Objective Large Aperture Off-axis Aspherical Optical Elements (LAOAOE) have been increasingly demanded, such as space/ground-based large aperture telescopes, aerial optoelectronics or ground tracking & sighting instruments. Moreover, the requirements for the larger aperture and shorter processing cycle make it be the core problem to manufacture the large aperture off-axis aspheric optical elements with the highly efficient and high-precision manufacturing. For instance, the processing cycle for the LAOAOE with the diameter of 1 meter is required to be 2-3 months. As the highly efficient removal process for the LAOAOE, surface form accuracy and damage depth of precision grinding having directly determined the processing difficulty and processing cycle of the subsequent polishing processing. Therefore, the high precision grinding process of shape-performance control for LAOAOE are investigated in this paper. In other words, it is required to improve the surface form accuracy and reduce the depth of grinding damage, simultaneously. The numerical collaborative approximation of both items is needed to be achieved in the end.
Methods In terms of the surface form control, it was identified the main factors for the machine tool structure, which affect the surface form accuracy of low-frequency surfaces. To achieve collaborative control and accuracy optimization of process parameters, the investigations were conducted to explore the influence laws between the surface shape accuracy and the A-axis zero error, Y-axis alignment error, shape and size error of grinding wheel, grinding wheel path, Z-axis surface compensation and so on. For the performance control, the influence laws between the grinding damage depth and grinding parameters were obtained, and the mapping relationship between the grinding damage depth and grinding surface roughness were established. The suppression strategy of the subsurface damage strategies for LAOAOE was proposed in the end.
Results and Discussions Firstly, the form accuracy (PV) of the grinding surface was significantly affected by multiple factors. The A-axis zero error variation of 0.001° had led to the change of 5.47 μm (the theoretical value)/6.9 μm (the experimental value) in surface form accuracy (PV). The Y-axis alignment error variation of 0.07 mm had caused the change of 7.9 μm (the theoretical value)/9 μm (the experimental value) in surface form accuracy (PV). Surface form accuracy had also been significantly affected by the profile error of grinding wheel, grinding method and approach as well as the Z-axis error compensation. For the reasons as above, the improvement of grinding surface form accuracy is subject to the collaborative control and optimization of the above factors. Moreover analysis based on indentation fracture mechanics revealed that there was a corresponding relationship between the grinding subsurface damage depth and surface roughness. When the damage depth was less than 5 μm in the experiment, the surface roughness Ra was below 30 nm and Rz lower than 0.25 μm, all of which could be used as the basis to control the grinding damage. Finally, after the shape and property-controlled grinding of off-axis aspheric lens with an aperture of 640 nm, the surface form accuracy could reach 3.1 μm with the surface roughness Ra less than 24 nm, Rz lower than 0.2 μm. According to the relationship between the surface roughness and the depth of the subsurface damage, the estimated depth of damaged layer was lower than 5 μm. It was verified that the subsequent polishing duration had been significantly shortened.
Conclusions For the LAOAOE, the grinding surface form accuracy can be efficiently improved by the deterministic analysis, control and compensation on the various factors affecting surface form accuracy. By mastering the mapping law between the grinding subsurface damage depth and surface roughness, the measurement on surface roughness can realize the indirect control of subsurface damage depth. Also, the combinatorial optimization of grinding process can achieve the efficient improvement and collaborative control of form property precision, which will lead to the significant reduction of polishing period for the optical elements with large aperture. It will be of great reference value for the efficient high-precision processing of optical elements with large aperture.
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