Polishing ultra-precision ground aspherical surfaces with MRF

The production of high-precision aspheres typically involves at least one extensive chemo-mechanical polishing process to remove subsurface damage after grinding. Especially when machining glasses that are prone to fracture from grinding, subsurface damage can be substantial. As a result, subsequent polishing times are high and it is challenging to maintain a reasonably low surface form error. In order to reduce processing times and increase process stability SwissOptic established an ultra-precision grinding process. This process operates in a ductile grinding mode that not only minimizes subsurface damage but also leads to surfaces with form errors well below one micrometer. The high surface quality after grinding makes it possible to omit chemo-mechanical polishing processes. Instead, we polish and form correct the ultra-precision ground aspherical surfaces in a single process step by applying magnetorheological finishing (MRF). In this paper, we demonstrate the feasibility of this approach based on the production of a fused silica asphere and a steep S-PHM52 asphere. We utilize QEDs Q-Flex 300 for MRF polishing. Very aggressive process parameters are used to keep processing times as low as possible. We find that polishing and form correction to an irregularity below 300 nm is feasible in less than 30 minutes. Depending on the desired quality of the aspheres, MRF polishing parameters can be adjusted.