Two important mechanisms damaging KH₂PO₄ crystal processed by ultraprecision fly cutting and their relationships with cutting parameters

Mid-frequency waviness and subsurface crack are two fundamental factors that damage KH₂PO₄ (KDP) crystal processed by ultraprecise fly cutting. In this paper, the motif theory and the Fourier model method are usedto analyze the influence of the two factors on the laser-induced damage threshold (LIDT) of KDP. Research results indicate that the modulation degrees increase nearly linearly when the waviness amplitude and subsurface crack depth increase, and, meanwhile, the LIDT tends to decrease. The two factors have different effects during different stages of KDP failure. The mean amplitudes of waviness and subsurface damage depth have similar changing regulations with different feeds. From the machining perspective, we need not necessarily know which is more dangerous, because when one factor is controlled, the other one will also be restrained at the same time. In general, smaller feed and cutting depth are benefits for improving the LIDT of KDP.