Fabrication of freeform progressive addition lenses using a self-developed long stroke fast tool servo

A long stroke fast tool servo (LFTS) system with a maximum stroke range of +/- 4 mm based on the voice coil actuator was developed and integrated on a diamond turning machine (DTM) for the fabrication of freeform progressive addition lenses (PALs). The LFTS utilized a three-ring cascade control system for the motion trajectory control of a diamond tool. The numerical control codes of the tool path were transferred from the 3D surface data with freeform features. The complete process chain from tool path generation and diamond machining to power measurement was experimentally demonstrated. The obtained surface form accuracy was 6.7 mu m (rms), and the measured power of the PAL agreed well with the theoretical design indicating the desirable processing capability of LFTS for the customized freeform spectacle optics.