Study On Ultra-precision Fluid Jet Polishing Technology And Process System Development

The most important breakthrough in the development of modern optical machining technology is the realization of deterministic material removal in the final optical polishing process using small tools,so that the polishing process can be accurately modeled and predicted,allowing reliable control of the process and effective correction of the surface form errors to achieve nanometer or sub-nanometer level accuracy.The deterministic removal of the surface form errors in the polishing process is realized by changing the dwell time of polishing tool along the tool path over the workpiece surface,whilst the variation of dwell time is mathematically solved using the surface error distribution model and the removal function model of the polishing tool.Previous research on the deterministic polishing technology mainly focuses on magnetorheological polishing and flexible bonnet polishing processes,whilst ion beam polishing technology is studied for the manufacture of optical components with extreme surface form accuracy requirements at a few nanometer or even sub-nanometer level.Compared to other polishing methods,fluid jet polishing(FJP)technology has its unique advantages and control difficulties.Research on FJP application is relatively few,current research works for FJP mainly focus on the material removal mechanism,removal function optimization and experimental study investigating the effects of process parameters on removal rate.In the present work,control methods of FJP process to achieve rapid and reliable deterministic removal,and effects of process parameters on the removal efficiency and removal contours are studied,which aims to explore the potential using abrasive water jet process to produce ultra-precision surfaces.In addition,tests are carried out using developed FJP control methods to manufacture ultra-smooth optical components,and a method for the rapid removal of rotary symmetric surface errors on the component surface is proposed.The main contents of the paper are as follows:As the cross sectional profile of the removal function produced in vertical jet polishing is typically a W shape,which can not be directly used in the process control algorithm to derive the dwell time map,a new concept of ‘virtual rotary function’ is proposed in this work to replace the use of a physical eccentric rotary device proposed by other researchers,which allows quick calculation of the dwell time and also simplifies the equipment structure.A ‘layer by layer’ dwell time computation method is also proposed to effectively improve the computing efficiency and avoid the convergence failure.A method to generate random tool paths is proposed,to overcome the occurrence of intermediate frequency surface waviness errors,often caused by the regular tool path design used in the traditional polishing and small tool polishing processes.Based on the technical requirements to achieve deterministic machining,a FJP test system with reliable jet pressure stability has been developed.Experimental study on the removal contour morphology and characteristics is carried out,adopting fixed point jet machining method,under different process parameters including jet pressure,jet angle and jet distance.Fluid flow simulations under different process parameters are conducted,analyzing the pressure and velocity distribution at the jet flow working region.The material removal mechanism in jet machining is analyzed by comparing the experimental results with the simulation results.The removal function(RF)in FJP is typically obtained from fixed point jet machining tests,whilst the measured 3D shapes of the removal function are often not in the ideal rotary symmetric form.Solutions to handle this issue are proposed.Algorithms to filter out the substrate form error and measurement noise and align the test piece in measurements before and after the jet machining are developed.An enhancement algorithm is proposed to improve the rotary symmetry of RF,with a correction factor added to the quality evaluation algorithm to study the change of the function quality with the jet time.The dynamic responses and moving accuracy of the worktable are measured,and their effects on the FJP polishing process are analyzed,which are used for the optimal selection of movement parameter settings in the following FJP machining tests and provides useful information for the optimal design of FJP equipment and control system in the future.Based on the above investigations,deterministic surface form correction tests are carried out and excellent optical surfaces with a few nanometer form accuracy RMS are succefully produced,which proves the effectiveness and correctness of the control methods and algorithms developed in this work.A quick removal method of the rotary symmetric error(RSE)on the workpiece surface is proposed,in which the tool nozzle and workpiece moves relatively as in an end-face turning process.Efficient improvement of the workpiece surface form accuracy is achieved using a relatively simple device.Two methods to extract the RSE from the measured surface error curves,i.e.,the average and minimum methods,are introduced and compared.The influence of the local jet rotation radius on the removal function of FJP is investigated experimentally.The algorithm for solving the dwell time curve in two-dimensional FJP process is derived.Deterministic FJP experiments are conducted to remove the RSE by means of the end-face turning movement.It shows that the proportion of RSE over the overall surface error is reduced from more than 60% to less than 20% on most workpieces,whilst the volume of the non-rotary symmetric error(non-RSE)fluctuates slightly before and after the FJP process.Using the average RSE extraction method and combining it with the zero-phase filtering method to filter the dwell time can effectively reduce the frequency of residual rotary error on the workpiece surface and avoid the increasing of PSD in mid-high frequency after polishing.At the end of this work,research on NC machining chain of aspheric optical lenses is carried out.As the material removal rates achieved in FJP are relatively low when comparing to other polishing methods,this part of research is to explore the potential position of FJP in the entire process chain.The results show that FJP has the possibility to be combined with CNC grinding and CNC polishing to realize rapid manufacture of ultra precision optical components.