BK7 optical glass is widely used as aerospace optical window. Precision grinding is a common method for machining BK7 optical elements. CVD diamond grinding wheel is a cutting tool with excellent wear resistance, which can realize high efficiency grinding for BK7. However, due to its small chip space, the surface quality will be affected by the large grinding force and difficult chip removal. In this paper, micro-grooves with different angles and widths were fabricated on CVD diamond grinding wheel surfaces by picosecond laser to improve the chip space. The grinding experiments of BK7 were carried out using these micro-structured wheels. The results show that the grinding force was reduced by 80% at most compared with the unstructured wheel, and the machined surface roughness was also decreased by up to 85%. The grinding force of structured CVD diamond grinding wheel decreases with the increase of the surface micro-structure groove angle. With the increase of groove width on the surface of structured CVD diamond grinding wheel, the surface roughness of the workpiece decreases first and then increases. When the groove width is 80μm, the surface roughness of the workpiece reaches the minimum value.
KEYWORDS: Silica, Optics manufacturing, Surface roughness, Optical components, Photovoltaics, Diamond, Spindles, Laser systems engineering, High power lasers, 3D metrology
Hard-brittle thin-walled tubular optics are widely used in the field of high-power solid-state lasers. The 3D accuracy requirement and the clamping deformation of the hard and brittle thin-walled structure pose a great challenge for ultra-precision manufacturing. In this paper, fused silica thin-walled tubular optics with the length of 100mm, the inner diameter of 42mm and the outer diameter of 50mm were fabricated successfully through ultra-precision grinding. The profile accuracy of PV1.6μm, the surface roughness of Ra10nm, the straightness of 1.64μm, the roundness of 1.62μm and the cylindricity of 2.13μm were achieved through in-place dressing of grinding wheel, on-machine measurement, off-line measurement, and the iterative compensation machining. The technical difficulties of ultra-precision manufacturing of hard brittle thin-walled optics have been overcome.
Cylindrical optical element is a common type of aspheric lens, which can realize the transmission, shaping and amplification of the beam in the optical system. It has a wide range of applications, such as linear detector illumination, holographic illumination, optical information processing, strong laser system and synchrotron radiation beamline. In this paper, an efficient robot-assisted rotation-revolution ultra-precision belt grinding technology was presented for cylindrical optical element. First, the structure of device and processing principle of the rotation-revolution belt grinding device was introduced. Then, based on the generalized Preston equation and Hertz contact theory, the removal function of the ultra-precision belt grinding was established. Finally, the cylindrical element of optical glass with a diameter of 50.09mm was ground. After 6 hours of processing, the cylindricity accuracy of the element was decreased from original 112.4μm to 4.6μm, and the surface roughness was decreased from 2.631μm to 3.085nm.
In consideration of the excellent property of SiC, the ground micro-structured surface quality is hard to meet the requirement,consequently the ultrasonic vibration assisted polishing (UVAP) of micro-structures of molds is proposed in this paper. Through the orthogonal experiment, the parameters of UVAP of micro-structures were optimized. The experimental results show that, abrasive polishing process, the effect of the workpiece feed rate on the surface roughness (Ra), groove tip radius (R) and material removal rate (MRR) of micro-structures is significant. While, the UVAP, the most significant effect factor for Ra, R and MRR is the ultrasonic amplitude of the ultrasonic vibration. In addition, within the scope of the polishing process parameters selected by preliminary experiments, ultrasonic amplitude of 2.5μm, polishing force of 0.5N, workpiece feed rate of 5 mm·min-1, polishing wheel rotational speed of 50rpm, polishing time of 35min, abrasive size of 100nm and the polishing liquid concentration of 15% is the best technology of UVAP of micro-structures. Under the optimal parameters, the ground traces on the micro-structured surface were removed efficiently and the integrity of the edges of the micro-structure after grinding was maintained efficiently.
Diamond wheel with sharp edge has small contour structures, which can lead to fast wear of wheel in the grinding process. Traditional truing methods are hard to apply to this kind of wheels. Therefore, as for the difficulty of precision truing of diamond wheel with sharp edge, the novel methods for resin and metal bonded diamond wheels with sharp edge are presented, respectively. In this experiment, a conditioning procedure with rare metal alloy block Ta was used to true the resin bonded diamond grinding wheel and in the same way Nb alloy block was utilized to complete rough truing of metal bonded diamond grinding wheel. Then a CNC truing technique with rotational green carbide (GC) truing stick was applied to precise truing of metal bonded diamond grinding wheel. Methods mentioned above were measured in order to evaluate the performance of truing. Geometric features of the wheel sharp edge were duplicated on the organic glass (PMMA) in order to measure and calculate the radius of the sharp edge. The edge radius of trued resin bonded wheel and metal bonded wheel is perceived as an important assessment. The experiments results revealed that the edge radius of 12.45μm for the resin bonded wheel and the edge radius of 30.17μm for the metal bonded wheel could be achieved.
KEYWORDS: 3D metrology, Scanning electron microscopy, Sensors, Diamond, Data acquisition, MATLAB, 3D acquisition, LabVIEW, Data processing, Microscopes
A method of non-contact, on-machine measurement of three dimensional surface topography of grinding wheels’ whole surface was developed in this paper, focusing on an electroplated coarse-grained diamond grinding wheel. The measuring system consists of a Keyence laser displacement sensor, a Keyence controller and a NI PCI-6132 data acquisition card. A resolution of 0.1μm in vertical direction and 8μm in horizontal direction could be achieved. After processing the data by LabVIEW and MATLAB, the 3D topography of the grinding wheel’s whole surface could be reconstructed. When comparing the reconstructed 3D topography of the grinding wheel’s marked area to its real topography captured by a high-depth-field optical digital microscope (HDF-ODM) and scanning electron microscope (SEM), they were very similar to each other, proving that this method is accurate and effective. By a subsequent data processing, the topography of every grain could be extracted and then the active grain number, the active grain volume and the active grain’s bearing ration could be calculated. These three parameters could serve as the criterion to evaluate the grinding performance of coarse-grained diamond grinding wheels. Then the performance of the grinding wheel could be evaluated on-machine accurately and quantitatively.
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