Abstract: The brittle fracture mechanism and surface formation mechanism are studied based on brittle fracture mechanics and tool-work piece interference principle in turning fluorophlogopite ceramics. The crack propagation angle and depth are predicted. The theoretical model of turning surface roughness is established to evaluate the surface quality of precision turning fluorophlogopite ceramics and improve the machining efficiency. The surface roughness in turning brittle materials consists of kinematical-geometrical roughness and brittle fracture roughness. Kinematical-geometrical roughness is mainly affected by tool geometry and feed rate， while brittle fracture roughness is mainly affected by cutting speed， cutting depth， cutting force and mechanical properties of ceramics. Experiment results show that surface roughness increases with the increase of feed rate and cutting depth， while decreases with the increase of cutting speed. Compared with the traditional geometric roughness model， the theoretical values of brittle model are closer to the experimental results， and theoretical values and experimental trends are identical.

Key words: surface roughness, theoretical model, brittle fracture mechanism, fluorophlogopite ceramics, turning