Optimising form accuracy in maching arrays of optically functional surfaces

The complexity of components used in recent optical technologies is ever increasing. In many applications like micro-lens arrays the optically functional surfaces have a smooth and steady geometry, but have an uncontinuous transition between the surrounding surface or other lenses. That type of non-rotationally symmetric surfaces can be machined by ultra precision diamond turning assisted by a Fast Tool Servo System (FTS), which extends the turning kinematics by a high frequency tool movement. To achieve high form accuracy and optical surface quality, two compensation approaches are discussed in this paper. The form error resulting from the geometry of the diamond tool's radius shaped cutting edge is corrected by the superposition of a compensation value. The form accuracy of the work piece beside the geometrical factors is further influenced by the accuracy of the FTS and the basis machine, especially regarding the dynamics of the axes. The transfer behaviour of the FTS is improved by applying a pre-control filter with significant order. Its extraction from measurements is shown in the paper. The positioning accuracy is mainly affected by the following error and the transfer behaviour of the axis. Using a pre-control filter, the following error of the axis can be reduced by 90 %. The combination with the radius tool compensation results in a rapid improvement of the form accuracy. Further improvements of the dynamic behaviour of the axis are possible with the application of a model-based control strategy. The dimensioning of such a modelbased system can be facilitated by a modification of the optimisation algorithm which was used to evaluate the pre-control filter. Instead of using ideal parameters for the geometry compensation of the radius tool, the measured geometry could be used to enhance the precision of the compensation, further.