Automated Optical Fabrication

Early work at UCL in automated optical fabrication in the 1990’s focused on the development of an active polishing tool, using an array of pressure actuators and custom load cells. The tool was deployed on the Grubb Parson 1m polishing machine, which moved the tool back and forth in a mechanically-defined path across the work-piece.
In those early days, key limitations were imposed by actuation technologies and the speed of available computers sufficiently compact to be installed on the polishing tool.

In those early days, key limitations were imposed by actuation technologies and the speed of available computers sufficiently compact to be installed on the polishing tool.

In practice, the tool was configured to create a central pressure ‘hot spot’, and the tool was rocked over the part to concentrate polishing action where it was needed. This concept led to the idea of the inflated membrane tool (‘bonnet’) which was spun-out into a start-up company Zeeko Ltd. The novel concept of precessing the tool rotation-axis provided a well-behaved and near-Gaussian removal profile (‘influence function’), and numerical optimization of the dwell-time of the influence function over the surface of the part enabled form-errors to be corrected. The resulting machine-tool provides full 7-axis CNC control of the tool with respect to the part, enabling the removal action to be concentrated precisely where it is needed.

Other complementary processes have also been developed. Fluid-jet polishing enables very small polishing spots, to address local defects or edges of parts. A range of ‘grolishing’ processes are intermediate between polishing and grinding and give higher removal rates and smoothing capability.

The first prototype Zeeko machine (200mm capacity) was installed at a defense company, and directly entered production. It was subsequently replaced with a larger Zeeko machine. The second machine to be built (of 600mm capacity) was installed originally at UCL under a research grant in producing astronomical optics, and subsequently re-located to the National Facility for Ultra Precision Surfaces at OpTIC in North Wales. Under the Basic Technology project “Ultra Precision Surfaces: A New Paradigm”, a 1.2m Zeeko machine was installed at OpTIC to join the 600mm machine. The Facility has since been enhanced with a 1.6m Zeeko machine for polishing the 1.4m prototype E-ELT mirror-segments, under the €5m contract from the European Southern Observatory

The resulting Facility, with its suite of metrology instrumentation, provides both an industrial-scale pilot plant for optical fabrication, and an R&D facility for developing novel surface-processes and associated computational techniques of the future. The main emphasis is likely to be on increasing end-to-endprocess-speed, controlling mid-spatial frequency defects (‘ripples’), and possibly, fundamental research into the physics, chemistry and mechanics of surface-removal at the nano-scale. Discussions are underway to develop processes for producing high-power laser optics, where laser-damage nucleated by surface-defects and sub-surface damage becomes a limiting factor. Plans are also being formulated to pick up on the original active lap concept in the context of modern actuation and sensing technologies, and the vastly greater computational power now available.

There remains a close relationship with Zeeko Ltd (Coalville, Leicestershire), which has located its subsidiary company Zeeko Research Ltd in the OpTIC building. This gives excellent opportunities for involvement of PhD students who wish to work across the university-company boundary

Page last modified on 03 dec 10 13:12 by A Peter Doel