Design and frequency control study of small-sized magnetorheological finishing device applied in optical manufacturing

With the continuous development of various advanced optical systems, there is an increasing demand for optical components with small apertures (generally ranging from a few millimeters to tens of millimeters in diameter), steep slopes, or even specific relief structures on their surfaces (such as continuous phase elements). These optical components all require high surface quality and shape accuracy. However, existing magnetorheological finishing (MRF) devices have larger dimensions, and the obtained removal function size is relatively larger than the size of small-diameter optical components, making it difficult to achieve high surface shape accuracy after processing. To achieve high-precision machining of small-aperture optical components and obtain higher frequency control capabilities, this study first designed and optimized a small-size MRF device with a 13 mm polishing wheel. Based on theoretical and finite element simulation analyses, the magnetic pole structure of the small-size MRF device was designed. The orthogonal experimental method was used to optimize the key parameters of the magnetic pole according to the magnetic field design requirements, and the device was constructed. Its performance and stability were experimentally analyzed, and the results showed that the peak magnetic field intensity in the polishing area of the device was about 230mT, which met the polishing requirements, and both its long-term and short-term processing stabilities met the design requirements. In addition, different sizes of removal functions were analyzed for frequency control ability, and theoretical and spectral analyses of the removal function indicated that the small-size removal function had better low-frequency error correction ability, while for mid-frequency band and mid-frequency ripple error, the opposite was true. The experimental results showed that when the optical component was shaped by 20 %, the low-frequency convergence capability of the 100 mm polishing device was only 18.72 %, much lower than that of the 13 mm polishing device, which was 52.51 %.Moreover, the mid-frequency ripple spectrum amplitude for the 13 mm polishing device was more than 15 times higher than that for the 100 mm polishing device when removing 50 nm material, and the midfrequency root mean square (RMS) increased significantly, deteriorating the surface mid-frequency error of the optical component. Therefore, when performing magnetorheological polishing on an optical component surface, a process of using large-size MRF devices for roughing combined with small-size MRF devices for finishing can ensure machining efficiency while controlling low-frequency and mid-frequency errors well. This study provides theoretical support for the magnetic pole design of MRF devices and broadens the frequency spectrum correction ability and application range of MRF processes, which has great application value.