Design of a rotary fast tool servo for ophthalmic lens fabrication

We have developed a novel fast tool servo and associated prototype diamond- turning machine for the production of plastic spectacle lenses. Our fast tool servo carries the cutting tool on a rotary arm and, thus, on a circular path, as opposed to straight line paths in conventional designs. The actuator, sensors, and bearings are standard elements that together allow experimentally demonstrated 500 m/s² instantaneous accelerations at the tool tip over a 3-cm range of cutting depth. We also describe in this paper new approaches we have developed for toolpath generation and calibration. The paper also presents associated control algorithms, because the controller must supply very high dynamic stiffness to the tool servo axis at multiples of the spindle frequency. This stiffness is achieved by means of repetitive control techniques. The new fast tool servo is shown to have great promise for machining asymmetric surfaces with large amplitude asymmetries.