Huang D (2015)
Publication Language: Chinese
Publication Type: Thesis
Publication year: 2015
Publisher: CNKI.NET
As the key component of new generation optical system, freeform optics has been widely used in the field of aerospace, biomedicine, optical communication and science instrument, etc. Comparing to traditional optical surfaces, the application of freeform optical surface will make the optical system more flexible in optical design, more excellent in optical performance, more compact in system structure and more convenient in system installation, which provides a best solution for high performance optical system. However, the high performance optical system usually calls for high-precision freeform optical surfaces with submicron form accuracy and nanometric surface finish. The complex shape and high precision surface quality bring challenges for optical manufacture.Ultra-precision polishing, an important machining process in optical surface manufacturing, focuses on the surface finishing, damage removal and form error correction. The surface quality of polished freeform optical surface has a close relationship with polish strategy, polishing path, dwell time, polishing slurry, tool shape, machine tool accuracy and surface measurement, etc. In this paper, the integrated polishing process system and polishing strategy for corrective polishing of the primary mirror and tertiary mirror, which are fabricated on one monolithic substrate and described by NURBS-based freeform surfaces, in an off-axis three-mirror imaging system are proposed firstly. And then, the polishing path is generated based on the4-axis CNC polishing platform and the material removal function is modeled for spherical polishing tool. Finally, the dwell time along the polishing path is calculated for material removal control.The proposed integrated polishing process system is consisted by the tool influence function module, the polishing path generation module and the dwell time calculation module, which are close related and served as the technical guide for the polishing process. Based on the proposed synchronous and asynchronous polishing strategy, the Concentric Circle Path (CCP), Approximately Concentric Circle Path (ACCP), Spiral Path (SP) and Cross Path (CP) are generated. During tool path generation, the NURBS based freeform surface is descreted into the Cartesian coordinate system according to the design surface, which is followed by the projection of driven path on driven plane onto the freeform surface. The tool contact path is generated by bicubic interpolation in the projection method. The normal vector on the tool contact points is calculated by construction of refined mesh grid on freeform surface. The polishing path is obtained by offsetting of the tool contact path along normal vector on tool contact point with a defined distance. To maintain a constant polishing posture, the angle position of B-axis corresponding to tool location point is determined by constructing a quadratic equation based on geometric analysis.The tool influence function of spherical polishing tool for polishing of the primary mirror and tertiary mirror in the off-axis three-mirror imaging system is modeled according to the Preston equation. The material removal functions for constant and variable feed speed polishing are constructed by the integral of tool influence function. To determine the dwell time along polishing path, the radial measurement lines are defined on freeform surface and the material removal equation system is established and solved by Nonnegative Constrained Least Squares method (NNLS). The material removal on the two measurement lines of the primary mirror is predicted and the validity and residual error of algorithm for dwell time calculation is proved.According to the polishing experiments on the4axis CNC polishing platform, the surface roughness of the primary mirror and tertiary mirror are reduced to Ra20nm and30nm respectively after20passes of polishing process and the form accuracy is reduced to PV0.3μm.
APA:
Huang, D. (2015). Investigation on Polishing Process for Freeform Optical Surfaces in an Off-axis Three Mirror System (Master thesis).
MLA:
Huang, Dengpeng. Investigation on Polishing Process for Freeform Optical Surfaces in an Off-axis Three Mirror System. Master thesis, CNKI.NET, 2015.
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