In spot welding, the electrodes wear out with increasing number of welds. Firstly, the electrode surfaces increase due to the constant striking and pressing of the electrodes onto the workpieces and, secondly, alloying occurs on the electrode surface, especially with coated materials. As a result, the current density is lower due to the larger surface area and the contact resistance is higher due to the alloys. In practice, this means that the welding spots are smaller. To achieve consistent welding quality, i.e. the same spot diameter, the welding current must be increased.
This can be done manually or automated with the help of the so-called stepper function. Based on the programmed basic current for new electrodes, the welding spot counter in the welding control increases the current according to the electrode wear at predefined counter values.
With older generation controllers, up to about the beginning / middle of the 90s, this was done step by step. When the pre-programmed counter value was reached, the current setpoint was increased by a likewise programmable value in per cent. The set current practically jumped up several times in the course of the electrode life. In the process, it moved from the adhesive limit to almost the splash limit. With modern controls, the setpoint is linearised between the individual support values and continuously ramped up.
This allows the natural wear process to be followed exactly and increases in tool life of up to 30 % with constant welding quality have been achieved. With both methods, it was of course necessary to determine the curves and thus the stepper values experimentally.
This is time-consuming and therefore very cost-intensive. In today’s automated manufacturing operations, the stepper function is used in conjunction with the capabilities of the cap cutters as electrode management. The welding control system can be used to conveniently organise milling, cap changing and the respective display with advance warning, etc. With the medium-frequency systems with integrated IQR control, cap wear is even automatically controlled today.