The First Aspheric Form and Texture Results From a Production Machine Embodying the Precession Process

We report on progress developing the 'Precession Process', that has recently been embodied for the firsttime in a fully-productionised aspheric polishing machine. We describe how the process uses inflatedpolishing tools of continuously-variable size and hardness. Despite the rapid tool-rotation needed togive high removal rates, the method produce well-behaved and near-Gaussian tool influence-functions,by virtue ofthe precession ofthe spin axis.

We then describe how form-errors are controlled. The method takes influence-function data and anerror-map as input, together with, uniquely, weighting factors for height and slope residuals andprocess-time. A numerical optimisation of the cost function with variable dwell-time, tool-path andtool-size is then performed. The advantages of this new technique are contrasted with conventional deconvolutionmethods. Results of form-control on aspheric surfaces are presented, with an interpretationin terms of spatial frequencies. We draw particular attention to control of form at the centre andperiphery of a work-piece.

Finally, we describe how Precession processing gives multi-directional rubbing of surfaces, and wepresent the superb texture achieved on samples.