Experimental study on planetary electric discharge machining of substrate of large-aperture silicon carbide (SiC) mirrors

Abstract

Silicon carbide (SiC) is widely used for fabricating large-aperture mirrors because of its excellent physical properties, such as a high elastic modulus, low density, and low-thermal-expansion coefficient. However, SiC mirrors are difficult to process because of the high rigidity and brittleness of the material. Mechanical grinding is often adopted in conventional processes. However, as a better alternative, we apply electric discharge machining (EDM) to process mirror substrates. Planetary EDM was improved relative to conventional EDM by adopting arrayed electrodes, high-speed and high-pressure liquid, water-based working fluid, and planetary machining. Applying planetary EDM to process SiC substrates can also yield excellent results. Theoretical studies have attributed the quality of the finished surface to the number of discharge corrosion cells, discharge current, and electrode parameter. This finding can be verified experimentally based on the flatness, surface roughness, and material removal rate. The substrate of a silicon carbide mirror with an aperture of 1230 mm is selected as the object to be processed, and planetary EDM is adopted as the machining method. The finished surface of the substrate has a flatness of 0.0153 mm and roughness Ra of 5.119 μm, and both these values satisfy the requirements of the mirror substrate. Planetary EDM has high machining efficiency, low cost, and the method is easy to implement.