Spot welding device and spot welding process

Abstract

Spot welding device (1) for spot welding a workpiece (100) in which a thinner plate (101) and a first thicker plate (102) and a second thicker plate (103), which have a greater stiffness than the thinner plate (101), are arranged sequentially one above the other from above, wherein the device (1) comprises: a base unit (3), a receiving unit (13) held by the base unit (3), and a first welding electrode (25) and a second welding electrode (35) which are held by the base unit (3) and are movable towards each other and away from each other in a facing position, wherein the workpiece (100) is selectively clamped between a combination of the second welding electrode (35), which rests against the thinner plate (101), and the receiving unit (13), which is located near the second welding electrode (35) and rests against the thinner plate (101), and the first welding electrode (25), which rests against the second thicker plate (103), wherein pressure is applied to the workpiece (100) by the first welding electrode (25) simultaneously with the clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100), and a combination of the first welding electrode (25) which rests against the thinner plate (101), and the receiving unit (13) which is located near the first welding electrode (25) and rests against the thinner plate (101), and the second welding electrode (35) which rests against the second thicker plate (103), whereby pressure is applied to the workpiece (100) by the second welding electrode (35) simultaneously with clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100).

Description

[0001] The present invention relates to devices and methods for spot welding a plate-shaped workpiece in which a thicker plate and a thinner plate, which have different stiffnesses, are arranged one above the other.

[0002] In the technique of joining superimposed plates, such as steel plates, spot welding is widely used, in which the plates are clamped between a pair of welding electrodes and a strong current is passed between the electrodes for a predetermined period of time, while welding pressure is applied to the plates to heat the joining zone to essentially a melting temperature in order to join the plates together.

[0003] Provided that the welding pressure applied by the welding electrodes and the duration of the current are constant during spot welding, the diameter of the weld nugget gradually increases with increasing current. However, applying excessively high current increases the amount of heat generated, potentially leading to a phenomenon called expulsion, in which molten metal is forced out from between the plates. Expulsion is the phenomenon of molten metal exploding at the joint due to overheating, resulting in defects such as holes or cracks in the weld nugget. Such discontinuities in the shape of the weld nugget and the metal structure can significantly reduce the joint strength and may also reduce the plate thickness at the joint.On the other hand, applying an excessively low current results in a small weld pool that does not produce a sufficiently strong joint. Furthermore, if the welding pressure is low, the contact area between the plates becomes small, resulting in an increased current density and expulsion due to overheating. If the welding pressure is excessively high, the contact area at the joint becomes large and the current density decreases, resulting in a reduced amount of heat generated. Consequently, a small weld pool is formed and a high welding force is not achieved.

[0004] It will now be based on Fig. Reference is made to Figure 13A, which shows the spot welding of a workpiece 100 in which a thinner plate 101 with low stiffness and small thickness is arranged on top of each other, and a first thicker plate 102 and a second thicker plate 103, which have a greater thickness and higher stiffness than the thinner plate 101, are arranged one above the other. As shown, the workpiece 100 is clamped between a movable electrode 111 and a stationary electrode 112 in such a state that the plates are in close contact with each other without a gap between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103. Current supply by a power supply 113 results in a substantially uniform current density over a current path between the movable electrode 111 and the stationary electrode 112.As a result, a satisfactory weld lens is formed in a region extending from the thinner plate 101 to the second thicker plate 103, thereby achieving the desired weld strength.

[0005] In fact, however, when the workpiece 100 is clamped and pressed by the movable electrode 111 and the stationary electrode 112, the thinner plate 101, which has a lower stiffness, and the first thicker plate 102 are bent upwards, leaving gaps between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103.

[0006] In this case, the contact area between the movable electrode 111 and the thinner plate 101 is increased as a result of the deflection of the thinner plate 101, while the contact areas at the connections between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103 are reduced by the gap.

[0007] As a result, the current density between the movable electrode 111 and the stationary electrode 112 is higher on the side of the second thicker plate 103 than on the side of the thinner plate 101, and the amount of local heat generated between the first thicker plate 102 and the second thicker plate 103 is greater than that generated between the thinner plate 101 and the first thicker plate 102.

[0008] Accordingly, a weld lens 105 is formed first at the connection between the first thicker plate 102 and the second thicker plate 103, as shown in Fig. 13A is shown, and the weld lens 105 gradually grows to weld the thinner plate 101 and the first thicker plate 102, as shown in Fig. Figure 13B shows the extent of penetration by the first, thicker plate 102 into the thinner plate 101. However, the weld strength is small, and the weld strength is unstable. Therefore, there is a risk that the thinner plate 101 will detach. Furthermore, the weld quality is variable. More precisely, this defect is more likely with increasing thickness of the first, thicker plate 102 and the second, thicker plate 103, as it becomes more difficult for the weld nugget 105 to reach between the first, thicker plate 102 and the thinner plate 101.

[0009] Another reason for the low penetration between the thinner plate 101 and the first thicker plate 102 and the resulting instability of the weld strength is the fact that, due to the small thickness of the thinner plate 101, heat is drawn into the movable electrode 111 when the movable electrode 111 is in contact with the thinner plate 101, with the result that the temperature on the side of the thinner plate 101 is not increased, thus preventing the formation of the weld nugget 105.

[0010] An exemplary mitigation of this problem is a spot welding process disclosed in the Japanese unexamined patent application JP 2003-251469A. According to this spot welding process, as in Fig. Figure 14 shows a workpiece 100 in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are arranged one on top of the other, spot-welded in such a way that the welding pressure (FU) of a movable electrode 125, which is arranged on the side of the thinner plate 101, is controlled such that it is less than the welding pressure (FL) of a stationary electrode 124, which is arranged on the side of the second thicker plate 103, thereby increasing the contact resistance at the connection between the thinner plate 101 and the first thicker plate 102, while decreasing the contact resistance at the connection between the first thicker plate 102 and the second thicker plate 103.In this way, an electric current between the movable electrode 125 and the stationary electrode 124 leads to an increased level of heat, which is generated at the connection between the thinner plate 101 and the first thicker plate 102. Thus, the weld strength between the thinner plate 101 and the first thicker plate 102 can be increased.

[0011] Fig. Figure 15 shows a setup of a spot welding gun used to perform the above procedure. As shown, a spot welding device 120 is attached to a wrist part 116 of a welding robot 115. The welding robot 115 is configured to spot weld a workpiece 100 by guiding the spot welding gun 120 to each of the welding points on the workpiece 100, which is held by a clamping device 118.

