Abstract:
There are provided a spot welding apparatus and a spot welding method. A workpiece is clamped between a combination of a second welding electrode and a receiving unit that abut against a thinner plate, and a first welding electrode that abuts against a second thicker plate, a pressure is applied to the workpiece by the first welding electrode, and a current is passed between the welding electrodes. Consequently, a satisfactory nugget is formed over from the thinner plate to the second thicker plate. Similarly, a workpiece is clamped between a combination of the first welding electrode and the receiving unit that abut against a thinner plate, and the second welding electrode that abuts against a second thicker plate, a pressure is applied to the workpiece by the second welding electrode. Consequently, a satisfactory nugget is formed over from the thinner plate to the second thicker plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese Patent Application No. 2011-065355 filed on Mar. 24, 2011, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to apparatuses and methods for spot welding a plate workpiece in which a thicker plate and a thinner plate having different rigidities are stacked one on top of the other. 
         [0004]    2. Description of the Related Art 
         [0005]    In the art of joining stacked plates such as steel plates, spot welding has been widely used in which the plates are clamped between a pair of welding electrodes and a large current is passed between the electrodes for a predetermined time period while applying a welding pressure to the plates in order to heat the joint zone to substantially a melting temperature, thereby joining the plates together. 
         [0006]    Provided that the welding pressure applied by the welding electrodes and the energization time in spot welding are constant, a nugget diameter gradually increases with an increase in current amperage. However, the application of an excessively large current increases the amount of generated heat, possibly resulting in a phenomenon called expulsion in which molten metal is expelled from between the plates. That is, expulsion is a phenomenon of molten metal exploding due to superheating at the joint zone and results in a nugget having defects such as holes and cracks. Such discontinuities in the nugget shape and the metal structure can be a factor that markedly decreases the joint strength as well as can be a factor reducing the plate thickness at the joint. On the other hand, the application of an excessively small current results in a small nugget that does not achieve sufficient joint strength. Further, when the welding pressure is low, the area of contact between the plates becomes small with the result that the current density is increased and expulsion is caused due to superheating. When the welding pressure is excessively high, the contact area at the joint becomes large and the current density is lowered with the result that the amount of generated heat is reduced. Consequently, the resultant nugget is small and does not achieve high weld strength. 
         [0007]    Reference is now made to  FIG. 13A  which illustrates spot welding of a workpiece  100  in which a thinner plate  101  having a low rigidity and a small thickness, and a first thicker plate  102  and a second thicker plate  103  having a larger thickness, namely, a higher rigidity than the thinner plate  101 , are stacked on top of one another. As illustrated, the workpiece  100  is clamped by a movable electrode  111  and a fixed electrode  112  in a condition such that the plates are in intimate contact without any clearance between the thinner plate  101  and the first thicker plate  102  and between the first thicker plate  102  and the second thicker plate  103 . Energization by a power supply  113  results in a substantially uniform current density throughout an energization path between the movable electrode  111  and the fixed electrode  112 . As a result, a satisfactory nugget is formed in a region ranging from the thinner plate  101  to the second thicker plate  103 , thereby achieving desired weld strength. 
         [0008]    In actuality, however, the low-rigidity thinner plate  101  and the first thicker plate  102  are bent upward when the workpiece  100  is clamped and pressed by the movable electrode  111  and the fixed electrode  112 , leaving clearances between the thinner plate  101  and the first thicker plate  102  and between the first thicker plate  102  and the second thicker plate  103 . 
         [0009]    In such a case, the area of contact between the movable electrode  111  and the thinner plate  101  is increased due to the deflection of the thinner plate  101 , while the contact areas at the joints between the thinner plate  101  and the first thicker plate  102  and between the first thicker plate  102  and the second thicker plate  103  are reduced by the clearances. 
         [0010]    As a result, the current density between the movable electrode  111  and the fixed electrode  112  becomes higher on the second thicker plate  103  side than on the thinner plate  101  side, and the amount of local heat generated between the first thicker plate  102  and the second thicker plate  103  is larger than that generated between the thinner plate  101  and the first thicker plate  102 . 
         [0011]    Consequently, a nugget  105  is formed first at the joint between the first thicker plate  102  and the second thicker plate  103  as illustrated in  FIG. 13A , and the nugget  105  gradually grows so as to weld the thinner plate  101  and the first thicker plate  102  as illustrated in  FIG. 13B . However, the amount of penetration through the first thicker plate  102  into the thinner plate  101  is small and the weld strength is unstable. Thus, there is a risk that the thinner plate  101  will become separated. Further, the weld quality is variable. In particular, this defect is more likely with an increase in thickness of the first thicker plate  102  and the second thicker plate  103  because it is more difficult for the nugget  105  to reach between the first thicker plate  102  and the thinner plate  101 . 
         [0012]    Another reason for the small penetration between the thinner plate  101  and the first thicker plate  102  and the consequent unstable weld strength is the fact that due to the small thickness of the thinner plate  101 , heat is drawn into the movable electrode  111  when the movable electrode  111  is in contact with the thinner plate  101  with the result that the temperature on the thinner plate  101  side is not increased, thereby preventing the formation of the nugget  105 . 
         [0013]    An exemplary remedy for this problem is a spot welding method disclosed in Japanese Unexamined Patent Application Publication No. 2003-251469. According to this spot welding method, as illustrated in  FIG. 14 , a workpiece  100  in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are stacked on top of one another is spot welded in a manner such that the welding pressure (FU) of a movable electrode  125  located on the thinner plate  101  side is controlled to be smaller than the welding pressure (FL) of a fixed electrode  124  positioned on the second thicker plate  103  side, whereby the contact resistance at the joint between the thinner plate  101  and the first thicker plate  102  is increased while the contact resistance at the joint between the first thicker plate  102  and the second thicker plate  103  is decreased. In this manner, energization between the movable electrode  125  and the fixed electrode  124  results in an increased amount of heat generated at the joint between the thinner plate  101  and the first thicker plate  102 . Thus, the weld strength between the thinner plate  101  and the first thicker plate  102  can be increased. 
