallic plate  344   c  on a metallic plate  342   c,  is spot-welded by using the auxiliary electrodes  334   a,    334   b.  The metallic plates  344   c,    342   c  are composed of a high tensile strength steel. As shown in  FIGS. 22 and 23 , in the case of not using the auxiliary electrodes  334   a,    334   b,  the melted portions  6  grows larger at the contact surface between the metallic plates  342   c,    344   c  (the high resistance workpieces  1 ,  2 ) in a relatively short time. Therefore, the spatter generation is liable to be caused. 
         [0290]    In contrast, as shown in  FIG. 37 , since the auxiliary electrodes  334   a,    334   b  are used in the third embodiment, a heated region  384  is formed at the contact surface between the metallic plates  342   c,    344   c,  and a heated region  386  is formed above the contact surface (i.e. in the vicinity of the auxiliary electrodes  334   a,    334   b  in the metallic plate  344   c ). This is because the metallic plate  344   c  is sufficiently heated by the branching current i 2  flow in the metallic plate  344   c.  Thus, also in this case, the heated region and hence a nugget  388  (see  FIG. 38 ) can be shifted closer to the auxiliary electrodes  334   a,    334   b.    
         [0291]    Consequently, the contact surface between the metallic plates  342   c,    344   c  is softened, thereby improving the sealing property. Thus, even when the current i 1  is continuously applied to form the sufficiently large nugget  388  as shown in  FIG. 38 , the spatter generation is hardly caused. 
         [0292]    Spot welding of a stacked body  340   d  shown in  FIG. 39  will be described below. The stacked body  340   d  is obtained by stacking a low resistance metallic plate  342   d  composed of a mild steel, high resistance metallic plates  344   d,    346   d  composed of a high tensile strength steel, and a low resistance metallic plate  390   d  composed of a mild steel in this order from below. The metallic plates  342   d,    390   d  has thicknesses smaller than those of the metallic plates  344   d,    346   d.    
         [0293]    The auxiliary electrodes  334   a,    334   b  are disposed in the vicinity of the upper tip  322 , and furthermore auxiliary electrodes  334   c,    334   d  are disposed in the vicinity of the lower tip  320 . The auxiliary electrodes  334   c,    334   d  are electrically connected to the positive terminal of the power source  350 , and thereby have a polarity opposite to that of the lower tip  320 . As shown in  FIG. 40 , to use the auxiliary electrodes  334   c,    334   d,  a bracket  392  and actuators  330   c,    330   d  may be disposed in the vicinity of the lower tip  320  in the same manner as the bracket  328  and the actuators  330   a,    330   b  in the vicinity of the upper tip  322 . The bracket  328  may be attached to the lower tip  320 . 
         [0294]    As shown in  FIG. 39 , the stacked body  340   d  is gripped between the upper tip  322  and the lower tip  320 . Before, at the same time as, or after the gripping, only the auxiliary electrodes  334   a,    334   b  are brought into contact with the metallic plate  390   d.  When the energization is started, the current i 1  flows from the upper tip  322  to the lower tip  320 , and the branching current i 2  flows from the upper tip  322  to the auxiliary electrodes  334   a,    334   b.  Then, nuggets  394 ,  396  are formed at the contact surfaces between the metallic plates  344   d,    346   d  and between the metallic plates  346   d,    390   d  respectively. 
         [0295]    Then, as shown in  FIG. 41 , the auxiliary electrodes  334   a,    334   b  are moved upward by the actuators  330   a,    330   b  and electrically disconnected from the upper tip  322  to eliminate the branching current i 2 . Before, at the same time as, or after the disconnection, the auxiliary electrodes  334   c,    334   d  are brought into contact with the metallic plate  342   d.  As a result, a branching current i 3  flows through the undermost metallic plate  342   d  from the auxiliary electrodes  334   c,    334   d  to the lower tip  320 . 
         [0296]    When the branching current i 2  vanishes, the growth of the nugget  96  is stopped. Meanwhile, the current i 1  continuously flows from the upper tip  322  to the lower tip  320 , and therefore the nugget  396  is grown larger at the contact surface between the metallic plates  344   d,    346   d.  Furthermore, another nugget  398  is formed at the contact surface between the metallic plates  342   d,    344   d  by the branching current i 3 . 
         [0297]    Then, as shown in  FIG. 42 , the auxiliary electrodes  334   c,    334   d  are separated from the metallic plate  342   d  to eliminate the branching current i 3 , whereby the growth of the nugget  398  is stopped. Thereafter, by continuously applying the current i 1 , only the nugget  396  at the contact surface between the metallic plates  44   d,    346   d  may be further grown larger and may be integrated with the nuggets  394 ,  398 . 
         [0298]    It is to be understood that the stacked body may contain five or more metallic plates. 
         [0299]    As shown in  FIG. 43 , a current may flow from the lower tip  320  on the metallic plate  342   a  to the upper tip  322  on the metallic plate  346   a.  Also in this case, the auxiliary electrodes  334   a,    334   b  on the metallic plate  346   a  have polarities opposite to that of the upper tip  322 . Thus, the lower tip  320  and the auxiliary electrodes  334   a,    334   b  are electrically connected to the positive terminal of the power source  350 , and the upper tip  322  is electrically connected to the negative terminal of the power source  350 . Consequently, the current i 1  flows from the lower tip  320  to the upper tip  322 , and the branching current i 2  flows from the auxiliary electrodes  334   a,    334   b  to the upper tip  322 . 
         [0300]    As shown in  FIG. 40 , the positively polarized lower tip  320  may be used as the first welding tip, the negatively polarized upper tip  322  may be used as the second welding tip, and the negatively polarized auxiliary electrodes  334   c,    334   d  may be disposed in the vicinity of the lower tip  320 . 
         [0301]    As shown in  FIG. 44 , the branching current i 2  may flow not only in the metallic plate  346   a  on the upper tip  322  but also in the metallic plate  344   a  located immediately beneath the metallic plate  346   a.    
         [0302]    As shown in  FIG. 45 , the actuators  330   a,    330   b  may be disposed not on the bracket  328  but on the gun body  316 . 
         [0303]    In any case, the auxiliary electrode is not particularly limited to the above-described two auxiliary electrodes  334   a,    334   b  having the long rod shape. For example, one, three, or more long rods may be used as the auxiliary electrodes. In the case of using three or more auxiliary electrodes, a plurality of the auxiliary electrodes may be contacted with and separated from the outermost metallic plate at the same time in the same manner as the two auxiliary electrodes  334   a,    334   b.  Each auxiliary electrode may have a ring shape surrounding the lower tip  320  or the upper tip  322 . 
