The growing interest in providing additional degrees of freedom to the design of high-end optical systems has led to an increased demand for freeform optical elements. The efficient fabrication of such elements requires a polishing process that provides high removal rates and a stable removal function while working with a relatively small spot size. Taking these constraints into consideration this paper focuses on the successful implementation of polishing processes applying the A-WPT (Advanced Wheel Polishing Tool) technology. In order to maintain perpendicularity towards the freeform surface to be polished, the A-WPT is run on a 6-axis machining system with an optimized kinematics set-up. Herein the use of a tip-tilt unit successfully suppresses the formation of singularities, e.g. close to the surface vortex when polishing along a raster tool path. First results for the pre-polishing of an off-axis parabolic surface made of Zerodur are analyzed and discussed.
The production of medium to large lenses (200 - 500 mm) is becoming increasingly important against the background of the semiconductor crisis. The value of a lens increases enormously through the entire value chain. The grinding, polishing and correction processes must be precisely coordinated in order to achieve highest levels of shape accuracy and surface finish. This leads to increasing demands with respect to the manufacturing equipment and processes. Not only a single step but the whole process chain needs to be addressed during optimization. It starts with the reduction of MSF errors during grinding and ends with well converging correction cycles during polishing. The very consequent design of ultra-precision grinding machines comprises hydrostatic bearings and a gantry-type machine base made from granite. The efficient pre-polishing of aspheres and freeforms demands for tools with high removal rates even at relatively small polishing spot sizes. The reliability and convergence of the correction cycles during polishing strongly depends on stable and predictable removal functions. For each step we identify the key challenges and introduce ways to meet them.
The production of medium to large lenses (200 - 500 mm) is becoming increasingly important against the background of the semiconductor crisis. The value of a lens increases enormously through the entire value chain. A large number of processes are necessary to achieve the final contour and quality. The grinding and polishing processes must be precisely coordinated in order to achieve all requirements. The polishing process is not always a controllable variable, since various chemical and mechanical influences come together and affect the process result. For this reason, it is important to control the grinding process. The question of how it is possible to improve the surface quality as much as possible without allowing any geometric deviations is central. The effect of different machine concepts and their advantages against the background of specific quality requirements is still unknown. Therefore, a comparison of two machine concepts and their effects on the grinding process, the component quality and the possibilities of polishing the generated surfaces will be analyzed in more detail. The focus is on the MCG500 and UPG500 machine concepts from OptoTech Optikmaschinen GmbH. The results show, that the 5-axis grinding machine MCG500 enables a high-quality grinding process, which allows to reach a PV inbetween 1.5 - 3 μm. Compared to the 4-axis machine UPG500, the MCG500 is used as a pre-grinding machine to achieve a PV of 0.4 - 1.5 μm with the help of the UPG500. In addition the, the sub surface damage is only 3 - 7 μm on the UPG500 compared to 10 - 20 μm on the MCG500. With the achieved tolerances of the UPG500, the subsequent polishing process is shortened by approx. 30% compared to the upstream grinding process by the MCG500 and at the same time the process reliability to achieve the final specifications increases.
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