| With the rapid development of science and technology,off-axis optical systems have been widely used in the fields of space optical remote sensing and photoelectric observation,which not only has a higher requirement for the design ability of optical system,but also a severe test for ultra-precision machining technology.Off-axis multi-reflection optical system has the characteristics of large field of view,wide band,high resolution and small volume,which makes the processing of off-axis multi-reflection optical system become a hot issue of related research institutions at China and abroad.The conventional machining of off-axis multi-reflection optical system usually adopts the method of manufacturing each mirror separately and then assembling and debugging.This machining method relies too much on the operating experience of the staff,resulting in inevitable assembly errors,and it is difficult to ensure the accuracy of the position between the mirrors,which seriously affects the overall imaging performance of the off-axis optical system.To ensure the machining accuracy and imaging performance of the off-axis optical system,an idea of integrated machining of off-axis multi-reflection optical mirror was proposed in this study.The technical route of integrated machining of off-axis multi-reflective free-surface was developed,and the design and simulation of relevant machining tools,the establishment of removal model and the optimization of process parameters were carried out.The main contents of this study are summarized as follows.1.Based on the influence of different machining tools on the surface roughness,an ultra-precision machining scheme for off-axis multi-reflection optical free-form surface was developed.An off-axis multi-reflection integrated optical free-form surface processing scheme consisting of three processing stages:polyurethane grinding,polishing sponge polishing and magnetorheological finishing,was designed based on the principle of coarse to fine processing of optical components and the influence of different processing methods on Ra.The feasibility test of the proposed machining scheme was carried out on Al6061 workpiece,and a super smooth surface with Ra of6.5nm was obtained.2.Two polyurethane grinding tools and two magnetorheological finishing（MRF）tools were developed based on the study of the geometric constraints of machining tools and off-axis multi-reflection integrated optical systems.Based on the structural constraints of the off-axis multi-reflection integrated optical system and the principle of maximizing processing efficiency,two kinds of polyurethane grinding tools with high wear resistance and high elasticity were designed.Then,the magnetic poles are arranged reasonably under the limitation of the size of the machining tools.A multi-magnetic pole exciting MRF tool with annular distribution of magnetic poles and a magnetorheological finishing tool coupling excitation of magnetic poles are developed respectively.The magnetic field distribution and rigidity of polishing tools are verified by finite element simulation method.3.The removal function of machining aluminum alloy 6061 with a polyurethane grinding tool and the dynamic compensation theory of machining path were established.A dynamic parametric removal model was established based on the classical Preston equation and Hertz contact theory for machining any position of each free-form reflector using the polyurethane grinding tool.The theory of dynamic compensation for machining path of off-axis multi-reflection integral optical free-form surface was proposed for the problem of real-time change of contact position between machining tool and workpiece,the relationship between machining tool size parameters and machining path compensation was studied,and the software of machining path compensation for integral multifaceted optical free-form surface with machining tool size as input parameter is developed.4.Based on the effect of magnetorheological fluid（MR Fluid）composition and proportion onΔRa%and the effect of magnetorheological finishing process parameters on removal depth,response surface method was used to optimize the main parameters.The polishing efficiency of MR Fluid using Ce O2 and Al2O3 as abrasiades was compared in order to configure MR Fluid with high removal efficiency.The experiments were set up using the response surface method to investigate the influence of abrasive vol%fraction,CIP vol%fraction and abrasive particle size onΔRa%,and the proportion of above components in MR Fluid was optimized usingΔRa%as evaluation index.The effects of rotational speed and polishing gap on the removal depth of the multi-pole exciting magnetorheological finishing tool and the pole-opposed arrangement magnetorheological finishing tool were investigated by single-factor and response surface experiments.5.The experiment of integrated machining of off-axis multi-reflection free-form surface was implemented according to the machining technology scheme.The off-axis quad-reflection optical system was processed using the machining tools and research theory developed in this paper.After four times of polyurethane iterative grinding,six times of sponge polishing and four times of MRF,an off-axis quad-reflection optical system with surface roughness was obtained.The Ra of the four mirrors after machining meets the surface quality requirements,which proves the validity and feasibility of this study. |
