Spot-welding method and spot-welding device

When spot-welding a workpiece including a thin plate, a first thick plate, and a second thick plate, the workpiece is clamped by a fixed electrode in contact with the second thick plate, a movable electrode in contact with the thin plate, and control-pressure applying unit set adjacent to the movable electrode and in contact with the thin plate. Pressure is applied to the second thick plate by the fixed electrode, and pressure and control pressure are respectively applied by the movable electrode and the control-pressure applying unit to the thin plate. The pressure from the fixed electrode is controlled to be smaller than the pressure from the movable electrode. The current density between the thin plate and the first thick plate increases.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2010-200643 filed on Sep. 8, 2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to spot-welding methods and spot-welding devices for spot-welding a workpiece having a stacked-plate structure constituted of stacked plate materials having different thicknesses.

2. Description of the Related Art

Generally, when joining together stacked plate materials, such as steel plates, a spot-welding technique is widely used. Specifically, this spot-welding technique involves using a pair of welding electrodes to clamp and apply pressure to the stacked plates, applying high current between the welding electrodes for a predetermined time, and then increasing the temperature at a joint section to substantially a melting temperature so as to form a nugget.

In spot-welding, the diameter of the nugget gradually increases with increasing current when the pressure applied by the two welding electrodes and the electricity application time are fixed. However, the calorific value becomes excessive when the current value is too high, causing expulsion and surface flash, in which molten metal spatters, to occur between the plate materials. Specifically, such expulsion and surface flash is an explosion phenomenon of the molten metal caused by overheating of the joint section and causes holes and cracks to form in the nugget. This results in discontinuity in the shape of the nugget and the metallographic structure, causing a reduced thickness of the joint section as well as a significant reduction in the strength thereof. In contrast, if the current is too low, sufficient joint strength cannot be obtained since the nugget becomes small in size. If the applied pressure is low, the contact area between the plate materials correspondingly decreases, leading to the occurrence of expulsion and surface flash caused by overheating due to an increased current density. On the other hand, an excessively applied pressure leads to an increase in the contact area at the joint section, resulting in a reduced current density and a reduced calorific value. As a result, the nugget is reduced in size and the weld strength is lowered.

Referring toFIG. 18A, when spot-welding a workpiece100constituted of three stacked plates, which are a thin plate101having low rigidity, and a first thick plate102and a second thick plate103that are thicker and more rigid than the thin plate101, a movable electrode121and a fixed electrode122are used to clamp the workpiece100therebetween in a state where the thin plate101and the first thick plate102as well as the first thick plate102and the second thick plate103are tightly attached to each other with no gap therebetween. Then, when a power source123applies electricity to the workpiece100via the movable electrode121and the fixed electrode122, clamping the workpiece100therebetween, the current density in an electric path between the movable electrode121and the fixed electrode122becomes substantially uniform so that a good nugget extending from the thin plate101to the second thick plate103is formed, thereby achieving required weld strength.

In actuality, however, when the workpiece100is clamped and pressed between the movable electrode121and the fixed electrode122, the thin plate101having low rigidity and the first thick plate102bend upward, causing gaps to form between the thin plate101and the first thick plate102as well as between the first thick plate102and the second thick plate103. In this case, the contact area between the movable electrode121and the thin plate101is increased due to the bending of the thin plate101, whereas the contact area of the joint section between the thin plate101and the first thick plate102as well as the contact area of the joint section between the first thick plate102and the second thick plate103are decreased due to the gaps.

Therefore, the current density at a joint section of the fixed electrode122at the side of the second thick plate103becomes higher than that at a joint section of the movable electrode121at the side of the thin plate101. This results in a larger local calorific value between the first thick plate102and the second thick plate103than between the thin plate101and the first thick plate102.

As a result, a nugget105is first formed at the joint section between the first thick plate102and the second thick plate103, as shown inFIG. 18A. Then, the nugget105gradually increases in size so that the thin plate101and the first thick plate102are ultimately welded to each other, as shown inFIG. 18B. However, because the amount of weld penetration between the thin plate101and the first thick plate102is small, the weld strength is unstable. Thus, the thin plate101may become delaminated, and the weld quality may vary from place to place. This problem is prominent especially with increasing thickness of the first thick plate102and the second thick plate103since this makes it difficult for the nugget105to reach the joint section between the first thick plate102and the thin plate101.

Another factor that makes the weld strength unstable due to a small amount of weld penetration between the thin plate101and the first thick plate102is a small thickness of the thin plate101. Specifically, such a thin plate101with a small thickness surrenders its heat to the movable electrode121by being in contact therewith and therefore does not increase in temperature, making it difficult to form the nugget105.

Japanese Unexamined Patent Application Publication No. 2003-251468 discloses an example of a spot-welding method as a countermeasure against this problem. Specifically, as shown inFIG. 19, when spot-welding the workpiece100constituted of stacked plates, i.e., the thin plate101, the first thick plate102, and the second thick plate103, the tip diameter of the movable electrode121that comes into contact with the thin plate101is made smaller than the tip diameter of the fixed electrode122that comes into contact with the second thick plate103so that the contact area between the thin plate101and the movable electrode121is smaller than the contact area between the second thick plate103and the fixed electrode122. Thus, the current density in the electric path between the movable electrode121and the fixed electrode122gradually decreases from the movable electrode121towards the fixed electrode122. As a result, the calorific value between the thin plate101and the first thick plate102becomes larger so that a good nugget is formed, whereby the weld strength between the thin plate101and the first thick plate102is increased.

Japanese Unexamined Patent Application Publication No. 2003-251469 discloses another spot-welding method. Specifically, as shown inFIG. 20, when spot-welding the workpiece100constituted of three stacked plates, i.e., the thin plate101, the first thick plate102, and the second thick plate103, a pressure FU from a movable electrode135located at the thin plate101side is set to be smaller than a pressure FL from a fixed electrode134located at the second thick plate103side so that the contact resistance between the thin plate101and the first thick plate102increases, whereas the contact resistance between the first thick plate102and the second thick plate103decreases. Thus, when electricity is applied between the movable electrode135and the fixed electrode134, the calorific value at the joint section between the thin plate101and the first thick plate102is increased, thereby increasing the weld strength between the thin plate101and the first thick plate102.

FIG. 21illustrates the configuration of a spot-welding device used for implementing this method. Specifically, a spot-welding device130is attached to a wrist portion126of a welding robot125. The welding robot125spot-welds the workpiece100by moving the spot-welding device130to each spot-welding position of the workpiece100supported by a clamper128.

The spot-welding device130includes a base132that is supported in a vertically movable manner by a linear guide131fixed to a support bracket127attached to the wrist portion126. The base132is provided with a C-shaped yoke133that extends downward therefrom. A lower end of the C-shaped yoke133is provided with the fixed electrode134.

A pressure actuator136, such as a servomotor, is attached to an upper-end of the base132. The movable electrode135is attached to a lower end of a rod137that is moved in the vertical direction by the pressure actuator136, such that the movable electrode135faces the fixed electrode134. A servomotor138is attached to an upper end of the support bracket127. By actuating the servomotor138, the base132is moved in the vertical direction via a ball-screw mechanism.

Based on teaching data preliminarily stored in a controller (not shown), the pressure FU from the movable electrode135located at the thin plate101side is set to be smaller than the pressure FL from the fixed electrode134(FU<FL).

In order to set the pressure FU from the movable electrode135to be smaller than the pressure FL from the fixed electrode134(FU<FL) in this manner, the controller first uses the servomotor138to move the base132upward so as to bring the fixed electrode134into contact with the lower surface of the workpiece100, and also uses the pressure actuator136to move the movable electrode135downward so as to bring the movable electrode135into contact with the upper surface of the workpiece100. In this case, the pressure of the pressure actuator136is uniformly applied to the movable electrode135and the fixed electrode134via the base132and the C-shaped yoke133.

Subsequently, the base132is lifted upward by the servomotor138. This upward lifting of the base132causes the pressure FL from the fixed electrode134to increase by an amount equivalent to how much the base132is lifted upward, whereby the pressure FU from the movable electrode135becomes smaller than the pressure FL from the fixed electrode134(FU<FL).

As a result, when electricity is applied between the movable electrode135and the fixed electrode134, the current density at the joint section between the thin plate101and the first thick plate102increases, causing the calorific value to become relatively higher than the calorific value at the joint section between the first thick plate102and the second thick plate103. Consequently, a good nugget extending from the thin plate101to the second thick plate103without an uneven amount of weld penetration is formed, thereby ensuring the weld strength.

In Japanese Unexamined Patent Application Publication No. 2003-251468 described above, the tip diameter of the movable electrode121that comes into contact with the thin plate101is made smaller than the tip diameter of the fixed electrode122that comes into contact with the second thick plate103so that the current density in the electric path between the movable electrode121and the fixed electrode122gradually decreases from the movable electrode121towards the fixed electrode122, thereby increasing the weld strength between the thin plate101and the first thick plate102.

However, the current density in the electric path between the movable electrode121and the fixed electrode122varies depending on the pressures from the movable electrode121and the fixed electrode122, the thicknesses of the thin plate101, the first thick plate102, and the second thick plate103, and the shape or the area of the workpiece100. This makes it difficult to ensure uniform weld quality. Furthermore, using various movable electrodes121and fixed electrodes122having different tip diameters in an interchangeable manner in accordance with the thicknesses of the thin plate101, the first thick plate102, and the second thick plate103and the shape or the area of the workpiece100is not practical since it is extremely troublesome and can possibly lead to a significant decrease in productivity. In addition, the preparation and management of such various movable electrodes121and fixed electrodes122having different tip diameters require a large amount of management cost.

In Japanese Unexamined Patent Application Publication No. 2003-251469, on the other hand, the spot-welding device130is moved to each spot-welding position of the workpiece100supported by the clamper128so as to bring the fixed electrode134into contact with the second thick plate103of the workpiece100and to bring the movable electrode135into contact with the thin plate101. Moreover, the pressure FU from the movable electrode135is set to be smaller than the pressure FL from the fixed electrode134by lifting the base132upward so that the current density between the thin plate101and the first thick plate102becomes relatively higher. Thus, a sufficient calorific value can be ensured at the joint section between the thin plate101and the first thick plate102, thereby achieving an increased amount of weld penetration and increased weld strength.

