Rotary Fast-Tool Servo for Diamond Turning of Asymmetric Optics

Single-point diamond turning machines, such as lathes, are used to produce optically-clear parts. Conventional lathes carry the workpiece on a high-speed spindle and the tool on a slow axis allowing only rotationally symmetric shapes, such as spheres or paraboloids, to be machined. Asymmetric optics cannot be made using a conventional lathe, and are usually produced in a multi-step process of grinding and polishing. The diamond turning of asymmetric optics requires a fast-tool servo, which tracks a trajectory of higher frequency than the spindle RPM.

We have built a prototype turning machine specialized for the production of plastic spectacle lenses. This machine features a novel rotary fast-tool servo, which can track trajectories at frequencies up to 500 Hz, and accelerations of 500 m/s^2. Thus the machine can turn 100 mm diameter toric lenses having surface asymmetries of up to 3 cm in depth. The control system consists of a conventional inner position loop, feedforward filtering, and repetitive control. This high-bandwidth, high-accuracy system allows us to turn a toric surface with tracking errors of less than 2 microns.