High electrode force, current and heat deform the electrode contact surfaces with increasing number of welding spots. If the welding current is kept the same during production, the current density decreases and the welding quality deteriorates. In addition to geometric wear, the “alloying” between the electrode and the material/coating of the joining part must be taken into account. This usually results in poorly conducting and deformed contact surfaces. The attainable tool life depends very much on the material to be welded, type and material of the electrodes, cooling conditions, welding parameters, cycle time, etc.
As a rule, several 1000 points can be achieved (but in extreme cases also e.g. only 10). Countermeasures to compensate for electrode wear are indispensable for stable and quality-compliant production. In principle, the use of direct current is advantageous over alternating current. The direct current characteristic reduces electrode wear. In practice, the stepper function of the welding control is also used. This increases the welding current depending on the spot counter in order to achieve a constant current density. In addition, many automated applications involve the use of milling cutters.
Milling is usually used to keep the electrode diameter constant. There are also variants of the milling cutter where, in addition to the diameter of the electrodes, the electrode surface is also treated. Then the milling cutter compensates for both the enlargement and deformation of the electrode contact area and the alloying on the electrode surface. In addition, there are other forms of electrode machining such as cap rolling, but this will be dealt with in another section.