Generally, when spot welding with the same electrode on both sides of the weld, the resulting weld nugget is smaller than the electrode face diameter. To obtain a nugget larger than the electrode face diameter extra growth time and or heat is applied to accommodate additional growth. Pulsing is a common method used to do this.
Without some extra time or heat it is not common to see nuggets larger than the electrode face. That is not to say it is not impossible. If one works out a schedule that has the heat over a long enough period with sufficient heat to grow the nugget without creating expulsion – IT CAN BE DONE!
If your schedule is producing a nugget larger than the electrode face once in a while, it must be close or at the ideal condition for nugget growth. There are likely variations in the operation.
There could be facility variations causing pressure or voltage fluctuations that allow this growth to occur occasionally. A drop in pressure will create more heat at the interface. Are you losing air pressure when all the plant equipment is on at the same time? Do you need an air surge tank near the welder? Good or bad pressure, what is the condition of the cylinders? Are they in need of rebuild? Are the servos functioning properly?
A drop in voltage does the opposite. If the control compensates for voltage this is not an issue. Don’t forget that faulty cables or shunts can be the same as voltage drops. The schedule might normally produce the large nugget but because of bad cables be failing and usually producing a smaller nugget.
For proper maintenance a current meter and force gauge are highly recommended to know what the welding conditions are from day to day or shift to shift.
CURRENT METER DIGITAL FORCE GAUGE
Equipment maintenance and close observation of the schedule and electrodes will find a variable that is creating a change at the electrodes and the variation in the nugget size.
Reference: RWMA Resistance Welding Manual 4th Edition
AWS Standard: AWS J1.2 Guide to Installation and Maintenance of Resistance Welding Machines
Tuffaloy Resistance Welding Products Catalog