[0012] The spot welding gun 120 has a base unit 122, which is mounted vertically for movement by means of a linear guide 121. The guide guide is fixed to a gun holder 117, which is attached to the wrist part 116. The base unit 122 is provided with a C-shaped support yoke 123 that extends downwards from the base unit. A stationary electrode 124 is arranged at the lower end of the C-shaped support yoke 123.

[0013] A pressure generator 126, for example a servo motor, is attached to the upper end of the base unit 122. The movable electrode 125, opposite the stationary electrode 124, is attached to the lower end of a rod 127, which is vertically movable by means of the pressure generator 126. A servo motor 128 is mounted at the upper end of the gripper holding device 117. The base unit 122 is vertically movable by means of a worm drive mechanism driven by the servo motor 128.

[0014] According to learning data stored in a control unit which is not shown, the welding pressure (FU) of the movable electrode 125, which is located on the side of the thinner plate 101, is controlled to be smaller than the welding pressure (FL) of the stationary electrode 124 (FU < FL).

[0015] To ensure that the welding pressure (FU) of the movable electrode 125 is lower than the welding pressure (FL) of the stationary electrode 124 (FU < FL), the control unit first activates the servomotor 128 to raise the base unit 122, bringing the stationary electrode 124 into contact with the lower surface of the workpiece 100. Simultaneously, the control unit activates the pressure generator 126 to lower the movable electrode 125 so that it comes into contact with the upper surface of the workpiece 100. In this case, the welding pressure exerted by the pressure generator 126 acts uniformly on the movable electrode 125 and the stationary electrode 124 via the base unit 122 and the C-shaped support yoke 123.

[0016] Next, the base unit 122 is raised by the servomotor 128. Raising the base unit 122 increases the welding pressure (FL) of the stationary electrode 124. As a result, the welding pressure (FU) of the movable electrode 125 becomes lower than the welding pressure (FL) of the stationary electrode 124 (FU < FL).

[0017] Accordingly, when current flows between the movable electrode 125 and the stationary electrode 124, a high current density is achieved at the junction between the thinner plate 101 and the first thicker plate 102, and the amount of heat generated at the junction is increased relative to the amount of heat generated at the junction between the first thicker plate 102 and the second thicker plate 103. Thus, a uniform and satisfactory weld nugget is formed in a region extending from the thinner plate 101 to the second thicker plate 103, and high weld strength is ensured.

[0018] According to the Japanese unexamined patent application JP 2003 - 251 469 A, the spot welding gun 120 is moved to each of the welding points on the workpiece 100, which is held by the clamping device 118. The stationary electrode 124 is brought into contact with the second, thicker plate 103 of the workpiece 100, and the movable electrode 125 is brought into contact with the thinner plate 101. The base unit 122 is raised to control the welding pressure (FU) of the movable electrode 125 so that it is lower than the welding pressure (FL) of the stationary electrode 124. In this way, the current density between the thinner plate 101 and the first, thicker plate 102 is relatively increased to ensure that a suitable amount of heat is generated at the junction between the thinner plate 101 and the first, thicker plate 102. The degree of penetration is thus increased, and the weld strength is improved.

[0019] However, it is necessary that the clamping device 118, which clamps the workpiece 100, withstands a heavy load when the welding pressure (FU) of the movable electrode 125 is controlled to be lower than the welding pressure (FL) of the stationary electrode 124 by moving the base unit 122, while the workpiece 100, clamped by the clamping device 118, is clamped between the stationary electrode 124 and the movable electrode 125. If the clamping position of the workpiece 100 by the clamping device 118 is very far from the welding position, i.e., the position of a weld point, the workpiece 100 will be deformed, causing variations in the welding pressure (FL) of the stationary electrode 124 and the welding pressure (FU) of the movable electrode 125.As a result, it is difficult to reliably ensure a suitable contact resistance between the thinner plate 101 and the first thicker plate 102, and between the first thicker plate 102 and the second thicker plate 103. This creates a risk that the current density at the joints in the workpiece 100 will not be uniform, thus reducing the quality of the spot weld. Furthermore, the above method does not allow the use of a spot welding gun that incorporates a compensating function between the robot wrist and the base unit to accommodate movements that utilize a reaction force generated by the welding pressure. Therefore, the available spot welding guns are limited.

[0020] JP H06 - 15 880 U describes a spot welding device for a welding adhesive with a hollow, column-shaped crimp guide.

[0021] DE 11 2010 002 236 B4 describes a resistance welding process for resistance welding a stacked assembly of at least three workpieces, containing a thinnest workpiece of the smallest thickness, which is arranged on an outermost side of the stacked assembly.

[0022] In view of the problems described above according to the prior art, the invention is based on the objective of providing a spot welding device and a spot welding process with which a workpiece in which a thicker plate and a thinner plate with different stiffnesses are arranged one above the other can be spot welded with stable weld quality.

[0023] This problem is solved according to one aspect of the invention by a spot welding device for spot welding a workpiece in which a thinner plate with low stiffness and a first thicker plate and a second thicker plate, which have higher stiffness and a greater thickness than the thinner plate, are arranged sequentially one above the other, wherein the device has a base unit, a receiving unit held by the base unit and a first welding electrode and a second welding electrode, which are held by the base unit and are movable towards each other and away from each other in a manner directed towards each other.The workpiece is optionally clamped between a combination of the second welding electrode, which rests against the thinner plate, and the holding unit, which is next to the second welding electrode and rests against the thinner plate, and the first welding electrode, which rests against the second thicker plate, whereby pressure is applied to the workpiece by the first welding electrode simultaneously with the clamping and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.The workpiece is further optionally clamped between a combination of the first welding electrode, which rests against the thinner plate, and the holding unit, which is arranged next to the first welding electrode and rests against the thinner plate, and the second welding electrode, which rests against the second thicker plate, whereby pressure is exerted on the workpiece by means of the second welding electrode simultaneously with the clamping and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.

