A Cutting Strategy with Continuously Varying Stepover to Suppress the Diffraction Effect of Ultraprecision Machined Surfaces

Diffraction effect has been troubling optical elements fabricated by ultra-precision machining for decades, which results in energy loss and imaging quality degradation. Such effect is assumed to result from the periodic micro textures similar to diffraction gratings which are generated during machining. To provide a possible solution, a continuously varying stepover cutting method is designed in this study to disrupt the periodic tool marks during slow tool servo turning. Tiny harmonic variations are integrated into the spiral cutting motion. The influence of amplitude and period of the variation on the resultant surface topography is investigated. A theoretical model is established to predict the periodic micro textures fabricated by the proposed method. Machining experiments with various magnitudes and periods are designed and conducted. Both measured topography and power spectral density analysis demonstrates disrupted textures with multiple frequencies. Suppressed diffraction effect is observed on machined surface. It is indicated by the results that variation with relatively small amplitude is conducive to suppressing the diffraction effect of ultra-precision machined optical elements while maintaining surface finish.

Keywords: Ultra-precision machining, Diffraction effect, Periodic surface texture, Continuously varying stepover.

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