         [0304]    Though the branching current i 2  flows from the upper tip  322  (or the lower tip  320 ) to the auxiliary electrodes  334   a,    334   b  in the third embodiment, the auxiliary electrodes  334   a,    334   b  may be electrically isolated from the power source  350 , and the spot welding may be carried out without the branching current i 2 . In this case, the auxiliary electrodes  334   a,    334   b  act only as the pressing members. 
         [0305]    Also in this case, the total pressing force is distributed from the upper tip  322  to the lower tip  320  as shown in  FIG. 27 . The contact area between the metallic plates  342   a,    344   a  is larger than without the pressing by the auxiliary electrodes  334   a,    334   b  (see  FIG. 4 ). Therefore, the contact resistance and the current density at the contact surface between the metallic plates  342   a,    344   a  are increased, and the generated amount of Joule heating (i.e. the amount of generated heat) is increased. Consequently, the heated region and hence the nugget are grown to a sufficient size in the vicinity of the contact surface between the metallic plates  342   a,    344   a.    
         [0306]    Furthermore, though the C-type welding gun is used in the third embodiment, the welding gun may be a so-called X-type gun. In this case, the lower tip  320  and the upper tip  322  may be mounted on a pair of openable and closable chucks respectively. When the chucks are opened or closed, the lower tip  320  and the upper tip  322  are moved away from or close to each other. 
         [0307]    A welding apparatus (indirect feeding type welding apparatus) according to a fourth embodiment will be described below. 
         [0308]      FIG. 46  is a side view of essential features of an indirect feeding type welding apparatus  430  according to the fourth embodiment. The indirect feeding type welding apparatus  430  has a first welding gun  432 , to which a welding current is supplied, a second welding gun  436  for welding a stacked body  434   a,  and an external feed terminal  438  for transferring the welding current from the first welding gun  432  to the second welding gun  436 . 
         [0309]    The first welding gun  432  is a so-called C-type gun having an approximately C-shaped fixed arm  441  under a gun body  440 . A lower electrode  442  is disposed on the lower end of the fixed arm  441 , and extends toward the gun body  440 . 
         [0310]    The gun body  440  contains a ball screw mechanism (not shown) for displacing a holder  446  having an upper electrode  444  in the vertical direction of  FIG. 46 . Specifically, the holder  446  is disposed on the end of a displacement shaft  448 , which projects from the gun body  440  and extends toward the lower electrode  442 . The displacement shaft  448  is displaced by a ball screw in the ball screw mechanism in the vertical direction of  FIG. 46 , and thus the upper electrode  444  is displaced by the holder  446 . 
         [0311]    In the fourth embodiment, the upper electrode  444  has a positive (+) polarity, the lower electrode  442  has a negative (−) polarity. Thus, the upper electrode  444  and the lower electrode  442  are electrically connected to positive and negative terminals of a power source  450  (see  FIG. 48 ) respectively. 
         [0312]    The external feed terminal  438  has conductive terminals  452   a,    452   b  and an insulator  454  interposed therebetween. The upper electrode  444  is brought into contact with the conductive terminal  452   a,  while the lower electrode  442  is brought into contact with the conductive terminal  452   b.  The external feed terminal  438  further has an auxiliary terminal  456  electrically connected to the conductive terminal  452   a.    
         [0313]    The second welding gun  436  has a gun arm  462 , which contains a first arm member  458  and a second arm member  460  combined to form an approximately X-shape. The first arm member  458  and the second arm member  460  are swung about the intersection thereof, and the gun arm  462  is opened and closed by the swing. 
         [0314]    Specifically, as shown in  FIG. 46 , an open/close cylinder  464  is disposed on the right end of the first arm member  458  as an open/close mechanism for opening and closing the gun arm  462 . The open/close cylinder  464  has an open/close rod  466 , which extends in the downward direction of  FIG. 46  and is connected to the right end of the second arm member  460 . Therefore, when the open/close rod  466  is moved forward or backward in the vertical direction of  FIG. 46 , the first arm member  458  and the second arm member  460  are moved close to or away from each other, whereby the gun arm  462  is closed or opened. 
         [0315]    The left ends of the first arm member  458  and the second arm member  460  are articulated downward and upward in the vertical direction respectively, and thereby extend facing each other. An upper tip  468  used as a first welding tip and a lower tip  470  used as a second welding tip are disposed on the facing ends respectively. 
         [0316]    A main part of the first arm member  458  is shown in an enlarged view of  FIG. 47 . A substantially plate-shaped bracket  472  composed of an insulator is attached to the body of the upper tip  468 . The bracket  472  has a through-hole  474 , which has a diameter approximately equal to the body diameter of the upper tip  468 . The body of the upper tip  468  is inserted and fitted into the through-hole  474 . 
         [0317]    Auxiliary electrodes  476   a,    476   b,  which act as pressing members, are disposed on the bracket  472  and extend parallel to the upper tip  468 . The auxiliary electrodes  476   a,    476   b  contains electrode bodies  478   a,    478   b,  flanges  480   a,    480   b  extending diametrically outward, relatively small-diameter shafts  482   a,    482   b,  and terminals  484   a,    484   b,  and the components are arranged in this order in the upward direction of  FIG. 46 . 
         [0318]    The bracket  472  further has other through-holes  486   a,    486   b  in the vicinity of the through-hole  474 . The small-diameter shafts  482   a,    482   b  are inserted into the through-holes  486   a,    486   b  respectively. 
         [0319]    Coil springs  488   a,    488   b  are attached to the small-diameter shafts  482   a,    482   b.  The lower and upper ends of the coil springs  488   a,    488   b  are in contact with the tops of the flanges  480   a,    480   b  and the bottom of the bracket  472  respectively. When the electrode bodies  478   a,    478   b  are brought into contact with the stacked body  434   a,  the coil springs  488   a,    488   b  are compressed. On the other hand, when the electrode bodies  478   a,    478   b  are separated from the stacked body  434   a,  the coil springs  488   a,    488   b  are returned and act to apply elastic forces for displacing the auxiliary electrodes  476   a,    476   b  away from the bracket  472 . 
         [0320]    As described in detail hereinafter, the distances Z 5 , Z 6  (see  FIG. 48 ) between the upper tip  468  and the auxiliary electrodes  476   a,    476   b  are controlled to achieve an appropriate pressure distribution in a metallic plate  504   a  and a metallic plate  502   a  located immediately beneath the metallic plate  504   a.    
