Abstract: A jig assembly for mounting a tool is disclosed in which the tool is held generally stationary in the jig. The jig assembly is used in a machining process in which the workpiece is mounted at the end of a movable robot arm and is manipulated with respect to the generally stationary machine tool. The machine tool, which may be a rotating tool, is rotated about its longitudinal axis and the workpiece is manipulated by the robot arm so as to contact the rotating rool thereby enabling it to perform its machining function. The use of this jig enables the application of a substantially constant force between the workpiece and the rotating tool in order to machine each of the workpieces to very small tolerances. The tool holder is attached to a balance beam having balance weights attached to both ends. The beam pivots about a generally horizontal pivot axis extending generally through its midpoint.