[0024] According to the above device, the workpiece is clamped between a combination of the second welding electrode and the holding unit, which rest against the thinner plate, and the first welding electrode, which rests against the second, thicker plate. Pressure is applied to the workpiece by means of the first welding electrode. The welding pressure from the first welding electrode is applied to the second, thicker plate of the workpiece, and the welding pressure from the second welding electrode and the holding unit is applied to the thinner plate. As a result, the welding pressure of the second welding electrode, which is located on the side of the thinner plate, is less than the welding pressure of the first welding electrode, which is located on the side of the second, thicker plate.As a result, when a current is passed between the first and second welding electrodes, the current density at the junction between the thinner plate and the first, thicker plate is relatively increased, and a satisfactory weld nugget is formed in a region extending from the thinner plate to the second, thicker plate, with uniform penetration. Similarly, the workpiece is clamped between a combination of the first welding electrode and the receiving unit, which rests against the thinner plate, and the second welding electrode, which rests against the second, thicker plate, and pressure is exerted on the workpiece by the second welding electrode.The welding pressure from the second welding electrode is applied to the second, thicker plate of the workpiece, while the welding pressure from the first welding electrode and the receiving unit are applied to the thinner plate. As a result, the welding pressure from the first electrode, located on the side of the thinner plate, is lower than the welding pressure from the second electrode, which is positioned on the side of the second, thicker plate. Consequently, when current is passed between the first and second electrodes, the current density at the junction between the thinner and thicker plates is relatively increased, forming a satisfactory weld nugget in a region extending from the thinner plate to the thicker plate, with uniform penetration. This improves the weld quality of the workpiece.

[0025] Furthermore, the device can continuously spot weld workpieces with a thinner plate, a first thicker plate, and a second thicker plate in different arrangements without requiring significant changes to the welding device's position. For example, a three-layer workpiece in which a thinner plate, a first thicker plate, and a second thicker plate are arranged sequentially from the bottom up, and a three-layer workpiece in which a thinner plate, a first thicker plate, and a second thicker plate are arranged sequentially from the top up, can be continuously spot welded without significant changes to the welding device's position.

[0026] The above problem is solved according to another aspect of the invention by a spot welding device for spot welding a workpiece, in which a thinner plate and a first thicker plate and a second thicker plate, which have a higher stiffness than the thinner plate, are arranged sequentially one above the other, the device comprising a base unit, a receiving unit held by the base unit, a first welding electrode which is movable to both a retracted position, a first pressing position and a second pressing position by means of a first pressure generator which is held by the base unit, and a second welding electrode which is movable to both a retracted position, a first pressing position and a second pressing position in an arrangement opposite to the first welding electrode by means of a second pressure generator which is held by the base unit.The workpiece is clamped between a combination of the second welding electrode, which is positioned in the first pressing position, and the receiving unit, which lies next to the second welding electrode and rests against the thinner plate, and the first welding electrode is positioned in the first pressing position, with pressure being applied to the workpiece from the first welding electrode simultaneously with the clamping, and a current being passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.The workpiece is clamped between a combination of the first welding electrode, which is positioned in the second press position, and the receiving unit, which is located next to the first welding electrode and rests against the thinner plate, and the second welding electrode, which is positioned in the second press position, whereby pressure is applied to the workpiece by means of the second welding electrode simultaneously with the clamping, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.

[0027] According to the above device, the workpiece is clamped between a combination of the second welding electrode in the first pressing position and the holding unit, which rest against the thinner plate, and the first welding electrode in the first pressing position, which rests against the second, thicker plate. Pressure is applied to the workpiece by means of the first welding electrode in the first pressing position. The welding pressure from the first welding electrode in the first pressing position is applied to the second, thicker plate of the workpiece, and the welding pressure from the second welding electrode and that from the holding unit is applied to the thinner plate. As a result, the welding pressure of the second welding electrode, which is located on the side of the thinner plate, is less than the welding pressure of the first welding electrode, which is positioned on the side of the second, thicker plate.As a result, when a current is passed between the first and second welding electrodes, the current density at the junction between the thinner plate and the first, thicker plate is relatively increased, and a satisfactory weld nugget is formed in a region extending from the thinner plate to the second, thicker plate, with uniform penetration. Similarly, the workpiece is clamped between a combination of the first welding electrode in the second clamping position and the receiving unit, which rests against the thinner plate, and the second welding electrode in the second clamping position, which rests against the second, thicker plate. Pressure is then applied to the workpiece by means of the second welding electrode in the second clamping position.The welding pressure from the second welding electrode is applied to the second, thicker plate of the workpiece, while the welding pressure from the first welding electrode and the receiving unit are applied to the thinner plate. As a result, the welding pressure from the first electrode, located on the side of the thinner plate, is lower than the welding pressure from the second electrode, which is positioned on the side of the second, thicker plate. Consequently, when current is passed between the first and second electrodes, the current density at the junction between the thinner and thicker plates is relatively increased, forming a satisfactory weld nugget in the region extending from the thinner plate to the thicker plate with uniform penetration. This improves the weld quality of the workpiece.

[0028] The above problem is solved according to another aspect of the invention by a spot welding device for spot welding a workpiece in which a plurality of thicker plates are arranged one above the other and a thinner plate and the thicker plates are arranged one above the other, wherein the device comprises a base unit, a receiving unit which is held by the base unit and can rest against the thinner plate, a first welding electrode which is movable relative to the receiving unit by means of a first pressure generator which is held by the base unit, and a second welding electrode which is movable relative to the receiving unit by means of a second pressure generator which is held by the base unit.The workpiece is clamped under pressure by means of the holding unit, the first welding electrode and the second welding electrode, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.

[0029] According to the above device, the workpiece is clamped under pressure by the receiving unit, which rests against the thinner plate, the first welding electrode, which can be moved by the first pressure generator, and the second welding electrode, which can be moved by the second pressure generator, in such a way that the welding pressure applied to a pair consisting of the thinner plate and the adjacent thicker plate is less than the welding pressure applied to each pair of thicker plates. As a result, when a current is passed between the first and second welding electrodes, the current density at the junction between the thinner plate and the adjacent thicker plate is increased, and a satisfactory weld nugget is formed in a region extending from the thinner plate to the thicker plates, with uniform penetration.The welding quality of the welded workpiece can thus be improved.

[0030] The above problem is solved according to another aspect of the invention by a spot welding method for spot welding a workpiece in which a plurality of thicker plates are arranged one above the other and a thinner plate is arranged with the thicker plates above the other, wherein the thinner plate is caused to rest against a receiving unit which is held by a base unit, and the workpiece is clamped under pressure between a first welding electrode, which is movable by means of a first pressure generator which is held by the base unit, and a second welding electrode, which is movable by means of a second pressure generator which is held by the base unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure in order to spot weld the workpiece.