         [0321]    The intersection of the first arm member  458  and the second arm member  460  is fixed by a jig  490  to support the gun arm  462 . The upper electrode  444  and the upper tip  468  are electrically connected by the conductive terminal  452   a  and a lead  492 , and the lower electrode  442  and the lower tip  470  are electrically connected by the conductive terminal  452   b  and a lead  494 . The auxiliary electrodes  476   a,    476   b  are electrically connected with the lower electrode  442  by a lead  496 , an ON/OFF switch  498 , the auxiliary terminal  456 , and the conductive terminal  452   a.  Consequently, the upper tip  468  has a positive (+) polarity as well as the upper electrode  444 , and the lower tip  470  and the auxiliary electrodes  476   a,    476   b  have a negative (−) polarity as well as the lower electrode  442 . 
         [0322]    The stacked body  434   a  to be welded contains three metallic plates  500   a,    502   a,    504   a  arranged upwardly in this order. Each of the metallic plates  500   a,    502   a  has a thickness D 5  (e.g. about 1 to 2 mm), and the metallic plate  504   a  has a thickness D 6  smaller than the thickness D 5  (e.g. about 0.5 to 0.7 mm). Thus, the metallic plates  500   a,    502   a  have the same thickness, and the metallic plate  504   a  is thinner than the metallic plates  500   a,    502   a.  In other words, the metallic plate  504   a  has the smallest thickness among the three metallic plates  500   a,    502   a,    504   a  in the stacked body  434   a.    
         [0323]    For example, each of the metallic plates  500   a,    502   a  is a high resistance workpiece made of a so-called high tensile strength steel, such as a high-performance high tensile strength steel sheet JAC590, JAC780, or JAC980 (defined according to the Japan Iron and Steel Federation Standard). For example, the metallic plate  504   a  is a low resistance workpiece made of a so-called mild steel, such as a high-performance steel sheet JAC270 for press-forming (defined according to the Japan Iron and Steel Federation Standard). The metallic plates  500   a,    502   a  may be made of the same or different metal materials. 
         [0324]    The stacked body  434   a  to be welded is interposed between the lower tip  470  and the upper tip  468 , and is energized by the lower tip  470  and the upper tip  468 . In the energization, the lower tip  470  is brought into contact with the undermost metallic plate  500   a,  and the upper tip  468  and the auxiliary electrodes  476   a,    476   b  are brought into contact with the uppermost metallic plate  504   a.  As described above, the auxiliary electrodes  476   a,    476   b  have polarities opposite to that of the upper tip  468  though all the components are brought into contact with the uppermost metallic plate  504   a  in the stacked body  434   a.    
         [0325]    In this structure, the open/close cylinder  464 , the power source  450 , and the ON/OFF switch  498  are electrically connected to a gun controller  506  serving as a control means (see  FIG. 48 ). Thus, the operation, actuation, and deactuation of the open/close cylinder  464 , the power source  450 , and the ON/OFF switch  498  are controlled by the gun controller  506 . 
         [0326]    The indirect feeding type welding apparatus  430  of the fourth embodiment is basically constructed as described above. Operations and advantages of the indirect feeding type welding apparatus  430  will be described below in relation to a spot welding method. 
         [0327]    In the spot welding method for welding the stacked body  434   a,  i.e. for joining the metallic plates  500   a,    502   a  to each other as well as joining the metallic plates  502   a,    504   a  to each other, first the stacked body  434   a  is located between the lower tip  470  and the upper tip  468 . Of course, in this step, the open/close rod  466  of the open/close cylinder  464  is moved backward, so that the gun arm  462  is in the opened state (see  FIG. 46 ). 
         [0328]    Then, the open/close cylinder  464  is actuated by the gun controller, and the open/close rod  466  is moved forward, whereby the left ends of the first arm member  458  and the second arm member  460  are moved close to each other. Thus, the gun arm  462  is closed. Consequently, as shown in  FIG. 49 , the lower tip  470  is brought into contact with the metallic plate  500   a,  and the upper tip  468  is brought into contact with the metallic plate  504   a,  whereby the stacked body  434   a  is gripped between the lower tip  470  and the upper tip  468 . At the same time, the auxiliary electrodes  476   a,    476   b  are brought into contact with the metallic plate  504   a.    FIG. 48  is a schematic vertical cross-sectional view of the main part in the indirect feeding type welding apparatus  430  in this step. 
         [0329]    The distances Z 5 , Z 6  between the upper tip  468  and the auxiliary electrodes  476   a,    476   b  are controlled such that as shown in  FIG. 50 , a portion pressed by the upper tip  468  exhibits the highest surface pressure, and portions pressed by the auxiliary electrodes  476   a,    476   b  exhibit the second highest surface pressure, at the contact surface between the metallic plates  504   a,    502   a.  The distance Z 5  is preferably equal to the distance Z 6 . 
         [0330]    In other words, at the contact surface, some portions exhibit surface pressures lower than the above high pressures obtained due to the upper tip  468  and the auxiliary electrodes  476   a,    476   b.  Consequently, a pressing force distribution shown in  FIG. 50  is achieved. The distribution will be described in detail below. 
         [0331]    The gun controller  506  controls the moving force of the open/close cylinder  464  such that the total pressing force (F 1 +F 2 +F 3 ) of the upper tip  468  and the auxiliary electrodes  476   a,    476   b  against the metallic plate  504   a  is well balanced with the pressing force (F 4 ) of the lower tip  470  against the metallic plate  500   a.  By this control, the total pressing force (F 1 +F 2 +F 3 ) applied to the stacked body  434   a  in the arrow Y 1  direction is made approximately equal to the pressing force (F 4 ) applied to the stacked body  434   a  in the arrow Y 2  direction. The pressing force F 2  is preferably equal to the pressing force F 3 . 
         [0332]    In this case, the relation of F 1 &lt;F 4  is satisfied. Therefore, as schematically shown in  FIG. 48 , in the stacked body  434   a,  the total pressing force of the lower tip  470  and the upper tip  468  acts on a wider (larger) area in a position closer to the lower tip  470  than the upper tip  468 . Thus, the force acting on the contact surface between the metallic plates  502   a,    504   a  is smaller than the force acting on the contact surface between the metallic plates  500   a,    502   a.  In a case where the distances Z 5 , Z 6  are excessively small, the stacked body  434   a  does not have the above described portions, which exhibit surface pressures lower than the high pressures obtained due to the upper tip  468  and the auxiliary electrodes  476   a,    476   b.  In this case, the appropriate distribution is hardly achieved. 