         [0014]      FIG. 15  illustrates a structure of a spot welding gun that is used in performing the above method. As illustrated, a spot welding apparatus  120  is mounted on a wrist  116  of a welding robot  115 . The welding robot  115  is configured to spot weld the workpiece  100  while moving the spot welding gun  120  to each of the welding sites on the workpiece  100  that is supported by a clamper  118 . 
         [0015]    The spot welding gun  120  has a base unit  122  that is vertically movably supported by a linear guide  121  fixed to a gun-supporting bracket  117  that is attached to the wrist  116 . The base unit  122  is fitted with a C-shaped yoke  123  that extends downward. The fixed electrode  124  is disposed on the lower tip of the C-shaped yoke  123 . 
         [0016]    A pressure actuator  126  such as a servomotor is mounted on the upper end of the base unit  122 . The movable electrode  125  opposing the fixed electrode  124  is attached to the lower end of a rod  127  that is vertically movable by the pressure actuator  126 . A servomotor  128  is mounted on the upper end of the gun-supporting bracket  117 . The base unit  122  is movable vertically by a ball screw mechanism by driving of the servomotor  128 . 
         [0017]    In accordance with teaching data stored in a controller which is not shown, the welding pressure (FU) of the movable electrode  125  located on the thinner plate  101  side is controlled to be smaller than the welding pressure (FL) of the fixed electrode  124  (EU&lt;FL). 
         [0018]    In order to control the welding pressure (FU) of the movable electrode  125  to be smaller than the welding pressure (FL) of the fixed electrode  124  (FU&lt;FL), the controller first actuates the servomotor  128  so as to lift the base unit  122  and bring the fixed electrode  124  into contact with the lower surface of the workpiece  100 . At the same time, the controller actuates the pressure actuator  126  so as to lower the movable electrode  125  into contact with the upper surface of the workpiece  100 . In this case, the welding pressure applied by the pressure actuator  126  acts evenly on the movable electrode  125  and the fixed electrode  124  via the base unit  122  and the C-shaped yoke  123 . 
         [0019]    Next, the base unit  122  is lifted by the servomotor  128 . By lifting the base unit  122 , the welding pressure (FL) of the fixed electrode  124  is increased in accordance with lifting of the base unit  122 . As a result, the welding pressure (FU) of the movable electrode  125  becomes smaller than the welding pressure (FL) of the fixed electrode  124  (FU&lt;FL). 
         [0020]    Consequently, when a current is passed between the movable electrode  125  and the fixed electrode  124 , a high current density is achieved at the joint between the thinner plate  101  and the first thicker plate  102  and the amount of heat generated at the joint is increased relative to the amount of heat generated at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, a uniform and satisfactory nugget is formed in a region ranging from the thinner plate  101  to the second thicker plate  103 , and high weld strength is ensured. 
         [0021]    According to Japanese Unexamined Patent Application Publication No. 2003-251469, the spot welding gun  120  is moved to each of the welding sites on the workpiece  100  that is supported by the clamper  118 ; the fixed electrode  124  is caused to abut against the second thicker plate  103  of the workpiece  100  and the movable electrode  125  is caused to abut against the thinner plate  101 ; and the base unit  122  is lifted to control the welding pressure (FU) of the movable electrode  125  to be smaller than the welding pressure (FL) of the fixed electrode  124 . In this manner, the current density between the thinner plate  101  and the first thicker plate  102  is relatively increased to ensure that an appropriate amount of heat is generated at the joint between the thinner plate  101  and the first thicker plate  102 . Thus, the amount of penetration is increased and the weld strength is improved. 
         [0022]    However, the clamper  118  that clamps the workpiece  100  is required to withstand a heavy load when the welding pressure (FU) of the movable electrode  125  is controlled to be smaller than the welding pressure (FL) of the fixed electrode  124  by moving the base unit  122  while the workpiece  100  that is clamped by the clamper  118  is pinched between the fixed electrode  124  and the movable electrode  125 . In the case where the position of the clamping of the workpiece  100  by the clamper  118  is very far from the welding position, namely, the position of a welding site, the workpiece  100  is distorted and deformed, whereby variations in the welding pressure (FL) of the fixed electrode  124  and the welding pressure (FU) of the movable electrode  125  are caused. As a result, it is difficult to stably ensure an appropriate contact resistance between the thinner plate  101  and the first thicker plate  102  and between the first thicker plate  102  and the second thicker plate  103 . Thus, there is a risk that the current density at the joints in the workpiece  100  will be nonuniform and the quality of the spot weld will be lowered. Further, in the above method, a spot welding gun that has an equalizing function between the robot wrist and the base unit to allow for movements utilizing a reactive force produced by the welding pressure cannot be adopted. Thus, usable spot welding guns are limited. 
       SUMMARY OF THE INVENTION 
       [0023]    In view of the problems in the art described above, it is an object of the invention to provide a spot welding apparatus and a spot welding method whereby a workpiece in which a thicker plate and a thinner plate having differing rigidities are stacked one on top of the other can be spot welded with a stable weld quality. 
