Research On Brittle Ductile Transition And Machining Quality In Ultrasonic Vibration Assisted Grinding Of BK7 Optical Glass

Due to the superior physical and chemical performance of optical glass material,they are widely used in the fields of optical instruments,new energies,bi ological medical treatment and aerospace,the performances of photo-permeability and refraction would be directly influenced by machined surface and subsurface qualities.Because of the characteristics of high brittleness and low fracture toughness of optical glass material,the cracks,even the brittle fracture would be easily induced into grinding process.To meet the need of high qualities and low damages of optical glass components in actual application,t he manufacturing process of optical glass components always consists of precision grinding,ultra-precision lapping and polishing.The improving of machining qualities in grinding process and the reducing of machined surface roughness and subsurface damages are both helpful to decrease the machining times of subsequent procedure of ultra-precision lapping and polishing,they are also helpful to improve the efficiency of whole manufacturing process.Ultrasonic vibration assisted grinding technology could change the kinematics of grinding tool in some degree in machining process,this technology has potential advantages in improving machining qualities,however,the researches of material removal,machined surface and subsurface qualities in ultrasonic vibration assisted grinding on optical glass still need to be improved.The researches of material removal,machined surface and subsurface qualities in ultrasonic vibration assisted grinding on optical glass have great theoretical significance and actual application value.The machined surface qualities would be directly influenced by the material removal modes.Because of the introducing of high frequency vibration,the material removal mechanism in ultrasonic vibration assisted grinding process is different from that in conventional grinding process.In order to research the influence of ultrasonic vibration on material removal and machining qualities in grinding of optical glass,this paper carried out the scratching tests with changing cutting depth by single diamond grain on BK7 optical glass,and analyzed the influence mechanism of ultrasonic vibration on the cracks propagation and powdering removal phenomenon of optical glass,the results showed that,the ultrasonic vibration could reduce the degree of cracks propagation and the phenomenon of powdering re moval to some degree;the experimental results of critical cutting depth on different machining conditions showed that,ultrasonic vibration was helpful to reduce the cutting force,and improve the critical cutting depth.Based on the above researches,by carrying out the Vicker's indentation tests,the fracture toughness of BK7 optical glass was calculated.Based on the calculation analysis of the influences of cutting depth on specific cutting energies in ductile and brittle removal modes,the prediction model of critical cutting depth of the brittle-ductile transition of BK7 optical glass was developed,and the corresponding verification tests were carried out,this could provide the theoretical basis for subsequent research and analysis of machined surface qualities and subsurface cracks damages.The researches of the influences and their factors of machined surface roughness are the basis of realizing machining of high qualit ies machined surfaces.This paper carried out the ultrasonic vibration assisted grinding experiments on BK7 optical glass,and researched the typical morphologies characteristics and their forming mechanisms of machined surfaces in ultrasonic vibration assisted grinding of this material,also analyzed the effect of processing parameters on the characteristics composing of surface morphologies in ultrasonic vibration assisted grinding of BK7 optical glass.Based on the analysis of the influences of processing parameters on machined surfaces morphologies,the prediction method of machined surface roughness was determined.By using this prediction method,the basic experimental data used in the modeling of machined surface roughness prediction were acquired through grinding experiments,and by applying Gaussian processing regression method,the prediction model of machined surface roughness was developed,the prediction model was validated to be effective through experiments,this provided the basis for subsequent optimization of processing parameters.The subsurface damages induced by machined process would directly influence the applying performance of optical glass components.This paper carried out the subsurface damages grinding experiments assisted by ultrasonic vibration on BK7 optical glass,the micro morphologies of subsurface cracks of optical glass were observed by using scanning electron microscope,and the inside connection between the randomness of the shapes and sizes of abrasive grains and the uncertainty of the propagation directions of median cracks was analyzed,the forming reasons of arc deviation and straight-line deflection of subsurface cracks were also analyzed.Through the subsurface damages grinding experiments assisted by ultrasonic vibration on BK7 optical glass,this paper researched the influences of ultrasonic vibration and machining parameters on the maximum subsurface cracks damages depth.Based on the above researches,the prediction model of maximum subsurface cracks damages depth was developed,the actual measurement values were used to compared with the prediction values,the efficiency of the proposed model was validated by the comparison results,this provided the basis for exploring the processing method to restrain subsurface damages,and also provided the basis for the subsequent optimization of processing parameters.Based on the above researches,the influences of grinding wheel and processing parameters on material removal rate in ultraosonic vibration assisted precision grinding were acquired through the calculation.By setting the surface roughness and the maximum subsurface cracks damages depth as restrain targets,the material removal rate as optimization objective,and setting the processing parameters as decision variables,the optimization of processing parameters were carried out by using multi-objectives non-dominated sorting genetic algorithm,the optimized grinding wheel and machining parameters in ultrasonic vibration assisted gridning of BK7 optical glass were achieved,the efficiency of the optimized parameters was validated through the grinding experiments.The results provided the basis for the selection of processing parameters in realizing high efficiency and low damages precision grindning assisted by ultrasonic vibration on optical glass material.