[0031] According to the above procedure, the workpiece is clamped under pressure by means of the holding unit, which rests against the thinner plate, the first welding electrode, which can be moved by the first pressure generator, and the second welding electrode, which can be moved by the second pressure generator, in such a way that the welding pressure applied to a pair consisting of the thinner plate and the adjacent thicker plate is less than the welding pressure applied to each pair of thicker plates. As a result, when a current is passed between the first and second welding electrodes, the current density at the junction between the thinner plate and the adjacent thicker plate is increased, and a satisfactory weld nugget is formed in a region extending from the thinner plate to the thicker plates, with uniform penetration. The weld quality of the welded workpiece can thus be improved.

[0032] According to the invention, the workpiece is clamped between a combination of the second welding electrode and the receiving unit, which rests against the thinner plate, and the first welding electrode, which rests against the second, thicker plate. Pressure is exerted on the workpiece by means of the first welding electrode, resulting in the welding pressure of the second welding electrode, which is located on the side of the thinner plate, being less than the welding pressure of the first welding electrode, which is positioned on the side of the second, thicker plate. Consequently, when a current is passed between the first and second welding electrodes, a satisfactory weld nugget is formed in a region extending from the thinner plate to the second, thicker plate, with uniform penetration.Similarly, the workpiece is clamped between a combination of the first welding electrode and the holding unit, which rests against the thinner plate, and the second welding electrode, which rests against the second, thicker plate. Pressure is applied to the workpiece by means of the second welding electrode, with the welding pressure of the first electrode, located on the side of the thinner plate, being less than the welding pressure of the second electrode, which is positioned on the side of the second, thicker plate. As a result, when a current is passed between the first and second welding electrodes, a satisfactory weld nugget forms in the region extending from the thinner plate to the second, thicker plate, with uniform penetration. The weld quality of the welded workpiece can thus be improved.

[0033] The invention will be further illustrated below with reference to exemplary embodiments and the drawings in which Fig. 1 a view illustrating the structure of a spot welding device according to an embodiment of the invention, Fig. 2 a view of a main section in the direction of arrow II in Fig. 1 is, Fig. 3 is an exemplary view of the operation of a spot welding device, Fig. 4 An exemplary cross-sectional view for an operation of a main section in Fig. 3 is, Fig. 5 is an exemplary view of an operation of the spot welding device, Fig. 6 An exemplary sectional view for an operation of a main section in Fig. 5 is, Fig. Figure 7 is an exemplary schematic view of the operation of the spot welding device. Fig. Figure 8 is an exemplary view of an operation of the spot welding device. Fig. 9 An exemplary sectional view for an operation of a main section in Fig. 8 is, Fig. 10 is an exemplary view of an operation of the spot welding device, Fig. 11 An exemplary sectional view for an operation of a main section in Fig. 10 is, Fig. Figure 12 is an exemplary schematic view of the operation of the spot welding device. Fig. Figure 13A is a view that illustrates familiar spot welding, Fig. Figure 13B is a view illustrating familiar spot welding, Fig. 14 is a view that illustrates familiar spot welding, and Fig. Figure 15 is a view that illustrates familiar spot welding.

[0034] An embodiment of the present invention is described below with reference to the Fig. 1 to 12 described.

[0035] Fig. Figure 1 is a view that illustrates the setup of a spot welding device, and Fig. Figure 2 is a view of a main section, made along arrow II in Fig. 1. For the purpose of describing the spot welding device, the directions upwards and downwards are defined as follows: Fig. 1 is designated as the upward direction and the downward direction of the spot welding device.

[0036] Before describing the spot welding device 1, the workpiece 100 to be welded is first described with reference to Fig. 2 described. The workpiece 100 is a three-layer plate in which a thinner plate is arranged over one of two thicker plates arranged one above the other, for example a three-layer plate in which a thinner plate 101 with low stiffness and a small thickness and a first thicker plate 102 and a second thicker plate 103, which have a greater thickness and a higher stiffness than the thinner plate 101, are arranged sequentially one above the other.

[0037] As in the Fig. 1 and Fig. As shown in Figure 2, the spot welding device 1 is held by a compensating unit 2 via a wrist-type component attached to the tip of a welding robot, for example, a multi-joint robot, and which can be moved in three dimensions by the welding robot. The welding robot moves the spot welding device 1 sequentially to each of the predetermined welding locations on the workpiece 100, which is held in a predetermined position by means of an element such as a clamping device (not shown), in order to perform the spot welding of the workpiece 100.

[0038] The spot welding device 1 has a base unit 3. The base unit 3 is composed of a fixture mounting bracket 4, which is attached to the wrist part of the welding robot via the compensation unit 2, and clamping jaws 5, which are integrally attached to the fixture mounting bracket 4. The fixture mounting bracket 4 has a U-shaped cross-section.

[0039] This cross-section includes a mounting base section 4A, which is a rectangular flat plate connected to the compensating unit 2, and a pair of opposing clamp mounting sections 4B that extend downwards in an arc from both sides of the mounting base section 4A.

[0040] The clamping jaws 5 are mounted such that they face the corresponding clamping jaw mounting sections 4B of the device mounting bracket 4. Each clamping jaw 5 is an essentially T-shaped plate with a mounting body 6, which is a right-angled plate extending in one direction to and attached to the clamping jaw mounting section 4B, and a mounting arm mounting section 7 extending downward from the mounting body 6, and which are essentially integrally formed.

[0041] A fixing arm 10, a first pressure generator 20, a second pressure generator 30 and a welding transformer 40 are attached and held by means of the opposing clamp holders 5.

[0042] The fixing arm 10 is an essentially L-shaped element comprising a fixing arm body 11 with an essentially U-shaped cross-section extending downwards as a result of a rear end section 11a suspended between and connected to the ends of the fixing arm retaining sections 7 of the clamp holders 5, and further comprising a retaining section 12 extending arcuately from the tip of the retaining arm body 11. A receiving unit 13 is formed at a tip 12a of the retaining section 12. The receiving unit 13 defines a tube having a central axis extending in a vertical direction and an upper end 13a and a lower end 13b projecting from the retaining section 12.

