Finishing of optical moulds to λ/20 by automated corrective polishing

Tungsten carbide is widely used to make replication moulds for small aspheric and freeform optics, like camera lenses. Since aspheric generation by grinding generally fails to reach ultra-precision criteria, a time efficient method for finishing moulds is needed. Using a 7-axis CNC polishing machine, an on-machine tool forming technique and compensation software were developed to correctively polish a plano WC surface of diameter 25 mm from a form error above λ to below λ/20 (29 nm P-V) in under 1 h, while reducing roughness to 1.0 nm Ra. This automatic method and tooling are applicable to aspheric and freeform moulds.