A novel surface roughness model for potassium dihydrogen phosphate (KDP) crystal in oblique diamond turning

KDP crystal is a typical soft-brittle material, and the challenge of predicting its surface roughness at a wide range of feed rates in oblique cutting invokes the comprehensive consideration of various factors, e.g. the plastic behavior in ductile mode and the cracks effect in brittle mode. In this work, a novel theoretical roughness model, into which the components of kinematics, plastic side flow, materials elastic recovery and cracks effect are integrated, is proposed to reveal the underlying mechanisms of surface roughness variation in oblique diamond turning of KDP crystal. In modeling, the duplication effect of the active cutting edge contour is successfully used to calculate the component of kinematics, and an empirical expression with consideration of equivalent cutting edge radius in oblique cutting is reconstructed to estimate the materials elastic recovery. For the component of plastic side flow, the effect of the volume of unremoved materials and the scale coefficient of plastic side flow determined by feed rate and tool inclination angle are taken into account. Moreover, the component of cracks effect is quantified based on the relative length of crack (RLC) model that considers the material properties, tool geometries, process parameters and the suppression of hydrostatic pressure in the cutting area. In experiments, a method of oblique fly-cutting is employed to reduce the impact of material anisotropy on surface roughness. The results show that the RLC model can well predict the distribution of cracks, and the surface roughness model considering these four components has a satisfactory prediction accuracy.