In this paper, mechanical characteristics of KDP crystal anisotropy are analysed
theoretically. Vickers indentation experiments are adopted to validate the variation rule of hardness
and fracture toughness in different orientation of KDP crystal plane (100), and a model to calculate
critical cutting thickness of brittle-ductile transition is proposed for the KDP crystals. The result
shows that, on the crystal plane (100), the minimum value of critical cutting thickness of KDP
crystal in brittle-ductile transition appears in the direction [110], but the maximum appears in the
direction [010]. Finally, the ultra-precision machining of KDP crystal is performed, and the results
agree well with the theoretical conclusions. Super-smooth surface with a roughness RMS of 6.6nm
is reached as machined in the crystal direction [010], and 11.2nm to the direction [110].
The method of single point diamond turning is used to machine KDP crystal. A regression analysis is adopted to construct a prediction model for surface roughness and cutting force, which realizes the purposes of pre-machining design, prediction and control of surface roughness and cutting force. The prediction model is utilized to analyze the influences of feed, cutting speed and depth of cut on the surface roughness and cutting force. And the optimal cutting parameters of KDP crystal on such condition are acquired by optimum design. The optimum estimated values of surface roughness and cutting force are 7.369nm and 0.15N, respectively .Using the optimal cutting parameters, the surface roughness Ra, 7.927nm, and cutting force, 0.19N, are obatained.
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