Experimental Study on Ultra-Precision Turning of Freeform Optical Surfaces of Polymethyl Methacrylate with Nanometer Surface Roughness

The high performance of optical components is contingent upon the quality of their optical surfaces, thereby imposing elevated standards on the methodologies employed for their fabrication. This study involved experimental research on freeform optical surface elements of polymethyl methacrylate with nano-surface roughness. In this study, the effects of machining parameters of ultra-precision slow tool servo turning on the surface roughness of different types of areas of freeform optical surfaces in the finishing stage were analysed. Based on the analysis of ultra-precision turning test results for freeform optical surfaces, a novel evaluation method for surface quality is proposed to assess the overall uniformity of surface quality across the entire freeform optical surface. Building upon this proposed evaluation method for overall surface quality uniformity, the processing method of high-quality freeform optical surfaces is studied. The results show that in the finishing stage, the radial feed rate exerts the greatest influence on the surface roughness of the freeform optical surface, especially the surface roughness of the concave surface area. This will exacerbate the surface quality inhomogeneity of the freeform optical surface. Based on the analysis results, optimal machining parameters were selected for processing trials. Concurrently, additional machining tests were conducted to further validate the influence of radial feed rate. Ultimately, a nano-scale PMMA freeform optical surface with uniform overall surface quality was achieved. The variation in surface roughness in different regions of the optical freeform is regulated to stabilise within 2 nm on the surface of polymethyl methacrylate. The overall uniformity of surface quality across the entire freeform optical surface was maintained at a high level.