Design Of Small-aperture Magnetorheological Finishing Device And The Key Technology Research

The technology of magnetorheological finishing(MRF)is a deterministic machining method for optical surfaces,with many advantages,such as high convergence efficiency,low sub-surface damage and high machining precision.After many years' research,the technology becomes mature in domestic,especially in National University of Defense Technology(NUDT).NUDT can efficiently machine optical elements with mid-large diameter and mid-small radius of curvature by MRF.However,the wheel size is related to the diameter and radius curvature of optical elements.Therefore,the wheel diameter must be about 50 mm or even smaller when the device is used to machine small optical elements.If the wheel diameter is less than 50 mm,the design of polishing device will produce many challenges: polishing system for magnetic field design,structure design,liquid circulation stability and process stability.Based on the existing polishing device,this thesis designs a polishing device,whose diameter of polishing wheel is 20 mm to machine small-aperture,high-gradient optical components.The difficult problems of liquid circulation and processing technology are solved.This research is divided into the following parts:1.Based on three-dimension model of magnetic field distribution,the structure of magnetic field generating device is designed by theoretical calculation and simulation.Meanwhile,through optimizing the structure of nozzle and recovery equipment,the output and input of liquid were realized.So this is the foundation of polishing liquid stability circulation.2.Based on the theory of fluid mechanics,fluid state equation is established.In the equation,the main factors that influence fluid state and the relationship between these factors and pipeline parameters are determined.At the same time,according to the characteristics of the water-based magnetorheological fluid,sedimentation model of solid particles in the process of liquid is established.Combining with the model,delivery pipe is optimized.And then,the polishing device can run stability over ten hours and produce stability removal function.3.In order to solve the problem of producing mid and high frequency error on optical element surface by rule path during MRF polishing process,complex vibration platform is designed.The platform can significantly increase entropy value of machining path.When the path entropy value increases,mid and high frequency error will be restrained on the optical surface.The restraining effect is related to scan speed,vibrate frequency and process step.Through theoretical analysis and simulation,the matching relationship among the three factors is determined.4.According to the results of analysis and simulation,experiments on restraining mid and high frequency error degree,is carried out.The results show that mid and high frequency error of optical surface process by complex vibration platform is halved by raster path.Through power spectrum analysis,we find that the PSD curve changes on the optical surface processed by complex vibration platform is very smooth and has no obvious spike.