         [0024]    To achieve the above object, an aspect of the invention provides a spot welding apparatus for spot welding a workpiece in which a thinner plate having a small thickness, and a first thicker plate and a second thicker plate that have a higher rigidity and a larger thickness than the thinner plate are sequentially stacked on top of one another, the apparatus including a base unit, a receiving unit supported by the base unit, and a first welding electrode and a second welding electrode that are supported by the base unit and are movable toward and away from each other in an opposing relationship. The workpiece is clamped between a combination of the second welding electrode that abuts against the thinner plate and the receiving unit that is adjacent to the second welding electrode and abuts against the thinner plate, and the first welding electrode that abuts against the second thicker plate, a pressure is applied to the workpiece by the first welding electrode simultaneously with the clamping, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. The workpiece is clamped between a combination of the first welding electrode that abuts against the thinner plate and the receiving unit that is adjacent to the first welding electrode and abuts against the thinner plate, and the second welding electrode that abuts against the second thicker plate, a pressure is applied to the workpiece by the second welding electrode simultaneously with the clamping, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. 
         [0025]    According to the above apparatus, the workpiece is clamped between a combination of the second welding electrode and the receiving unit that abut against the thinner plate, and the first welding electrode which abuts against the second thicker plate, and a pressure is applied to the workpiece by the first welding electrode. The welding pressure from the first welding electrode is applied to the second thicker plate of the workpiece, and the welding pressure from the second welding electrode and that from the receiving unit are applied to the thinner plate with the result that the welding pressure of the second welding electrode located on the thinner plate side is smaller than the welding pressure of the first welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the first thicker plate is relatively increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Similarly, the workpiece is clamped between a combination of the first welding electrode and the receiving unit that abut against the thinner plate, and the second welding electrode which abuts against the second thicker plate, and a pressure is applied to the workpiece by the second welding electrode. The welding pressure from the second welding electrode is applied to the second thicker plate of the workpiece, and the welding pressure from the first welding electrode and that from the receiving unit are applied to the thinner plate with the result that the welding pressure of the first welding electrode located on the thinner plate side is smaller than the welding pressure of the second welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the first thicker plate is relatively increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Thus, the weld quality of the welded workpiece can be improved. 
         [0026]    Further, the apparatus can continuously spot weld workpieces having a thinner plate, a first thicker plate and a second thicker plate in different arrangements without the need of greatly changing the posture of the welding apparatus. For example, a three-ply workpiece in which a thinner plate, a first thicker plate and a second thicker plate are sequentially stacked from the bottom, and a three-ply workpiece in which a thinner plate, a first thicker plate and a second thicker plate are sequentially stacked from the top can be spot welded continuously without greatly changing the posture of the welding apparatus. 
         [0027]    To achieve the above object, another aspect of the invention provides a spot welding apparatus for spot welding a workpiece in which a thinner plate, and a first thicker plate and a second thicker plate having a higher rigidity than the thinner plate are sequentially stacked on top of one another, the apparatus including a base unit, a receiving unit supported by the base unit, a first welding electrode that is movable to any one of a retracted position, a first pressing position and a second pressing position by a first pressure actuator supported by the base unit, and a second welding electrode that is movable to any one of a retracted position, a first pressing position and a second pressing position in an opposing relationship with the first welding electrode by a second pressure actuator supported by the base unit. The workpiece is clamped between a combination of the second welding electrode that is positioned at the first pressing position and the receiving unit that is adjacent to the second welding electrode and abuts against the thinner plate, and the first welding electrode that is positioned at the first pressing position, a pressure is applied to the workpiece by the first welding electrode simultaneously with the clamping, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. The workpiece is clamped between a combination of the first welding electrode that is positioned at the second pressing position and the receiving unit that is adjacent to the first welding electrode and abuts against the thinner plate, and the second welding electrode that is positioned at the second pressing position, a pressure is applied to the workpiece by the second welding electrode simultaneously with the clamping, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. 
         [0028]    According to the above apparatus, the workpiece is clamped between a combination of the second welding electrode at the first pressing position and the receiving unit which abut against the thinner plate, and the first welding electrode at the first pressing position which abuts against the second thicker plate, and a pressure is applied to the workpiece by the first welding electrode at the first pressing position. The welding pressure from the first welding electrode at the first pressing position is applied to the second thicker plate of the workpiece, and the welding pressure from the second welding electrode and that from the receiving unit are applied to the thinner plate with the result that the welding pressure of the second welding electrode located on the thinner plate side is smaller than the welding pressure of the first welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the first thicker plate is relatively increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Similarly, the workpiece is clamped between a combination of the first welding electrode at the second pressing position and the receiving unit which abut against the thinner plate, and the second welding electrode at the second pressing position which abuts against the second thicker plate, and a pressure is applied to the workpiece by the second welding electrode at the second pressing position. The welding pressure from the second welding electrode is applied to the second thicker plate of the workpiece, and the welding pressure from the first welding electrode and that from the receiving unit are applied to the thinner plate with the result that the welding pressure of the first welding electrode located on the thinner plate side is smaller than the welding pressure of the second welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the first thicker plate is relatively increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Thus, the weld quality of the welded workpiece can be improved. 
         [0029]    To achieve the above object, another aspect of the invention provides a spot welding apparatus for spot welding a workpiece in which a plurality of thicker plates are stacked on top of one another and a thinner plate is stacked together with the thicker plates, the apparatus including a base unit, a receiving unit that is supported by the base unit and is to abut against the thinner plate, a first welding electrode that is movable relative to the receiving unit by a first pressure actuator supported by the base unit, and a second welding electrode that is movable relative to the receiving unit by a second pressure actuator supported by the base unit. The workpiece is clamped under pressure by the receiving unit, the first welding electrode and the second welding electrode, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. 
         [0030]    According to the above apparatus, the workpiece is clamped under pressure by the receiving unit that abuts against the thinner plate, the first welding electrode that can be moved by the first pressure actuator, and the second welding electrode that can be moved by the second pressure actuator, in a manner such that the welding pressure applied to a pair of the thinner plate and the adjacent thicker plate is smaller than the welding pressure applied to each pair of the thicker plates. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the adjacent thicker plate is increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the thicker plates with uniform penetration. Thus, the weld quality of the welded workpiece can be improved. 
