Experimental Study Of Wide Ribbon Wheel Magnetorheological Polishing For Sapphire Optical Curved Surface

The structural features and excellent optical properties of sapphire optical curved surface elements have been applied more and more widely on the optical equipment in the field of modern cutting-edge technology,which puts forward a higher demand for the optical surface quality in the meantime.There are still some challenges in the ultra-precision machining of sapphire optical curved surface elements.Magnetorheological finishing(MRF)has the characteristics of high precision,certainty and flexible contact,which can realize nearly zero sub-surface damage to the workpiece and the precision machining on nanometer scale.Therefore,the wide ribbon wheel magnetorheological finishing for sapphire optical curved surface is studied in this thesis.First of all,the principle of wide ribbon wheel magnetorheological finishing is analyzed according to the machining characteristics of single curvature optical surface.The magnetic field produced by series of permanent magnets causes the magnetorheological fluid to form a combined polishing ribbon on the external surface of the polishing wheel.The polishing wheel moves back and forth along the axis during the movement of machining of curved surface which can improve the processing efficiency and polishing effect.According to the characteristics of wide ribbon wheel MRF,the polishing path of square wave,triangle wave and sine wave was designed.The effects of three polishing paths are compared and analyzed,and the result shows that the Surface roughness of the workpiece processed by square wave's path are more evenly distributed,the average roughness Ra is lower and the surface topography is better.Secondly,using COMSOL software,the distribution of magnetic field lines,magnetic induction,axial flux density patterns along the surface of the polishing wheel under different magnet arrangement,magnet spacing,magnet dimensions was obtained respectively.The validity of the simulation results is verified by using the Digital Tesla meter to measure the relative eigenvalues.The result shows that multiple polished ribbons can be formed under different magnet arrangements;along the polishing wheel surface axial measurement values and simulation values are basically consistent;the distribution of magnetic induction lines and magnetic induction in different positions are not same;the distribution of magnetic induction lines and magnetic induction are different,among them the magnetic induction on the perimeter surface of the polishing wheel under the two kinds of magnet arrangement in radial magnetizing is unevenly distributed,the magnetic induction is higher with the axial magnetized poles arranged in the opposite direction;through fixed-point polishing test,it is found that the maximum removal depth of the material is 17?m and the lowest surface roughness is obtained with axial magnetized pole arranged in reverse direction.The size of ribbon,the strength of micro-grinding head are suitable and the maximum material removal capacity of fixed-point polishing is obtained when the When the distance between the magnets is 3mm and the size of the magnets is 6mm.Thirdly,The theoretical model of wheel-type magnetorheological polishing fluid was established according to the Herschel-Bulkley-Papanastasiou instantonal equation,Based on the design of crossbar slotted polishing wheels,spiral slotted polishing wheels and smooth polishing wheels,COMSOL software was used to model and simulate the effect of polishing wheels with different slotted structures on the fluid properties of magnetorheological fluids.Simulation analysis of the effect of different slot structure polishing wheels on the flux density mode,fluid flow rate and fluid pressure of magnetorheological fluid in the processing area,By comparing the fluid properties through simulation,the effect of spiral groove type polishing wheel is significantly improved compared with smooth polishing wheel,the average flux density mode of magnetorheological fluid is increased by 7.7%,the average fluid velocity is increased by 6.9%,the average radial shear is increased by 29.2%,and the maximum radial shear is increased by 23.6%.Three kinds of polishing wheels are used to polish the sapphire optical surface.The results show that there is little difference in polishing effect between the transverse groove polishing wheel and the smooth polishing wheel,the material removal rate was increased by 51.7% compared to a 32.5% reduction in the Surface roughness.Finally,based on the simulation and experiment to optimize the magnetic field and structure of groove on the surface of polishing wheel,the wide ribbon wheel magnetorheological finishing method was used to polish the sapphire optical surface with single curvature.The kinds of abrasives,the concentration of abrasives,the particle size of abrasives,the concentration of carbonyl iron powder,the rotation speed of polishing wheel,the machining gap and the machining time were selected for the systematic experiments to investigate the influence of different processing parameters on the polishing effect of sapphire optical surface with single curvature.After polishing the sapphire optical surface for 4 h,the RA value can be reduced from 5 nm to 0.53 nm when using the silica sol with the concentration of 20 wt.%,the particle size of 30 nm as abrasive,the concentration of carbonyl iron powder was 75 wt.%,the rotation speed of polishing wheel was 200r/min and the machining gap was0.8mm.