Form error compensation in the slow tool servo machining of freeform optics

Abstract

Advancements in diamond turning technology with tool servo configurations enables the generation of precise freeform surfaces. However, the profile accuracy is mainly limited due to non-availability of an efficient tool path compensation techniques and precise alignment methods. The aim of this study is focused on developing a tool path compensation routine for slow tool servo machining of freeform optics. A seven-order polynomial freeform surface, designed for hyperspectral imaging is selected for experimentation. Alignment strategy by utilizing the available fiducials is presented to ensure the precise re-mounting of surface during machining and metrology. The contact type profilometer is used to measure the fabricated surface by taking 25 numbers of two-dimensional raster scans at an interval of 0.5 mm. The scans are then stitched to get the 3D surface measurement. The residual form error map is used to compensate the tool path. Significant reduction in form error, i.e., from peak to valley (PV) of 9.27 to 0.75 μm with surface finish (Ra) of 11.82 nm, is achieved by performing four machining iterations of compensation. The simulation studies are also presented to investigate the effects of various misalignments on manufacturing accuracies. The developed compensation process is effective for fast convergence of form error and to manufacture the precise freeform optics for various imaging and non-imaging applications.