Laser assisted diamond turning of tungsten carbide and the material properties required to obtain optical surface finish suitable for lens molds

Abstract

Tungsten carbide is a material of interest to the optical molding industry because of its suitable thermal properties in molding at higher temperatures. Tungsten carbide is typically ground and polished as tool wear from conventional machining is too high to be feasible. Laser assisted machining developed through Micro-LAM has allowed direct machinability of this material. A machinability study was performed on five grades of tungsten carbide that have been specially developed for glass lens molds. The primary difference between the grades studied is the grain size. With advances in material technology, there is an ability to provide finer grain structures in binderless alloys of tungsten carbide. Standardized trials were then performed across the different grades to evaluate machinability and surface roughness using Laser Assisted Machining (LAM) on a Single Point Diamond Turning (SPDT) platform. The trials proved that there is a strong dependence and correlation of grain size versus final achievable surface finish after LAM turning. Larger grain materials have larger voids and gaps which may cause larger pull outs. These voids then must be polished in post-processing to get beneath the sub-surface damage which is a function of the void depth. Laser assisted machining of fine grain tungsten carbide can achieve a mirror-like surface finish suitable for optical molding applications with minimal post-polishing. Using this technology allows for producing tungsten carbide molds through a more deterministic process. Also, given the range of diamond tool sizes, this method is suitable for complex geometries such as those used in the molding of collimation optics for 5G applications or biomedical applications such as endoscopes.