In the optical systems field, key components such as spectroscopic elements often require the use of optical materials with high-refractive indices to achieve miniaturization and lightweight characteristics. However, high-refractive index optical materials have low machinability due to their brittle characteristic. In this study, we investigated the changes in surface characteristics during precision pattern machining of high-refractive index materials; specifically, a low fracture toughness, for use in grating spectroscopic elements. The experiment involved diamond turning for the primary machining, and for the secondary pattern machining, the tool rake angle, tool feed rate, and depth of cut were set as variable conditions. Surface roughness measurements and surface quality analyses were carried out using a white-light interferometer and tool microscopy. The results provide insights into the influence of conditions on the surface properties during the machining of high-refractive index materials for grating spectroscopic components. Under the machining conditions with a tool rake angle of -65o, tool feed rate of 5,000 mm/min, and a depth of cut 10 nm, the surface roughness of Ra 8.0 nm was achieved. Based on these findings, we plan to conduct further research on the mechanical fabrication of the blaze angle for grating spectroscopic components.
Advanced engineering ceramics have been highlighted mainly owing to their superior hardness, corrosion/wear resistance, and thermal insulation performances. However, they are usually very difficult-to-cut because of their high brittleness. In light of this, ultra-precision machining has been studied to perform ductile-regime cutting in the machining of ceramics. Ductile-regime cutting can feature a smoother surface, and lower subsurface damage as the dominant material response during cutting showed ductile behavior. Researchers have investigated promoting ductileregime cutting to improve the machinability of ceramics. In this study, various coating materials were applied to the workpiece surface, and their effects on machinability improvements were explored. A total of 6 surface coatings and lubricants were applied to soda-lime glass. The critical depth of cut (CDC), the depth where the ductile-brittle transition (DBT) occurred, was increased in all coatings and lubricants, with an improved ductile cutting regime. Experimental results showed that solid coatings were more effective than liquid lubricants in enhancing the ductile cutting regime. It was thought that solid coatings induced an additional downward force by resisting material deformation and chip evacuation, thus contributing to suppression of crack opening. It is expected that this research can contribute to the machinability improvements of brittle materials.
Germanium, an optical material, has high transmittance and refractive index and low light scattering in the infrared region, and research is being conducted to utilize it in various industrial fields. Various forms of optical lenses can be subjected to ultra-precision machining with high quality surface roughness, and they form accuracy through single point diamond turning (SPDT). In particular, the diamond tool with a negative rake angle and the u-LAM process that applies a 1,064 nm laser to the material have been studied to fabricate brittle materials into optical lenses. In this study, the effects of process parameters, such as laser power (W), spindle speed (RPM), feed rate (mm/min), and depth of cut (μm), on the surface roughness of a sub-nanometer scale and the occurrence of defects during the machining process were analyzed for Germanium materials. The process of removing these defects was also analyzed.
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Recently, interest in astronomy has increased internationally, and the technological development of lenses for large space telescopes is progressing. The multi-order diffractive engineered (MODE) lenses can make a large space telescope light and thin. However, because glass lenses are difficult to machine, we have adopted a method of molding at high temperature and high pressure. The STAVAX is commercially available chrome alloy stainless steel, and it is applied as various mold materials. The ultrasonic vibration cutting was adopted for ultra-precision machining because the tool wear is severe when cutting the STAVAX with a diamond tool. To achieve a flat surface for smooth ultrasonic vibration cutting, we performed a precise shape cutting using a CBN tool and confirmed and observed changes in the surface roughness and hardness depending on the cutting conditions. The ultrasonic vibration cutting was performed on the surface of the machine using a CBN tool, and the surface roughness was observed. It was confirmed that the surface roughness was impacted by the surface hardness. The specimens with low surface hardness showed the highest surface roughness at approximately 3 nm.
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