A novel machining approach of freeform multi-mirror mold via normal swing cutting

Abstract

Freeform optical elements have found wide applications in the fields of virtual /augmented reality (VR/AR), aerospace and biomedical engineering. Machining complex freeform surfaces on Nip mold with traditional Slow/Fast tool servo (STS/FTS) is difficult due to its large slope angles and multi-mirror structure. We developed a novel normal swing cutting method to solve these issues. The three translational motions were synchronized with the rotation of B axis to generate a normal swing cutting path for fabrication of freeform multi-mirror mold. The mathematical model of tool path for the normal swing cutting was established. A trial cutting process flow of spherical copper columns was carried out where the main error sources were identified and compensated before machining the freeform surface mold and the profile error of spherical copper columns after compensation machining was predicted accurately. To validate the effectiveness of the presented novel cutting method and trial cutting compensation, cutting experiments of a multi-mirror mold with freeform surface were conducted. As a result, the PV-values of the S1 and S2 freeform surface with error compensation machining were improved to 1.1 μm and 1.0 μm based on the on-machine measurement. The results were basically consistent with the surface errors obtained from off-machine measurements, which were 1.2 μm and 1.03 μm respectively. The surface roughness of three local areas was less than Sa 9 nm. This confirms the effectiveness of the developed novel machining methods for the complex freeform surface. Prospects of the new machining method were discussed to provide guidance to the future research.