         [0031]    To achieve the above object, another aspect of the invention provides a spot welding method for spot welding a workpiece in which a plurality of thicker plates are stacked on top of one another and a thinner plate is stacked together with the thicker plates, wherein the thinner plate is caused to abut against a receiving unit that is supported by a base unit and the workpiece is clamped under pressure between a first welding electrode that is movable by a first pressure actuator supported by the base unit and a second welding electrode that is movable by a second pressure actuator supported by the base unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece. 
         [0032]    According to the above method, the workpiece is clamped under pressure by the receiving unit that abuts against the thinner plate, the first welding electrode that can be moved by the first pressure actuator, and the second welding electrode that can be moved by the second pressure actuator, in a manner such that the welding pressure applied to a pair of the thinner plate and the adjacent thicker plate is smaller than the welding pressure applied to each pair of the thicker plates. As a result, when a current is passed between the first welding electrode and the second welding electrode, the current density at the joint between the thinner plate and the adjacent thicker plate is increased, and a satisfactory nugget is formed in a region ranging from the thinner plate to the thicker plates with uniform penetration. Thus, the weld quality of the welded workpiece can be improved. 
         [0033]    According to the invention, the workpiece is clamped between a combination of the second welding electrode and the receiving unit which abut against the thinner plate, and the first welding electrode which abuts against the second thicker plate, and a pressure is applied to the workpiece by the first welding electrode with the result that the welding pressure of the second welding electrode located on the thinner plate side is smaller than the welding pressure of the first welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Similarly, the workpiece is clamped between a combination of the first welding electrode and the receiving unit which abut against the thinner plate, and the second welding electrode which abuts against the second thicker plate, and a pressure is applied to the workpiece by the second welding electrode with the result that the welding pressure of the first welding electrode located on the thinner plate side is smaller than the welding pressure of the second welding electrode positioned on the second thicker plate side. As a result, when a current is passed between the first welding electrode and the second welding electrode, a satisfactory nugget is formed in a region ranging from the thinner plate to the second thicker plate with uniform penetration. Thus, the weld quality of the welded workpiece can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a view illustrating a structure of a spot welding apparatus according to an embodiment of the invention; 
           [0035]      FIG. 2  is a view of a main section taken on arrow II in  FIG. 1 ; 
           [0036]      FIG. 3  is an explanatory view for an operation of a spot welding apparatus; 
           [0037]      FIG. 4  is a sectional explanatory view for an operation of a main section in  FIG. 3 ; 
           [0038]      FIG. 5  is an explanatory view for an operation of the spot welding apparatus; 
           [0039]      FIG. 6  is a sectional explanatory view for an operation of a main section in  FIG. 5 ; 
           [0040]      FIG. 7  is a schematic explanatory view for an operation of the spot welding apparatus; 
           [0041]      FIG. 8  is an explanatory view for an operation of the spot welding apparatus; 
           [0042]      FIG. 9  is a sectional explanatory view for an operation of a main section in  FIG. 8 ; 
           [0043]      FIG. 10  is an explanatory view for an operation of the spot welding apparatus; 
           [0044]      FIG. 11  is a sectional explanatory view for an operation of a main section in  FIG. 10 ; 
           [0045]      FIG. 12  is a schematic explanatory view for an operation of the spot welding apparatus; 
           [0046]      FIG. 13A  is a view that briefly illustrates conventional spot welding; 
           [0047]      FIG. 13B  is a view that briefly illustrates conventional spot welding; 
           [0048]      FIG. 14  is a view that briefly illustrates conventional spot welding; and 
           [0049]      FIG. 15  is a view that briefly illustrates conventional spot welding. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0050]    An Embodiment of the present invention will be described hereinbelow with reference to  FIGS. 1 to 12 . 
         [0051]      FIG. 1  is a view illustrating a structure of a spot welding apparatus, and  FIG. 2  is a view of a main section taken along arrow II in  FIG. 1 . For convenience in the description of the spot welding apparatus, the upward direction and the downward direction in  FIG. 1  will be referred to as the upward direction and the downward direction in the spot welding apparatus. 
         [0052]    Prior to describing a spot welding apparatus  1 , a workpiece  100  to be welded will be described with reference to  FIG. 2 . The workpiece  100  is a three-ply plate in which a thinner plate is stacked onto one of two thicker plates that have been stacked one on top of the other, for example, a three-ply plate in which a thinner plate  101  having a low rigidity and a small thickness, and a first thicker plate  102  and a second thicker plate  103  that have a larger thickness and a higher rigidity than the thinner plate  101  are sequentially stacked on top of one another. 
         [0053]    As illustrated in  FIGS. 1 and 2 , the spot welding apparatus  1  is supported, via an equalizer unit  2 , by a wrist that is attached to the tip of an arm of a welding robot such as a multi-joint robot, and can be moved three dimensionally by the welding robot. The welding robot moves the spot welding apparatus  1  sequentially to each of predetermined welding sites on the workpiece  100  that is held at a predetermined position by a member such as a clamper which is not shown, thereby spot welding the workpiece  100 . 
         [0054]    The spot welding apparatus  1  has a base unit  3 . The base unit  3  is composed of an apparatus attachment bracket  4  that is attached to a wrist of the welding robot via the equalizer unit  2 , and gun brackets  5  that are integrally attached to the apparatus attachment bracket  4 . The apparatus attachment bracket  4  has a U-shaped cross section. This cross section includes an attachment base section  4 A that is a rectangular flat plate connected to the equalizer unit  2 , and a pair of opposing gun attachment sections  4 B that are curved downward from both sides of the attachment base section  4 A. 
