Material Removal Stability Of The Sub-Aperture Polishing For Large Aspheric Surface

Aspheric surface has the advantages of correcting system aberration,improving imaging quality and reducing the volume of the system.Therefore,it plays an important role in the optical projects such as high power laser engineering,large astronomical telescope project,extreme ultraviolet lithography engineering.In order to meet the requirements of aspheric optical elements for the optical projects,a large aspheric fabricating process has been proposed,which includes "ultra precision grinding and deterministic polishing".Indeed,the precision grinding is utilized for shape formation of the aspheric element firstly.Subsequently,deterministic polishing removes the defect layer produced by grinding,and deterministically corrects the surface error.Since deterministic polishing stage has a significant influence on the producing efficiency and accuracy of the aspheric surface,the corresponding technology is widely focused and quickly developed.At present,the varied sub-aperture polishing technology based on computer-controlled optical surfacing(CCOS),such as small tool polishing(STP),bonnet polishing(BP),magnetorheological finishing(MRF),ion beam figuring(IBF),is supposed to be the most frequently used technology in the deterministic polishing stage of aspheric surface to solve the problem of manufacturing precision and efficiency for large-aperture aspheric optical elements.The ideal removal model of CCOS is linear and time-invariant.However,in the actual process,the change of chemical mechanical characteristics will lead to the instability and distortion of tool influence function(TIF),which reduces the manufacturing precision and efficiency of aspheric surface.Therefore,the researches on the key impact factors of removal stability and the related control measures provide the basic support for improving the reliability of sub-aperture polishing process.For the distortion of TIF in the process of large-aperture aspheric optical components,this paper combine the methods of theoretical analysis,simulation verification,processing experiment.Base on the removal theory of sub-aperture polishing,the researches carry out for the stability of TIF.The main research and achievements are summarized as follows:(1)The basic theories of the aspheric optical elements including the distortion model of TIF and the theoretical influences on the surface error are introduced.The study provides the theoretical basis for the removal stability.(2)To solve the problem of removal instability inducing in the tool wear,the distortion rule of TIF and its effect on the removal characteristics in the high-efficiency polishing process is supposed to find out.The correlation of tool condition,polishing force and tool removal characteristic is investigate.In addition,the post-processing technology of worn tool by conditioning process is proposed,and the removal stability in the polishing process is improved.(3)To solve the problem of removal instability inducing in the the different surface curvature on aspheric surface,this paper carried out the investigation on the surface curvature effect to the bonnet-workpiece contact area,and presented a time-varying TIF modeling method,which can forecast the TIF for different polishing positions on aspheric surfaces.In addition,the dwell time compensation algorithm is proposed to restrain the spatial distortion of TIF,which provides a theoretical compensation method for CCOS process optimization.