Influence Of Diamond Tool On The Micro Surface Defects In Cutting Of KDP Crystal

Due to the excellent nonlinear optical characteristics,Potassium Dihydrogen Phosphate(KDP)crystal is widely used in optical components such as optoelectronic switches and laser frequency multiplier.However,because of its deliquescence,softness,high brittleness and other material properties,the efficient processing method right now is the single point diamond fly-cutting technology.But from the actual machining cases,the machined surface quality is unsteady because of the appearance of micro surface defects.Moreover,the poor quality of cutting edge also results in the unsteady tool cutting performance.Therefore,this paper is devoted to investigating the material removal mechanism and surface formation process in depth.At the same time,the action mechanism of factors related to diamond tools in processing is investigated.Firstly,by analyzing the cutting force and hydrostatic pressure in the cutting area,the relative length of crack(RLC)model of processed KDP surface is formulated.This model analyzes the spatial distance from cracks generated in the cutting area to the machined surface quantitatively.It reveals the influences of rake angle,inclination angle and processing parameters on the material removal mode of KDP crystal and micro surface defects.The correctness of RLC model is verified by intermittent cutting experiments and oblique cutting experiments.To suppress the generation of cracks and effectively control them,tool rake angles smaller than-25°are suggested.As for inclination angles,the range of 15°to 45°can effectively suppress the propagation of cracks and increase the critical feed rate for the cracks-free machined surface of KDP crystal.Secondly,to accurately describe the cutting deformation behavior of KDP crystal in finite element(FE).Via the nanoindentation results,this paper uses the inverse analysis combined with dimensional analysis and Oliver-Pharr fitting to acquire the material model parameters of KDP crystal.Based on this material model,a hybrid cutting model of FE-SPH(Smoothed Particle Hydrodynamics,SPH)is constructed to carry out the simulation works on tool rake angles and inclination angles.The hydrostatic pressure distribution at each rake angle and inclination angle is obviously distinctive,which has an important influence on the cracks generation and chip morphologies.In addition,the chip flows towards the direction of unmachined workpiece in oblique cutting,and the reduction of interference zone is beneficial to reduce the plastic side flow and chip tearing.However,multiple competitive factors determine the surface roughness.Thirdly,considering the soft-brittle characteristic,this paper establishes a theoretical surface roughness model in oblique cutting of KDP crystal,into which the components of arc-edge kinematics,plastic side flow,material elastic recovery and cracks effect are integrated.Through the oblique cutting experiments at a wide range of feed rates,the roughness model is comprehensively validated in both the ductile-mode range and brittle-mode range.The results show that the kinematics increases sharply at large inclination angles,the smallest amount of plastic side flow is obtained at 15°inclination angle.In the meantime the material elastic recovery increases as the inclination angle increases.Moreover,the component of cracks effect based on the oblique RLC model enables the prediction results to maintain high accuracy in both the plastic zone and brittle zone.Overall,when the inclination angle is set to 15°,the machined surface roughness is the smallest.Finally,this paper employs the atomic force microscope to detect and analyze the tool-edge micro defects.SPH cutting simulation is employed to study the effect of tool-edge micro defects on surface quality.Resultly,a passivation process is proposed to remove the tool-edge micro defects based on the simulation results.In passivation,the tool depression angle is optimized to ensure the sharpness of cutting edge.Combining with the re-sharpening of rake face,the composite-process manufacturing method of micro chamfered edge is explored.At last,the improvement of tool cutting performance after passivation is validated through cutting experiments of KDP crystal.Synthesizing the research results,this paper advances an oblique cutting method of KDP crystal by controlling the cutting edge quality.An ultra-smooth surface,i.e.R_a=1.36 nm,is processed,when a high-quality diamond tool with a chamfered edge is employed at the rake angle of-25°and the inclination angle of 15°.