Steppers are a means of changing the weld schedule to match the increase in the face size of the electrode as it mushrooms/wears during use. All spot-welding electrodes mushroom as they are used. As this face increases in size the original weld schedule is no longer sufficient to maintain a constant weld nugget size. The current density has decreased due to the increased electrode face area. To counteract this one can either dress the face back to original size or increase the weld schedule to match the face growth.
Today’s modern controls can be programed to match this growth. The programs are called stepper programs. Some step a little each weld. Others step a few amperes every fifty or one hundred welds. There are many variations with the same goal to match face growth with a relatively constant current density on the electrode face.
Development of the stepper schedule is frequently developed through trial and error. The goal is to watch the weld nugget change and adjust the schedule to compensate. After a modest number of trials one can develop the proper rate and frequency of current increase.
Steppers can be very successful in making a long production run without interruption during normal production hours. Ideally they will allow you to reach a break or lunch period to do maintenance when the interruption will not cause unscheduled down time.
Reference: RWMA - Resistance Welding Manual 4th Edition