         [0055]    The gun brackets  5  are attached so as to oppose the respective gun attachment sections  4 B of the apparatus attachment bracket  4 . Each gun bracket  5  is a substantially T-shaped plate in which a bracket body  6  that is a rectangular plate extending in a direction and is attached to the gun attachment section  4 B, and a fixing arm support section  7  that extends downward from the bracket body  6  are integrally formed. 
         [0056]    A fixing arm  10 , a first pressure actuator  20 , a second pressure actuator  30  and a welding transformer  40  are attached to and supported by the opposing gun brackets  5 . 
         [0057]    The fixing arm  10  is a substantially L-shaped member that has a fixing arm body  11  of a substantially U-shaped cross section which extends downward as a result of a rear end section  11   a  being hung between and coupled to the ends of the fixing arm support sections  7  of the gun brackets  5 , and further has a support section  12  which extends while curving from the tip of the fixing arm body  11 . A receiving unit  13  is formed in a tip  12   a  of the support section  12 . The receiving unit  13  defines a pipe that has a central axis extending in a vertical direction, and has an upper end  13   a  and a lower end  13   b  that project from the support section  12 . 
         [0058]    The first pressure actuator  20  has a cylinder device or a servomotor, in this embodiment a servomotor  21 , and a direct driving unit  22  that is constituted by a ball screw mechanism or the like. A rod  23  of the direct driving unit  22  is raised or lowered by driving of the servomotor  21 . A first electrode arm  24  is provided at the lower end of the rod  23  of the direct driving unit  22 . A first welding electrode  25  is attached at the tip of the first electrode arm  24  so as to be in a coaxial relationship with the central axis of the receiving unit  13  of the fixing arm  10 . According to this configuration, a top end  25   a  of the first welding electrode  25  can be moved by driving of the servomotor  21  of the first pressure actuator  20  among a retracted position at which the top end  25   a  is raised furthest upward from the upper end  13   a  of the receiving unit  13 , a first pressing position at which the top end  25   a  clamps the workpiece  100  in cooperation with the upper end  13   a  of the receiving unit  13  and applies a welding pressure to the workpiece  100 , and a second pressing position at which the top end  25   a  penetrates through the receiving unit  13  to reach the position of the lower end  13   b  of the receiving unit  13 . 
         [0059]    The second pressure actuator  30  has a cylinder device or a servomotor, in this embodiment a servomotor  31 , and a direct driving unit  32  that is constituted by a ball screw mechanism or the like. A rod  33  of the direct driving unit  32  is raised or lowered by driving of the servomotor  31 . At the lower end of the rod  33  of the direct driving unit  32 , an L-shaped second electrode arm  34  is provided which has a linear base section  34 A and a curved electrode holding section  34 B formed at the lower end of the base section  34 A. A second welding electrode  35  is attached at the electrode holding section  34 B of the second electrode arm  34  such that a top end  35   a  of the second welding electrode  35  has a coaxial relationship with the central axis of the receiving unit  13  of the fixing arm  10 . Further, an electrode arm slide member  36  is provided at the fixing arm support section  7  of the gun bracket  5 . The electrode arm slide member  36  guides the movement of the second electrode arm  34  by being in slidable contact with the base section  34 A of the second electrode arm  34 . 
         [0060]    According to this configuration, the second welding electrode  35  can be moved by driving of the servomotor  31  of the second pressure actuator  30  among a retracted position at which the second welding electrode  35  is lowered furthest downward from the lower end  13   b  of the receiving unit  13 , a second pressing position at which the second welding electrode  35  clamps the workpiece  100  in cooperation with the lower end  13   b  of the receiving unit  13  and the first welding electrode  25  at the second pressing position, and applies a welding pressure to the workpiece  100 , and a first pressing position at which the second welding electrode  35  penetrates through the receiving unit  13  to reach the position of the upper end  13   a  of the receiving unit  13  and clamps the workpiece  100  in combination with the upper end  13   a  of the receiving unit  13  and in cooperation with the first welding electrode  25  at the first pressing position. 
         [0061]    An output terminal  41  of the welding transformer  40  serving as a power supply is conductively connected to the first welding electrode  25  via, for example, a busbar  43  and the first electrode arm  24 . An output terminal  42  of the welding transformer  40  is conductively connected to the second welding electrode  35  via, for example, a busbar  44  and the second electrode arm  34 . 
         [0062]    A welding robot controller RC, which is not shown, stores teaching data for the welding robot. The teaching data include operation programs for sequentially spot welding the welding sites on the workpiece  100 , as well as data indicating the position and posture of the spot welding apparatus  1  at each of the welding sites, namely, the welding zones. A welding apparatus controller WC, which is not shown, stores operation programs for the welding apparatus  1 , as well as data for controlling the operations of the first pressure actuator  20 , the second pressure actuator  30  and the welding transformer  40 . 
         [0063]    Next, operations of the spot welding apparatus  1  will be described. For convenience, a first welding step will be described with reference to  FIGS. 3 to 7  which illustrate spot welding of a workpiece  100  that is a three-ply plate in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are stacked on top of one another in this order from the bottom, and a second welding step will be described thereafter with reference to  FIGS. 8 to 12  which illustrate spot welding of a workpiece  100  that is a three-ply plate in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are stacked on top of one another in this order from the top. 
         [0064]    In the first welding step in which a workpiece  100  including a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  that are stacked on top of one another in this order from the bottom is spot welded, a previously determined operation program is conducted whereby the first welding electrode  25  is held at the furthest upward position, namely, the retracted position, and the rotation of the servomotor  31  of the second pressure actuator  30  is controlled so as to move the second welding electrode  35  to the first pressing position at which the top end  35   a  of the second welding electrode  35  is at the same level as the upper end  13   a  of the receiving unit  13 . 
