New results extending the Precessions process to smoothing ground aspheres and producing freeform parts

D. D. Walker, A. T.H. Beaucamp, V. Doubrovski, C. Dunn, R. Freeman, G. McCavana, R. Morton, D. Riley, J. Simms, X. Wei

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

40 Citations (Scopus)


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.

Original languageEnglish
Title of host publicationOptical Manufacturing and Testing VI
EditorsH. Philip Stahl
Number of pages9
ISBN (Print)0819458740, 9780819458742
Publication statusPublished - 24 Aug 2005
Externally publishedYes
EventOptics and Photonics 2005 - San Diego, United States
Duration: 31 Jul 20054 Aug 2005

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


ConferenceOptics and Photonics 2005
Country/TerritoryUnited States
CitySan Diego


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