| Advanced optical system is an important part and key research object of the national optical engineering including laser fusion and space telescope,and the large aperture optical aspheric surface plays a key role in the advanced optical system.In order to effectively obtain large aperture optical aspheric surface with less damage and high surface accuracy,ultra precision grinding machine has become an important equipment for guaranteeing the mass production.The machining technology of large aperture optical aspheric surface usually is "high efficiency shaping grinding+deterministic polishing".If the high-quality surface is obtained in the grinding stage,it can not only improve the machining efficiency in the subsequent polishing stage,but also help to shorten the whole cycle of machining the large aperture optical workpieces.In order to achieve the efficiency goal of the whole process,our research group successfully developed the ultra precision grinding machine named UPG80,and the hydrostatic technology is adopted to ensure the machining accuracy of the machine.Due to that both the surface accuracy and the surface quality of the workpiece are directly affected by the precision and accuracy of the moving parts on the machine tool,this paper conducts the motion error analysis and the throttle control technology of hydrostatic guideway s.The main contents are as follows.(1)Firstly,the design scheme and overall layout of the ultra precision grinding machine UPG80 for machining large aperture optical aspheric are given,so the specific research object is determined.Then the theoretical analysis and experimental researches are carried out specifically.Taking the closed hydrostatic guideway with four oil pads as the analysis sample,according to the kinematics theory,the position and posture of the slider with four oil pads at any position in the closed guideway are described,then the quasi-static theoretical model of 2-DOF motion errors of the closed hydrostatic guideway can be established.The influence of the main performance parameters of PM flow controller on the motion accuracy is analyzed,and the variation characteristics of the motion accuracy with the total oil film clearance under different external force cases are studied.Finally,combined with the relative difference of the paired guide rails,the theoretical model with 2-DOF motion errors is improved further,and the influence of the relative difference between the paired guide rails on the motion accuracy is analyzed,the mechanism of the motion errors is reasonably explained as well.(2)Taking the moving hydrostatic parts in X axis and Z axis of UPG80 as the analysis objects.In the assembly stage of the machine,the theoretical modeling and the experimental researches of the motion errors in X axis are carried out.In the final process experiment stage of the machine,by considering the relative differences between the main guideway and the auxiliary guideway,the overall motion errors of the moving parts are studied and the corresponding mechanism is analyzed.In the assembly stage of the machine,the hydrostatic guideways in X axis are taken as the research object.Owing to that the guide rails in the assembly stage of the machine are ground by another machine directly,the guide rail profiles of the main and auxiliary guideways are highly similar with each other.Based on the analysis of these similar guide rail profiles,the moving table with six hydrostatic guide shoes is simplified to the slider with six oil pads(half worktable type),and the quasi-static model of the motion errors is established correspondingly.The motion straightness of the worktable in the whole stroke is measured by Renishaw laser interferometer and compared with the prediction results,the predicted results are in good agreement with the measured data.Finally,the sensitivity effect of the profile errors of the paired guide rails on the motion accuracy is discussed.In the final process experiment stage of the machine,the main and auxiliary guideways both are processed by manual precision scraping,so the relative differences between the main and auxiliary guideways do occur.Therefore,it is necessary to take the whole sliding platform as the analysis object and establish the 3-DOF motion errors model with twelve oil pads.The hydrostatic column in Z axis and the hydrostatic worktable in X axis are taken as the specific research samples,respectively.The motion errors measured by experiments and the theoretical prediction results are compared,and the motion error mechanism based on thermal effect is analyzed and discussed.This 3-DOF motion errors model has higher generality than before.(3)As the key element of hydrostatic technology,the restrictor is researched and developed by our team.Based on the constraint of motion error index and the design theory of hydrostatic bearing,the design of single-sided membrane self feedback variable restrictor is carried out,which is adapted with the pure hydrostatic guide shoe(HGS)having two recesses,so that the self designed HGS is formed.Through three specific research cases,the throttling performance of the self designed HGS is analyzed and compared.The simulation results prove the effectiveness of the designed single-sided membrane self feedback variable restrictor,the initial foundation of theoretical design is established.Next,the experimental research will be carried out on the actual sample of the self designed HGS.Through a single self designed HGS experiment,the manufacturing error and assembly error of the designed restrictor will be determined,and the reverse theoretical correction will be completed;Through the static characteristic experiment of the sliding platform equipped with self designed HGSs,the design experience can be summarized and the theoretical system will be formed.Then,an active variable restrictor is designed,which can adjust the internal resistance of the restrictor in real time,actively and pertinently through the piezoelectric actuator.The motion error compensation of the moving table with six oil pads(half worktable type)in the assembly stage of the machine is studied clearly.(4)Combined with the aforementioned research basis and design guidance on the motion accuracy of the hydrostatic guideways,the design objective of the grinding machine for large aperture optical aspheric is determined and optimized further by decomposing the efficiency requirements and accuracy characteristics of the processing technology route.The spatial errors of the machine in the stage of assembly are analyzed.Then the accuracy of each axis is tested experimentally.The influence law of the workpiece surface error and the adjustment direction of the whole machine are determined.After the debugging of the machine is completed,the process experiment is carried out.The machining experiment of the aspheric workpiece with the scale of 530mm × 530mm shows that,the surface accuracy results meet the desired requirements.According to the similarity region feature of workpiece surface error,it is necessary to conduct a more in-depth spatial kinematics coupling analysis on the motion accuracy variation based on the hydrostatic bearing theory,which will be one of the key works in the future. |
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