[0043] The first pressure generator 20 comprises a cylinder device or a servo motor, in this embodiment a servo motor 21, and a direct drive unit 22, which is represented by a worm drive mechanism or the like. A rod 23 of the direct drive unit 22 is raised or lowered by means of a drive from the servo motor 21. A first electrode arm 24 is provided at the lower end of the rod 23 of the direct drive unit 22. A first welding electrode 25 is attached to the tip of the first electrode arm 24, such that it is in a coaxial relationship with the central axis of the receiving unit 13 of the fixing arm 10.According to this configuration, an upper end 25a of the first welding electrode 25 can be moved by driving the servomotor 21 of the first pressure generator 20 between a retracted position, in which the upper end 25a is raised furthest from the upper end 13a of the receiving unit 13, a first pressing position, in which the upper end 25a clamps the workpiece 100 in conjunction with the upper end 13a of the receiving unit 13 and exerts a welding pressure on the workpiece 100, and a second pressing position, in which the upper end 25a penetrates through the receiving unit 13 to reach the position of the lower end 13b of the receiving unit 13.

[0044] The second pressure generator 30 comprises a cylinder device or a servo motor, in this embodiment a servo motor 31, and a direct drive unit 32, which is represented by a worm drive mechanism or the like. A rod 33 of the direct drive unit 32 is raised or lowered by means of a drive of the servo motor 31. At the lower end of the rod 33 of the direct drive unit 32, an L-shaped second electrode arm 34 is provided, which has a straight base section 34A and a curved electrode holding section 34B, which is formed at the lower end of the base section 34A. A second welding electrode 35 is attached to the electrode holding section 34B of the second electrode arm 34 such that an upper end 35a of the second welding electrode 35 has a coaxial relationship with respect to the central axis of the receiving unit 13 of the fixing arm 10.Furthermore, an electrode arm sliding element 36 is provided on the mounting arm holding section 7 of the clamp holder 5. The electrode arm sliding element 36 guides the movement of the second electrode arm 34 by being in sliding contact with the base section 34A of the second electrode arm 34.

[0045] According to this configuration, the second welding electrode 35 can be moved by driving the servomotor 31 of the second pressure generator 30 between a retracted position, in which the second welding electrode 35 is lowered furthest from the lower end 13b of the receiving unit 13, a second pressing position, in which the second welding electrode 35 clamps the workpiece 100 in conjunction with the lower end 13b of the receiving unit 13 and the first welding electrode 25 in the second pressing position, and applies a welding pressure to the workpiece 100, and a first pressing position, in which the second welding electrode 35 penetrates through the receiving unit 13 to reach the position of the upper end 13a of the receiving unit 13 and clamps the workpiece 100 in combination with the upper end 13a of the receiving unit 13 and in conjunction with the first welding electrode 25 in the first pressing position.

[0046] An output terminal 41 of the welding transformer 40, which serves as a power source, is conductively connected to the first welding electrode 25 via, for example, a power terminal 43 and the first electrode arm 24. An output terminal 42 of the welding transformer 40 is conductively connected to the second welding electrode 35 via, for example, a power terminal 44 and the second electrode arm 34.

[0047] A welding robot control unit RC, not shown, stores learning data for the welding robot. This learning data includes operating programs for the sequential spot welding of the weld points on workpiece 100, as well as data indicating the position and orientation of the spot welding device 1 at each of the weld points, namely the welding zones. A welding device control unit WC, also not shown, stores operating programs for the welding device 1, as well as data for controlling the operation of the first pressure generator 20, the second pressure generator 30, and the welding transformer 40.

[0048] Next, the functions of spot welding device 1 will be described. For clarity, a first welding step will be described with reference to the Fig. Figures 3 to 7, which show spot welding of a workpiece 100, which is a three-layer plate in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are arranged one above the other in this form from below, and a second welding step is subsequently described with reference to the Fig. Figures 8 to 12 illustrate the spot welding of a workpiece 100, which is a three-layer plate in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are arranged one above the other in this order from top to bottom.

[0049] In the first welding step, in which a workpiece 100, comprising a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 arranged one above the other in this order from bottom to top, is spot welded, a previously defined operating program is executed, whereby the first welding electrode 25 is held in the uppermost position, namely the retracted position, and the rotation of the servomotor 31 of the second pressure generator 30 is controlled such that the second welding electrode 35 is moved to the first press position, in which the tip end 35a of the second welding electrode 35 is at the same height as the upper end 13a of the receiving unit 13.

[0050] In this state, where the first welding electrode 25 and the second welding electrode 35 are separated, namely the first welding electrode 25 is in the retracted position and the second welding electrode 35 is in the first pressing position, the welding robot control unit RC actuates the welding robot according to the predetermined program, thereby moving the spot welding device 1 to a welding point on the workpiece 100 such that the upper end 13a of the receiving unit 13 is positioned in the fixing arm 10 and the upper end 35a of the second welding electrode 35 is positioned so that it rests against the lower surface of the workpiece 100, namely the thinner plate 101, as shown in Fig. 3 shown.

[0051] In this state, in which the spot welding device 1 is positioned in the welding position, as shown in Fig. As shown in Figure 4, the lower surface of the thinner plate 101 of the workpiece 100 is in contact with the upper end 35a of the second welding electrode 35 and the upper end 13a of the receiving unit 13, which defines a ring near the upper end 35a of the second welding electrode 35. On the other hand, the tip end 25a of the first welding electrode 25 faces the second, thicker plate 103 with a gap between them.

[0052] Next, as in the Fig. As shown in Figures 5 to 7, the servomotor 21 of the first pressure generator 20 is controlled, thereby moving the first welding electrode 25 from the retracted position to the first pressing position to be in pressure contact with the second, thicker plate 103, while the upper end 13a of the receiving unit 13 and the upper end 35a of the second welding electrode 35 rest against the thinner plate 101 of the workpiece 100. In this way, the welding zone of the workpiece 100 is clamped under pressure between the upper end 35a of the second welding electrode 35 and the upper end 25a of the first welding electrode 25.

[0053] In the above state, where the first welding electrode 25 exerts welding pressure on the second, thicker plate 103, while the upper end 35a of the second welding electrode 35 and the upper end 13a of the receiving unit 13 are in contact with the thinner plate 101 of the workpiece 100, welding pressure is applied downwards from the first welding electrode 25 to the second, thicker plate 103 via, for example, the first electrode arm 24. Furthermore, welding pressure is applied upwards from the second welding electrode 35 to the thinner plate 101 via, for example, the base unit 3 and the second electrode arm 34, and is also applied upwards from the receiving unit 13, located next to the second welding electrode 35, to the thinner plate 101 via the fixing arm 10. This process is shown schematically in Fig. 7 shown.