         [0065]    In this state in which the first welding electrode  25  and the second welding electrode  35  are distant from each other, namely, the first welding electrode  25  is at the retracted position and the second welding electrode  35  is at the first pressing position, the welding robot controller RC operates the welding robot in accordance with the predetermined program whereby the spot welding apparatus  1  is moved to a welding site on the workpiece  100  such that the upper end  13   a  of the receiving unit  13  in the fixing arm  10  and the top end  35   a  of the second welding electrode  35  are positioned in an abutting relationship with the lower surface of the workpiece  100 , namely, the thinner plate  101 , as illustrated in  FIG. 3 . 
         [0066]    In this state in which the spot welding apparatus  1  has been positioned at the welding position, as illustrated in  FIG. 1 , the lower surface of the thinner plate  101  of the workpiece  100  is in contact with the top end  35   a  of the second welding electrode  35  and the upper end  13   a  of the receiving unit  13  that defines a ring in the vicinity of the top end  35   a  of the second welding electrode  35 . On the other hand, the tip end  25   a  of the first welding electrode  25  opposes the second thicker plate  103  with a space therebetween. 
         [0067]    Next, as illustrated in  FIGS. 5 to 7 , while the upper end  13   a  of the receiving unit  13  and the top end  35   a  of the second welding electrode  35  are in an abutting relationship with the thinner plate  101  of the workpiece  100 , the servomotor  21  of the first pressure actuator  20  is driven whereby the first welding electrode  25  is moved from the retracted position toward the first pressing position so as to be in pressed contact with the second thicker plate  103 . In this manner, the welding zone of the workpiece  100  is clamped under pressure between the top end  35   a  of the second welding electrode  35  and the top end  25   a  of the first welding electrode  25 . 
         [0068]    In the above state in which the first welding electrode  25  applies a welding pressure to the second thicker plate  103  while the top end  35   a  of the second welding electrode  35  and the upper end  13   a  of the receiving unit  13  are in an abutting relationship with the thinner plate  101  of the workpiece  100 , the welding pressure from the first pressure actuator  20  is applied downward from the first welding electrode  25  to the second thicker plate  103  via, for example, the first electrode arm  24 . Further, the welding pressure is applied upward from the second welding electrode  35  to the thinner plate  101  via, for example, the base unit  3  and the second electrode arm  34 , and is also applied upward from the receiving unit  13  that is adjacent to the second welding electrode  35  to the thinner plate  101  via the fixing arm  10 . This operation is schematically illustrated in  FIG. 7 . 
         [0069]    In the above case, the welding pressure of the first pressure actuator  20  acts on the first welding electrode  25  via, for example, the first electrode arm  24 , as well as on the second welding electrode  35  via, for example, the base unit  3  and the second electrode arm  34 , and on the receiving unit  13  via the fixing arm  10 . Here, the welding pressure FU applied to the second thicker plate  103  by the first welding electrode  25  is equal to the total of the welding pressure FL and the welding pressure Fα that are applied to the thinner plate  101  by the second welding electrode  35  and the receiving unit  13 , respectively (FU=FL+Fα). 
         [0070]    In the above manner, the workpiece  100  is clamped stably by the welding pressure FU that is applied downward to the second thicker plate  103  side by the first welding electrode  25 , and the welding pressure FL and the welding pressure Fα that are applied upward to the thinner plate  101  side by the second welding electrode  35  and the receiving unit  13 , respectively. 
         [0071]    As described above, the welding pressure FU is applied to the second thicker plate  103  by the first welding electrode  25 , and the welding pressure FL and the welding pressure Fα are applied to the thinner plate  101  by the second welding electrode  35  and the receiving unit  13 , respectively. In the welding zone of the workpiece  100 , the welding pressure FL that is applied to the thinner plate  101  by the second welding electrode  35  is of a quantity obtained by subtracting the welding pressure Fα by the receiving unit  13  from the welding pressure FU by the first welding electrode  25  (FL=FU−Fα). 
         [0072]    As described above, the welding pressure FL of the second welding electrode  35  that is positioned on the thinner plate  101  side is controlled to be smaller than the welding pressure FU of the first welding electrode  25  that is positioned on the second thicker plate  103  side (FL&lt;FU). As a result, the contact pressure at the joint between the thinner plate  101  and the first thicker plate  102  becomes smaller than the contact pressure at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, the contact resistance between the thinner plate  101  and the first thicker plate  102  is relatively increased, and the contact resistance between the first thicker plate  102  and the second thicker plate  103  is relatively decreased. 
         [0073]    Here, a case will be considered in which the receiving unit  13  of the fixing arm  10  is not provided. When the first pressure actuator  20  is operated to bring the first welding electrode  25  into pressed contact with the second thicker plate  103  while the second welding electrode  35  is in an abutting relationship with the thinner plate  101  of the workpiece  100 , the welding zone of the workpiece  100  is clamped and pressed between the second welding electrode  35  and the first welding electrode  25 . In this case, however, the welding pressure of the first pressure actuator  20  acts evenly on the first welding electrode  25  and the second welding electrode  35  via, for example, the second electrode arm  34 . That is, the welding pressure FU that is applied to the second thicker plate  103  by the first welding electrode  25  is equal to the welding pressure FL that is applied to the thinner plate  101  by the second welding electrode  35 . 
         [0074]    Referring back to the embodiment described so far, the first welding electrode  25  and the second welding electrode  35  are then energized for a predetermined time by the welding transformer  40  to perform spot welding in the state in which the workpiece  100  is clamped under pressure by the first welding electrode  25  and a combination of the second welding electrode  35  and the receiving unit  13  in a manner such that the welding pressure FL of the second welding electrode  35  that is positioned on the thinner plate  101  side is smaller than the welding pressure FU of the first welding electrode  25  that is positioned on the second thicker plate  103  side. When the first welding electrode  25  and the second welding electrode  35  are being energized, the contact resistance and the current density at the joint between the thinner plate  101  and the first thicker plate  102  are relatively increased while the contact resistance between the first thicker plate  102  and the second thicker plate  103  is maintained low. As a result, the amount of heat generated at the joint between the thinner plate  101  and the first thicker plate  102  is relatively increased to the amount of heat generated at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, a current flows with a uniform current density from the thinner plate  101  to the second thicker plate  103 , and a satisfactory nugget is formed in a region ranging from the thinner plate  101  to the second thicker plate  103 , thereby ensuring high weld strength of the thinner plate  101 . 
