Theoretical And Experimental Study On Fluid Hydrodynamic Fixed Abrasive Grinding

Modern optical technology is developing in a high way with the basic support from all kinds of optics elements.The high quality optical element is based on the accurancy of manufacturing.Nowadays,there are many manufacturing methods after the developing for a long time.A high quality optical element can be achieved by combining some of the manufacturing methods,which can meet the specific machining accurancy.However,optical processing technology has been in constant development.At the same time,the exploration and attempt for new methods is running all the way.The background of this research is the requirements for large aspheric optical elements processing in national major projects.This rewearch proposed fluid hydrodynamic fixed abrasive grinding(FHFAG)processing method to improve the machining efficiency while keeing the machining quality.Especially,it fills the gap from pre-grinding surface to ultra-smooth surface rapid in polishing process for largeaperture aspheric optical elements machining.The analysis for its machining mechanism has been done based on theoretical and experimental research that we can achieve a kind of decisive processing method.At the same time,the analysis can also be the basic support for FHFAG to applying it into practice.At first,the specific grinding pad was taken as the research sub-object.We have made some real grinding pad based on the research of the basic composition and manufacturing methods.A new cubic cell model was proposed to simulate the topography of the grinding pad's surface that the cutting depth distribution of the particles on the pad was achieved.In order to verify the accuracy of the simulation results,a comparative experiment was designed and carried out.The surface topography of the polished pad was captured by optical microscope.At the same time,the surface topography image of the grinding pad was processed by special digitization method,and the actual observation data of the height of the particles were obtained.The experimental results and simulation results can meet very well.Secondly,the liquid film between the workpiece and the grinding pad was taken as another research sub-object.In this part,the physical model was taken into a abstracting mathematical model preliminarily.The static grinding pressure distribution equation is derived based on the transient Reynolds equation of isothermal condition in cylindrical coordinates and other basic equations of physical conditions in actual machining.The above equations were discretized in the prescribed solution domain by using the display difference scheme.The initial solution of the basic liquid film dynamic pressure distribution was achieved by using the low relaxation iteration method under the given initial boundary conditions and processing parameters.Then the influence of different machining parameters on pressure distribution was analyzed,and the boundary of parameters was preliminarily explored.The results showed that the wedge gap between the abrasive particles and the workpiece would cause a large pressure peak value,accompanied by some nodes with the pressure value of zero.Rotation speed of the grinding tool has a great influence on the dynamic pressure change and the overall load-carrying capacity of the liquid film in the clearance.The inlet pressure has a great influence on the pressure distribution of the liquid film too,but it is not continuous.It is limited to use it to change the load-bearing capacity of the liquid film.The influence of the inlet flow on the pressure distribution of the liquid film is very complex.It is not a good choice to change the load-bearing capacity of the liquid film simply by using it.The liquid film was studied further into a dynamic boundry condition.We have made a full coupling analysis of interstitial liquid film and many processing elements.According to the machining mechanism of hard and brittle materials represented by fused silica glass,the whole variation of normal polishing force of polishing pad under complex working conditions was obtained.Based on Hertz contact theory,the deformation distribution of polishing pad under different working conditions was analyzed due to the combined action of contact force and liquid film pressure.The coupling relationship between the above intermediate variables and the whole input and output parameters was clarified.Based on the previous calculation,the whole operation was decoupled with a convergent result.It is shown that the whole complex machining process is actually a dynamic system,which can reach the equilibrium state.According to the analysis,a linear dynamic pressure groove was designed.The distribution of basic liquid film thickness and dynamic pressure under different processing conditions were calculated based on specific orthogonal parameters.The mapping relation between basic liquid film thickness and processing parameters was obtained by inverse fitting.The maximum error between the experimental results and the theoretical results is 30% without considering the abrasive wear.It can be concluded that the theoretical analysis can accurately predict the balanced machining model.Finally,the removal efficiency of FHFAG was studied,and the theoretical model of the removal efficiency of the grinding pad was established.Based on the removal mechanism of hard and brittle materials,the mapping relationship between the removal volume and the cutting depth of single and multiple particles were established respectively.Combining with the trajectory of abrasive particles in different motion forms and the distribution of the cutting depth under different processing parameters,the overall removal model of FHFAG under complex working conditions was achieved.At the same time,the removal model validation experiments were designed and implemented,and the removal model of fixed-point motion and planetary motion were validated respectively under different processing parameters.The experimental results were in good agreement with the theoretical analysis results.In fixed-point removal,a "double-peak" shape of the removal curve appears,in planetary motion,it has a stronger Gaussian-like characteristics compared with the traditional CCOS calculation method.Fluid dynamic pressure,consolidation abrasive polishing,gap liquid film coupling characteristics,quantitative prediction,polishing pad modeling.