         [0333]    In a case where the relation of F 1 =F 4  is satisfied without using the auxiliary electrodes  476   a,    476   b , a force distribution shown in  FIG. 51  is achieved in the stacked body  434   a  by the lower tip  470  and the upper tip  468 . As shown in  FIG. 50 , in this case, the total force acts uniformly over the stacked body  434   a  from the upper tip  468  to the lower tip  470 . In other words, the force acting on the contact surface between the metallic plates  502   a,    504   a  is equal to the force acting on the contact surface between the metallic plates  500   a,    502   a.    
         [0334]    In  FIGS. 48 and 51 , at the contact surface between the metallic plates  502   a,    504   a,  an area, on which the force acts, is represented by a thick solid line. As is clear from the comparison between  FIGS. 48 and 51 , the area, on which the force acts, is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Thus, the metallic plate  504   a  has an area pressed against the metallic plate  502   a,  and the area is smaller under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . In other words, the contact area between the metallic plates  504   a,    502   a  is smaller under the condition of F 1 &lt;F 4 . 
         [0335]    In the fourth embodiment, when the total pressing force is distributed from the upper tip  468  to the lower tip  470  as shown in  FIG. 50  to achieve the smaller contact area between the metallic plates  502   a,    504   a,  a reaction force is generated in the direction from the stacked body  434   a  toward the upper tip  468 . The auxiliary electrodes  476   a,    476   b  are subjected to the reaction force. 
         [0336]    After the pressing force distribution is achieved, the gun controller  506  sends a control signal to the power source  450 . When the power source  450  receives the control signal, the power source  450  acts to supply a welding current. The welding current flows from the upper electrode  444  connected to the positive terminal, through the lower electrode  442 , to the negative terminal. 
         [0337]    The welding current flows from the upper electrode  444 , through the conductive terminal  452   a,  the lead  492 , and the upper tip  468 , to the metallic plate  504   a.  Therefore, as shown in  FIGS. 52 and 53 , a current i 1  flows in the direction from the upper tip  468  to the lower tip  470 . This is because the lower tip  470  is connected to the negative terminal of the power source  450  by the lead  494 , the conductive terminal  452   b,  and the lower electrode  442  as described above. 
         [0338]    The contact surface between the metallic plates  500   a,    502   a  and the contact surface between the metallic plates  502   a,    504   a  are heated by Joule heating generated due to the current i 1 , whereby heated regions  510 ,  512  are formed respectively. 
         [0339]    As described above, the contact area between the metallic plates  504   a,    502   a  is smaller in  FIG. 48  than in  FIG. 51 . Therefore, the contact resistance and the current density at the contact surface between the metallic plates  502   a,    504   a  are higher in  FIG. 48  than in  FIG. 51  (i.e. under the condition of F 1  &lt;F 4  than under the condition of F 1 =F 4 ). Thus, the generated amount of Joule heating (i.e. the amount of generated heat) is larger under the condition of F 1 &lt;F 4  than under the condition of F 1 =F 4 . Consequently, under the condition of F 1 &lt;F 4 , as shown in  FIG. 52 , the heated region  510  in the vicinity of the contact surface between the metallic plates  500   a,    502   a  and the heated region  512  in the vicinity of the contact surface between the metallic plates  502   a,    504   a  are grown to approximately the same size. 
         [0340]    Also the auxiliary electrodes  476   a,    476   b  having the negative polarities are in contact with the metallic plate  504   a.  Therefore, in addition to the current i 1 , a branching current i 2  flows from the upper tip  468  toward the auxiliary electrodes  476   a,    476   b  (see  FIGS. 52 and 53 ). 
         [0341]    Thus, in the fourth embodiment, the branching current i 2  is generated not in the metallic plates  500   a,    502   a  but in the metallic plate  504   a.  As a result, the metallic plate  504   a  exhibits a larger current value in this method as compared to conventional spot welding methods using only the upper tip  468  and the lower tip  470 . 
         [0342]    In this method, as shown in  FIG. 54 , another heated region  514  different from the heated region  512  is formed in the metallic plate  504   a.  The heated region  514  is grown with time and then integrated with the heated region  512 . The contact surface between the metallic plates  502   a,    504   a  is subjected to heat from both of the integrated heated regions  512 ,  514 . 
         [0343]    The contact surface between the metallic plates  500   a,    502   a  and the contact surface between the metallic plates  502   a,    504   a  are heated to a sufficient temperature and melted by the heated regions  510 ,  512 ,  514 . Thus obtained melted portions are cooled and solidified, whereby nuggets  516 ,  518  are formed between the metallic plates  500   a,    502   a  and between the metallic plates  502   a,    504   a  respectively. Though the nuggets  516 ,  518  are shown in  FIG. 54  to facilitate understanding, the nuggets  516 ,  518  are in the liquid-phase states of the melted portions during the energization. Such melted portions are shown in this manner also in the following drawings. 
         [0344]    The nugget  518  between the metallic plates  502   a,    504   a  can be grown further larger by increasing the pressing forces F 2 , F 3  of the auxiliary electrodes  476   a,    476   b.  However, the size of the nugget  518  tends to become saturated at certain levels of the pressing forces F 2 , F 3 . In other words, the nugget  518  is hardly grown larger than a certain size by excessively increasing the pressing forces F 2 , F 3 . Furthermore, in the case of excessively increasing the pressing forces F 2 , F 3 , the pressing force F 1  has to be excessively lowered in order to balancing the total force of the pressing forces F 1 , F 2 , F 3  with the pressing force F 4 . As a result, the size of the nugget  516  between the metallic plates  500   a,    502   a  is reduced. 
         [0345]    Consequently, it is preferred that the difference between the pressing force F 1  of the upper tip  468  and the pressing forces F 2 , F 3  of the auxiliary electrodes  476   a,    476   b  is determined in view of maximizing the sizes of the nuggets  516 ,  518 . 
         [0346]    As the ratio of the branching current i 2  is increased, the heated region  514  can be made larger. However, when the ratio of the branching current i 2  is excessively high, the current value of the current i 1  is reduced, whereby the sizes of the heated regions  510 ,  512  are reduced. Thus, the size of the nugget  516  is liable to be reduced, while the size of the nugget  518  becomes saturated. The ratio of the branching current i 2  is preferably selected in view of growing the nugget  516  to a sufficient size under the current i 1 . 
