Resistance of the spot weld

THE OUTPUT of resistance-welding machines is generally determined by Joule´s law W=RI² where R is the resistance of the heated metal being welded and I is the current flowing during the welding operation. With the majority of machines, welding current is determined by the internal impedance of the machine, the resistance of the weld being but a small part of the impedance of the entire circuit.¹ Therefore the value of R is the only variable which can change the output at the electrodes of a given machine for a given internal impedance. Designers of welding machines are familiar with the value of R for different welding jobs,² however, there has been a lack of systematic study of this resistance. This paper indicates one method of evaluating spot weld resistance. Investigations were restricted to the most important case, the welding of the ordinary mild-steel sheets.