High-Precision Grinding of Thin and Large Optical Workpieces with the Kinematic Support

In fabrication of a thin and large optical element, grinding with the conventional fixture cannot achieve high precision. When such a thin and large workpiece is tightly fixed to the grinder table, the inconsistency in the form of the contact surfaces and thermal expansion of them produce unexpected stress on the workpiece. After finishing grinding with the fixture, the deformation of the ground surface would happen, accompanied by the release of the stress. This paper proposes a stress-free fixture method by removing over-constraint from the fixture structure. Corrective grinding is executed by calculating the deformation of the workpiece due to the load of the grinding wheel. Furthermore, the fixture enables on-machine measurement by an interferometer above the grinder by replicating the same condition as when the optical element is in use. We achieved a precision of RMS 0.30 mu m on a one-metersize glass ceramic of 60 mm in thickness.