         [0347]    For example, the ratio between the current i 1  and the branching current i 2  can be controlled by changing the distances Z 5 , Z 6  between the upper tip  468  and the auxiliary electrodes  476   a,    476   b  (see  FIG. 48 ) as described above. The ratio between the current i 1  and the branching current i 2  is preferably e.g. 70:30. 
         [0348]    In the process of forming the melted portion, the metallic plate  504   a  is pressed against the metallic plate  502   a  by the auxiliary electrodes  476   a,    476   b.  The metallic plate  504   a  having a low rigidity can be prevented by such pressing from warping and thus from separating from the metallic plate  502   a  during the energization (heating). Thus, spatter scattering of the softened melted portion from a gap between the metallic plates  504   a,    502   a  can be prevented. 
         [0349]    The melted portion and hence the nugget  518  are grown with the passage of time as long as the energization is continued. Therefore, the nugget  518  can be sufficiently grown by performing the energization over an appropriate time. 
         [0350]    The current value of the current i 1  in the metallic plates  500   a,    502   a  is smaller than that in a conventional spot welding method. Therefore, the amount of generated heats of the metallic plates  500   a,    502   a  can be prevented from excessively increasing in the process of growing the melted portion (the nugget  518 ) between the metallic plates  502   a,    504   a.  Consequently, the apparatus is capable of eliminating the possibility of the spatter generation. 
         [0351]    In this process, a melted portion to be solidified into the nugget  516  is formed by the current i 1  between the metallic plates  500   a,    502   a.  When the branching current i 2  is continuously applied, the total amount of the current i 1  is reduced, and the heated region  510  and hence the nugget  516  are liable to be reduced in size, as compared to the case without the branching current i 2 . 
         [0352]    Therefore, in the case of further increasing the size of the nugget  516 , the ON/OFF switch  498  is opened by the gun controller  506  as shown in  FIGS. 55 and 56 . As a result, the auxiliary electrodes  476   a,    476   b  are electrically disconnected from the auxiliary terminal  456  to eliminate the branching current i 2 , so that the heated region  514  (see  FIG. 54 ) disappears. 
         [0353]    In this process, the energization of the upper tip  468  and the lower tip  470  is continued. Thus, the metallic plates  500   a,    502   a  are under a common spot welding condition. Since the current value of the current i 1  is increased after the dissipation of the branching current i 2 , the Joule heating value is increased in the high resistance metallic plates  500   a,    502   a,  whereby the heated region  510  is expanded and further heated to a higher temperature. The contact surface between the metallic plates  500   a,    502   a  is heated to a sufficient temperature and melted by the heated region  510  having the higher temperature, and the melted portion (the nugget  516 ) is grown larger. 
         [0354]    Thereafter, the energization may be continued until the melted portion (the nugget  516 ) grows sufficiently, e.g. until the melted portion for forming the nugget  516  is integrated with the melted portion for forming the nugget  518  as shown in  FIG. 56 . The relation between the energization time and the growth of the nugget  516  may be confirmed in advance by a spot welding test using test pieces. 
         [0355]    The contact surface between the metallic plates  500   a,    502   a  is preheated by the heated region  510  formed by the conduction of the current i 1  while the nugget  518  is grown between the metallic plates  502   a,    504   a.  Therefore, the affinity of the metallic plates  500   a,    502   a  with each other is improved before the melted portion to be converted to the nugget  516  is grown larger. Consequently, the spatter generation is hardly caused. 
         [0356]    As described above, in the fourth embodiment, the spatter generation can be prevented in both of the process of growing the nugget  518  between the metallic plates  502   a,    504   a  and the process of growing the nugget  516  between the metallic plates  500   a,    502   a.    
         [0357]    After the melted portion is sufficiently grown in a predetermined time, the energization is stopped as shown in  FIG. 57 . The energization may be stopped by separating the upper electrode  444  from the conductive terminal  452   a  or by stopping the welding current supply to the upper electrode  444 . 
         [0358]    The open/close cylinder  464  is actuated, and the open/close rod  466  is moved backward, whereby the gun arm  462  is opened. As a result, as shown in  FIG. 58 , the upper tip  468  and the lower tip  470  are moved away from each other, and are separated from the stacked body  434   a.  At the same time, the auxiliary electrodes  476   a,    476   b  are separated from the metallic plate  504   a.  The auxiliary electrodes  476   a,    476   b  are returned to the original positions under the elastic force of the coil springs  488   a,    488   b  (see  FIG. 47 ). 
         [0359]    The operations from the start to the end of the spot welding method are performed under the control of the gun controller  506 . 
         [0360]    When the energization is stopped in the above manner, the heating of the metallic plates  500   a,    502   a  is stopped. The obtained melted portion is cooled and solidified with the passage of time, whereby the metallic plates  500   a,    502   a  are joined to each other by the nugget  516 . 
         [0361]    Consequently, in the stacked body  434   a,  the metallic plates  500   a,    502   a  are joined to each other, and the metallic plates  502   a,    504   a  are joined to each other, to obtain a bonded article as a final product. 
         [0362]    The bonded product is excellent in the bonding strengths between the metallic plates  500   a,    502   a  and between the metallic plates  502   a,    504   a.  This is because the nugget  516  between the metallic plates  502   a,    504   a  is sufficiently grown under the branching current i 2  flowing in the metallic plate  504   a  as described above. 
         [0363]    As described above, in the fourth embodiment, the nugget  518  between the metallic plates  502   a,    504   a  can be grown to a size approximately equal to that of the nugget  516  between the metallic plates  500   a,    502   a  while preventing the spatter generation, whereby the bonded product can be produced with the excellent bonding strength between the metallic plates  502   a,    504   a.    
         [0364]    The indirect feeding type welding apparatus  430  can be prepared only by attaching the bracket  472  having the auxiliary electrodes  476   a,    476   b  to the upper tip  468  in a known indirect feeding type welding apparatus. Thus, the indirect feeding type welding apparatus  430  can be prevented from having a complicated or large structure due to the auxiliary electrodes  476   a,    476   b.  Therefore, even in a case where an intricately-shaped object is welded, the object can be located in a desired welding position without interference from the auxiliary electrodes  476   a,    476   b  and the upper tip  468 . 
         [0365]    The object to be welded is not limited to the stacked body  434   a.  The number, the materials, and the thicknesses of the metallic plates may be variously changed in the stacked body. Several specific examples will be described below. 
