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Off Axis Grinding Machine

The classical approach to the fabrication of optical systems takes advantage of their symmetric shape. The optical centre of the element to be generated is placed at the centre of rotation of a rotary table and the operations, be they grinding with a fixed abrasive wheel, loose abrasive machining or polishing, proceed in a rotationally symmetric fashion.

Design trends in the large optics field have progressed toward systems incorporating large, off-axis aspheric optical elements. These elements may be used in groups to form a single large “parent” optical surface whilst other systems utilise off-axis elements individually to provide compact, wide field-of-view imaging systems. In general, off-axis elements give optical system designers another set of options to provide critical specialised systems for ground-based, airborne and space-borne optical applications.

NGST demonstrator mirror segment

NGST demonstrator mirror segment by Eastman Kodak

Production optical segments are typically hexagonal and range in size up to a maximum of 2.5m across. The machine designed and built by Cranfield Precision is capable of grinding large glass elements with spherical and aspheric surfaces to satisfy the stringent needs of optical imagining systems.

Minimum Design Constraints

To generate, by use of a grinding wheel of partial spherical form, any three dimensional surfaces within the above capacity and described in X Y Z co-ordinates to the accuracy levels discussed below. This requirement includes the ability to traverse across the surface in any combination of motions and to create the edge profile.

To measure the surface of the part in X Y Z co-ordinates prior to grinding and at intervals between grinding operations. This feature is needed to enable the surface topography of the workpiece to be determined to design the process strategy for minimum component cycle time. It also allows the effect of grinding wheel wear to be taken into account and corrected for as the grinding process proceeds.

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