The Design and Manufacture of a Large CNC Grinding Machine for Off-Axis Mirror Segments

Abstract

The trend in the design of large optics is towards systems employing off-axis aspheric elements frequently employed in groups to form a single large parent optical surface. This gives the optical system designer more options to work with but as the off-axis elements are not suitable for manufacture by classical methods of optical fabrication, creates the need for advanced methods of manufacture. To meet this requirement Cranfield Precision Engineering Ltd. have designed and are at an advanced stage of manufacture of a large three axis CNC machine for grinding and in-situ metrology of optical elements. Machine capacity is 2.5 m × 2.5 m × 0.61 m resulting in a machine of overall dimensions 8 m × 6 m × 5 m and overall weight 130 tonnes. Overall accuracy performance is achieved by use of an unusual metrology frame concept combined with a multiple path laser interferometer system of 2.5 nanometer resolution.