         [0366]    In a stacked body  434   b  shown in  FIG. 59 , a metallic plate  502   b  having the smallest thickness is interposed between metallic plates  500   b,    504   b.  For example, the metallic plate  500   b  is a high resistance workpiece composed of a high tensile strength steel, and the metallic plates  502   b,    504   b  are low resistance workpieces composed of a mild steel. 
         [0367]    In a case where the stacked body  434   b  is spot-welded only by the upper tip  468  and the lower tip  470 , the contact surface between the metallic plates  500   b,    502   b  is melted first. This is because the metallic plate  500   b  is the high resistance workpiece, whereby the contact resistance between the metallic plates  500   b,    502   b  is higher than that between the metallic plates  502   b,    504   b.  Therefore, when the energization of the upper tip  468  and the lower tip  470  is continued to sufficiently grow a nugget at the contact surface between the metallic plates  502   b,    504   b,  the spatter generation may be caused at the contact surface between the metallic plates  500   b,    502   b.    
         [0368]    In contrast, as shown in  FIG. 56 , according to the second embodiment using the auxiliary electrodes  476   a,    476   b,  heated regions  520 ,  522  are formed at the contact surface between the metallic plates  500   b,    502   b  and the contact surface between the metallic plates  502   b,    504   b  respectively. This is because the contact surface between the metallic plates  502   b,    504   b  is sufficiently heated by the branching current i 2  in the metallic plate  504   b  in the same manner as the above stacked body  434   a.    
         [0369]    Consequently, nuggets  524 ,  526  are formed as shown in  FIG. 60 . After the branching current i 2  has vanished, the current i 1  may be continuously applied. In this case, for example, as shown in  FIG. 61 , a sufficiently larger nugget  528  can be developed over the contact surface between the metallic plates  500   b,    502   b  and the contact surface between the metallic plates  502   b,    504   b.    
         [0370]    As is clear from the above explanations of the spot welding of the stacked assemblies  434   a,    434   b,  by using the auxiliary electrodes  476   a,    476   b,  the heated regions and hence the nuggets can be shifted closer to the auxiliary electrodes  476   a,    476   b.    
         [0371]    Though the metallic plate  500   b  is composed of the high tensile strength steel and the metallic plates  502   b,    504   b  are composed of the mild steel in the above example, of course, the combination of the materials are not particularly limited thereto. 
         [0372]    In  FIG. 62 , a stacked body  434   c,  which is provided by stacking a metallic plate  502   c  on a metallic plate  500   c,  is spot-welded by using the auxiliary electrodes  476   a,    476   b.  The metallic plates  502   c,    500   c  are composed of a high tensile strength steel. 
         [0373]    In the case of not using the auxiliary electrodes  476   a,    476   b,  because the metallic plates  500   c,    502   c  are high resistance workpieces, a large amount of Joule heating is generated in the vicinity of the contact surface therebetween during the energization. Therefore, a melted portion grows larger in a relatively short time in the vicinity of the contact surface, so that the melted portion is liable to be scattered (the spatter generation is liable to be caused). 
         [0374]    In contrast, as shown in  FIG. 62 , since the auxiliary electrodes  476   a,    476   b  are used in the fourth embodiment, a heated region  530  is formed at the contact surface between the metallic plates  500   c,    502   c,  and a heated region  532  is formed above the contact surface (i.e. in the vicinity of the auxiliary electrodes  476   a,    476   b  in the metallic plate  502   c ). This is because the metallic plate  502   c  is sufficiently heated by the branching current i 2  flow in the metallic plate  502   c.  Thus, also in this case, the heated region and hence a nugget  534  (see  FIG. 63 ) can be shifted closer to the auxiliary electrodes  476   a,    476   b.    
         [0375]    Consequently, the contact surface between the metallic plates  500   c,    502   c  is softened, thereby improving the sealing property. Thus, even when the current i 1  is continuously applied to form the sufficiently large nugget  534  as shown in  FIG. 63 , the spatter generation is hardly caused. 
         [0376]    In addition, it is to be understood that the stacked body may contain four or more metallic plates. 
         [0377]    As shown in  FIG. 64 , the branching current i 2  may be eliminated not by opening the ON/OFF switch  498  but by separating the auxiliary electrodes  476   a,    476   b  from the metallic plate  504   a  (the outermost workpiece). In this case, a displacement mechanism for displacing the auxiliary electrodes  476   a,    476   b  (such as an air cylinder) may be disposed on the bracket  472 , and the auxiliary electrodes  476   a,    476   b  may be moved upward away from the metallic plate  504   a  by the displacement mechanism. The displacement mechanism can be controlled by the gun controller  506 . 
         [0378]    Furthermore, as shown in  FIGS. 65 and 66 , a changing-over switch  536  may be used instead of the ON/OFF switch  498 . In this case, the changing-over switch  536  acts to form a current pathway between the auxiliary electrodes  476   a,    476   b  and the auxiliary terminal  456  (see  FIG. 65 ) or a current pathway between the lower tip  470  and the auxiliary terminal  456  (see  FIG. 66 ). In an initial stage of the welding, as shown in  FIG. 65 , the welding current is supplied to the upper electrode  444 , conducted through the upper tip  468 , the metallic plate  504   a,  the auxiliary electrodes  476   a,    476   b,  the changing-over switch  536 , the auxiliary terminal  456 , and the conductive terminal  452   b,  and introduced to the lower electrode  442 . 
         [0379]    Thus, in the initial stage, the current does not flow in the thickness direction of the stacked body  434   a,  so that only an internal portion of the metallic plate  504   a  and hence a portion in the vicinity of the contact surface between the metallic plates  502   a,    504   a  is heated. 
         [0380]    After a predetermined time has elapsed, as shown in  FIG. 66 , the changing-over switch  536  is switched, whereby the current pathway is formed between the lower tip  470  and the auxiliary terminal  456 . Consequently, the welding current is supplied to the upper electrode  444 , conducted through the stacked body  434   a  in the thickness direction from the upper tip  468  to the lower tip  470 , further transferred through the auxiliary terminal  456  and the conductive terminal  452   b,  and introduced to the lower electrode  442 . 
         [0381]    In this stage, the melted portions and hence the nuggets are grown in the vicinity of the contact surface between the metallic plates  500   a,    502   a  and in the vicinity of the contact surface between the metallic plates  502   a,    504   a.  Since the metallic plates  500   a,    502   a  have a high contact resistance, a large amount of Joule heating is generated in the vicinity of the contact surface therebetween, and the contact surface is sufficiently heated. Though the metallic plates  502   a,    504   a  have a low contact resistance, the contact surface therebetween has been heated, and the melted portion is readily formed in the vicinity of the contact surface. 
