A spot welding machine is a mechanical device that uses the principle of double-sided, double-point overcurrent welding. During operation, two electrodes press the workpiece so that the two layers of metal form a certain contact resistance under the pressure of the two electrodes.
When the welding current flows from one electrode to the other electrode, it forms an instantaneous thermal fusion at the two contact resistance points. The welding current also flows instantaneously from the other electrode along the two workpieces to this electrode to form a circuit, without damaging the internal structure of the workpiece being welded.