Controllable spiral magnetorheological finishing (CSMRF) method for complex structural surface manufacturing

In advanced optical systems, new functions that are difficult to achieve by ordinary optical elements can be achieved by designing the complex surface structure of optical elements. Taking Continuous Phase Plate (CPP) as an example, it has broad application prospects in beam shaping, compensation and modulation. At present, magnetorheological polishing is the main method for polishing CPP. However, due to the influence of tool influence function (TIF) size and processing efficiency, it is difficult to further improve the accuracy of the surface relief structure after processing, which restricts the further improvement of the performance of the optical system. In order to further improve the figuring ability of existing magnetorheological polishing, this study proposes a novel controllable spiral magnetorheological finishing (CSMRF) method. Firstly, the figuring ability of spiral TIFs is analyzed. By analyzing the surface shape structure of CPP, the matching strategy between surface shape error and spiral angle of TIF is established based on genetic algorithm. And the TIF of CSMRF is dynamically compensated, so as to improve the figuring ability of CSMRF. Secondly, through the simulation figuring experiment of double sinusoidal surface with different periodic structures, the control ability of spiral TIFs to different spatial wavelengths is analyzed. The simulation results show that the spiral TIF has better control effect on the frequency band above 8 mm spatial wavelength under the TIF. On this basis, a CSMRF frequency-division processing method is proposed. The CPP of 338.4 mm × 338.4 mm is experimentally processed. Compared with the original method, the low-frequency surface error is reduced from RMS 37.984 nm to RMS 31.64 nm, which verifies the effectiveness of this method in improving the low-frequency surface error figuring ability. Therefore, this work provides theoretical and technical support for the CSMRF processing and manufacturing of high-precision complex surface optical elements, and has great application value.