However, in order to set the pressure FU from the movable electrode135to be smaller than the pressure FL from the fixed electrode134by moving the base132in a state where the workpiece100clamped by the clamper128is held and pressed between the fixed electrode134and the movable electrode135, a large load is required on the clamper128that clamps the workpiece100. On the other hand, when the clamped position of the workpiece100by the clamper128and the welding position, i.e., the spot-welding position, of the workpiece100are distant from each other, the workpiece100becomes bent. This causes the pressure FL from the fixed electrode134and the pressure FU from the movable electrode135to become unbalanced, making it difficult to ensure stable contact resistance between the thin plate101and the first thick plate102and stable contact resistance between the first thick plate102and the second thick plate103. This can possibly result in variations in the current density at the joint sections of the workpiece100, leading to reduced spot-welding quality. Furthermore, this method is limited to specific spot-welding devices since the method cannot be used in a spot-welding device that has an equalizing function between the base and the wrist portion of the robot for allowing for movement in response to a reaction force generated during the welding and pressing process.

SUMMARY OF THE INVENTION

Accordingly, in view of the circumstances described above, a first object of the present invention is to provide a spot-welding method and a spot-welding device that can achieve uniform and stable weld quality when a workpiece having a three-stacked-plate structure formed by stacking a thin plate over one of two stacked thick plates is spot-welded.

A second object of the present invention is to provide a spot-welding method and a spot-welding device that can achieve uniform and stable weld quality when a workpiece having a four-stacked-plate structure formed by stacking two thin plates respectively over opposite faces of two stacked thick plates is spot-welded.

A spot-welding method according to a first aspect of the present invention for achieving the first object is for spot-welding a workpiece including a thin plate, a first thick plate, and a second thick plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the thin plate. The spot-welding method includes clamping and pressing the workpiece by using a first welding electrode that is brought into contact with the second thick plate, a second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and control-pressure applying unit that is set adjacent to the second welding electrode and is brought into contact with the thin plate; and spot-welding the workpiece by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed.

Accordingly, when spot-welding the workpiece having a three-stacked-plate structure including the thin plate, the first thick plate, and the second thick plate, the workpiece is clamped and pressed by using the first welding electrode that is brought into contact with the second thick plate, the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and the control-pressure applying unit that is set adjacent to the second welding electrode and is brought into contact with the thin plate. Thus, the pressure from the first welding electrode is applied to the second thick plate of the workpiece, whereas the pressure from the second welding electrode and the control pressure from the control-pressure applying unit set adjacent to the second welding electrode are applied to the thin plate, whereby the pressure from the second welding electrode located at the thin plate side is set to be smaller than the pressure from the first welding electrode located at the second thick plate side. Consequently, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the joint section between the thin plate and the first thick plate relatively increases. As a result, a good nugget extending from the thin plate to the second thick plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

In the spot-welding method according to the first aspect of the present invention, FL=FU+Fα may be satisfied in the state where the workpiece is clamped and pressed, where FL denotes a pressure applied to the second thick plate by the first welding electrode, FU denotes a pressure applied to the thin plate by the second welding electrode, and Fα denotes a control pressure applied to the thin plate by the control-pressure applying unit.

Accordingly, in the state where the workpiece is clamped and pressed, the pressure FL applied to the second thick plate by the first welding electrode is equal to the sum of the pressure FU applied by the second welding electrode and the control pressure Fα applied by the control-pressure applying unit to the thin plate (FL=FU+Fα). Thus, the workpiece is stably clamped and held by the first welding electrode, the second welding electrode, and the control-pressure applying unit, and the pressure FU applied to the thin plate by the second welding electrode is equal to a difference between the pressure FL applied to the second thick plate from the first welding electrode and the control pressure Fα from the control-pressure applying unit. Consequently, the pressure applied to the thin plate by the second welding electrode can be set to be smaller than the pressure applied to the second thick plate by the first welding electrode.

A spot-welding method according to a second aspect of the present invention for achieving the second object is for spot-welding a workpiece including a first thin plate, a first thick plate, a second thick plate, and a second thin plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the first thin plate and the second thin plate. The spot-welding method includes a first welding step for clamping and pressing the workpiece by using a first welding electrode that is brought into contact with the second thin plate, a first control presser that is set adjacent to the first welding electrode and is brought into contact with the second thin plate, and a second welding electrode that faces the first welding electrode and is brought into contact with the first thin plate, and spot-welding the workpiece by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed; and a second welding step for moving the first control presser away from the second thin plate while maintaining the workpiece in a clamped state between the first welding electrode and the second welding electrode, setting a second control presser adjacent to the second welding electrode and bringing the second control presser into contact with the first thin plate, and spot-welding the workpiece by applying electricity between the second welding electrode and the first welding electrode in a state where the workpiece is clamped and pressed by the first welding electrode, the second welding electrode, and the second control presser.

Accordingly, in the first welding step, the workpiece is clamped and pressed by the first welding electrode and the first control presser that are located at the second thin plate side, and by the second welding electrode that is located at the first thin plate side, so that the pressure applied to the second thin plate by the first welding electrode is set to be smaller than the pressure applied to the first thin plate by the second welding electrode. Thus, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the joint section between the second thin plate and the second thick plate becomes relatively higher than the current density at the joint section between the first thick plate and the first thin plate, whereby the amount of weld penetration between the second thin plate and the second thick plate increases. In the second welding step, on the other hand, the workpiece is clamped and pressed by the second welding electrode and the second control presser that are located at the first thin plate side, and by the first welding electrode that is located at the second thin plate side, so that the pressure applied to the first thin plate by the second welding electrode is set to be smaller than the pressure applied to the second thin plate by the first welding electrode. Thus, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the joint section between the first thin plate and the first thick plate becomes relatively higher than the current density at the joint section between the second thick plate and the second thin plate, whereby the amount of weld penetration between the first thin plate and the first thick plate increases. Consequently, a good nugget extending from the first thin plate to the second thin plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

In the spot-welding method according to the second aspect of the present invention, FU=FL+Fα may be satisfied in the state where the workpiece is clamped and pressed in the first welding step, where FL denotes a pressure applied to the second thin plate by the first welding electrode, Fα denotes a control pressure applied to the second thin plate by the first control presser, and FU denotes a pressure applied to the first thin plate by the second welding electrode. Moreover, FL=FU+Fβ may be satisfied in the state where the workpiece is clamped and pressed in the second welding step, where FL denotes the pressure applied to the second thin plate by the first welding electrode, FU denotes the pressure applied to the first thin plate by the second welding electrode, and Fβ denotes a second control pressure applied to the first thin plate by the second control presser.

Accordingly, in the state where the workpiece is clamped and pressed in the first welding step, the sum of the pressure FL applied by the first welding electrode and the control pressure Fα applied by the first control presser to the second thin plate is equal to the pressure FU applied to the first thin plate by the second welding electrode (FU=FL+Fα). Thus, the workpiece is stably clamped by the first welding electrode, the first control presser, and the second welding electrode, and the pressure applied to the second thin plate by the first welding electrode can be set to be smaller than the pressure applied to the first thin plate by the second welding electrode. Furthermore, in the state where the workpiece is clamped and pressed in the second welding step, the sum of the pressure FU applied by the second welding electrode and the second control pressure Fβ applied by the second control presser to the first thin plate is equal to the pressure FL applied to the second thin plate by the first welding electrode (FL=FU+Fβ). Thus, the workpiece is stably clamped by the first welding electrode, the second welding electrode, and the second control presser, and the pressure applied to the first thin plate by the second welding electrode can be set to be smaller than the pressure applied to the second thin plate by the first welding electrode.

A spot-welding device according to a third aspect of the present invention for achieving the first object is configured to spot-weld a workpiece including a thin plate, a first thick plate, and a second thick plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the thin plate. The spot-welding device includes a first welding electrode that faces the second thick plate; a second welding electrode that is coaxially aligned with the first welding electrode and that faces the thin plate of the workpiece so as to clamp and press the workpiece together with the first welding electrode; and control-pressure applying unit that is set adjacent to the second welding electrode and applies a control pressure to the thin plate. The workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thick plate, the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and the control-pressure applying unit, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed.

Accordingly, the workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thick plate, the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and the control-pressure applying unit that is set adjacent to the second welding electrode and is brought into contact with the thin plate. Thus, the pressure from the first welding electrode is applied to the second thick plate of the workpiece, whereas the pressure from the second welding electrode and the control pressure from the control-pressure applying unit are applied to the thin plate, whereby the pressure from the second welding electrode located at the thin plate side is set to be smaller than the pressure from the first welding electrode located at the second thick plate side. Consequently, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the joint section between the thin plate and the first thick plate increases. As a result, a good nugget extending from the thin plate to the second thick plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

A spot-welding device according to the third aspect of the present invention for achieving the first object is configured to spot-weld a workpiece including a thin plate, a first thick plate, and a second thick plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the thin plate. The spot-welding device includes a base; a first welding electrode that is supported by the base and is contactable with the second thick plate; a pressure actuator that is attached to the base and supports a second welding electrode via a rod, the pressure actuator being capable of moving the second welding electrode between a retreated position located away from the workpiece and a pressing position where the second welding electrode is brought into contact with the thin plate so as to clamp the workpiece together with the first welding electrode and to apply pressure onto the workpiece; and control-pressure applying unit that is set adjacent to the second welding electrode on the workpiece and applies a control pressure to the thin plate. The workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thick plate, the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and the control-pressure applying unit, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed.

Accordingly, the workpiece is clamped and pressed by using the first welding electrode that is brought into contact with the second thick plate, the control-pressure applying unit that is set adjacent to the second welding electrode and is brought into contact with the thin plate, and the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate. Thus, the pressure from the pressure actuator is applied to the thin plate from the second welding electrode and is also applied to the second thick plate from the first welding electrode via the base. Moreover, the control pressure from the control-pressure applying unit is applied to the thin plate. Therefore, the pressure from the second welding electrode located at the thin plate side is set to be smaller than the pressure from the first welding electrode located at the second thick plate side.

