One example of the present disclosure relates to a method of processing first and second workpieces. The method includes constraining the second workpiece; abutting first and second workpieces together to form a joint; applying first and second selectively adjustable forces against the first workpiece to bias the first workpiece against the second workpiece; friction stir welding the first and second workpieces along the joint; maintaining the forces equal to each other while friction stir welding; and automatically varying at least one of the forces responsive to thermal size variations of the first and second workpieces. An apparatus for carrying out this method is also provided.