*pim.messelink@fisba.ch; phone +41 71 282 33 12; fax +41 71 282 31 30; www.fisba.ch & www.fh-aargau.ch Optimization of Fluid Jet Polishing CNC tool design Wilhelmus A. C. M. Messelink* a , Reto Wäger a , Mark Meeder b , Herbert Looser a , Torsten Wons b , Kurt C. Heiniger a , Oliver W. Fähnle b a Fachhochschule Nordwestschweiz, Klosterzelgstrasse 2, CH-5210, Windisch b Fisba Optik AG, Rorschacher Strasse 268, CH-9016 St. Gallen ABSTRACT 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 nozzles 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 specifically developed 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. Keywords: Polishing, Shaping, Optical Fabrication, Fluid Jet Polishing 1. INTRODUCTION A variation of the Fluid Jet Polishing (FJP) technique is arbitrarily named Jules Verne (JV). Similar to PACE (Plasma Assisted Chemical Etching) a small cup-wheel like nozzle is positioned close to the glass surface to be processed. FJP slurry is fed into the nozzle along its axis of symmetry, and because of the circular gap between tool and workpiece the suspension is radially accelerated to high velocities depending on the working pressure and the standoff between the nozzle and the glass surface. Thus a FJP like material removal is achieved along the edge of the rotating nozzle, see Figure 1. The interaction between solid particles at high kinetic energy and penetrated surfaces was already reported by different authors [5-7]. 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]. 2. SIMULATIONS The geometry of the JV nozzle determines the flow conditions between the tool and the surface being polished. To simulate the place of erosion on the glass, caused by the slurry flow, the nozzle geometry needs to be drawn, meshed and implemented in the CFD software. Several characteristic nozzle geometries have been selected for analysis. In order to reduce computing time, the geometries were simulated solely over a sector of 45°, see Figure 2. After simulating the flow conditions in the nozzle with a 2-equation turbulence model, particles, typically 1 μm in diameter, were simulated to be fed through the slurry inlet, starting at random positions. These particles were assumed to follow the fluid flow absolutely, because the effect of the drag force acting on the particles is large with respect to the difference of inertia between fluid and abrasive. To simulate the particles impacting on the glass surface and predict material wear, Finnie’s erosion model has been used [4]. To be able to predict the material wear downstream of the gap, the flow needs to be simulated in this region as well. This results in a two phase model simulation (suspension expanding into surrounding air). But the results of different experimental tests, not presented in this paper, have shown,