Study on the surface formation mechanism and theoretical model of brittle surface roughness in turning machinable ceramics

Machinable ceramics are typical brittle materials, and their surface formation mechanism during machining has not been well revealed. Currently, there are few theoretical models of surface roughness for turning brittle materials. Through experiments with turning machinable ceramics, the surface formation mechanism of brittle materials is investigated from macro and micro perspectives. The combined action of tool and workpiece interference, elastic recovery, and crushing craters creates the machined surface of brittle materials. The initial surface angle and length, the deflection surface angle and length, and the extension length of the upward-extending surface were calculated based on the biaxial stress analytical method, the fracture criterion of the stress intensity factor, and the energy conservation principle. By quantitatively describing the cross-sectional shape of crushing craters, a brittle surface roughness model is established with the combination of actual cutting layer parameters, tool angle, material properties, tool-workpiece interference relationship, and elastic recovery. The experimental validation results show that the brittleness model is more accurate and consistent in trend compared to the traditional geometric model and the theoretical model of crushing with simplified crushing crater shapes. The new model can be utilized as a basis for the theoretical prediction of surface roughness in the cutting of brittle materials.