Research On Ultrasonic Vibration Assisted Fixed Abrasive Machining Of CaF₂ Crystal

CaF₂ crystal has excellent optical properties,mechanical properties and physical and chemical stability,and is widely used in laser,lithography,astronomy,aerial detection and other fields.The deep ultraviolet lithography and high-energy laser systems have strict requirements on crystal purity,structural defects and surface quality of CaF₂ crystal.CaF₂ crystal is soft and brittle;fixed abrasive machining usually generates micro scratches on its surface.In order to optimize the surface quality and increase the material removal rate,the method of vibration assisted fixed abrasive machining was proposed.The main completed work and research results are as follows:（1）Single abrasive vibration assisted fixed abrasive machining simulation.The influence of vibration direction,frequency and amplitude on the surface morphology was studied.The results show that compared with radial vibration and elliptical vibration,axial vibration leads to largest deformation.Axial vibration plays a significant role in the machining process and strengthen the workpiece surface.The force state and uniformity of elliptical vibration are better than that of radial vibration.For axial vibration,the depth of the section decreases gradually with the frequency increasing,so when the frequency is 40 kHz the surface morphology is best;with the increase of vibration amplitude,the surface morphology and uniformity become worse.（2）Ultrasonic vibration assisted fixed abrasive machining method optimization.A method of ultrasonic vibration assisted fixed abrasive machining was proposed.The effects of two process parameters on material removal rate and surface roughness were studied by single factor experiments.Based on the problems existing in the processing,the ultrasonic vibration assisted fixed abrasive machining method was optimized.Compared with the results before optimization,the material removal rate of lapping is increased by 189%and surface roughness is decreased by 11%.Compared with the results before optimization,the material removal rate of polishing is increased by 50%and surface roughness is reduced by 44%.The research demonstrates that after optimization,the stability of ultrasonic vibration assisted fixed abrasive processing is improved,and the workpiece trajectory is complex.The material removal rate of lapping and polishing CaF₂ crystal is improved,and surface quality is optimized.A method of high-efficiency,low-damage ultrasonic vibration-assisted fixed abrasive processing was obtained,which paves the way for CaF₂ crystals polishing.（3）Ultrasonic vibration assisted fixed abrasive polishing of CaF₂ crystal.The effects of four process parameters on the material removal rate and surface roughness of ultrasonic vibration assisted fixed abrasive polishing CaF₂ crystal were studied by orthogonal experiments.The material removal rate of CaF₂ crystal polishing under the optimal parameters is 324 nm/min,and surface roughness Sa is1.92 nm.Compared with the fixed abrasive polishing without vibration assisted,the material removal rate is increased by 57%and surface roughness is decreased by 35%.Ultrasonic vibration assisted can increase material removal rate and enhance surface quality by cavitation and intermittent contact in fixed abrasive polishing process.