A new ultra precision large optics grinding machine, BoX® has been developed at Cranfield University. BoX® is
located at the UK's Ultra Precision Surfaces laboratory at the OpTIC Technium. This machine offers a rapid
and economic solution for grinding large off-axis aspherical and free-form optical components.
This paper presents an analysis of subsurface damage assessments of optical ground materials produced using
diamond resin bonded grinding wheels. The specific materials used, Zerodur® and ULE® are currently under
study for making extremely large telescope (ELT) segmented mirrors such as in the E-ELT project.
The grinding experiments have been conducted on the BoX® grinding machine using wheels with grits sizes of
76 μm, 46 μm and 25 μm. Grinding process data was collected using a Kistler dynamometer platform. The
highest material removal rate (187.5 mm3/s) used ensures that a 1 metre diameter optic can be ground in less
than 10 hours. The surface roughness and surface profile were measured using a Form Talysurf. The subsurface
damage was revealed using a sub aperture polishing process in combination with an etching technique.
These results are compared with the targeted form accuracy of 1 μm p-v over a 1 metre part, surface roughness
of 50-150 nm RMS and subsurface damage in the range of 2-5 μm. This process stage was validated on a 400
mm ULE® blank and a 1 metre hexagonal Zerodur® part.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.