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A new ultra precision large optics grinding machine, BoX®has been developed atCranfield University. BoX®islocated at the UK's Ultra Precision Surfaceslaboratory at the OpTIC Technium. This machine offers a rapidand economicsolution for grinding large off-axis aspherical and free-form opticalcomponents.This paper presents an analysis of subsurface damage assessments ofoptical ground materials produced usingdiamond resin bonded grinding wheels. Thespecific materials used, Zerodur®and ULE®are currently understudy for makingextremely large telescope (ELT) segmented mirrors such as in the E-ELTproject.The grinding experiments have been conducted on the BoX®grinding machineusing wheels with grits sizes of76 μm, 46 μm and 25 μm. Grinding process datawas collected using a Kistler dynamometer platform. Thehighest material removalrate (187.5 mm3/s) used ensures that a 1 metre diameter optic can be ground inlessthan 10 hours. The surface roughness and surface profile were measured usinga Form Talysurf. The subsurfacedamage was revealed using a sub aperturepolishing process in combination with an etching technique.These results arecompared with the targeted form accuracy of 1 μm p-v over a 1 metre part,surface roughnessof 50-150 nm RMS and subsurface damage in the range of 2-5 μm.This process stage was validated on a 400mm ULE®blank and a 1 metre hexagonal Z

Sub-surface damage issues for effective fabrication of large optics