In this study, our aim is to improve the efficiency of the optical manufacturing process employing magnetorheological finishing (MRF) by quantitatively analyzing the MRF response characteristics that vary according to the type and size of low-spatial frequency. Dimension-variable patterns were devised based on the dimension of the tool influence function (TIF), consisting of two types: a width-variable pattern and a height-variable pattern. These dimension-variable patterns were utilized as input data for the MRF corrective polishing system. The resulting residual figure error of the patterns generated through the MRF corrective polishing system was calculated and expressed as output data. Furthermore, to quantify the MRF response characteristics for low-spatial frequency, the relative error is presented by comparing the input data and output data. The results indicate that the MRF polishing performance for low-spatial frequency is influenced by both the type and size of the frequency, and these trends can assist in devising sophisticated and efficient MRF strategies for manufacturing ultra-precision optical surfaces.
Polycrystalline zinc sulfide (ZnS) is a widely used optical material especially for infrared applications. After ultra-precision diamond turning (DT) machining of the material in ductile regime, surface roughness of a few nanometers is achievable. However, the traces of crystal structure were not only visible on the DT machined surface but it also showed the steep ups and downs on the surface profile, although the same depth-of-cut was applied to the whole surface. In this study, it was hypothesized that the ups and downs on the surface profile might be affected by the crystallographic orientation and elastic recovery of each grain on the surface of ultra-precision DT machined ZnS. The relationship between the crystallographic orientation and elastic recovery of the material was investigated using electron microscopy with electron back-scattered diffraction (EBSD) and nanoscale scratch tester. The results showed certain grain orientations exhibit higher levels of elastic recovery, leading to increased surface roughness. These results highlight the importance of considering crystallographic effects in the machining process of ZnS to achieve desired surface quality.
Optics for accelerators require extremely low surface roughness (SR) to achieve high reflectivity due to the very short wavelengths of light used. Diamond turning (DT) is one of the leading machining processes for manufacturing optical components, and is widely used because of its high material removal rate and the ability to obtain optical surfaces with a SR of a few nm or less. There have been many studies on the prediction of SR in the DT process, and many SR prediction models have been proposed for the expression of it. However, when the SR is nm or less, the proposed SR prediction models show different results from the actual results. Aluminum has been used as a material for optical components used in the DT process due to its excellent machinability and high reflectivity, and it is a material that has been used as an actual processing material when proposing the SR prediction model. In order to use aluminum mirrors as optical components for accelerators, their SR must be generated to nm or less. In this study, we have compared and analyzed the nanometer SR of two aluminum materials to fabricate Al mirrors used in infrared optical systems for accelerators.
Zinc sulfide (ZnS) exhibits unique properties that make it highly suitable for applications as a multi-wavelength optical element. Its transmittance shows significant improvements in the visible light and near-infrared regions when subjected to high-temperature and high-pressure processes. However, the mechanical cutting of ZnS is limited due to its brittleness and polycrystalline properties. In this study, we propose the feasibility of employing ultrasonic vibration cutting to enable the mechanical cutting of polycrystalline zinc sulfide. The objectives are to analyze both the ductile mode machining and brittle fracture behavior while addressing the issue of spring back encountered during the diamond turning process. Analysis is conducted on cutting parameters, including vibration amplitude, spindle speed, cutting depth, and feed rate, to evaluate their impact on the cutting process. The results showed that we establish ultrasonic vibration cutting can indeed lead to improved cutting quality under specific conditions. We demonstrated the potential of ultrasonic vibration cutting as a promising alternative technique for the precision machining of polycrystalline zinc sulfide. The resolution of the spring back problem represents a significant advancement, potentially enabling the manufacturing of high-quality optical elements using polycrystalline material.
In this paper, we propose a fabrication process based on Magneto-Rheological Finishing (MRF) for a reflective Spiral Phase Plate (SPP) with a continuous surface. The front surface of a nickel-plated aluminum disk is machined by diamond turning as a plane mirror, and spiral structure with low topological charge is generated by sub-aperture polishing tools, i.e., MRF. Interferometers are used to analyze the smoothness of the spiral structure, steepness of the center step and surface roughness of the random areas for the fabricated SPP. The results indicate that the direct-polishing approach can be a promising technique to fabricate high-precision SPPs.
Single Point Diamond Turning (SPDT) has the potential to cost-effectively manufacture optical materials such as metals and plastic types. However, SPDT generally leaves tool marks on the machined surfaces, which creates problems that can deteriorate the optical performance. Several processes have been studied to eliminate the tool marks caused by SPDT, but it was difficult to carry out without the additional defects like sub-surface damages and other tool marks. To overcome this weakness, we investigated the Magneto-Rheological Finishing (MRF) process to effectively remove the periodic micro structures without surface deterioration for optical performance. The workpiece used in the experiment is a mirror plated with electroless nickel-phosphorus. Through the processing of the SPDT, an initial surface gets periodic tool marks, which have a height of 1.1 μm and a pitch of 20 μm. We studied on the reduction rate of the turning marks by the MRF process with some different conditions of uniform removal. The quantitative analysis of the surface roughness and residual marks was performed using a scanning low-coherence interferometer and through the Power Spectral Density (PSD) respectively. The results showed that reduction rates of tool marks depend on the angles (0, 45, and 90 degs) between the turning direction of the tool marks and the rotation direction of MR wheel. In the case of 45 degs, it indicated the fastest reduction rate.
A magneto-rheological finishing (MRF) process for the post-treatment of diamond turning is presented to remove the periodic micro structures and sub-surface damages with improvement of the original figure and surface roughness. An off-axis aspherical mirror with electro-less nickel-phosphorus plated surface was machined by a Single point diamond turning machine (SPDTM) and MRF polisher. The machined surfaces were examined by a scanning low-coherence interferometer, and the technique of Fast Fourier Transformation (FFT) and Power Spectrum Density (PSD) were introduced to evaluate the residual diamond turning marks on the turned and polished surfaces. The turning marks, which was clearly visible on the diamond turned surface, were absolutely removed after MR process, and the surface roughness of the machined surface was improved from 6 nm(Sa) and 7 nm (Sq) to 2 nm(Sa) and 3 nm (Sq). Consequently, the experimental results indicate that MRF is suitable for removing periodic micro-patterns caused by diamond turning process with the progress of the original figure and surface roughness.
An off-axis optical system can effectively avoid some problems, such as aberrations, shielded area created by the secondary mirror and a narrow field of view (FOV), while an on-axis optical system has the problems. Inspired by the consideration, the off-axis optical system is generally used for hyperspectral sensors and telescopes. However, there are several obstacles limiting the productivity of the off-axis optics in fabrication and measurement processes. In this study, to overcome this weakness, we suggests a new fabrication technique using a customized jig, not separated from the work-piece. A convex aspheric mirror and the off-axis mirror are fabricated by Single Point Diamond Turning Machine (SPDTM) for comparison analysis of surface state. The mirrors are made from aluminum (Al6061-T6) and used for the reflectors of a coastal water remote sensing system. We show fast machining and simple measurement in comparison with traditional off-axis single machining and measurement, provide performance results, such as form accuracy and surface roughness measured by both contact 3D profilometer (UA3P) and non-contact 3D profiler (CCI-Optics). The customized ultra-precision machining process can be effectively used for complex off-axis mirror fabricating.
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