Prediction,Compensation And Evaluation Of Machined Error In Micro Milling Of Freeform Surface

With more requirement for imaging quality in the fields of aerospace,communication,and biomedical,freeform optics has been applied more broadly in recent years.To meet the growing market demand,more and more freeform optics are mass produced by high precision die.The cutting scale in micro milling of freeform optics die is small(feed per tooth range from 0.1 to 10 microns),and hence the diameter of micro end mill is also small(range from 0.1 to 0.8 millimeters).As the cutter diameter is small,and the die materials are typical difficult to cut materials,such as corrosion resistant optical die steel,nickel base alloy,etc.,the large cutter deflection occur easily.The machined error induced by tool deflection is significant factor that affect the machined precision of freeform optics die.In addition,due to the small cutting scale,the size effect in micro milling is remarkable,which poses a great challenge to the precise modeling of cutting forces and machined error in micro milling.Therefore,development of accurate model for cutting forces and machined error in micro milling with size effect is significant.This thesis deals with the cutting forces and machined error in micro milling of freeform optics die,and the main contents are as follows,The accurate cutting forces model has been developed with the effect of edge radius and material strengthening.Based on moment balance analysis of chip,a new iterative calculation method for effective rake angle,shear angle and friction angle is proposed considering the negative rake angle phenomenon induced by edge radius.A modified Johnson-Cook constitutive model is presented to estimate shear flow stress that considers material strengthening effect.Based on the classical oblique cutting theory and slip line field theory,the accurate analytical prediction of cutting forces in micro milling is achieved,which considers the shearing forces derived from shearing action and ploughing forces resulted from ploughing action.In addition,the influence of edge radius,material strengthening,nominal rake angle and cutting speed on micro milling cutting forces are investigated in this thesis.The prediction and compensation model for tool deflection and machined error in micro milling of freeform surface have been established including the coupled effect of run-out and tool deflection feedback.The measurement device is designed to measure the run-out by using the laser displacement sensor.The cutting geometry model of micro milling including instantaneous uncut chip thickness and cutting state of cutting edge element are developed considering the coupled effect of run-out,exact trochoidal trajectory and tool deflection feedback.The iterative prediction algorithm for cutting forces and tool deflection in micro milling of freeform surface is presented,and the accurate prediction of tool deflection in micro milling is achieved.The coupling mechanism and influence rule of run-out and tool deflection feedback on micro milling cutting forces and tool deflection is explained.The run-out and steady tool deflection at the instant that cutting edge pass the cutter contact point are projected to the outside normal vector of freeform surface,and thus the machined error in micro milling of freeform surface is obtained.Based on such a prediction model,the compensation model for machined error in micro milling of freeform surface is developed by introduced the multilevel iterative algorithm.The influence rule of milling strategies,path interval and local inclination angle on micro milling machined error is investigated.Based on investigation of geometry characteristic of freeform surface and scale feature of machined error in micro milling of freeform surface,the outlier in measured surface induced by machined error or measured error is filtered by using the mean curvature,and the region recognition and coarse matching algorithm is proposed by introducing a new two-dimension geometric shape descriptor.The iterative closest point algorithm is used to implement the fine matching,and thus the machined error is obtained.A three axis ultra-precision CNC milling machine is developed to serve as experimental platform,and the design,selection and adjustment of mechanical structure and servo control system are carried out to ensure the motion accuracy of machine.Based on the proposed model of cutting forces and machined error in micro milling of freeform surface,and the developed ultra-precision machine,the high precision micro milling of some typical freeform optics,such as F-theta,artificial compound eyes,etc.,are achieved.It provides a good theoretical basis and technical support for micro milling of freeform optics.