         [0075]    After the completion of the welding, the first pressure actuator  20  is operated so as to move the first welding electrode  25  from the first pressing position to the retracted position whereby the workpiece  100  is released from being clamped by the first welding electrode  25  and a combination of the second welding electrode  35  and the receiving unit  13 . 
         [0076]    Next, the second welding step will be described with reference to  FIGS. 8 to 12 . In this step, a workpiece  100  that is a three-ply plate in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are stacked on top of one another in this order from the top is spot welded. 
         [0077]    In performing the spot welding of the workpiece  100  including the thinner plate  101 , the first thicker plate  102  and the second thicker plate  103  that are stacked on top of one another in this order from the top, a previously determined operation program is conducted whereby the second welding electrode  35  is held at the furthest downward position, namely, the retracted position, and the rotation of the servomotor  21  of the first pressure actuator  20  is controlled so as to move the first welding electrode  25  to the second pressing position at which the top end  25   a  of the first welding electrode  25  is of the same level as the lower end  13   b  of the receiving unit  13 . 
         [0078]    In this state in which the second welding electrode  35  and the first welding electrode  25  are distant from each other, namely, the second welding electrode  35  is at the retracted position and the first welding electrode  25  is at the second pressing position, the welding robot is operated in accordance with the predetermined program whereby the spot welding apparatus  1  is moved to a welding site on the workpiece  100  such that the lower end  13   b  of the receiving unit  13  in the fixing arm  10  and the top end  25   a  of the first welding electrode  25  are positioned in an abutting relationship with the upper surface of the workpiece  100 , namely, the thinner plate  101 , as illustrated in  FIG. 8 . 
         [0079]    In this state in which the spot welding apparatus  1  has been positioned in the welding position, as illustrated in  FIG. 9 , the upper surface of the thinner plate  101  of the workpiece  100  is in contact with the top end  25   a  of the first welding electrode  25  and the lower end  13   b  of the receiving unit  13  that defines a ring in the vicinity of the top end  25   a  of the first welding electrode  25 . On the other hand, the tip end  35   a  of the second welding electrode  35  opposes the second thicker plate  103  with a space therebetween. 
         [0080]    Next, as illustrated in  FIGS. 10 and 11 , while the lower end  13   b  of the receiving unit  13  and the top end  25   a  of the first welding electrode  25  are in an abutting relationship with the thinner plate  101  of the workpiece  100 , the servomotor  31  of the second pressure actuator  30  is driven whereby the second welding electrode  35  is moved from the retracted position toward the second pressing position so as to be in pressed contact with the second thicker plate  103 . In this manner, the welding zone of the workpiece  100  is clamped under pressure between the top end  25   a  of the first welding electrode  25  and the top end  35   a  of the second welding electrode  35   
         [0081]    In the above state in which the second welding electrode  35  applies a welding pressure to the second thicker plate  103  while the top end  25   a  of the first welding electrode  25  and the lower end  13   b  of the receiving unit  13  are in an abutting relationship with the thinner plate  101  of the workpiece  100 , the welding pressure from the second pressure actuator  30  is applied upward from the second welding electrode  35  to the second thicker plate  103  via, for example, the second electrode arm  34 . Further, the welding pressure is applied downward from the first welding electrode  25  to the thinner plate  101  via, for example, the base unit  3  and the first electrode arm  24 , and is also applied downward from the receiving unit  13  that is adjacent to the first welding electrode  25  to the thinner plate  101  via the fixing arm  10 . This operation is schematically illustrated in  FIG. 12 . 
         [0082]    In the above case, the welding pressure of the second pressure actuator  30  acts on the second welding electrode  35  via, for example, the second electrode arm  34 , as well as on the first welding electrode  25  via, for example, the base unit  3  and the first electrode arm  24 , and on the receiving unit  13  via the fixing arm  10 . Here, the welding pressure FL applied to the second thicker plate  103  by the second welding electrode  35  is equal to the total of the welding pressure FU and the welding pressure Fα that are applied to the thinner plate  101  by the first welding electrode  25  and the receiving unit  13 , respectively (FL=FU+Fα). 
         [0083]    In the above manner, the workpiece  100  is clamped stably by the welding pressure FL that is applied upward to the second thicker plate  103  side by the second welding electrode  35 , and the welding pressure FU and the welding pressure Fα that are applied downward to the thinner plate  101  side by the first welding electrode  25  and the receiving unit  13 , respectively. 
         [0084]    In the welding zone of the workpiece  100 , the welding pressure FL is applied to the second thicker plate  103  by the second welding electrode  35 , and the welding pressure FU is applied to the thinner plate  101  by the first welding electrode  25  that is of a quantity obtained by subtracting the welding pressure Fα by the receiving unit  13  from the welding pressure FL by the second welding electrode  35  (FU=FL−Fα). 
         [0085]    As described above, the welding pressure FU of the first welding electrode  25  that is positioned on the thinner plate  101  side is controlled to be smaller than the welding pressure FL of the second welding electrode  35  that is positioned on the second thicker plate  103  side (FU&lt;FL). As a result, the contact pressure at the joint between the thinner plate  101  and the first thicker plate  102  becomes smaller than the contact pressure at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, the contact resistance between the thinner plate  101  and the first thicker plate  102  is relatively increased, and the contact resistance between the first thicker plate  102  and the second thicker plate  103  is relatively decreased. 
