Partly random surface micro textures acquired by continuously varying stepover cutting to suppress the diffraction effect

Abstract

Diffraction effect has been troubling optical elements fabricated by ultra-precision machining for decades, which results in energy loss and imaging quality degradation. Recently, such an effect is attributed to the periodic micro textures on machined surfaces which are similar to diffraction gratings. And such textures result from the constant stepover of current turning processes. To address the issue, this study proposed a continuously varying stepover cutting (CVSC) method to disrupt the periodic surface textures during tool servo turning. Continuously varying stepover was realized by integrating harmonic variations into the conventional Archimedean driving spiral along the feed direction. The linear chirp signal, which possesses both smoothness and varying frequency, was adopted as the variation signal. A theoretical surface topography model and a diffraction model were established to predict the three-dimensional surface textures and resultant diffraction effect respectively. An X-axis tracking trial was conducted to identify the potential negative effect on following accuracy. Experimental investigations including cutting experiments, surface topography analysis and diffraction tests were conducted to validate the proposed CVSC. Disrupted surface textures and dispersed spatial frequency were observed and coincided well with simulation results. And the effect of CVSC in suppressing the diffraction of machined surfaces was verified by diffraction tests. It is indicated that moderately varying stepover is conducive to suppressing the diffraction effect of ultra-precision machined optical elements while maintaining acceptable surface finish.