Consequently, when electricity is applied between the second welding electrode and the first welding electrode, the current density between the thin plate and the first thick plate relatively increases. As a result, a good nugget extending from the thin plate to the second thick plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

The control-pressure applying unit may include a control presser and a control-pressure actuator that is attached to the base and supports the control presser. The control-pressure actuator may be capable of moving the control presser between a retreated position located away from the workpiece and a pressing position where the control presser is set adjacent to the second welding electrode and brought into contact with the thin plate so as to apply the control pressure thereto. Furthermore, the control-pressure actuator may be an air cylinder attached to the base. Furthermore, the control-pressure applying unit may be a coil spring having a base end and a tip end. In this case, the coil spring may be attached to the rod by fitting the base end around the rod, and the tip end may protrude from a tip end of the second welding electrode when the coil spring is in an unloaded state.

A spot-welding device according to a fourth aspect of the present invention for achieving the second object is configured to spot-weld a workpiece including a first thin plate, a first thick plate, a second thick plate, and a second thin plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the first thin plate and the second thin plate. The spot-welding device includes a first welding electrode that faces the second thick plate; a second welding electrode that is coaxially aligned with the first welding electrode and that faces the first thin plate of the workpiece so as to clamp and press the workpiece together with the first welding electrode; and a first control presser that is set adjacent to the first welding electrode and applies a first control pressure to the second thin plate, and a second control presser that is set adjacent to the second welding electrode and applies a second control pressure to the first thin plate. The workpiece is clamped and pressed by the first welding electrode and the first control presser that are brought into contact with the second thin plate, and by the second welding electrode that faces the first welding electrode and is brought into contact with the first thin plate, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed. The workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thin plate, and by the second welding electrode and the second control presser that face the first welding electrode and are brought into contact with the first thin plate, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed.

Accordingly, in the first welding step, the workpiece is clamped and pressed by the first welding electrode and the first control presser that are located at the second thin plate side, and by the second welding electrode that is located at the first thin plate side, so that the pressure applied to the second thin plate by the first welding electrode is set to be smaller than the pressure applied to the first thin plate by the second welding electrode. Thus, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the contact section between the second thin plate and the second thick plate becomes relatively higher than the current density at the contact section between the first thick plate and the first thin plate, thereby allowing for an increased amount of weld penetration and ensuring the weld strength between the second thin plate and the second thick plate. In the second welding step, the workpiece is clamped and pressed by the second welding electrode and the second control presser that are located at the first thin plate side, and by the first welding electrode that is located at the second thin plate side, so that the pressure applied to the first thin plate by the second welding electrode is set to be smaller than the pressure applied to the second thin plate by the first welding electrode. Thus, when electricity is applied between the second welding electrode and the first welding electrode, the current density at the contact section between the first thin plate and the first thick plate becomes relatively higher than the current density at the contact section between the second thick plate and the second thin plate, thereby allowing for an increased amount of weld penetration and ensuring the weld strength between the first thin plate and the first thick plate. Consequently, a good nugget extending from the first thin plate to the second thin plate without unevenness is formed, whereby the weld quality for the workpiece is enhanced.

A spot-welding device according to the fourth aspect of the present invention for achieving the second object is configured to spot-weld a workpiece including a first thin plate, a first thick plate, a second thick plate, and a second thin plate that are sequentially stacked, the first thick plate and the second thick plate being thicker than the first thin plate and the second thin plate. The spot-welding device includes a base; a first welding electrode that is supported by the base and is contactable with the second thick plate; a pressure actuator that is attached to the base and supports a second welding electrode via a rod, the pressure actuator being capable of moving the second welding electrode between a retreated position located away from the workpiece and a pressing position where the second welding electrode is brought into contact with the first thin plate so as to clamp the workpiece together with the first welding electrode and to apply pressure onto the workpiece; and control-pressure applying unit including a first control presser that is set adjacent to the first welding electrode on the workpiece and applies a first control pressure to the second thin plate, and a second control presser that is set adjacent to the second welding electrode on the workpiece and applies a second control pressure to the first thin plate. The workpiece is clamped and pressed by the first welding electrode and the first control presser that are brought into contact with the second thin plate, and by the second welding electrode that faces the first welding electrode and is brought into contact with the first thin plate, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed. The workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thin plate, and by the second welding electrode and the second control presser that face the first welding electrode and are brought into contact with the first thin plate, and the workpiece is spot-welded by applying electricity between the second welding electrode and the first welding electrode in the state where the workpiece is clamped and pressed.

Accordingly, the workpiece is clamped and pressed by the first welding electrode that is brought into contact with the second thin plate, the first control presser that is set adjacent to the first welding electrode and is brought into contact with the second thin plate, and the second welding electrode that faces the first welding electrode and is brought into contact with the first thin plate. Thus, the pressure from the pressure actuator is applied to the first thin plate from the second welding electrode and is also applied to the second thin plate from the first welding electrode via the base. Moreover, the first control pressure from the first control presser is applied to the second thin plate. Therefore, the pressure from the first welding electrode located at the second thin plate side is set to be smaller than the pressure from the second welding electrode located at the first thin plate side.

Consequently, when electricity is applied between the second welding electrode and the first welding electrode, the current density between the second thin plate and the second thick plate increases, whereby a good nugget extending from the second thin plate to the second thick plate without an uneven amount of weld penetration is formed. Furthermore, the workpiece is clamped and pressed by the second welding electrode that is brought into contact with the first thin plate, the second control presser that is set adjacent to the second welding electrode and is brought into contact with the first thin plate, and the first welding electrode that faces the second welding electrode and is brought into contact with the second thin plate. Thus, due to the second control pressure applied to the first thin plate by the second control presser, the pressure applied to the first thin plate by the second welding electrode is set to be smaller than the pressure applied to the second thin plate by the first welding electrode. Therefore, when electricity is applied between the second welding electrode and the first welding electrode, the current density between the first thin plate and the first thick plate increases. As a result, a good nugget extending from the first thin plate to the first thick plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

The control-pressure applying unit may include the first control presser, the second control presser, and a control-pressure actuator that is attached to the base and supports the first control presser and the second control presser. The control-pressure actuator may be capable of moving the first control presser and the second control presser selectively between a first pressing position and a second pressing position. The first pressing position is where the second control presser is located away from the first thin plate and where the first control presser is set adjacent to the first welding electrode and is brought into contact with the second thin plate so as to apply the first control pressure to the second thin plate. The second pressing position is where the first control presser is located away from the second thin plate and where the second control presser is set adjacent to the second welding electrode and is brought into contact with the first thin plate so as to apply the second control pressure to the first thin plate.

According to the present invention, when spot-welding the workpiece having a three-stacked-plate structure including the thin plate, the first thick plate, and the second thick plate, the workpiece is clamped and pressed by using the first welding electrode that is brought into contact with the second thick plate, the second welding electrode that faces the first welding electrode and is brought into contact with the thin plate, and the control-pressure applying unit that is set adjacent to the second welding electrode and is brought into contact with the thin plate. Thus, the pressure from the first welding electrode is applied to the second thick plate of the workpiece, whereas the pressure from the second welding electrode and the control pressure from the control-pressure applying unit set adjacent to the second welding electrode are applied to the thin plate, whereby the pressure from the second welding electrode located at the thin plate side is set to be smaller than the pressure from the first welding electrode located at the second thick plate side. Consequently, when electricity is applied between the second welding electrode and the first welding electrode, the current density between the thin plate and the first thick plate relatively increases. As a result, a good nugget extending from the thin plate to the second thick plate without an uneven amount of weld penetration is formed, whereby the weld quality for the workpiece is enhanced.

Furthermore, according to the present invention, when spot-welding the workpiece having a four-stacked-plate structure including the first thin plate, the first thick plate, the second thick plate, and the second thin plate, the first thick plate and the second thick plate being thicker than the first thin plate and the second thin plate, the workpiece is clamped and pressed by the first welding electrode and the first control presser that are located at the second thin plate side, and by the second welding electrode located at the first thin plate side. Therefore, in the clamped section between the first welding electrode and the second welding electrode, the contact pressure between the second thin plate and the second thick plate is smaller than the contact pressure between the first thick plate and the first thin plate. When electricity is applied between the second welding electrode and the first welding electrode, the current density at the joint section between the second thin plate and the second thick plate becomes relatively higher than the current density at the joint section between the first thick plate and the first thin plate, thereby ensuring the weld strength of the second thin plate and the second thick plate. Moreover, the workpiece is clamped and pressed by the second welding electrode and the second control presser that are located at the first thin plate side, and by the first welding electrode located at the second thin plate side. Therefore, in the clamped section between the second welding electrode and the first welding electrode, the contact pressure between the first thin plate and the first thick plate is smaller than the contact pressure between the second thick plate and the second thin plate. When electricity is applied between the second welding electrode and the first welding electrode, the current density at the contact section between the first thin plate and the first thick plate becomes relatively higher than the current density at the contact section between the second thick plate and the second thin plate, thereby ensuring the weld strength of the first thin plate and the first thick plate. Consequently, a good nugget extending from the first thin plate to the second thin plate without unevenness is, formed, whereby the weld quality for the workpiece is enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A first embodiment of the present invention will be described below with reference toFIGS. 1 to 8.FIG. 1illustrates the configuration of a spot-welding device.FIG. 2illustrates a relevant part of the spot-welding device.

InFIG. 1, reference numeral1denotes a welding robot, reference numeral10denotes a spot-welding device supported by the welding robot1, and reference numeral100denotes a workpiece to be welded.

Before describing the welding robot1and the spot-welding device10, the workpiece100will be described first. The workpiece100has a three-stacked-plate structure formed by stacking a thin plate over one of two stacked thick plates. For example, in the following order from the top, the three-stacked-plate structure includes a thin plate101having low rigidity, and a first thick plate102and a second thick plate103that are thicker and more rigid than the thin plate101.

The welding robot1is, for example, an articulated robot and has a base portion fixed to the ground (not shown), a plurality of arms2, and a wrist3attached to an end of the arms2. The welding robot1is capable of three-dimensionally moving the spot-welding device10supported by the wrist3via an equalizer unit4. The equalizer unit4is interposed between the spot-welding device10and the wrist3and supports the spot-welding device10to the arms2in a movable manner in response to a reaction force generated when the spot-welding device10applies pressure onto the workpiece100.

The welding robot1sequentially moves the spot-welding device10to each preset spot-welding position, that is, a weld section, of the workpiece100held at a predetermined position by a clamper (not shown) so as to spot-weld the workpiece100.

The spot-welding device10includes a base11attached to the wrist3via the equalizer unit4. A C-shaped yoke13extending downward is attached to the base11. A fixed electrode14serving as a first welding electrode is attached to a lower end of the C-shaped yoke13.

