Abstract: A spin-welding apparatus utilizing a servo motor assembly has a closed-loop control circuit for rotating one part of the workpiece being welded relative to the other. In use, the servo motor immediately rotates the first part up to a set speed at which welding will occur, and a power cylinder moves the first part into frictional contact with the second part. Contact between the two parts is observed by a microprocessor interrogating a signal from the servo output corresponding to an increase in the load on the servo motor due to frictional drag between the parts, with "contact" being identified as surpassing a predetermined threshold error or variance value between the closed-loop system's input and output values. The microprocessor then counts a predetermined number of revolutions, and terminates rotation of the servo motor and first part.