| Single Point Diamond Turning (SPDT) is an ultra-precision machining process. Compared to conventional optical machining, SPDT has several outstanding advantages such as high efficiency, high precision accuracy, and high repeatability accuracy. Therefore, SPDT has been widely used for manufacturing ultra-precision optical elements. Brittle optical crystal materials such as Potassium Dihydrogen Phosphate (KDP) crystals and calcium fluoride (CaF2) crystals are widely used in the high-power laser systems, infrared systems and ultra-precision lithography systems and other areas of extensive and important applications. Using the traditional method of grinding, polishing to machine soft brittle optical materials such as KDP crystals and CaF2crystals, impurity particles can be easily embedded in the machined surface, this causes deterioration of surface quality. While using SPDT to machine KDP and CaF2brittle optical crystal materials can overcome such problems, and play the advantages of the SPDT.Surface roughness of optical components is one of the crucial indicators, which is significant to the performance of the optical system. How to control the surface roughness is the key, and one of the difficulties in the study of SPDT machining of brittle optical materials.Surface roughness in SPDT is affected by a number of factors, which collectively contribute to the final finish of diamond-turned surface. This paper analyses the dominant factors affecting surface roughness in SPDT. These factors can be categorized into five aspects, including the material properties of the workpieces, the diamond tools, the cutting process parameters, the equipment and the environment condition. Considering the mechanism of SPDT, the generation of surface roughness is closely related to the material properties of workpieces, such as anisotropy, impurity, inclusions and thermal properties. The conditions of the tool such as the rake angle, the nose radius, the tool cutting edge waviness and the wear exert significant influence on the surface roughness. The cutting process parameters, including the feed rate, the spindle speed and the depth of cut, influence the surface roughness as well, and the cutting conditions can be optimized for given materials and workpieces. The usage of mist have also to be considered. By analyzing the factors affecting the surface roughness of SPDT machining brittle optical crystal materials, a processing control method is proposed.Based on the processing control method, this paper studies on the control of surface roughness of SPDT machining KDP crystal and CaF2crystal, takes a series of cutting experiments on the KDP crystal and CaF2crystals, and analyses the relationship between each cutting parameter and surface roughness. An adaptive processing parameters optimization method is proposed.Once the workpiece is given, then a diamond tool with appropriate geometry parameters can be chosen, and the correct usage of the liquid spray can be decided according to the thermal properties of the workpiece material. Based on the relationship between the cutting parameters and surface roughness, we can design experiments to establish an optimized combination of cutting parameters to control the surface roughness, and some successful control of the surface topography had been achieved. The surface roughness Sq of diamond machined surface of KDP crystal workpiece is1.5nm, and the surface roughness Sq of diamond machined surface of GaF2crystal workpiece is3.86nm。... |
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