         [0382]    As described above, the nugget can be sufficiently grown between the adjacent metallic plates also by changing the current flow direction in the initial stage and the following stage of the welding. Consequently, the welded assembly can be produced with the excellent bonding strength. 
         [0383]    Though the branching current i 2  flows from the upper tip  468  to the auxiliary electrodes  476   a,    476   b  in the fourth embodiment, the auxiliary electrodes  476   a,    476   b  may be electrically isolated from the power source  450 , and the spot welding may be carried out without the branching current i 2 . In this case, the auxiliary electrodes  476   a,    476   b  act only as the pressing members. 
         [0384]    Also in this case, the total pressing force is distributed from the upper tip  468  to the lower tip  470  as shown in  FIG. 48 . The contact area between the metallic plates  504   a,    502   a  is larger than the case without being pressed by the auxiliary electrodes  476   a,    476   b  (see  FIG. 49 ). Therefore, the contact resistance and the current density at the contact surface between the metallic plates  502   a,    504   a  are increased, and the generated amount of Joule heating (i.e. the amount of generated heat) is increased. Consequently, the heated region and hence the nugget are grown to a sufficient size in the vicinity of the contact surface between the metallic plates  502   a,    504   a.    
         [0385]    A first support tip and a support pressing member may be disposed between the stacked body and the upper tip  468  (and the auxiliary electrodes  476   a,    476   b  (pressing members)), and a second support tip may be disposed between the stacked body and the lower tip  470 . A fifth embodiment, which uses the components e.g. in the welding of the stacked body  434   a,  will be described below. 
         [0386]      FIG. 67  is a front view of the essential features of an indirect feeding type welding apparatus having an upper support tip  550  (first support tip), support pressing members  552   a,    552   b,  and a lower support tip  553  (second support tip). In this indirect feeding type welding apparatus, a bracket  554  is attached to the body of the upper tip  468 . The bracket  554  has a through-hole  555 , which has a diameter approximately equal to the body diameter of the upper tip  468 . The body of the upper tip  468  is inserted and fitted into the through-hole  555 . 
         [0387]    Specifically, two actuators  556   a,    556   b  are disposed in the bracket  554 . The auxiliary electrodes  476   a,    476   b,  which act as pressing members, project from tubes  558   a,    558   b  in the actuators  556   a,    556   b  and extend parallel to the upper tip  468 . The auxiliary electrodes  476   a,    476   b  are displaced by the actuators  556   a,    556   b  close to and away from the lower tip  470 . Thus, the actuators  556   a,    556   b  act as displacement mechanisms for displacing the auxiliary electrodes  476   a,    476   b  and as pressing force generation/control mechanisms for generating and controlling pressing forces of the auxiliary electrodes  476   a,    476   b.    
         [0388]    The upper support tip  550  and the support pressing members  552   a,    552   b  are interposed between the metallic plate  504   a  of the stacked body  434   a  and the upper tip  468  (and the auxiliary electrodes  476   a,    476   b ). The upper support tip  550  and the support pressing members  552   a,    552   b  are disposed on a first open/close bracket  560  supported by an open/close mechanism (not shown). The first open/close bracket  560  is composed of an insulator. 
         [0389]    Long wide pressing parts  562 ,  564 ,  566  are disposed on the tops of the upper support tip  550  and the support pressing members  552   a,    552   b  respectively. The pressing parts  562 ,  564 ,  566  are conductors. 
         [0390]    As shown in  FIG. 68 , the lower ends of the upper tip  468  and the auxiliary electrodes  476   a,    476   b  are brought into contact with the upper surfaces of one ends of the pressing parts  562 ,  564 ,  566  respectively. The upper support tip  550  and the support pressing members  552   a,    552   b  project from the lower surfaces of the other ends of the pressing parts  562 ,  564 ,  566 . 
         [0391]    The lower support tip  553  is disposed on a second open/close bracket  567  supported by the open/close mechanism. The lower support tip  553  is interposed between the lower tip  470  and the metallic plate  500   a  of the stacked body  434   a.  Also the second open/close bracket  567  is composed of an insulator. 
         [0392]    A pressing part  568  is disposed on the bottom of the lower support tip  553 . In the structure of  FIG. 68 , the top of the lower tip  470  is brought into contact with the lower surface of one end of the pressing part  568 . The lower support tip  553  projects from the upper surface of the other end of the pressing part  568 . 
         [0393]    The advantages of this structure will be described below. 
         [0394]    For example, there is a case where a stacked body  434   d  shown in  FIG. 69 , which contains a shaped workpiece  572  having a vertical wall  570 , has to be welded at an angle of  FIG. 70 . In this case, as is clear from  FIG. 70 , when only the upper tip  468  and the auxiliary electrodes  476   a,    476   b  are used, the auxiliary electrode  476   a  may interfere with the vertical wall  570 , and the auxiliary electrode  476   b  may be insufficiently contacted with the stacked body  434   d,  disadvantageously. 
         [0395]    In contrast, the upper support tip  550 , the support pressing members  552   a,    552   b,  and the lower support tip  553  are used in this embodiment. Therefore, by appropriately controlling the lengths of the pressing parts  562 ,  564 ,  566 ,  568 , etc. as shown in  FIG. 68 , the support pressing member  552   a  can be prevented from interfering with the vertical wall  570 , and the support pressing member  552   b  can be sufficiently contacted with the stacked body  434   d.    
         [0396]    In the case of using the indirect feeding type welding apparatus for welding the stacked body  434   a  (see  FIG. 67 ), the first open/close bracket  560  and the second open/close bracket  567  are closed, whereby the upper support tip  550 , the support pressing members  552   a,    552   b,  and the lower support tip  553  are located in the vicinity of the welding position. Thereafter, the gun arm  462  is closed by the open/close cylinder  464  in the same manner as above, so that the upper tip  468  and the lower tip  470  are moved close to each other. Consequently, the upper tip  468  and the lower tip  470  are brought into contact with the upper surfaces of one ends of the pressing parts  564 ,  568 . 
         [0397]    Meanwhile, the actuators  556   a,    556   b  are driven by the gun controller, whereby the auxiliary electrodes  476   a,    476   b  are lowered toward the stacked body  434   a.  The auxiliary electrodes  476   a,    476   b  are brought into contact with the upper surfaces of one ends of the pressing parts  562 ,  566 . The auxiliary electrodes  476   a,    476   b  may be contacted with the pressing parts  562 ,  566  before, at the same time as, or after the contact of the upper tip  468  and the lower tip  470  with the pressing parts  564 ,  568 . 
