Research On Grinding Forces And Machining Damages In Ultrasonic Vibration Grinding Of BK7 Optical Glass

Optical glass covers a broad range of applications in scientific and engineering fields in recent years,such as aerospace,national defense,and energy engineering,because of its perfect physical,chemical,and optical characteristics.Unfortunately,the high hardness and low fracture toughness properties of optical glass would easily induce micro-crack damages on the surface and subsurface of the components during the conventional grinding process.The performances and lifetime of optical components would be significantly affects by these damages,which also limits the development and application of the optical glass.Ultrasonic vibration grinding is a new composite processing technology for addressing this issue,which combines the principles and features of ultrasonic machining and conventional grinding.Through introducing the ultrasonic vibration on the grinding tools,it changes the kinematic of abrasive grits and affects the material removal process of brittle materials.Ultrasonic vibration grinding is considered a feasible method to realize precise and efficient processing of hard and brittle materials because it provides positive effects in terms of reducing grinding forces,improving machined quality,and enhancing machining efficiency.However,the damage formation and material removal mechanisms during ultrasonic vibration grinding of BK7 optical glass have not been fully revealed up to date,and the influences mechanisms of micro-interactions between grits and workpiece on the grinding forces and subsurface damage also have not been clarified.Therefore,the research of the material removal and machining damage in ultrasonic vibration grinding of BK7 optical glass has important theoretical awareness and engineering practical values for the realization of low damage ultrasonic vibration grinding of optical glass components.Due to the axial high-frequency ultrasonic vibration,the kinematics properties of abrasive grits are changed,which has directly influence on the manufacturing damages formation and material removal process of BK7 optical glass materials.Consequently,the kinematics of abrasive grits in ultrasonic vibration grinding were firstly analyzed in this thesis,and it was found that there were two different kinds of special effects,i.e.,periodic hammering and repeated polishing,on the workpiece surface resulting from abrasive grits at different positions.In order to investigate the influences of these two effects on the damage formation and material removal mechanisms of BK7 optical glass.A series of multi-indentation and ultrasonic vibration-assisted scratching experiments of BK7 optical glass were designed and performed.The experimental results showed that there were two material removal modes for the optical glass including plastic-flowing deformation and brittle fracture,and the ultrasonic vibration could inhibit the propagation of median cracks and promote the propagation of lateral cracks.It could help to increase the ductile-brittle transition critical cutting depth and the proportion of ductile removal in the grinding process of BK7 optical glass.On this basis,the internal elastic-plastic stress field models under quasi-static loading and dynamic scratch loading of optical glass were developed.The microscopic mechanisms from abrasive grit loading to crack damage formation and then to material removal were revealed from the perspective of internal stress-driven based on these model analyses.This provided the theoretical basis for subsequent research on grit-workpiece interactions and the grinding force model.The grit and workpiece interactions in ultrasonic vibration grinding were the coupling effect results of the cutting motion of the stochastically distributed grits and ultrasonic vibration.In this thesis,an approach of simulating and analysis the ultrasonic vibration grinding process was proposed to study the influences of these two factors.A grinding wheel surface morphology model containing randomly distributed grits was established based on the measurement of wheel surface.Combining the kinematic analysis of grits and wheel surface model,the cutting trajectory equation of multiple grits during ultrasonic vibration grinding in a global coordinate system was constructed.Furthermore,the theoretical model of undeformed chip thickness generated in the grinding process was deduced to establish the geometrical relationship between stochastically distributed characteristics of grits and cutting chip thickness.Then,a new method for determining the micro-interaction stages between grits and workpiece by using undeformed chip thickness was proposed to provide the theoretical basis for subsequent modelling of grinding force.The grinding force was regarded as the macroscopic physical performance of grits-workpiece interactions,and also a significant parameter to evaluate the grinding process and quality.Based on the calculation of grinding force components from different interactions of grits-workpiece,the grinding force predicted models for ultrasonic vibration side and end grinding were established and verified by the grinding experiments.Then,the influences of ultrasonic and grinding parameters on the force signal were analyzed,which provided the evidence for the prediction of subsurface damage and the optimization of process parameters.Surface and subsurface damages would seriously affect the performance and lifetime of BK7 optical glass components,and the formation of such damages was directly related to the micro-interaction of grits-workpiece.Therefore,the influences of ultrasonic vibration on the machined surface and subsurface qualities during ultrasonic vibration grinding process were analyzed in this thesis,and the inner relationship between subsurface crack formation and grinding force was explored based on the indentation fracture mechanics.Meanwhile,a predicted model of the maximum depth for subsurface crack damages was built on the basis of above research.Finally,an optimization scheme of ultrasonic vibration grinding parameters considering the predicted models of machined quality evaluation parameters was proposed.The optimal parameter groups that reached the machined quality requirements were obtained,and then the effectiveness of this method was verified by grinding experiments.The research results of this thesis provide technical support and theoretical basis for realizing low-damage and high-precision ultrasonic vibration grinding of BK7 optical glass.