[0054] In the above case, the welding pressure of the first pressure generator 20 acts on the first welding electrode 25 via, for example, the first electrode arm 24, as well as on the second welding electrode 35 via, for example, the base unit 3 and the second electrode arm 34, and on the receiving unit 13 via the fixing arm 10. Here, the welding pressure FU exerted by the first welding electrode 25 on the second, thicker plate 103 is equal to the sum of the welding pressure FL and the welding pressure Fα exerted on the thinner plate 101 by the second welding electrode 35 and the receiving unit 13, respectively (FU = FL + Fα).

[0055] In this way, the workpiece 100 is stably clamped by the welding pressure FU, which is applied downwards to the side of the second thicker plate 103 by the first welding electrode 25, and the welding pressure FL and the welding pressure Fα, which are applied upwards to the side of the thinner plate 101 by the second welding electrode 35 and the receiving unit 13 respectively.

[0056] As previously described, the welding pressure FU is applied by the first welding electrode 25 to the second, thicker plate 103, and the welding pressure FL and the welding pressure Fα are applied by the second welding electrode 35 and the receiving unit 13 to the thinner plate 101. In the welding zone of the workpiece 100, the welding pressure FL applied by the second welding electrode 35 to the thinner plate 101 has a magnitude obtained by subtracting the welding pressure Fα applied by the receiving unit 13 from the welding pressure FU of the first welding electrode 25 (FL = FU - Fα).

[0057] As previously described, the welding pressure FL of the second welding electrode 35, which is positioned on the side of the thinner plate 101, is controlled such that it is lower than the welding pressure FU of the first welding electrode 25, which is positioned on the side of the second, thicker plate 103 (FL < FU). As a result, the contact pressure at the junction between the thinner plate 101 and the first, thicker plate 102 is lower than the contact pressure at the junction between the first, thicker plate 102 and the second, thicker plate 103. Thus, the contact resistance between the thinner plate 101 and the first, thicker plate 102 is relatively increased, and the contact resistance between the first, thicker plate 102 and the second, thicker plate 103 is relatively decreased.

[0058] Here, a case is considered in which the receiving unit 13 of the fixing arm 10 is not provided. When the first pressure generator 20 is actuated to bring the first welding electrode 25 into contact with the second, thicker plate 103, while the second welding electrode 35 rests against the thinner plate 101 of the workpiece 100, the welding zone of the workpiece 100 is clamped and pressed between the second welding electrode 35 and the first welding electrode 25. In this case, however, the welding pressure of the first pressure generator 20 acts uniformly on the first welding electrode 25 and the second welding electrode 35 via, for example, the second electrode arm 34. This means that the welding pressure FU exerted by the first welding electrode 25 on the second, thicker plate 103 is equal to the welding pressure FL applied by the second welding electrode 35 to the thinner plate 101.

[0059] Returning once again to the embodiment described so far, the first welding electrode 25 and the second welding electrode 35 are energized by the welding transformer 40 for a predetermined period of time in order to carry out a spot weld in the state in which the workpiece 100 is clamped under pressure by the first welding electrode 25 and a combination of the second welding electrode 35 and the receiving unit 13 in such a way that the welding pressure FL of the second welding electrode 35, which is positioned on the side of the thinner plate 101, is less than the welding pressure FU of the first welding electrode 25, which is positioned on the side of the second thicker plate 103.When the first welding electrode 25 and the second welding electrode 35 are energized, the contact resistance and current density at the junction between the thinner plate 101 and the first thicker plate 102 are relatively increased, while the contact resistance between the first thicker plate 102 and the second thicker plate 103 is kept low. As a result, the amount of heat generated at the junction between the thinner plate 101 and the first thicker plate 102 is relatively higher than the amount of heat generated at the junction between the first thicker plate 102 and the second thicker plate 103.A current thus flows with a uniform current density from the thinner plate 101 to the second thicker plate 103 and a satisfactory weld nugget is formed in a region extending from the thinner plate 101 to the second thicker plate 103, thereby ensuring a high weld strength of the thinner plate 101.

[0060] After the welding process is completed, the first pressure generator 20 is actuated to move the first welding electrode 25 from the first pressing position to the retracted position, thereby releasing the workpiece 100 from the clamping by the first welding electrode 25 and the combination of the second welding electrode 35 and the receiving unit 13.

[0061] Next, the second welding step will be described with reference to the Fig. 8 to 12. In this step, a workpiece 100 is spot-welded, which is a three-layer plate with a thinner plate 101, a first thicker plate 102 and a second thicker plate 103, which are arranged on top of each other in this order.

[0062] When spot welding the workpiece 100, which has the thinner plate 101, the first thicker plate 102 and the second thicker plate 103 arranged one above the other in this order from top to bottom, a previously defined operating program is executed, whereby the second welding electrode 35 is held in the lowest position, namely the retracted position, and the rotation of the servomotor 21 of the first pressure generator 20 is controlled so that the first welding electrode 25 is moved to the second press position, in which the upper end 25a of the first welding electrode 25 is at the same height as the lower end 13b of the receiving unit 13.

[0063] In this state, where the second welding electrode 35 and the first welding electrode 25 are far apart, that is, the second welding electrode 35 is in the retracted position and the first welding electrode 25 is in the second pressing position, the welding robot is actuated in accordance with the predetermined program, whereby the spot welding device 1 is moved to a welding point on the workpiece 100 such that the lower end 13b of the receiving unit 13 is brought into the fixing arm 10 and the upper end 25a of the first welding electrode 25 is brought into contact with the upper surface of the workpiece 100, namely the thinner plate 101, as shown in Fig. 8 shown.

[0064] In this state, in which the spot welding device 1 is positioned in the welding position, as shown in Fig. As shown in Figure 9, the upper surface of the thinner plate 101 of the workpiece 100 is in contact with the tip end, or upper end 25a, of the first welding electrode 25 and the lower end 13b of the receiving unit 13, which defines a ring near the upper end 25a of the first welding electrode 25. On the other hand, the tip end 35a of the second welding electrode 35 faces the second, thicker plate 103, with a gap between them.