         [0398]    Of course, the thrust forces of the actuators  556   a,    556   b  and the driving force of the open/close cylinder  464  are controlled such that the total pressing force (F 1 ′+F 2 ′+F 3 ′) of the upper tip  468  and the auxiliary electrodes  476   a,    476   b  against the metallic plate  504   a  is well balanced with the pressing force (F 4 ′) of the lower support tip  553  against the metallic plate  500   a.  By this control, the total pressing force (F 1 ′+F 2 ′+F 3 ′) applied to the stacked body  434   a  in the arrow Y 1  direction is made approximately equal to the pressing force (F 4 ′) applied to the stacked body  434   a  in the direction of the arrow Y 2 . Consequently, a pressing force distribution equal to that of  FIGS. 48 and 49  is achieved. 
         [0399]    After the pressing force distribution is achieved, the gun controller  506  sends a control signal to the power source  450 . When the power source  450  receives the control signal, the power source  450  acts to supply a welding current. The welding current flows from the upper electrode  444  connected to the positive terminal, through the lower electrode  442 , to the negative terminal. 
         [0400]    The welding current flows from the upper electrode  444 , through the conductive terminal  452   a,  the lead  492 , the upper tip  468 , the pressing part  564 , and the upper support tip  550 , to the metallic plate  504   a.  Furthermore, the current is transferred through the metallic plates  502   a,    500   a,  the lower support tip  553 , and the pressing part  568 , to the lower tip  470 . At the same time, a current flows through the metallic plate  504   a,  the support pressing members  552   a,    552   b,  and the pressing parts  562 ,  566 , to the auxiliary electrodes  476   a,    476   b.  Thus, as shown in  FIG. 71 , a current i 1  flows in the direction from the upper support tip  550  (the upper tip  468 ) to the lower support tip  553  (the lower tip  470 ), and a branching current i 2  flows in the direction from the upper support tip  550  (the upper tip  468 ) to the support pressing members  552   a,    552   b  (the auxiliary electrodes  476   a,    476   b ). 
         [0401]    The metallic plates  500   a,    502   a  and the metallic plates  502   a,    504   a  are heated by Joule heating generated due to the current i 1  and the branching current i 2 , whereby heated regions  574 ,  576  are formed respectively. 
         [0402]    Also in this case, a sufficiently large amount of Joule heating is generated in the vicinity of the contact surface between the metallic plates  504   a,    502   a.  This is because the contact area between the metallic plates  504   a,    502   a  is smaller (i.e. the contact resistance is higher) in this case as compared with the case of using only the upper tip  468  and the lower tip  470  for gripping the stacked body  434   a  (see  FIG. 50 ). Consequently, a nugget  578  in the vicinity of the contact surface between the metallic plates  500   a,    502   a  and a nugget  580  in the vicinity of the contact surface between the metallic plates  502   a,    504   a  are grown to approximately the same size. 
         [0403]    After the completion of the welding, the gun arm  462  is opened, whereby the upper tip  468 , the auxiliary electrodes  476   a,    476   b,  and the lower tip  470  are separated from the upper support tip  550 , the support pressing members  552   a,    552   b,  and the lower support tip  553  respectively. Furthermore, the first open/close bracket  560  and the second open/close bracket  567  are opened, whereby the upper support tip  550 , the support pressing members  552   a,    552   b,  and the lower support tip  553  are separated from the stacked body  434   a.  The upper support tip  550 , the support pressing members  552   a,    552   b,  and the lower support tip  553 , separated from the stacked body  434   a,  may be returned to the original positions by a coil spring or the like. 
         [0404]    Also in this embodiment, only the upper tip  468  and the lower tip  470  may be energized in the welding, while the electric power is not supplied to the support pressing members  552   a,    552   b.  In this case, for example, the support pressing members  552   a,    552   b  may be composed of an insulator, and the auxiliary electrodes  476   a,    476   b  may be electrically inactivated. 
         [0405]    In the fourth and fifth embodiments, the current flows in the direction from the upper tip  468  on the metallic plate  504   a  to the lower tip  470  on the metallic plate  500   a.  However, the current may flow in the opposite direction as shown in  FIG. 72 . Also in this case, the auxiliary electrodes  476   a,    476   b  on the metallic plate  504   a  have polarities opposite to that of the upper tip  468 . Thus, the lower electrode  442  is electrically connected to the positive terminal of the power source  450 , whereby the lower tip  470  and the auxiliary electrodes  476   a,    476   b  have a positive (+) polarity. On the other hand, the upper electrode  444  is electrically connected to the negative terminal of the power source  450 , whereby the upper tip  468  has a negative (−) polarity. Consequently, the current i 1  flows from the lower tip  470  to the upper tip  468 , and the branching current i 2  flows from the auxiliary electrodes  476   a,    476   b  to the upper tip  468 . 
         [0406]    Of course, also in the case of using the upper support tip  550  and the support pressing members  552   a,    552   b  (see  FIGS. 67 and 71 ), the current may flow from the support pressing members  552   a,    552   b  to the upper support tip  550 . 
         [0407]    As shown in  FIG. 73 , the branching current i 2  may flow not only in the metallic plate  504   a  in contact with the upper tip  468  but also in the metallic plate  502   a  located immediately beneath the metallic plate  504   a.    
         [0408]    Even after the energization from the upper tip  468  to the auxiliary electrodes  476   a,    476   b  or from the upper support tip  550  to the support pressing members  552   a,    552   b  is stopped, the stacked body may be continuously pressed by the auxiliary electrodes  476   a,    476   b  or the support pressing members  552   a,    552   b.  In this case, for example, the increased contact area is maintained between the metallic plates  502   a,    504   a.  Therefore, the nugget between the metallic plates  502   a,    504   a  can be readily grown even under the current i 1  flow. 
         [0409]    In any case, the auxiliary electrode is not particularly limited to the above-described two auxiliary electrodes  476   a,    476   b  having the long rod shape. For example, one, three, or more long rods may be used as the auxiliary electrodes. In the case of using three or more auxiliary electrodes, a plurality of the auxiliary electrodes may be contacted with and separated from the outermost metallic plate at the same time in the same manner as with the two auxiliary electrodes. Each auxiliary electrode may have a ring shape surrounding the lower tip  470  or the upper tip  468 .