Effect of tool inclination on surface quality of KDP crystal processed by micro ball-end milling

Micro-milling has been considered as the most promising method to repair the micro-defects on the surface of KH₂PO₄ (KDP) crystal. However, acquiring an ultra-smooth repaired surface by ball-end milling remains a longstanding challenge for KDP crystal due to its soft-brittle properties. In micro ball-end milling of KDP crystal, tool inclination angle has a remarkable effect on the quality of machined surface. Therefore, picking out an optimal tool inclination angle plays a great role in guaranteeing the ductile-mode machining and improving the surface quality of brittle KDP crystal. In this work, the effect of tool inclination on the brittle–ductile transition and surface quality of micro-milled KDP crystal were investigated. A theoretical model considering the tool inclination direction and angle was proposed to calculate the undeformed chip thickness (UCT) and cutting speed involved in the micro ball-end milling process. Besides, micro groove experiments were conducted to evaluate the change rule of the brittle–ductile transition and surface quality related to the tool inclination. The experimental results agree well with the theoretical results, which shows that the evolution of surface quality with respect to the tool inclination depends on the competitive mechanisms between UCT and cutting speed. Inclining the cutter in the tool feed direction (positive inclination angle) and increasing the tool inclination angle both contribute to the ductile cutting of KDP crystal. A + 45° inclination angle is the optimal angle for the micro ball-end milling of KDP crystal and the best surface roughness value achieved could be up to 35.3 nm.