[0065] Next, as in the Fig. 10 and Fig. Figure 11 shows that while the lower end 13b of the receiving unit 13 and the upper end 25a of the first welding electrode 25 are in contact with the thinner plate 101 of the workpiece 100, the servomotor 31 of the second pressure generator 30 is activated, thereby moving the second welding electrode 35 from the retracted position to the second pressing position in order to come into contact with the second thicker plate 103. In this way, the welding zone of the workpiece 100 is clamped under pressure between the upper end 25a of the first welding electrode 25 and the upper end 35a of the second welding electrode 35.

[0066] In the above state, where the second welding electrode 35 exerts welding pressure on the second, thicker plate 103, while the upper end 25a of the first welding electrode 25 and the lower end 13b of the receiving unit 13 are in contact with the thinner plate 101 of the workpiece 100, welding pressure is exerted upwards from the second welding electrode 35 onto the second, thicker plate 103 by the second pressure generator 30, for example, via the second electrode arm 34. Additionally, welding pressure is exerted downwards from the first welding electrode 25 onto the thinner plate 101 via, for example, the base unit 3 and the first electrode arm 24, and welding pressure is also exerted downwards from the receiving unit 13, which is located near the first welding electrode 25, onto the thinner plate 101 via the fixing arm 10. This process is shown schematically in Fig. 12 shown.

[0067] In the above case, the welding pressure of the second pressure generator 30 acts on the second welding electrode 35, for example, the second electrode arm 34, and also on the first welding electrode 25, for example, the base unit 3 and the first electrode arm 24, and the receiving unit 13 via the fixing arm 10. Here, the welding pressure FL exerted by the second welding electrode 35 on the second thicker plate 103 is equal to the sum of the welding pressure FU and the welding pressure Fα applied to the thinner plate 101 by the first welding electrode 25 and the receiving unit 13, respectively (FL = FU + Fα).

[0068] In the above manner, the workpiece 100 is stably clamped by the welding pressure FL, which is applied upwards to the side of the second thicker plate 103 via the second welding electrode 35, and the welding pressure FU and the welding pressure Fα, which are applied downwards to the side of the thinner plate 101 by the first welding electrode 25 and the receiving unit 13.

[0069] In the welding zone of the workpiece 100, the welding pressure FL is applied by the second welding electrode 35 to the second thicker plate 103, and the welding pressure FU is applied to the thinner plate 101 by the first welding electrode 25, which has a size obtained by subtracting the welding pressure Fα by the receiving unit 13 from the welding pressure FL of the second welding electrode 35 (FU = FL - Fα).

[0070] As previously described, the welding pressure FU of the first welding electrode 25, which is positioned on the side of the thinner plate 101, is controlled to be lower than the welding pressure FL of the second welding electrode 35, which is positioned on the side of the second, thicker plate 103 (FU < FL). As a result, the contact pressure at the junction between the thinner plate 101 and the first, thicker plate 102 is lower than the contact pressure at the junction between the first, thicker plate 102 and the second, thicker plate 103. Thus, the contact resistance between the thinner plate 101 and the first, thicker plate 102 is relatively increased, and the contact resistance between the first, thicker plate 102 and the second, thicker plate 103 is relatively decreased.

[0071] Next, the first welding electrode 25 and the second welding electrode 35 are energized by the welding transformer 40 for a specific duration to perform spot welding in the state in which the workpiece 100 is clamped under pressure by the second welding electrode 35 and a combination of the first welding electrode 25 and the receiving unit 13 in such a way that the welding pressure FU of the first welding electrode 25, which is positioned on the side of the thinner plate 101, is less than the welding pressure FL of the second welding electrode 35, which is positioned on the side of the second thicker plate 103.When the first welding electrode 25 and the second welding electrode 35 are energized, the contact resistance and current density at the junction between the thinner plate 101 and the first thicker plate 102 are relatively increased, while the contact resistance between the first thicker plate 102 and the second thicker plate 103 is kept low. As a result, the amount of heat generated at the junction between the thinner plate 101 and the first thicker plate 102 is relatively higher than the amount of heat generated at the junction between the first thicker plate 102 and the second thicker plate 103.Thus, current flows with a uniform current density from the thinner plate 101 to the second thicker plate 103, and a satisfactory weld nugget is formed in a region extending from the thinner plate 101 to the second thicker plate 103, thereby ensuring high weld strength of the thinner plate 101.

[0072] After welding is completed, the second pressure generator 30 is actuated so that the second welding electrode 35 is moved from the second pressing position to the retracted position, thereby releasing the workpiece 100 from the clamping by the second welding electrode 35 and a combination of the first welding electrode 25 and the receiving unit 13.

[0073] Next, the welding robot is activated to withdraw the spot welding device 1 from the welding point on the workpiece 100 and to move the spot welding device 1 to a next welding point on the workpiece 100.

[0074] According to the embodiment described above, the spot welding device 1 is configured to spot weld a workpiece 100, which is a three-layer plate. The thinner plate 101 has low stiffness, while the first, thicker plate 102 and the second, thicker plate 103, which have higher stiffness than the thinner plate 101, are stacked on top of each other. The spot welding device 1 comprises the receiving unit 13, which is attached to the base unit 3 via the fixing arm 10; the first welding electrode 25, which can be moved between the retracted position, the first pressing position, and the second pressing position by means of the first pressure generator 20; and the second welding electrode 35, which can be moved between the retracted position, the second pressing position, and the first pressing position by means of the second pressure generator 30.The spot welding device 1 is configured such that the workpiece 100 is clamped between a combination of the second welding electrode 35 in the first pressing position and the holding unit 13, and the first welding electrode 25 in the first pressing position, which faces the second welding electrode 35. As a result, the welding pressure FL and the welding pressure Fα are applied to the thinner plate 101 by means of the second welding electrode 35 and the holding unit 13, respectively, and the welding pressure FU is applied to the second, thicker plate 103 by the first welding electrode 25. In this way, the contact pressure at the connection between the thinner plate 101 and the first, thicker plate 102 is controlled so that it is lower than the contact pressure at the connection between the first, thicker plate 102 and the second, thicker plate 103.As a result, when the first welding electrode 25 and the second welding electrode 35 are energized, the current density at the junction between the thinner plate 101 and the first thicker plate 102 is relatively increased compared to the current density at the junction between the first thicker plate 102 and the second thicker plate 103. Therefore, a satisfactory weld nugget is formed in an expansion region extending from the thinner plate 101 to the second thicker plate 103, with uniform penetration, thus ensuring high weld strength of the thinner plate 101.The spot welding device 1 is configured to perform spot welding in such a way that the workpiece 100 is clamped between a combination of the first welding electrode 25 at the second press position and the holding unit 13, and the second welding electrode 35 at the second press position, with the result that the welding pressure FU and the welding pressure Fα are applied to the thinner plate 101 by the first welding electrode 25 and the holding unit 13, respectively, and the welding pressure FL is applied to the second, thicker plate 103 by the second welding electrode 35. In this way, the contact pressure at the connection between the thinner plate 101 and the first, thicker plate 102 is controlled to be lower than the contact pressure at the connection between the first, thicker plate 102 and the second, thicker plate 103.As a result, when the first welding electrode 25 and the second welding electrode 35 are energized, a satisfactory weld nugget is formed in an area extending from the thinner plate 101 to the second, thicker plate 103 with uniform penetration, thus ensuring high weld strength of the thinner plate 101. A stable weld quality can therefore be achieved. According to this configuration, the spot welding device 1 can continuously spot weld a workpiece 100, which has the thinner plate 101, the first thicker plate 102, and the second thicker plate 103 in different arrangements, without the need for major changes in the position of the welding device 1.