A cylinder unit, a servomotor, or a pressure actuator15with a servomotor as a driving source in this embodiment is attached to an upper end of the base11. A rod16that is movable toward and away from the fixed electrode14along the axis of the fixed electrode14by actuating the pressure actuator15protrudes downward from the base11. A movable electrode17serving as a second welding electrode that is disposed facing the fixed electrode14and that is movable toward and away from the fixed electrode14along the axis thereof is attached to the tip end of the rod16. Thus, the movable electrode17can be moved toward and away from the fixed electrode14between a pressing position corresponding to a descended end and a retreated position corresponding to an ascended end by actuating the pressure actuator15. Specifically, at the retreated position, the movable electrode17is positioned away from the fixed electrode14. At the pressing position, the movable electrode17is positioned near towards the fixed electrode14so as to clamp the workpiece100together with the fixed electrode14and to apply pressure onto the workpiece100.

The base11is provided with control-pressure applying unit20that applies control pressure to a position adjacent to the movable electrode17, that is, a position near the welding position, on the thin plate101of the workpiece100clamped and pressed between the fixed electrode14and the movable electrode17.

As shown inFIG. 1and inFIG. 2illustrating an enlarged perspective view of the relevant part, the control-pressure applying unit20includes air cylinders21and23serving as a pair of control-pressure actuators extending parallel to the moving direction of the rod16. The air cylinders21and23are separated from each other with the rod16therebetween and have their base ends attached to opposite sides of the base11at the C-shaped yoke13side. The air cylinders21and23selectively supply air from an air supply source31to respective expansion air chambers or contraction air chambers via an air-supply switch valve32so as to expand or contract cylinder rods22and24respectively protruding from the tip ends of the air cylinders21and23, and also maintain the air in the contraction air chambers or the expansion air chambers so as to hold the cylinder rods22and24at the corresponding position.

A connection plate25is bridged between tip ends22aand24aof the cylinder rods22and24protruding from the tip ends of the air cylinders21and23, respectively. The connection plate25is provided with a strip-like workpiece holder26whose base end is connected to a central section of the connection plate25, that is, a section of the connection plate25between the tip ends22aand24aof the cylinder rods22and24, and whose tip end extends away from the C-shaped yoke13. The tip end of the workpiece holder26is provided with an electrode insertion section27into which the movable electrode17and the fixed electrode14can be inserted. The electrode insertion section27is formed by cutting out the end of the workpiece holder26into an arc shape or a recessed shape. Moreover, two regulators28and29respectively protrude from opposite sides of the electrode insertion section27. The lower surfaces of the regulators28and29are respectively provided with convex regulating surface pieces28aand29aserving as control pressers and whose top portions protrude in a semispherical shape. The upper surfaces of the regulators28and29are respectively provided with convexed regulating surface pieces28band29b. The regulating surface pieces28aand29aand the regulating surface pieces28band29bare preferably adjacent to the axis of the movable electrode17and are preferably provided at symmetric positions with respect to the axis of the movable electrode17.

The air cylinders21and23include a first-retreated-position detection sensor S1that detects a first retreated position at which the cylinder rods22and24of the air cylinders21and23are contracted so that the tip ends thereof are ascended; a first-pressing-position detection sensor S2that detects a first pressing position, which corresponds to an expanded position of the air cylinders21and23in a state where the regulating surface pieces28aand29aare in pressure contact from above with the upper surface of the workpiece100clamped by the fixed electrode14and the movable electrode17; a second-retreated-position detection sensor S3that detects a second retreated position at which the cylinder rods22and24of the air cylinders21and23are expanded so that the tip ends thereof are descended; and a second-pressing-position detection sensor S4that detects a second pressing position, which corresponds to an expanded position of the air cylinders21and23in a state where the regulating surface pieces28band29bare in pressure contact from below with the lower surface of the workpiece100clamped by the fixed electrode14and the movable electrode17.

A welding-robot controller RC stores teaching data for the welding robot1. The teaching data contains an operation program for sequentially spot-welding the welding spots of the workpiece100, and spot-welding positions, that is, the positions and orientations of the spot-welding device10for when spot-welding the welding spots. A welding-device controller WC contains an operation program for the spot-welding device10and performs operation control of the air-supply switch valve32on the basis of the detection results of the first-retreated-position detection sensor S1, the first-pressing-position detection sensor S2, the second-retreated-position detection sensor S3, and the second-pressing-position detection sensor S4.

For example, when the first-retreated-position detection sensor S1detects that the air cylinders21and23have reached the first retreated position due to contraction of the air cylinders21and23from an expanded state as a result of air supplied to the respective contraction air chambers, the air-supply switch valve32is switched to the first retreated position on the basis of the detection signal. Thus, the supply of air to the contraction air chambers is discontinued, and the air in the contraction air chambers is maintained therein, thereby holding the air cylinders21and23at the first retreated position.

When the air-supply switch valve32is switched from the first retreated position so that the air in the contraction air chambers is discharged therefrom and air is supplied to the expansion air chambers, the air cylinders21and23expand and reach the first pressing position. As the first-pressing-position detection sensor S2detects that the air cylinders21and23have reached the first pressing position, the air-supply switch valve32is switched to the first pressing position on the basis of the detection signal. Thus, the supply of air to the expansion air chambers is discontinued, and the air in the expansion air chambers is maintained therein, thereby holding the air cylinders21and23at the first pressing position.

When the second-retreated-position detection sensor S3detects that the air cylinders21and23have reached the second retreated position due to expansion of the air cylinders21and23as a result of air supplied to the respective expansion air chambers, the air-supply switch valve32is switched to the second retreated position on the basis of the detection signal. Thus, the supply of air to the expansion air chambers is discontinued, and the air in the expansion air chambers is maintained therein, thereby holding the air cylinders21and23at the second retreated position. When the air-supply switch valve32is switched from the second retreated position so that the air in the expansion air chambers is discharged therefrom and air is supplied to the contraction air chambers, the air cylinders21and23contract and reach the second pressing position. As the second-pressing-position detection sensor S4detects that the air cylinders21and23have reached the second pressing position, the air-supply switch valve32is switched to the second pressing position on the basis of the detection signal. Thus, the supply of air to the contraction air chambers is discontinued, and the air in the contraction air chambers is maintained therein, thereby holding the air cylinders21and23at the second pressing position.

Next, the operation of the spot-welding device10will be described. For the sake of convenience, a first description corresponding to when the workpiece100to be spot-welded has a three-stacked-plate structure including the thin plate101, the first thick plate102, and the second thick plate103in that order from the top will be provided below with reference toFIGS. 3A to 3G,FIG. 4, andFIG. 5. Specifically,FIGS. 3A to 3Gillustrate operation steps of the spot-welding device10,FIG. 4is an operation diagram thereof, andFIG. 5is an operation diagram of a comparative example. Then, a second description corresponding to when the workpiece100has a three-stacked-plate structure including the thin plate101, the first thick plate102, and the second thick plate103in that order from the bottom will be provided below with reference toFIGS. 6A to 6GandFIG. 7. Specifically,FIGS. 6A to 6Gillustrate operation steps of the spot-welding device10, andFIG. 7is an operation diagram thereof.

With regard to spot-welding the workpiece100including the thin plate101, the first thick plate102, and the second thick plate103in that order from the top, a state in which the movable electrode17is at a retreated position located away from the fixed electrode14and in which the workpiece holder26of the control-pressure applying unit20is held near the movable electrode17is confirmed, more specifically, a state in which the air cylinders21and23are expanded and are detected not to be at the first retreated position by the first-retreated-position detection sensor S1is confirmed in accordance with a preset operation program, as shown inFIG. 3A. In other words, when the position of the workpiece holder26is confirmed, the air-supply switch valve32is switched in response to an operation signal from the welding-device controller WC so as to start supplying air to the contraction air chambers of the air cylinders21and23, thereby causing the air cylinders21and23to contract. The contraction of the air cylinders21and23causes the workpiece holder26to ascend above the tip end of the movable electrode17. Then, when the air cylinders21and23contracted as shown inFIG. 3Breach the first retreated position, the first-retreated-position detection sensor S1detects the air cylinders21and23. Thus, the air-supply switch valve32is switched to the first retreated position so that the supply of air to the contraction air chambers of the air cylinders21and23is discontinued, and the air in the contraction air chambers is maintained therein. Consequently, the workpiece holder26is held at the first retreated position.

Subsequently, when it is confirmed that the air cylinders21and23are at the first retreated position on the basis of the detection signal from the first-retreated-position detection sensor S1, the welding-robot controller RC actuates the welding robot1so as to move the spot-welding device10to a spot-welding position of the workpiece100in accordance with a preset program. Consequently, a weld section of the workpiece100is positioned between the fixed electrode14and the movable electrode17as well as between the regulating surface pieces28aand29a, and the fixed electrode14is positioned at a specific position of the second thick plate103that corresponds to the spot-welding position, as shown inFIG. 3C.

In the state where the spot-welding device10is positioned at the welding position, the tip end of the fixed electrode14is in contact with the second thick plate103of the workpiece100from below, whereas the tip end of the movable electrode17and the regulating surface pieces28aand29aface the thin plate101with a gap therebetween, as shown inFIG. 3C.

Subsequently, referring toFIG. 3D, the pressure actuator15is actuated in the state where the fixed electrode14is in contact with the second thick plate103of the workpiece100, so that the movable electrode17is moved from the retreated position toward a pressing position to approach the fixed electrode14, whereby the movable electrode17comes into pressure contact with the thin plate101. Thus, the pressure of the pressure actuator15is applied to the fixed electrode14and the movable electrode17via the base11, whereby the weld section of the workpiece100is clamped and pressed between the movable electrode17and the fixed electrode14.

On the other hand, the air-supply switch valve32is switched so that the air in the contraction air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the expansion air chambers. This causes the air cylinders21and23to expand and the workpiece holder26to descend, whereby the regulating surface pieces28aand29aare set adjacent to the movable electrode17and come into pressure contact with the thin plate101of the workpiece100from above. Moreover, when the expanded air cylinders21and23reach the first pressing position, the first-pressing-position detection sensor S2detects the air cylinders21and23. Then, the air-supply switch valve32is switched to the first pressing position so that the supply of air to the expansion air chambers of the air cylinders21and23is discontinued, and the air in the expansion air chambers is maintained therein. Consequently, the workpiece holder26is held at the first pressing position.

