Wilhelmus Messelink

Prepolishing and finishing of optical surfaces using fluid jet polishing

The footprint of the Fluid Jet Polishing process is determined by the shape of the nozzle as well as by the orientation of the slurry beam with respect to the local surface normal. Besides, no tool wear occurs and the footprint remains constant during the manufacturing process allowing shape corrections in a deterministic way. To that aim, FJP has been implemented on a CNC machine and applied for both shaping of previously polished aspheres and polishing of fine ground a-spheres. In this paper, results will be presented showing the application of FJP as a sub-aperture shape correction method. Besides, experimental data will be reported demonstrating FJPs capability of polishing previously fine ground surfaces. The wear rate depends on the sharpness of the abrasives and their kinetic energy. It can thus be adjusted by various parameters, among others the applied pressure, slurry concentration and abrasive sizes. In this paper, an additional process parameter is identified allowing the application of the same polishing compound for wear rates ranging from nanometers to micrometers. This large wear range is achieved by mixing a well controlled amount of gas into the slurry flow allowing the abrasives to travel at higher speeds.

Development and optimization of FJP tools and their practical verification

This article presents the recent achievements with Jules Verne, a sub-aperture polishing technique closely related to Fluid Jet Polishing [1]. Whereas FJP typically applies a nozzle stand-off distance of millimeters to centimeters, JV uses a stand-off distance down to 50 μm. The objective is to generate a non-directional fluid flow parallel to the surface, which is specifically suited to reduce the surface roughness [2, 3]. Different characteristic Jules Verne nozzle geometries have been designed and numerically simulated using Computational Fluid Dynamics (CFD). To verify these simulations, the flow of fluid and particles of these nozzles has been visualized in a measurement setup developed specifically for this purpose. A simplified JV nozzle geometry is positioned in a measurement setup and the gap between tool and surface has been observed by an ICCD camera. In order to be able to visualize the motion of the abrasives, the particles have been coated with fluorescence. Furthermore, these nozzles have been manufactured and tested in a practical environment using a modified polishing machine. The results of these laboratory and practical tests are presented and discussed, demonstrating that the CFD simulations are in good agreement with the experiments. It was possible to qualitatively predict the material removal on the processed glass surface, due to the implementation of appropriate erosion models [4, 5] in the CFD software.