         [0086]    Next, the first welding electrode  25  and the second welding electrode  35  are energized for a predetermined time by the welding transformer  40  to perform spot welding in the state in which the workpiece  100  is clamped under pressure by the second welding electrode  35  and a combination of the first welding electrode  25  and the receiving unit  13  in a manner such that the welding pressure FU of the first welding electrode  25  that is positioned on the thinner plate  101  side is smaller than the welding pressure FL of the second welding electrode  35  that is positioned on the second thicker plate  103  side. When the first welding electrode  25  and the second welding electrode  35  are being energized, the contact resistance and the current density at the joint between the thinner plate  101  and the first thicker plate  102  are relatively increased while the contact resistance between the first thicker plate  102  and the second thicker plate  103  is maintained low. As a result, the amount of heat generated at the joint between the thinner plate  101  and the first thicker plate  102  is relatively increased to the amount of heat generated at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, a current flows with a uniform current density from the thinner plate  101  to the second thicker plate  103 , and a satisfactory nugget is formed in a region ranging from the thinner plate  101  to the second thicker plate  103 , thereby ensuring high weld strength of the thinner plate  101 . 
         [0087]    After the completion of the welding, the second pressure actuator  30  is operated so as to move the second welding electrode  35  from the second pressing position to the retracted position whereby the workpiece  100  is released from being clamped by the second welding electrode  35  and a combination of the first welding electrode  25  and the receiving unit  13 . 
         [0088]    Next, the welding robot is operated so as to retract the spot welding apparatus  1  from the welding site on the workpiece  100  and to move the spot welding apparatus  1  to a next welding site on the workpiece  100 . 
         [0089]    According to the embodiment described above, the spot welding apparatus  1  is configured to be capable of spot welding the workpiece  100  that is a three-ply plate in which the thinner plate  101  of low rigidity and the first thicker plate  102  and the second thicker plate  103  having a higher rigidity than the thinner plate  101  are stacked on top of one another. The spot welding apparatus  1  includes the receiving unit  13  that is attached to the base unit  3  via the fixing arm  10 ; the first welding electrode  25  that can be moved among the retracted position, the first pressing position and the second pressing position by the first pressure actuator  20 ; and the second welding electrode  35  that can be moved among the retracted position, the second pressing position and the first pressing position by the second pressure actuator  30 . The spot welding apparatus  1  is configured such that the workpiece  100  is clamped between a combination of the second welding electrode  35  at the first pressing position and the receiving unit  13 , and the first welding electrode  25  at the first pressing position that opposes the second welding electrode  35 , with the result that the welding pressure FL and the welding pressure Fα are applied to the thinner plate  101  by the second welding electrode  35  and the receiving unit  13 , respectively, and the welding pressure FU is applied to the second thicker plate  103  by the first welding electrode  25 . In this manner, the contact pressure at the joint between the thinner plate  101  and the first thicker plate  102  is controlled to be smaller than the contact pressure at the joint between the first thicker plate  102  and the second thicker plate  103 . As a result, when the first welding electrode  25  and the second welding electrode  35  are energized, the current density at the joint between the thinner plate  101  and the first thicker plate  102  is relatively increased to the current density at the joint between the first thicker plate  102  and the second thicker plate  103 . Thus, a satisfactory nugget is formed in an extensive region ranging from the thinner plate  101  to the second thicker plate  103  with uniform penetration, thereby ensuring high weld strength of the thinner plate  101 . The spot welding apparatus  1  is also configured to perform spot welding in a manner such that the workpiece  100  is clamped between a combination of the first welding electrode  25  at the second pressing position and the receiving unit  13 , and the second welding electrode  35  at the second pressing position, with the result that the welding pressure FU and the welding pressure Fα are applied to the thinner plate  101  by the first welding electrode  25  and the receiving unit  13 , respectively, and the welding pressure FL is applied to the second thicker plate  103  by the second welding electrode  35 . In this manner, the contact pressure at the joint between the thinner plate  101  and the first thicker plate  102  is controlled to be smaller than the contact pressure at the joint between the first thicker plate  102  and the second thicker plate  103 . As a result, when the first welding electrode  25  and the second welding electrode  35  are energized, a satisfactory nugget is formed in an extensive region ranging from the thinner plate  101  to the second thicker plate  103  with uniform penetration, thereby ensuring high weld strength of the thinner plate  101 . Thus, stable weld quality can be achieved. According to this configuration, the spot welding apparatus  1  can continuously spot weld workpieces  100  having the thinner plate  101 , the first thicker plate  102  and the second thicker plate  103  in different arrangements without the need of greatly changing the posture of the welding apparatus  1 . For example, a three-ply workpiece  100  in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are sequentially stacked from the bottom, and a three-ply workpiece  100  in which a thinner plate  101 , a first thicker plate  102  and a second thicker plate  103  are sequentially stacked from the top can be spot welded continuously without greatly changing the posture of the welding apparatus  1 . 
         [0090]    The scope of the present invention is not limited to the embodiment described above, and various modifications can occur insofar as they are within the spirit of the invention. For example, the servomotors  21  and  31  used for the first pressure actuator  20  and the second pressure actuator  30  may be replaced by air cylinders. The direct driving units  22  and  32  may be constituted by a gear mechanism or the like instead of the ball screw mechanism adopted in the above embodiment. 
         [0091]    Further, although the receiving unit  13  has been described as having a pipe shape with the upper end  13   a  and the lower end  13   b , the shape may be changed to various shapes such as a semi-broken pipe shape or a semi-circular convex shape curved outward from the support section  12  of the fixing arm  10  in accordance with the shape or other conditions of the workpiece  100 . 
         [0092]    The workpiece  100  is not limited to a three-ply plate and may contain four or more plates as long as thicker plates and thinner plate(s) having different rigidities are stacked on top of one another.