In this state where the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17, and the regulating surface pieces28aand29aset adjacent to the movable electrode17apply pressure to the thin plate101due to the air cylinders21and23, a pressure FL from the fixed electrode14is applied to the second thick plate103of the workpiece100from below, and a pressure FU from the movable electrode17and a control pressure Fα from the regulating surface pieces28aand29a, set adjacent to the movable electrode17, due to the air cylinders21and23are applied to the thin plate101, as shown inFIG. 4which is a schematic operation diagram.

In this case, the pressure of the pressure actuator15and the control pressure of the air cylinders21and23are applied to the movable electrode17and the regulating surface pieces28aand29a, and to the fixed electrode14via the base11and the C-shaped yoke13. The pressure FL applied to the second thick plate103by the fixed electrode14is equal to the sum of the pressure FU applied by the movable electrode17and the control pressure Fα applied by the regulating surface pieces28aand29ato the thin plate101(FL=FU+Fα).

Consequently, the workpiece100is stably clamped and held by the pressure FL applied to the second thick plate103by the fixed electrode14from below, the pressure FU applied to the thin plate101by the movable electrode17from above, and the control pressure Fα applied to the thin plate101by the regulating surface pieces28aand29afrom above.

In the weld section of the workpiece100, on the other hand, the pressure FL from the fixed electrode14is applied to the second thick plate103, whereas the pressure FU applied to the thin plate101by the movable electrode17is equal to a difference between the pressure FL from the fixed electrode14and the control pressure Fα from the regulating surface pieces28aand29a(FU=FL−Fα).

By setting the pressure FU from the movable electrode17located at the thin plate101side to be smaller than the pressure FL from the fixed electrode14located at the second thick plate103side (FU<FL), the contact pressure at the joint section between the thin plate101and the first thick plate102becomes smaller than the contact pressure at the joint section between the first thick plate102and the second thick plate103. Consequently, the contact resistance between the thin plate101and the first thick plate102relatively increases, whereas the contact resistance between the first thick plate102and the second thick plate103relatively decreases.

For example, supposing that the spot-welding device10is not equipped with the control-pressure applying unit20, the pressure actuator15is actuated in a state where the fixed electrode14is in contact with the second thick plate103of the workpiece100so as to bring the movable electrode17into pressure contact with the thin plate101, thereby clamping and pressing the weld section of the workpiece100between the fixed electrode14and the movable electrode17. Referring toFIG. 5schematically illustrating a comparative example, the pressure of the pressure actuator15is uniformly applied to the movable electrode17and the fixed electrode14via the base11and the C-shaped yoke13so that the pressure FL is applied to the second thick plate103by the fixed electrode14and the pressure FU is applied to the thin plate101by the movable electrode17.

Next, in the state where the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17, and the control pressure from the regulating surface pieces28aand29ais applied to the workpiece100such that the pressure FU from the movable electrode17located at the thin plate101side is smaller than the pressure FL from the fixed electrode14located at the second thick plate103side, welding is performed by applying electricity between the movable electrode17and the fixed electrode14for a predetermined time, as shown inFIG. 3E. When the electricity is applied between the movable electrode17and the fixed electrode14, the contact resistance at the joint section between the thin plate101and the first thick plate102relatively increases and the current density becomes higher, whereas the contact resistance-between the first thick plate102and the second thick plate103is maintained at a small value. Thus, the calorific value at the joint section between the thin plate101and the first thick plate102becomes relatively higher than the calorific value at the joint section between the first thick plate102and the second thick plate103. Consequently, a good nugget extending from the thin plate101to the second thick plate103without uneven current density is formed, thereby ensuring the weld strength of the thin plate101.

After the welding process is completed, the pressure actuator15is actuated so that the movable electrode17is moved from the pressing position towards the retreated position, thereby releasing the clamped state of the workpiece100between the fixed electrode14and the movable electrode17. On the other hand, the air-supply switch valve32is switched so that the air in the expansion air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the contraction air chambers, thereby causing the air cylinders21and23to contract. The contraction of the air cylinders21and23causes the workpiece holder26to ascend above the tip end of the movable electrode17. Then, when the air cylinders21and23contracted as shown inFIG. 3Freach the first retreated position, the first-retreated-position detection sensor S1detects the air cylinders21and23. Thus, the air-supply switch valve32is switched to the first retreated position so that the supply of air to the contraction air chambers of the air cylinders21and23is discontinued, and the air in the contraction air chambers is maintained therein. Consequently, the workpiece holder26is held at the first retreated position.

Subsequently, when it is confirmed that the air cylinders21and23are at the first retreated position on the basis of the detection signal from the first-retreated-position detection sensor S1, the welding robot1is actuated, as shown inFIG. 3G, so as to move the spot-welding device10away from the current workpiece100to a spot-welding position of a subsequent workpiece100.

Next, the second description corresponding to when the workpiece100has a three-stacked-plate structure including the thin plate101, the first thick plate102, and the second thick plate103in that order from the bottom will be provided below with reference toFIGS. 6A to 6Gillustrating the operation steps of the spot-welding device10andFIG. 7illustrating the operation thereof.

With regard to spot-welding the workpiece100including the thin plate101, the first thick plate102, and the second thick plate103in that order from the bottom, when a state in which the movable electrode17is at a retreated position located away from the fixed electrode14and in which the workpiece holder26of the control-pressure applying unit20is held near the movable electrode17is confirmed, more specifically, when a state in which the air cylinders21and23are detected not to be at the second retreated position by the second-retreated-position detection sensor S3is confirmed in accordance with a preset operation program, as shown inFIG. 6A, the air-supply switch valve32is switched so as to start supplying air to the expansion air chambers of the air cylinders21and23, thereby causing the air cylinders21and23to expand.

The expansion of the air cylinders21and23causes the workpiece holder26to descend below the tip end of the fixed electrode14. Then, the second-retreated-position detection sensor S3detects that the air cylinders21and23expanded as shown inFIG. 6Bhave reached the second retreated position. Thus, the air-supply switch valve32is switched to the second retreated position so that the supply of air to the expansion air chambers of the air cylinders21and23is discontinued, and the air in the expansion air chambers is maintained therein. Consequently, the workpiece holder26is held at the second retreated position.

Subsequently, when it is confirmed that the air cylinders21and23are at the second retreated position on the basis of the detection signal from the second-retreated-position detection sensor S3, the welding robot1is actuated so as to move the spot-welding device10to a spot-welding position of the workpiece100in accordance with a preset program. Consequently, a weld section of the workpiece100is positioned between the fixed electrode14and the movable electrode17as well as between the regulating surface pieces28band29b, and the fixed electrode14is positioned in contact with a specific position of the thin plate101that corresponds to the spot-welding position, as shown inFIG. 6C.

In the state where the spot-welding device10is positioned at the welding position, the tip end of the fixed electrode14is in contact with the thin plate101of the workpiece100from below, whereas the tip end of the movable electrode17faces the second thick plate103with a gap therebetween, and the regulating surface pieces28aand29aface the thin plate101with a gap therebetween, as shown inFIG. 6C.

Subsequently, referring toFIG. 6D, the pressure actuator15is actuated in the state where the fixed electrode14is in contact with the thin plate101of the workpiece100, so that the movable electrode17is moved from the retreated position toward a pressing position, whereby the movable electrode17comes into pressure contact with the second thick plate103. Thus, the weld section of the workpiece100is clamped and pressed between the movable electrode17and the fixed electrode14.

On the other hand, the air-supply switch valve32is switched so that the air in the expansion air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the contraction air chambers. This causes the air cylinders21and23to contract and the workpiece holder26to ascend, whereby the regulating surface pieces28band29bare set adjacent to the fixed electrode14and come into pressure contact with the thin plate101of the workpiece100from below. Moreover, when the contracted air cylinders21and23reach the second pressing position, the second-pressing-position detection sensor S4detects the air cylinders21and23. Then, the air-supply switch valve32is switched to the second pressing position so that the supply of air to the contraction air chambers of the air cylinders21and23is discontinued, and the air in the contraction air chambers is maintained therein. Consequently, the workpiece holder26is held at the second pressing position.

In this state where the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17, and the regulating surface pieces28band29bapply control pressure to the thin plate101due to the air cylinders21and23, the pressure FU from the movable electrode17is applied to the second thick plate103of the workpiece100from above, and the pressure FL from the fixed electrode14and the control pressure Fα from the regulating surface pieces28band29bdue to the air cylinders21and23are applied to the thin plate101from below, as shown inFIG. 7which is a schematic operation diagram.

In this case, the pressure of the pressure actuator15is applied to the movable electrode17and to the fixed electrode14via the base11and the C-shaped yoke13, and the control pressure of the air cylinders21and23is applied to the regulating surface pieces28band29band to the movable electrode17via the base11. The pressure FU applied to the second thick plate103by the movable electrode17is equal to the sum of the pressure FL applied by the fixed electrode14and the control pressure Fα applied by the regulating surface pieces28band29bto the thin plate101(FU=FL+Fα). Consequently, the workpiece100is stably held by the fixed electrode14, the movable electrode17, and the regulating surface pieces28band29b.

On the other hand, in the weld section of the workpiece100, the pressure FU from the movable electrode17is applied to the second thick plate103, whereas the pressure FL applied to the thin plate101by the fixed electrode14is equal to a difference between the pressure FU from the movable electrode17and the control pressure Fα from the regulating surface pieces28band29b(FL=FU−Fα).

By setting the pressure FL from the fixed electrode14located at the thin plate101side to be smaller than the pressure FU from the movable electrode17located at the second thick plate103side (FL<FU), the contact pressure at the joint section between the thin plate101and the first thick plate102becomes smaller than the contact pressure at the joint section between the first thick plate102and the second thick plate103. Consequently, the contact resistance between the thin plate101and the first thick plate102relatively increases, whereas the contact resistance between the first thick plate102and the second thick plate103relatively decreases.

