Objective   With the increasing requirement of satellite imaging quality, the aperture of the main mirror of the optical system is also increasing, and the large-aperture mirror must be designed with lightweight structure. In order to improve the processing efficiency, the main mirror needs to be roughed before polishing to remove the surface allowance during the sintering casting of the mirror embryo, but the ultra-thin and lightweight design of the mirror makes roughing very difficult. On the one hand, the ultra-thin mirror increases the brittleness of the mirror itself, and it is easy to crack the mirror surface by excessive stress during processing. On the other hand, the design of the lightweight hole on the back of the mirror will reduce the stiffness and natural frequency of the mirror, resulting in the mirror easily resonating with the processing system during the milling process, so that the mirror is destroyed. At present, manual grinding or ultrasonic vibration milling methods are generally used for rough machining, but these two processing methods have low surface removal efficiency and long processing cycle, which can't meet the needs of space optical systems for development cycle. Therefore, it is necessary to establish a milling method with higher surface removal efficiency and accuracy. For this purpose, an ultra-precision milling technology of large-aperture ultra-lightweight SiC mirror is established in this paper.

  Methods  A five-axis high-efficiency milling ultra-precision machining method was proposed by using finite element analysis. Through the analysis of the resonance mechanism in the milling process of the mirror, the causes of resonance were explained (Fig.4). The finite element analysis method was adopted to simulate and verify that the mirror would not be damaged and the system would not have resonance during the machining process (Fig.6-8). An ultra-lightweight SiC mirror was quickly milling (diameter is 510 mm, the wall thickness is 4 mm, the lightweight rate is 92%) with the support of designed ring tooling (Fig.10).

  Results and Discussions   The mirror is machined by means of parallel feed cutter, and the shape of the mirror is measured by interferometer. The initial Peak-to-Valley (PV) value of the mirror was 956.1 μm (Fig.11), the mirror removal amount is 1 mm, and the processing time is only 48 h, which is 90% lower than that of manual grinding. The PV value of surface is 3.5 μm (Fig.11), which meets the requirement that the PV value before mirror polishing should be better than 4 μm. The experimental results show that the scheme is feasible and can be used for high-efficiency and high-precision machining of large-caliber ultra-lightweight mirror

  Conclusions  Aiming at the problem that the surface roughness of large-caliber ultra-thin lightweight silicon carbide mirror is difficult and the cycle is long, a five-axis high-efficiency ultra-precision milling method is proposed. Using the milling method of parallel cutter feeding, the annular tool was used to assist the edge support of the mirror, and the advanced CAE simulation technology was used to verify the fast milling process of the large diameter Ф510 mm lightweight silicon carbide mirror. Simulation and testing results show that compared with manual grinding, this processing method can reduce the processing cycle by 90% under the premise of ensuring the processing accuracy, and there is no mirror damage and system resonance in the processing process. This method has been successfully used in the rough machining of large-diameter ultra-thin lightweight silicon carbide mirror, which can replace the traditional grinding method and can be used as a technical reference for other large-diameter mirror machining.