Surface Finish and Diamond Tool Wear when machining PMMA and PC Optics

摘要

Optical devices are extremely important since they play a critical role in optical recording and display. Single point diamond turning is one of the most common methods to create plastic optics. Diamond turning of plastics is influenced by a wide variety of factors such as the glass transition temperature of the polymer, other material properties and operator controlled cutting conditions. Since diamond is one of the hardest materials in nature and polymers are relatively soft, little tool wear is expected. But the optics industry claims that tool wear is a major problem. Most of the optical industry uses Poly (methyl methacrylate) (PMMA) and Polycarbonate (PC) for creating optics. The objective of this research is to optimize machining parameters (such as feed, depth of cut, cutting speed and rake angle) to produce optical surface quality (RMS Surface finish ＜ 10 nm) while minimizing tool wear for the two materials. A wide range of experiments were performed on the two materials by varying machining parameters and measuring worn tools using the Electron Beam Induced Deposition (EBID) technique in the Scanning Electron Microscope (SEM). In the experimental conditions used, PMMA was found to have better surface finish than PC when machined with a zero-rake angle diamond tool. Polycarbonate was found to wear the tool more than PMMA under similar cutting conditions. It was also found that Polycarbonate is more sensitive to chip management and chip geometry than PMMA. Detailed effects of all machining parameters for the two materials were studied.