Spot welding apparatus and spot welding method

1. A spot welding apparatus for spot welding a workpiece in which a thinner plate, and a first thicker plate and a second thicker plate that have a higher rigidity than the thinner plate are sequentially stacked on top of one another, the apparatus comprising:

a base unit;

a first welding electrode and a second welding electrode that are supported by the base unit and are movable toward and away from each other in an opposing relationship; and

a receiving unit supported by a fixing arm which is arranged between the first welding electrode and the second welding electrode on the base unit, the receiving unit including an upper end and a lower end for clamping the workpiece,

wherein a central axis of the receiving unit is attached so as to be in a coaxial relationship with the first welding electrode and the second welding electrode,

wherein, in a case where the thinner plate is nearest the second welding electrode, the workpiece is clamped between a combination of the second welding electrode that abuts against the thinner plate and the upper end of the receiving unit that is adjacent to the second welding electrode and abuts against the thinner plate, and the first welding electrode that abuts against the second thicker plate, a pressure is applied to the workpiece only by the first welding electrode simultaneously with the clamping by the second welding electrode and the upper end of the receiving unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece, and

wherein, in a case where the thinner plate is nearest the first welding electrode, the workpiece is clamped between a combination of the first welding electrode that abuts against the thinner plate and the lower end of the receiving unit that is adjacent to the first welding electrode and abuts against the thinner plate, and the second welding electrode that abuts against the second thicker plate, a pressure is applied to the workpiece only by the second welding electrode simultaneously with the clamping by the first welding electrode and the lower end of the receiving unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece.

2. A spot welding apparatus for spot welding a workpiece in which a thinner plate, and a first thicker plate and a second thicker plate having a higher rigidity than the thinner plate are sequentially stacked on top of one another, the apparatus comprising:

a base unit;

a first welding electrode that is movable to any one of a retracted position, a first pressing position and a second pressing position by a first pressure actuator supported by the base unit;

a second welding electrode that is movable to any one of a retracted position, a first pressing position and a second pressing position in an opposing relationship with the first welding electrode by a second pressure actuator supported by the base unit; and

a receiving unit supported by a fixing arm which is arranged between the first welding electrode and the second welding electrode on the base unit, the receiving unit including an upper end and a lower end for clamping the workpiece,

wherein a central axis of the receiving unit is attached so as to be in a coaxial relationship with the first welding electrode and the second welding electrode,

wherein, in a case where the thinner plate is nearest the second welding electrode, the workpiece is clamped between a combination of the second welding electrode that is positioned at the first pressing position and the upper end of the receiving unit that is adjacent to the second welding electrode and abuts against the thinner plate, and the first welding electrode that is positioned at the first pressing position, a pressure is applied to the workpiece only by the first welding electrode simultaneously with the clamping by the second welding electrode and the upper end of the receiving unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece, and

wherein, in a case where the thinner plate is nearest the first welding electrode, the workpiece is clamped between a combination of the first welding electrode that is positioned at the second pressing position and the lower end of the receiving unit that is adjacent to the first welding electrode and abuts against the thinner plate, and the second welding electrode that is positioned at the second pressing position, a pressure is applied to the workpiece only by the second welding electrode simultaneously with the clamping by the second welding electrode and the lower end of the receiving unit, and a current is passed between the first welding electrode and the second welding electrode while the workpiece is clamped under pressure to spot weld the workpiece.

3. The spot welding apparatus according to claim 1 , wherein, in the case where the thinner plate is nearest the second welding electrode, a distal end surface of the second welding electrode and a distal end surface of the receiving unit are co-planar such that they simultaneously contact the thinner plate.

4. The spot welding apparatus according to claim 1 , wherein, in the case where the thinner plate is nearest the first welding electrode, a distal end surface of the first welding electrode and a distal end surface of the receiving unit are co-planar such that they simultaneously contact the thinner plate.

5. The spot welding apparatus according to claim 1 , wherein, in the case where the thinner plate is nearest the second welding electrode, a pressure applied by the first welding electrode to the second thicker plate is substantially equal to a sum of a pressure applied by the second welding electrode and a pressure applied by the receiving unit on the thinner plate.

6. The spot welding apparatus according to claim 1 , wherein, in the case where the thinner plate is nearest the first welding electrode, a pressure applied by the second welding electrode to the second thicker plate is substantially equal to a sum of a pressure applied by the first welding electrode and a pressure applied by the receiving unit on the thinner plate.

7. The spot welding apparatus according to claim 2 , wherein, in the case where the thinner plate is nearest the second welding electrode, a distal end surface of the second welding electrode and a distal end surface of the receiving unit are co-planar such that they simultaneously contact the thinner plate.

8. The spot welding apparatus according to claim 2 , wherein, in the case where the thinner plate is nearest the first welding electrode, a distal end surface of the first welding electrode and a distal end surface of the receiving unit are co-planar such that they simultaneously contact the thinner plate.

9. The spot welding apparatus according to claim 2 , wherein, in the case where the thinner plate is nearest the second welding electrode, a pressure applied by the first welding electrode to the second thicker plate is substantially equal to a sum of a pressure applied by the second welding electrode and a pressure applied by the receiving unit on the thinner plate.

10. The spot welding apparatus according to claim 2 , wherein, in the case where the thinner plate is nearest the first welding electrode, a pressure applied by the second welding electrode to the second thicker plate is substantially equal to a sum of a pressure applied by the first welding electrode and a pressure applied by the receiving unit on the thinner plate.