On-machine self-calibration method for compensation during precision fabrication of 900-mm-diameter zerodur aspheric mirror

An effective on-machine self-calibration compensation grinding method for the fabrication of a 900-mm-diameter zerodur aspheric mirror is presented in this paper. The proposed independent on-machine measuring system consists of a linear motion and a Heidenhain CT60 length gauge. Measurement errors caused by the length gauge’s tilt angle and offset would decrease the compensation grinding accuracy. The length gauge tilt angle and offset could be determined by using the workpiece itself, as the ground workpiece has excellent symmetry and small axial runout values. Three points on the workpiece concentric circle are sampled to determine the tilt angle. Multiple points along the X-axis are also sampled to determine the offset between the length gauge probe and the workpiece grinding center. Trial measurement experiment of the 180-mm K9 workpiece showed an excellent coherence with the PGI1250 measuring results. Based on the proposed self-calibration method, the raw measured profile error curve is firstly smoothened and then spline fitted to generate the compensated grinding path. The grinding experiment indicated that the profile error could be improved to be less than 5 μm from 40 μm for the 900-mm zerodur mirror after four compensation grinding cycles.