In the state where the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17, and the control pressure Fα from the regulating surface pieces28band29bis applied to the workpiece100, welding is performed by applying electricity between the movable electrode17and the fixed electrode14for a predetermined time, as shown inFIG. 6E. When the electricity is applied between the movable electrode17and the fixed electrode14, the contact resistance at the joint section between the thin plate101and the first thick plate102relatively increases and the current density becomes higher, whereas the contact resistance between the first thick plate102and the second thick plate103relatively decreases. Thus, the calorific value at the joint section between the thin plate101and the first thick plate102becomes relatively higher than the calorific value at the joint section between the first thick plate102and the second thick plate103. Consequently, a good nugget N extending from the thin plate101to the second thick plate103without an uneven amount of weld penetration is formed, thereby ensuring the weld strength of the thin plate101.

After the welding process is completed, the pressure actuator15is actuated so that the movable electrode17is moved from the pressing position towards the retreated position, thereby releasing the clamped state of the workpiece100between the fixed electrode14and the movable electrode17, as shown inFIG. 6F. On the other hand, the air-supply switch valve32is switched so that the air in the contraction air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the expansion air chambers, thereby causing the air cylinders21and23to expand. The expansion of the air cylinders21and23causes the workpiece holder26to descend below the tip end of the fixed electrode14. Then, when the air cylinders21and23expanded as shown inFIG. 6Freach the second retreated position, the second-retreated-position detection sensor S3detects the air cylinders21and23. Thus, the air-supply switch valve32is switched to the second retreated position so that the supply of air to the expansion air chambers of the air cylinders21and23is discontinued, and the air in the expansion air chambers is maintained therein. Consequently, the workpiece holder26is held at the second retreated position.

Subsequently, when it is confirmed that the air cylinders21and23are at the second retreated position on the basis of the detection signal from the second-retreated-position detection sensor S3, the welding robot1is actuated, as shown inFIG. 6G, so as to move the spot-welding device10away from the spot-welding position of the current workpiece100and then to move the spot-welding device10to a spot-welding position of a subsequent workpiece100.

According to the first embodiment having the above-described configuration, the spot-welding device10that spot-welds the workpiece100having a three-stacked-plate structure including the thin plate101having low rigidity, and the first thick plate102and the second thick plate103that are more rigid than the thin plate101includes the fixed electrode14provided at the base11via the C-shaped yoke13, the movable electrode17that is provided at the base11and that is moved toward and away from the fixed electrode14by the pressure actuator15, and the control-pressure applying unit20that applies control pressure to the weld section of the thin plate101of the workpiece100clamped and pressed between the fixed electrode14and the movable electrode17. The pressure FL and the pressure FU are applied to the workpiece100from the fixed electrode14and the movable electrode17, respectively, and the control pressure Fα is applied to near the welding position of the workpiece100, so that the contact pressure between the thin plate101and the first thick plate102is controlled to be lower than the contact pressure between the first thick plate102and the second thick plate103, whereby the current density at the joint section between the thin plate101and the first thick plate102becomes relatively higher than the current density at the joint section between the first thick plate102and the second thick plate103when electricity is applied to the movable electrode17and the fixed electrode14. Consequently, a good nugget extending from the thin plate101to the second thick plate103without uneven weld penetration is formed, thereby ensuring the weld strength of the thin plate101. In particular, since the workpiece100clamped and pressed between the fixed electrode14and the movable electrode17receives the pressure FL from the fixed electrode14, the pressure FU from the movable electrode17, and the control pressure Fα to the joint section of the workpiece100, good nuggets extending from the thin plate101to the second thick plate103without unevenness can be formed at various welding positions without being affected by a clamping position where the workpiece100is clamped, thereby ensuring the weld quality.

The present invention is not limited to this embodiment and permits various modifications without departing from the scope of the invention. For example, although the above embodiment is directed to an example in which the air cylinders21and23are used as control-pressure actuators, a servomotor or the like may be used as an alternative.

Furthermore, although the convexed regulating surface pieces28a,29a,28b, and29bserve as an example of control pressers in the above description, various modifications are permissible in accordance with the shape of the workpiece100. For example, referring toFIG. 8corresponding toFIG. 2, the control pressers may alternatively be defined by a semi-arc-shaped regulating surface piece28A and a semi-arc-shaped regulating surface piece28B formed on the connection plate25bridged between the tip ends22aand24aof the cylinder rods22and24respectively protruding from the tip ends of the air cylinders21and23. Specifically, the semi-arc-shaped regulating surface piece28A extends along the electrode insertion section27from the lower surface of the regulators28and29that are formed along the electrode insertion section27, which is formed by cutting out the end of the protruding workpiece holder26into an arc shape or a recessed shape. The semi-arc-shaped regulating surface piece28B extends along the electrode insertion section27from the upper surface of the regulators28and29.

Second Embodiment

A second embodiment of the present invention will be described below with reference toFIGS. 9 to 11.FIG. 9illustrates the configuration of a spot-welding device, FIG.10illustrates a relevant part of the spot-welding device, andFIG. 11is an operation diagram thereof. Components inFIGS. 9 to 11that correspond to those inFIGS. 1 and 2are given the same reference numerals, and detailed descriptions thereof will be omitted.

In a spot-welding device40according to this embodiment, the control-pressure applying unit20in the first embodiment is replaced by control-pressure applying unit41supported by the rod16.

As shown inFIGS. 9 and 10, the rod16of the spot-welding device40has a columnar shape and includes a base end16A protruding downward from the base11and having a relatively large diameter, a columnar shaft16C having a smaller diameter than the base end16A and extending continuously from and coaxially with the base end16A via a step16B, and a shank16D formed at the tip end of the shaft16C. The movable electrode17is fitted to the shank16D. The shaft16C has a diameter larger than that of the movable electrode17.

The control-pressure applying unit41is defined by a cylindrical elastic member fittable around the shaft16C and given an insulating coating. In this embodiment, the control-pressure applying unit41is defined by a coil spring42.

The coil spring42has a base end42athat is in abutment with the step16B, and a base-end segment42bthat is fitted around the shaft16C so that the coil spring42is attached to the rod16, and a tip-end segment42cserving as a control presser. In an unloaded state, the tip-end segment42chas an effective length protruding from the tip end of the movable electrode17.

In the spot-welding device40equipped with the control-pressure applying unit41, the pressure actuator15is actuated in a state where the fixed electrode14is in contact with the second thick plate103of the workpiece100so as to move the movable electrode17toward a pressing position and to bring the movable electrode17into pressure contact with the thin plate101, whereby a weld section of the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17. This causes the tip-end segment42cof the coil spring42to be set annularly adjacent to the tip end of the movable electrode17and to come into contact with the thin plate101, whereby the coil spring42becomes compressed. Thus, the tip-end segment42cin pressure contact with the workpiece100due to a reaction force of the coil spring42applies a control pressure Fα to the thin plate101along the periphery of the movable electrode17.

In this state where the workpiece100is clamped and pressed between the fixed electrode14and the movable electrode17, and the control pressure Fα is applied to the thin plate101by the coil spring42, a pressure FL from the fixed electrode14is applied to the second thick plate103of the workpiece100from below, and a pressure FU from the movable electrode17and the control pressure Fα from the coil spring42are applied to the thin plate101, as shown inFIG. 11which is a schematic operation diagram. In this case, the pressure FL applied to the second thick plate103by the fixed electrode14is equal to the sum of the pressure FU applied by the movable electrode17and the control pressure Fα applied by the coil spring42to the thin plate101(FL=FU+Fα). In the weld section of the workpiece100, the pressure FL from the fixed electrode14is applied to the second thick plate103, whereas the pressure FU applied to the thin plate101by the movable electrode17is equal to a difference between the pressure FL from the fixed electrode14and the control pressure Fα from the coil spring42(FU=FL−Fα).

By setting the pressure FU from the movable electrode17located at the thin plate101side to be smaller than the pressure FL from the fixed electrode14located at the second thick plate103side (FU<FL), the contact resistance between the thin plate101and the first thick plate102relatively increases and the current density becomes higher when electricity is applied between the movable electrode17and the fixed electrode14, whereas the contact resistance between the first thick plate102and the second thick plate103is maintained at a small value. Thus, the calorific value between the thin plate101and the first thick plate102becomes relatively higher than the calorific value between the first thick plate102and the second thick plate103. Consequently, a good nugget N extending from the thin plate101to the second thick plate103without uneven weld penetration is formed, thereby ensuring the weld quality.

Accordingly, the configuration of the control-pressure applying unit41is simple, lightweight, and compact, as compared with that of the control-pressure applying unit20in the first embodiment. Thus, the spot-welding device40can be used in an area where the workspace is relatively limited, thereby allowing for an increased spot-weldable range. Furthermore, the control pressure Fα can be readily adjusted by changing the specification of the repulsive force of the coil spring42.

Third Embodiment

A third embodiment of the present invention will be described below with reference toFIGS. 12 to 17.FIG. 12illustrates the configuration of a spot-welding device,FIG. 13illustrates a relevant part of the spot-welding device, andFIGS. 14A to 14Hillustrate operation steps thereof. Components inFIGS. 12 to 14Hthat correspond to those inFIGS. 1 and 2are given the same reference numerals, and detailed descriptions thereof will be omitted.

InFIG. 12, reference numeral1denotes a welding robot, reference numeral50denotes a spot-welding device supported by the welding robot1, and reference numeral110denotes a workpiece to be spot-welded.

Before describing the welding robot1and the spot-welding device50, the workpiece110will be described first. The workpiece110has a four-stacked-plate structure formed by stacking thin plates over opposite faces of two stacked thick plates. For example, the four-stacked-plate structure includes a first thick plate102and a second thick plate103having high rigidity and stacked one on top of the other, and a first thin plate101and a second thin plate104having low rigidity and respectively stacked over opposite faces of the first thick plate102and the second thick plate103.

The welding robot1is, for example, an articulated robot and has a plurality of arms2and a wrist3attached to an end of the arms2. The welding robot1is capable of three-dimensionally moving the spot-welding device50supported by the wrist3via an equalizer unit4. The welding robot1sequentially moves the spot-welding device50to each preset spot-welding position, that is, a weld section, of the workpiece110held at a predetermined position by a clamper (not shown) so as to spot-weld the workpiece110.

The spot-welding device50includes a base11attached to the wrist3via the equalizer unit4. A fixed electrode14is attached to a lower end of a C-shaped yoke13, which is attached to the base11.

