Investigation on critical cutting depth in milling of wave-transmitting Si3N4 ceramics

Wave-transmitting Si3N4 ceramics exhibits high levels of brittleness, and the milling surface inevitably exists defects. Critical cutting depth is an important reference parameter to improve the surface quality and processing efficiency in milling. A theoretical mathematical model has been proposed to predict the critical cutting depth in the study. The contact relationship between the tool cutting edge and the ceramics surface is analyzed, and the non-uniform bearing crack system is established. The micro cracks propagation critical condition has been put as the criterion of critical cutting depth. Through the model calculation, the critical cutting depth is 0.38 mm, and it is confirmed the critical cutting depth is between 0.3 and 0.4 mm with experimental results. The model predictions are consistent well with the experimental results. In addition, the theoretical model shows that the critical cutting depth decreases exponentially with the increase of the material hardness, while the increase of fracture toughness, tool helix angle and side edge rear angle, leads to critical cutting depth rising. The theoretical model can be applied to evaluate the critical cutting depth of brittle ceramics in milling.