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Proceedings Paper

New results extending the Precessions process to smoothing ground aspheres and producing freeform parts
Author(s): D. D. Walker; A. T. H. Beaucamp; V. Doubrovski; C. Dunn; R. Freeman; G. McCavana; R. Morton; D. Riley; J. Simms; X. Wei
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Paper Abstract

Zeeko's Precession polishing process uses a bulged, rotating membrane tool, creating a contact-area of variable size. In separate modes of operation, the bonnet rotation-axis is orientated pole-down on the surface, or inclined at an angle and then precessed about the local normal. The bonnet, covered with standard polishing cloth and working with standard slurry, has been found to give superb surface textures in the regime of nanometre to sub-nanometre Ra values, starting with parts directly off precision CNC aspheric grinding machines. This paper reports an important extension of the process to the precision-controlled smoothing (or 'fining') operation required between more conventional diamond milling and subsequent Precession polishing. The method utilises an aggressive surface on the bonnet, again with slurry. This is compared with an alternative approach using diamond abrasives bound onto flexible carriers attached to the bonnets. The results demonstrate the viability of smoothing aspheric surfaces, which extends Precessions processing to parts with inferior input-quality. This may prove of particular importance to large optics where significant volumes of material may need to be removed, and to the creation of more substantial aspheric departures from a parent sphere. The paper continues with a recent update on results obtained, and lessons learnt, processing free-form surfaces, and concludes with an assessment of the relevance of the smoothing and free-form operations to the fabrication of off-axis parts including segments for extremely large telescopes.

Paper Details

Date Published: 24 August 2005
PDF: 9 pages
Proc. SPIE 5869, Optical Manufacturing and Testing VI, 58690E (24 August 2005); doi: 10.1117/12.617067
Show Author Affiliations
D. D. Walker, Univ. College London (United Kingdom)
Zeeko Ltd. (United Kingdom)
A. T. H. Beaucamp, Zeeko Ltd. (United Kingdom)
V. Doubrovski, Zeeko Ltd. (United Kingdom)
C. Dunn, Univ. College London (United Kingdom)
R. Freeman, Zeeko Ltd. (United Kingdom)
G. McCavana, Zeeko Ltd. (United Kingdom)
R. Morton, Zeeko Ltd. (United Kingdom)
D. Riley, Zeeko Ltd. (United Kingdom)
J. Simms, Zeeko Ltd. (United Kingdom)
X. Wei, Zeeko Ltd. (United Kingdom)

Published in SPIE Proceedings Vol. 5869:
Optical Manufacturing and Testing VI
H. Philip Stahl, Editor(s)

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