Abstract: A model and a workpiece are rotated synchronously, and a stylus is disposed for relative movement toward or away from the model in a direction perpendicular to the axis of rotation thereof for contact therewith. The amount of displacement of the stylus from its neutral point which occurs while it contacts the model is detected by a tracer head and is divided into a pair of components lying in a plane perpendicular to the axis of rotation of the model which are along the direction of said relative movement and along a direction orthogonal thereto.