A pressure actuator15is attached to an upper end of the base11. A movable electrode17disposed facing the fixed electrode14is attached to the tip end of a rod16. The rod16is movable toward and away from the fixed electrode14along the axis of the fixed electrode14by actuating the pressure actuator15. Thus, the movable electrode17is capable of moving toward and away from the fixed electrode14between a retreated position corresponding to an ascended end and a pressing position corresponding to a descended end. Specifically, at the retreated position, the movable electrode17is positioned away from the fixed electrode14by actuating the pressure actuator15. At the pressing position, the movable electrode17clamps the workpiece110together with the fixed electrode14.

The base11is provided with control-pressure applying unit60that further applies control pressure to the workpiece110clamped and pressed between the fixed electrode14and the movable electrode17.

As shown inFIG. 12andFIG. 13illustrating an enlarged perspective view of the relevant part, the control-pressure applying unit60includes air cylinders21and23serving as a pair of control-pressure actuators separated from each other with the rod16therebetween and having their base ends attached to opposite sides of the base11at the C-shaped yoke13side. The air cylinders21and23selectively supply air from an air supply source31to respective expansion air chambers or contraction air chambers via an air-supply switch valve32so as to expand or contract cylinder rods22and24, and also maintain the air in the contraction air chambers or the expansion air chambers so as to hold the cylinder rods22and24at the corresponding position.

A lower first connection plate61and an upper second connection plate65are bridged between tip ends of the cylinder rods22and24protruding from the tip ends of the air cylinders21and23. The first connection plate61and the second connection plate65are disposed facing each other in the vertical direction with a gap therebetween into which the workpiece110can be inserted.

The first connection plate61is provided with a strip-like first workpiece holder62whose base end is connected to a central section of the first connection plate61, that is, a section of the first connection plate61between the tip ends of the cylinder rods22and24, and whose tip end extends away from the C-shaped yoke13. The tip end of the first workpiece holder62is provided with an electrode insertion section63into which the movable electrode17and the fixed electrode14can be inserted. The electrode insertion section63is formed by cutting out the end of the first workpiece holder62into an arc shape or a recessed shape. Moreover, two regulators64A and64B respectively protrude from opposite sides of the electrode insertion section63. The upper surfaces of the regulators64A and64B are respectively provided with convexed regulating surface pieces64aand64bserving as control pressers.

Likewise, the second connection plate65is provided with a strip-like second workpiece holder66whose base end is connected to a central section of the second connection plate65and whose tip end extends away from the C-shaped yoke13. The tip end of the second workpiece holder66is provided with an electrode insertion section67into which the movable electrode17and the fixed electrode14can be inserted. The electrode insertion section67is formed by cutting out the end of the second workpiece holder66into an arc shape or a recessed shape. Moreover, two regulators68A and68B respectively protrude from opposite sides of the electrode insertion section67. The lower surfaces of the regulators68A and68B are respectively provided with convexed regulating surface pieces68aand68bserving as control pressers.

The air cylinders21and23include a retreated-position detection sensor S5, a first-pressing-position detection sensor S6, and a second-pressing-position detection sensor S7. Specifically, the retreated-position detection sensor S5detects a retreated position, which is an expanded position of the cylinder rods22and24of the air cylinders21and23. At the retreated position, the regulating surface pieces64aand64bface the lower surface of the second thin plate104of the workpiece110, which is clamped between the fixed electrode14and the movable electrode17, with a gap therebetween, and the regulating surface pieces68aand68bface the upper surface of the first thin plate101with a gap therebetween. The first-pressing-position detection sensor S6detects a first pressing position, which is an expanded position of the air cylinders21and23in a state where the regulating surface pieces64aand64bare in pressure contact from below with the second thin plate104of the workpiece110clamped between the fixed electrode14and the movable electrode17. The second-pressing-position detection sensor S7detects a second pressing position, which is an expanded position of the air cylinders21and23in a state where the regulating surface pieces68aand68bare in pressure contact from above with the first thin plate101of the workpiece110clamped between the fixed electrode14and the movable electrode17.

A welding-robot controller RC stores teaching data for the welding robot1. The teaching data contains an operation program for sequentially spot-welding the welding spots of the workpiece100, and spot-welding positions, that is, the positions and orientations of the spot-welding device10for when spot-welding the welding spots. A welding-device controller WC contains an operation program for the spot-welding device50and performs operation control of the air-supply switch valve32on the basis of the detection results of the retreated-position detection sensor S5, the first-pressing-position detection sensor S6, and the second-pressing-position detection sensor S7.

For example, when the air cylinders21and23expand due to air supplied to the respective expansion air chambers and the retreated-position detection sensor S5detects that the air cylinders21and23have reached the retreated position, the air-supply switch valve32is switched to the retreated position on the basis of the detection signal. Thus, the supply of air to the expansion air chambers is discontinued, and the air in the expansion air chambers is maintained therein, thereby holding the air cylinders21and23at the retreated position. When the air-supply switch valve32is switched from the retreated position so that the air in the expansion air chambers is discharged therefrom and air is supplied to the contraction air chambers, the air cylinders21and23contract and reach the first pressing position. As the first-pressing-position detection sensor S6detects that the air cylinders21and23have reached the first pressing position, the air-supply switch valve32is switched to the first pressing position on the basis of the detection signal. Thus, the supply of air to the contraction air chambers is discontinued, and the air in the contraction air chambers is maintained therein, thereby holding the air cylinders21and23at the first pressing position. When the air-supply switch valve32is switched from the first pressing position so that the air in the contraction air chambers is discharged therefrom and air is supplied to the expansion air chambers, the air cylinders21and23expand and reach the second pressing position. As the second-pressing-position detection sensor S7detects that the air cylinders21and23have reached the second pressing position, the air-supply switch valve32is switched to the second pressing position on the basis of the detection signal. Thus, the supply of air to the expansion air chambers is discontinued, and the air in the expansion air chambers is maintained therein, thereby holding the air cylinders21and23at the second pressing position.

Next, the operation of the spot-welding device50will be described with reference toFIGS. 14A to 14Hillustrating the operation steps of the spot-welding device50, andFIGS. 15 and 16.

With regard to spot-welding the workpiece110, when it is confirmed in a first welding step that the movable electrode17is at the retreated position located away from the fixed electrode14in accordance with a preset operation program and that the retreated-position detection sensor S5of the control-pressure applying unit60has not detected the air cylinders21and23at the retreated position, as shown inFIG. 14A, the air-supply switch valve32is switched in response to an operation signal from the welding-device controller WC so as to start supplying air to the expansion air chambers of the air cylinders21and23, thereby causing the air cylinders21and23to expand. If the retreated-position detection sensor S5detects the air cylinders21and23at the retreated position during the expansion, the supply of air is discontinued at that time. If the air cylinders21and23are not detected at the retreated position, the air cylinders21and23expand until the first workpiece holder62descends below the tip end of the fixed electrode14. Then, the supply of air to the contraction air chambers of the air cylinders21and23is commenced, thereby causing the air cylinders21and23to contract. As the air cylinders21and23reach the retreated position during this time, as shown inFIG. 14B, the retreated-position detection sensor S5detects the air cylinders21and23at the retreated position. The air-supply switch valve32discontinues the supply of air to the contraction air chambers of the air cylinders21and23, and the first workpiece holder62and the second workpiece holder66are held at the retreated position due to cylinder braking.

Subsequently, when it is confirmed that the air cylinders21and23are at the retreated position on the basis of the detection signal from the retreated-position detection sensor S5, the welding robot1is actuated so as to move the spot-welding device50to a spot-welding position of the workpiece110in accordance with a preset program. Consequently, a joint section of the workpiece110is positioned between the fixed electrode14and the movable electrode17as well as between the regulating surface pieces64aand64band between the regulating surface pieces68aand68b, and the fixed electrode14is positioned in contact with a specific position of the second thin plate104that corresponds to the spot-welding position, as shown inFIG. 14C.

In the state where the spot-welding device50is positioned at the welding position, the tip end of the fixed electrode14is in contact with the second thin plate104of the workpiece110from below, whereas the tip end of the movable electrode17and the regulating surface pieces68aand68bface the first thin plate101with a gap therebetween, and the regulating surface pieces64aand64bface the second thin plate104with a gap therebetween, as shown inFIG. 14C.

Subsequently, referring toFIG. 14D, the pressure actuator15is actuated in the state where the fixed electrode14is in contact with the second thin plate104of the workpiece110, so that the movable electrode17is moved from the retreated position toward the pressing position, whereby the movable electrode17comes into pressure contact with the first thin plate101. Thus, the pressure of the pressure actuator15is applied to the fixed electrode14and the movable electrode17via the base11and the C-shaped yoke13, whereby the weld section of the workpiece110is clamped and pressed between the movable electrode17and the fixed electrode14.

On the other hand, the air-supply switch valve32is switched so that the air in the expansion air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the contraction air chambers. This causes the air cylinders21and23to contract and the first workpiece holder62and the second workpiece holder66to ascend, whereby the regulating surface pieces64aand64bprovided on the upper surface of the first workpiece holder62are set adjacent to the tip end of the fixed electrode14and come into pressure contact with the second thin plate104of the workpiece110from below. Moreover, when the contracted air cylinders21and23reach the first pressing position, the first-pressing-position detection sensor S6detects the air cylinders21and23. Then, the air-supply switch valve32is switched to the first pressing position so that the supply of air to the contraction air chambers of the air cylinders21and23is discontinued, and the air in the contraction air chambers is maintained therein.

In this state where the workpiece110is clamped and pressed between the fixed electrode14and the movable electrode17, and the regulating surface pieces64aand64bapply pressure to the second thin plate104due to the air cylinders21and23, a pressure FL from the fixed electrode14and a first control pressure Fα from the regulating surface pieces64aand64b, set adjacent to the tip end of the fixed electrode14, due to the air cylinders21and23are applied to the second thin plate104of the workpiece110from below, and a pressure FU from the movable electrode17is applied to the first thin plate101, as shown in FIG.15which is a schematic operation diagram.

In this case, as schematically shown inFIG. 15, the pressure of the pressure actuator15is applied to the movable electrode17and to the fixed electrode14via the base11and the C-shaped yoke13, and the control pressure of the air cylinders21and23is applied to the regulating surface pieces64aand64b. The sum of the pressure FL applied by the fixed electrode14and the first control pressure Fα applied by the regulating surface pieces64aand64bto the second thin plate104is equal to the pressure FU applied to the first thin plate101by the movable electrode17(FU=FL+Fα). Consequently, the workpiece110is stably clamped by the fixed electrode14, the movable electrode17, and the regulating surface pieces64aand64b.