[0075] For example, a three-layer workpiece 100, in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are arranged sequentially on top of each other from below, and a three-layer workpiece 100, in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are arranged sequentially on top of each other from above, can be spot-welded continuously without a major change in the position of the welding device 1.

[0076] The scope of the present invention is not limited to the embodiment described above, and numerous modifications are possible within the scope of the invention. For example, the servomotors 21 and 31 used for the first print generator 20 and the second print generator 30 can be replaced by air cylinders. The direct drive units 22 and 32 can be replaced by a gear mechanism or the like instead of the worm drive mechanism used in the embodiment described above.

[0077] Furthermore, although the receiving unit 13 has been described as having a tubular shape with the upper end 13a and the lower end 13b, the shape can be changed to various shapes such as a partially broken tubular shape or a semicircular convex shape curved outwards from the holding section 12 of the fixing arm 10, in accordance with the shape or other conditions of the workpiece 100.

[0078] The workpiece 100 is not limited to a three-layer plate and can have four or more plates, as long as thicker plates and a thinner plate or thinner plates with different stiffnesses are arranged on top of each other.

Claims

[1] Spot welding device (1) for spot welding a workpiece (100) in which a thinner plate (101) and a first thicker plate (102) and a second thicker plate (103), which have a greater stiffness than the thinner plate (101), are arranged sequentially from above one above the other, wherein the device (1) comprises: a base unit (3), a receiving unit (13) held by the base unit (3), and a first welding electrode (25) and a second welding electrode (35) which are held by the base unit (3) and are movable towards each other and away from each other in a facing position, wherein the workpiece (100) is selectively clamped between a combination of the second welding electrode (35), which rests against the thinner plate (101), and the receiving unit (13), which is located near the second welding electrode (35) and rests against the thinner plate (101), and the first welding electrode (25), which rests against the second thicker plate (103), wherein pressure is applied to the workpiece (100) by the first welding electrode (25) simultaneously with the clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100), and a combination of the first welding electrode (25) which rests against the thinner plate (101), and the receiving unit (13) which is located near the first welding electrode (25) and rests against the thinner plate (101), and the second welding electrode (35) which rests against the second thicker plate (103), whereby pressure is applied to the workpiece (100) by the second welding electrode (35) simultaneously with clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100). [2] Spot welding device (1) for spot welding a workpiece (100) in which a thinner plate (101) and a first thicker plate (102) and a second thicker plate (103), which have a higher stiffness than the thinner plate (101), are arranged sequentially on top of each other, wherein the device (1) comprises: a base unit (3), a receiving unit (13) held by the base unit (3), a first welding electrode (25) which is movable into a retracted position, a first pressing position and a second pressing position by means of a first pressure generator (20) which is held by the base unit (3), and a second welding electrode (35) which is movable into a retracted position, a first pressing position and a second pressing position in a position facing the first welding electrode (25) by means of a second pressure generator (30) which is held by the base unit (3), wherein the workpiece (100) is selectively clamped between a combination of the second welding electrode (35), which is positioned at the first press position, and the receiving unit (13), which is located near the second welding electrode (35) and rests against the thinner plate (101), and the first welding electrode (25), which is positioned in the first pressing position, whereby pressure is applied to the workpiece (100) by the first welding electrode (25) simultaneously with the clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100) and (100) a combination of the first welding electrode (25) positioned in the second press position and the receiving unit (13) located near the first welding electrode (25) and resting against the thinner plate (101), and the second welding electrode (35), which is positioned at the second press position, wherein pressure is applied to the workpiece (100) by the second welding electrode (35) simultaneously with clamping and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100). [3] Spot welding device (1) for spot welding a workpiece (100) in which a plurality of thicker plates (102, 103) are arranged one above the other and a thinner plate (101) is arranged together with the thicker plates (102, 103) one above the other, wherein the device (1) comprises: a base unit (3), a receiving unit (13) which is held by the base unit (3) and is designed to rest against the thinner plate (101), a first welding electrode (25) which is movable relative to the receiving unit (13) by means of a first pressure generator (20) which is held by the base unit (3), and a second welding electrode (35) which is movable relative to the receiving unit (13) by means of a second pressure generator (30) which is held by the base unit (3), wherein the workpiece (100) is clamped under pressure by means of the receiving unit (13), the first welding electrode (25) and the second welding electrode (35) and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100). [4] Spot welding method for spot welding a workpiece (100) in which a plurality of thicker plates (102, 103) are arranged one above the other and a thinner plate (101) is arranged one above the thicker plates (102, 103) in the other, wherein the thinner plate (101) is caused to rest against a receiving unit (13) which is held by a base unit (3), and the workpiece (100) is clamped under pressure between a first welding electrode (25) which is movable by a first pressure generator which is held by the base unit (3), and a second welding electrode (35) which is movable by a second pressure generator (30) which is held by the base unit (3), and a current is passed between the first welding electrode (25) and the second welding electrode (35) while the workpiece (100) is clamped under pressure in order to spot weld the workpiece (100).

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