Research On The Key Technologies Of Direct Optical Polishing For Aluminum Mirror

Aspheric mirror with large diameter has been widely applied in modern optical systems. With aluminum alloy as the material, the metallic mirror has a high reflectivity over a broad spectral range from the infrared, through visible light, and into the ultraviolet light. Meanwhile, the aluminum alloy material is characterized by good processing performance, light weight, innocuity, high thermal conductivity and low price. Aluminum alloy has been the major buttress material used in many optical systems, especially in aerospace applications. Using the same aluminum alloy material for both optical components and supporting structure can avoid the thermal stress caused by the changing temperature of different materials. Therefore, aluminum mirrors occupies an important position in modern optical system.Single point diamond turning can achieve good quality and precision of the surface of aluminum alloy. However, the disadvantage of SPDT is that it leaves tool marks on the mirror surface which affects image quality. A common method processed in aluminum mirror is the surface coating, and then the subsequent polishing. However, the difference in thermal expansion coefficient between the aluminum body and the film results in the change of temperature, which will have a "bimetal" effect that will affect the use of the optical system. Due to the characteristic of metal aluminum being relatively soft, ordinary polishing will scratch the surface of aluminum mirror. And the surface oxidation reaction will happen when the metal aluminum mirror is being polished because of the lively chemical properties of metal aluminum, thus reduce surface reflection properties. In this paper, a new polishing process will be studied, that is, to directly optically polish the metal aluminum mirror. It can not only eliminate the turning grain aluminum on the mirror surface but also further improve the surface accuracy of aluminum mirror.The main research work includes:1 The mirror polishing aluminum removal principle has been analyzed. In the polishing process, the mechanical and the chemical reactions have been generated on the surface of the aluminum mirror and those two reactions manage to achieve the purpose of polishing the surface. The experiment investigated the p H of the environment and the impact of oxidant on direct polishing of metal aluminum mirror. According to the results of the surface roughness values, the metal aluminum mirror should be polished under non-oxidant alkaline environment. Directing at polishing discs for different materials and different types of abrasive, direct polishing experiments are carried out on aluminum mirrors. By analyzing and comparing the roughness of the aluminum mirror surface after polishing, we can choose the most suitable polishing disc and polishing abrasive.2. The relevant processing parameters affecting the surface quality of aluminum mirror are also studied. The experiment applies a single parameter method to directly polish aluminum mirror. According to the surface roughness values gained from the experiment, the reasonable value range of direct pressure, rotational speed, and p H value for direct polishing of aluminum mirror can be determined. Orthogonal experiment method is used in aluminum mirror polishing experiments. According to the orthogonal design principles, we can select multiple sets of parameter combinations from the reasonable value range of pressure, rotational speed, p H value. The optimal combination of parameters of polishing aluminum mirror can also be obtained through the analysis and the treatment of the results and data of the experiment. Adopting the best combination of parameters to direct polish the metal aluminum mirror, we can produce metal aluminum mirror surface better than the one produced by single point diamond turning.3. Computer control technology is applied to directly polish the aluminum mirror. Under the control of computer, polishing and modification experiments were conducted on the flat aluminum mirror and aspherical mirror, which shows that polishing can greatly improve the surface accuracy after single point diamond turning, eliminate the turning grain on the aluminum mirror surface and improve the surface quality; under the computer's control, the workpieces with non-rotational symmetric surface were processed and polished after firstly being processed by ordinary CNC machine tool, and the ability of the studied polishing process for aluminum mirror can be further validated from the results that the rough surface was directly polished to optical mirror.