Patent ID: 7145097

Claim:
A welding gun for spot welding a workpiece whereupon a weld nugget is formed that expands and then becomes plastic during the spot welding process, the welding gun comprising: two electrodes, at least one electrode being movable with respect to the other electrode; a power source operative to provide welding power to the electrodes for forming the weld nugget; an electric servo actuator operative to move the at least one movable electrode toward the other electrode such that the electrodes contact opposite sides of the workpiece, the actuator operative to apply a predetermined amount of force to the workpiece; a force sensor in communication with at least one of the two electrodes, the force sensor operative to detect an increase in force on the at least one electrode when the weld nugget expands and a decrease in force when the weld nugget becomes plastic, the force sensor further operative to produce a signal indicative of the increase and decrease in force; and a controller in communication with the force sensor and the power source, the controller operative to receive the signal from the force sensor and to cause the power source to stop providing welding power to the electrodes after the force sensor detects the decrease in force due to the weld nugget becoming plastic; wherein the gun does not include a brake operable to prevent the actuator from being back driven.