Mid-spatial frequency reduction via zero-depth of cut rapid-feed passes in face-turning

The single point diamond turning process (SPDT) is used widely in creating optical grade mirror surfaces on several engineering materials ranging from polymers, and metals, to brittle materials such as silicon and germanium. In visual optic mirror applications, mid-spatial frequency (MSF) errors generated during the SPDT process interfere with the visible spectrum of light thereby affecting the image quality. To overcome these errors, a post-processing operation of polishing the optical mirrors is required. The post-processing step not only increases the complexity of the manufacturing process but also leads to minor geometrical form changes in the mirror which affects performance. To avoid post-processing and minimize MSFs formed during turning- a novel method to modify the toolpath during the machining process has been proposed in this paper. The suggested toolpath strategy comprises two consecutive operations: i) employing variable low feed rates with the specified depth of cut (DoC) and ii) executing rapid traverse rates with zero depth of cut for a predetermined number of passes. The effectiveness of the proposed strategy is tested by carrying out facing experiments in a micro-precision CNC lathe. The power spectral density (PSD) content of the machined surface is then analyzed to check for any improvement in the frequency characteristics. The results show that the frequency errors generated by the toolpath in normal turning operations can be minimized, distributing the resulting PSD peak over a wide range of spatial frequencies. From the PSD plots, it is observed that there is a decrease of 77% and 85.82% in the peak intensity values when compared with surfaces machined at constant feedrates of 150 μm/rev and 200 μm/rev respectively. This method can be applied to nanoprecision SPDT machines to improve the surface quality and to eliminate the MSF errors of the visual optical grade mirrors without the need for post-processing.