On the other hand, in the weld section of the workpiece110, the pressure FU from the movable electrode17is applied to the first thin plate101, whereas the pressure FL applied to the second thin plate104by the fixed electrode14is equal to a difference between the pressure FU from the movable electrode17and the first control pressure Fα from the regulating surface pieces64aand64b(FL=FU−Fα).

Accordingly, the pressure FL from the fixed electrode14is set to be smaller than the pressure FU from the movable electrode17(FL<FU).

In this state where the workpiece110is clamped and pressed between the fixed electrode14and the movable electrode17, and the control pressure from the regulating surface pieces64aand64bis applied to the workpiece110such that the pressure FL from the fixed electrode14located at the second thin plate104side is smaller than the pressure FU from the movable electrode17located at the first thin plate101side, welding is performed by applying electricity between the movable electrode17and the fixed electrode14for a predetermined time, as shown inFIG. 14E. When the electricity is applied between the movable electrode17and the fixed electrode14for this welding process, the current density at the joint section between the second thin plate104and the second thick plate103becomes larger and the calorific value thereof becomes relatively higher than the calorific value at the joint section between the first thin plate101and the first thick plate102. Consequently, a good nugget N1extending with a large amount of weld penetration from the joint section between the second thin plate104and the second thick plate103to the joint section between the first thick plate102and the second thick plate103is formed, thereby ensuring the weld strength of the second thin plate104and the second thick plate103.

After the welding process is completed, the air-supply switch valve32is switched in a second welding step so that the air in the contraction air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the expansion air chambers, thereby causing the air cylinders21and23to expand, as shown inFIG. 14F. This causes the first workpiece holder62to descend and move away from the second thin plate104of the workpiece110, and the second workpiece holder66to descend so that the regulating surface pieces68aand68bare set adjacent to the tip end of the movable electrode17and come into pressure contact with the first thin plate101of the workpiece110from above. Moreover, when the expanded air cylinders21and23reach the second pressing position, the second-pressing-position detection sensor S7detects the air cylinders21and23. Then, the air-supply switch valve32is switched to the second pressing position so that the supply of air to the expansion air chambers of the air cylinders21and23is discontinued, and the air in the expansion air chambers is maintained therein.

In this state where the workpiece110is clamped and pressed between the fixed electrode14and the movable electrode17, and the regulating surface pieces68aand68bapply pressure to the first thin plate101due to the air cylinders21and23, the pressure FL from the fixed electrode14is applied to the second thin plate104from below, and the pressure FU from the movable electrode17and a second control pressure Fβ from the regulating surface pieces68aand68bdue to the air cylinders21and23are applied to the first thin plate101, as shown inFIG. 16which is a schematic operation diagram.

In this case, the pressure FL applied to the second thin plate104by the fixed electrode14is equal to the sum of the pressure FU applied by the movable electrode17and the second control pressure Fβ applied by the regulating surface pieces68aand68bto the first thin plate101(FL=FU+Fβ). Consequently, the workpiece110is stably clamped by the fixed electrode14, the movable electrode17, and the regulating surface pieces68aand68b.

On the other hand, in the weld section of the workpiece110, the pressure FL from the fixed electrode14is applied to the second thin plate104, whereas the pressure FU applied to the first thin plate101by the movable electrode17is equal to a difference between the pressure FL from the fixed electrode14and the second control pressure Fβ from the regulating surface pieces68aand68b(FU=FL−Fβ).

Accordingly, the pressure FU from the movable electrode17is set to be smaller than the pressure FL from the fixed electrode14(FU<FL).

In this state where the workpiece110is clamped and pressed between the fixed electrode14and the movable electrode17, and the second control pressure from the regulating surface pieces68aand68bis applied to the workpiece110such that the pressure FU from the movable electrode17located at the first thin plate101side is smaller than the pressure FL from the fixed electrode14located at the second thin plate104side, welding is performed by applying electricity between the movable electrode17and the fixed electrode14for a predetermined time, as shown inFIG. 14G. When the electricity is applied between the movable electrode17and the fixed electrode14for this welding process, the current density at the joint section between the first thin plate101and the first thick plate102becomes relatively higher than the current density at the joint section between the second thin plate104and the second thick plate103. Consequently, a good nugget N2with a large amount of weld penetration is formed at the joint section between the first thin plate101and the first thick plate102, thereby ensuring the weld strength of the first thin plate101and the first thick plate102.

Specifically, the first welding step involves actively forming the nugget N1between the second thin plate104and the second thick plate103by reducing the contact pressure between the second thin plate104and the second thick plate103and applying electricity between the movable electrode17and the fixed electrode14, thereby ensuring the weld strength. Subsequently, the second welding step involves actively forming the nugget N2in the contact section between the first thin plate101and the first thick plate102by reducing the contact pressure between the first thin plate101and the first thick plate102and applying electricity between the movable electrode17and the fixed electrode14, thereby ensuring the weld strength. Consequently, the weld strength at the weld section of the workpiece110having a four-stacked-plate structure including the first thin plate101, the first thick plate102, the second thick plate103, and the second thin plate104can be ensured, thereby ensuring the weld quality.

After the welding process is completed, the pressure actuator15is actuated so that the movable electrode17is moved from the pressing position towards the retreated position, thereby releasing the clamped state of the workpiece110between the fixed electrode14and the movable electrode17, as shown inFIG. 14H. On the other hand, the air-supply switch valve32is switched so that the air in the expansion air chambers of the air cylinders21and23is discharged therefrom and air is supplied to the contraction air chambers, thereby causing the air cylinders21and23to contract. When the contracted air cylinders21and23reach the retreated position, the retreated-position detection sensor S5detects the air cylinders21and23. Thus, the air-supply switch valve32is switched to the retreated position so that the supply of air to the contraction air chambers of the air cylinders21and23is discontinued, and the air in the contraction air chambers is maintained therein. Consequently, the first workpiece holder62and the second workpiece holder66are held at the retreated position.

Subsequently, when it is confirmed that the air cylinders21and23are at the retreated position on the basis of the detection signal from the retreated-position detection sensor S5, the welding robot1is actuated so as to move the spot-welding device50away from the spot-welding position of the current workpiece110to a spot-welding position of a subsequent workpiece110.

According to the third embodiment having the above-described configuration, the spot-welding device50that spot-welds the workpiece110having a four-stacked-plate structure formed by stacking the first thin plate101and the second thin plate104having low rigidity respectively over opposite faces of the first thick plate102and the second thick plate103having high rigidity includes the fixed electrode14provided at the base11via the C-shaped yoke13, the movable electrode17that is provided at the base11and that is moved toward and away from the fixed electrode14by the pressure actuator15, and the control-pressure applying unit60that selectively applies control pressure to near the welding positions in the first thin plate101and the second thin plate104of the workpiece110clamped and pressed between the fixed electrode14and the movable electrode17. In the workpiece110clamped and pressed between the fixed electrode14and the movable electrode17, the pressure FL and the pressure FU are applied to the workpiece110from the fixed electrode14and the movable electrode17, respectively, and the first control pressure Fα is applied to near the welding position of the second thin plate104, so that the contact pressure between the second thin plate104and the second thick plate103is controlled to be lower than the contact pressure between the first thick plate102and the first thin plate101, whereby the calorific value at the joint section between the second thin plate104and the second thick plate103becomes relatively higher than the calorific value at the joint section between the first thick plate102and the first thin plate101when electricity is applied to the movable electrode17and the fixed electrode14. Therefore, the weld strength of the second thin plate104and the second thick plate103is ensured.

Similarly, the second control pressure Fβ is applied to the first thin plate101so that the contact pressure between the first thin plate101and the first thick plate102becomes lower than the contact pressure between the second thick plate103and the second thin plate104, whereby the current density at the joint section between the first thin plate101and the first thick plate102becomes relatively higher than the current density at the joint section between the second thick plate103and the second thin plate104when electricity is applied to the movable electrode17and the fixed electrode14. Therefore, the weld strength of the first thin plate101and the first thick plate102is ensured, thereby ensuring the weld quality for the workpiece110having a four-stacked-plate structure formed by stacking the first thin plate101and the second thin plate104having low rigidity respectively, over opposite faces of the first thick plate102and the second thick plate103having high rigidity. In particular, since the workpiece110clamped and pressed between the fixed electrode14and the movable electrode17can receive the pressure FL from the fixed electrode14, the pressure FU from the movable electrode17, and the control pressure to near the welding position of the workpiece110, good nuggets extending from the first thin plate101to the second thin plate104without unevenness can be formed at various welding positions without being affected by a clamping position where the workpiece110is clamped, thereby ensuring the weld strength of the first thin plate101.

The present invention is not limited to this embodiment and permits various modifications without departing from the scope of the invention. For example, although the above embodiment is directed to an example in which the air cylinders21and23are used as control-pressure actuators, a servomotor or the like may be used as an alternative.

Furthermore, although the convexed regulating surface pieces64a,64b,68a, and68bserve as an example of control pressers in the above description, various modifications are permissible in accordance with the shape of the workpiece110. For example, referring toFIG. 17corresponding toFIG. 13, the control pressers may alternatively be defined by a semi-arc-shaped regulating surface piece70and a semi-arc-shaped regulating surface piece71. Specifically, the semi-arc-shaped regulating surface piece70protrudes upward along the electrode insertion section63from the upper surface of the regulators64A and64B that are formed along the electrode insertion section63, which is formed by cutting out the end of the first workpiece holder62into an arc shape or a recessed shape. The semi-arc-shaped regulating surface piece71protrudes downward along the electrode insertion section67from the lower surface of the regulators68A and68B that are formed along the electrode insertion section67, which is formed by cutting out the end of the second workpiece holder66into an arc shape or a recessed shape.

Furthermore, in the first embodiment and the second embodiment, a pressing position where the desired control pressure Fα or Fβ can be obtained is preliminarily calculated on the basis of the thickness of the workpiece and the pressures FL and FU, and the air cylinders are controlled such that the regulating surface pieces are positioned at the calculated pressing position. Alternatively, the amount and the pressure of air to be supplied to the corresponding air chambers of the air cylinders may be controlled as parameters or the pressure between the regulating surface pieces and the thin plate may be controlled while directly detecting the pressure, so long as the desired control pressure Fα or Fβ can be obtained from the regulating surface pieces serving as control pressers by using the control-pressure applying unit.