Research On Ultra-precision Dual-axis Wheel Polishing Technology Using A Semi-rigid Polishing Wheel

CCOS(computer-controlled optical surfacing)based deterministic polishing techniques,combining predictable material removal function and 4-dimemsional control(x,y,z space and dwell time),provide important process capabilities of surface form correction for making ultra-precision optical components with nanometer level form accuracy.Various CCOS techniques have been developed for the ultra-precision machining of optical components in plano,spherical and aspherical forms.Current CCOS polishing techniques typically feature a soft tool-workpiece contact,resulting in very low material remo val and long process cycle time,and also a high requirement on the quality of pre-machined workpiece surface.Prior rough and semi-finish lapping or polishing steps are necessary to change the surface fro m ground status to mirror status.Typical CCOS techniques,such as tilted bonnet polishing and wheel polishing,tend to leave directional texture on the component surface,reducing the service performance of the optical components produced,and also restricting the further convergence of surface roughness.This dissertation proposes a novel high-efficiency dual-axis wheel polishing(DAWP)technique using a semi-rigid polishing wheel,designed to achieve high material removal rate and high convergence rate of surface roughness.Quick polishing of ground surface to mirror status and surface form correction can be achieved by using the same polishing tool,the process chain can be significant ly simplified by using the DAWP technique,with much improved manufacturing efficiency.Typical optical materials,BK7 and fused silica,are selected in the research.The research aspects of the dissertation include the principle of DAWP,tool influence function modeling and the tool's material removal law,establishment of theoretical model for the prediction of mid-spatial frequency ripples generation,mechanism o f rapid convergence of high-spatial frequency surface characteristics and uniform texture acquisition,and establishment of comprehensive mathematical models for the prediction and description of surface texture and roughness generation in DAWP process.The main research works conducted are listed as follows:1.The system configuration and working principle of the proposed DAWP system are introduced.The structure and the mechanical properties of the used semi-rigid polishing wheel are studied.And the motion precision and thermal balance issue of the DAWP tool are also studied and controlled.2.Tool influence function(TIF)of the DAWP tool is modeled based on finite element analysis(FEA),using material properties of the polyurethane pad obtained from material property tests.Verification tests and material removal tests under different process parameters are carried out using the DAWP system developed.3.A dynamical removal model(DRM)based on the spatial kinematic ana lysis and polishing material removal theory is proposed,to investigate the actual material removal characteristics in dual-axis wheel polishing(DAWP).The model is developed to calculate the overall material removal profile(low-spatial-frequency error,LSF)and the middle-spatial-frequency(MSF)errors(ripples),simultaneously.A series of straight line path polishing tests on BK7 glass are carried out to investigate the effect of the processing parameters on the generated MSF errors and also verify the validit y of the proposed DRM.4.The mechanism of rapid convergence of high-spatial frequency surface characteristics of BK7 optical glass,and the mechanism of uniform surface with nano-scale roughness acquisition by using DAWP process are experimentally investigated.The effects of process parameters and polishing modes on the surface characteristics of TIF area,and the relationship between TIF topographies and polished surface topographies are systematically analyzed.5.Two comprehensive mathematical models are proposed to provide a better scientific understanding of the surface texture generation in DAWP process.The first model,based on the abrasive wear theory,abrasives distribution investigation and accumulative removal theory,predict s the micro-topography and roughness generated within the single-axis TIF area.On the basis of this model,and combined with the polishing path and feed motion parameters,another surface micro-topography generation model is also built to predict the micro-topography and roughness generated on polished workpiece surface in actual DAWP process(both for single and dual-axis modes).A series of fixed spot polishing tests and uniform polishing tests on BK7 glass are carried out to analyze the influence of polishing parameters and motion modes on the generated surface texture and roughness,and also verify the validity and feasibility of the proposed models.6.In the uniform polishing test under co-rotating speed of 80 r/min,after only one-pass polishing,a 99.74%improvement o n surface roughness from Ra 485.99 nm to Ra 1.243 nm and a MRR of 1.614 mm~3/min have been achieved on a BK7 ground surface.The present work shows that the semi-rigid DAWP process has great potential to achieve excellent surface quality on optical components with ultra-high process efficiency.