Longitudinal micro-waviness (LMW) formation mechanism on large optical surface during ultra-precision fly cutting

During fly cutting of large aperture flat optical components, mid-spatial frequency micro-waviness is formed on the machined surface, which greatly affects the optical performance of the components. This study aims to explore the mechanism of formation of micro-waviness perpendicular to the feed direction, which is called longitudinal micro-waviness (LMW). A finite element model (FEM) of the ultra-precision fly cutting spindle system is developed to analyze the axial vibrations of the tool nose. The analysis shows that the formation of LMW is the result of coupling of the response of the spindle system and its higher order natural frequency response under the excitation of the tangential cutting force. The vibration frequency of the tool nose is closely related to the structure of the spindle system. By changing the stiffness of the cutting head, the frequency is significantly changed, and the LMW formed on the machined surfaces has a spatial period of 39.2 mm, which is out of the range of mid-spatial frequency waviness. This result has great significance in the design of fly cutting machines, and can be used to improve the quality of the optical surface and thereby improve the performance of large aperture flat optical devices.