High-performance grinding of a 2-m scale silicon carbide mirror blank for the space-based telescope

Silicon carbide (SiC) is a competitive candidate material for building the space-based reflecting mirrors. However, SiC is also a typical difficult-to-machine material due to its extreme hardness. When SiC workpiece is machined by grinding, the wheel wears rapidly which leads to a deterioration of surface form accuracy. Grinding efficiency also becomes extremely low due to the frequent truing and dressing of grinding wheels. To achieve high-performance grinding process capable of producing accurate surface at high grinding efficiency, an ultrasonic vibration-assisted fix-point grinding technology was developed in this study. Wheel wear observation indicated that the wheel needs not to be dressed during the whole grinding cycle. Moreover, a laser tracker was used to achieve an on-machine measurement of the surface form error. A CNC tool microset was adopted to evaluate the wheel wear amount. On the basis of the above two results, surface form error could be predicted before grinding, and thus, an in-process compensation of surface form error was carried out. Using the above-mentioned grinding strategies, a SiC mirror blank with an aperture diameter of 2 m was successfully ground to a form accuracy of 18 μm in peak-to-valley (PV). Therefore, this work provides an efficient and economical solution for grinding the large-scale SiC aspherical surfaces.