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SPECIAL SECTION ON THE FLUID MECHANICS AND RHEOLOGY OF NONLINEAR MATERIALS AT THE MACRO, MICRO AND NANO SCALE

Magnetorheological Jet (MR $JetTM$) Finishing Technology

[+] Author and Article Information
William I. Kordonski

QED Technologies® , Inc., 1040 University Avenue, Rochester, NY 14607kordonski@qedmrf.com

Aric B. Shorey, Marc Tricard

QED Technologies® , Inc., 1040 University Avenue, Rochester, NY 14607

J. Fluids Eng 128(1), 20-26 (Apr 17, 2005) (7 pages) doi:10.1115/1.2140802 History: Received May 05, 2004; Revised April 17, 2005

Abstract

Conformal (or freeform) and steep concave optics are important classes of optics that are difficult to finish using conventional techniques due to mechanical interferences and steep local slopes. One suitable way to polish these classes of optics is by using a jet of abrasive/fluid mixture. The energy required for polishing may be supplied by the radial spread of a liquid jet, which impinges a surface to be polished. Such fluid flow may generate sufficient surface shear stress to provide material removal in the regime of chemical mechanical polishing. Once translated into a polishing technique, this unique tool may resolve a challenging problem of finishing steep concave surfaces and cavities. A fundamental property of a fluid jet is that it begins to lose its coherence as the jet exits a nozzle. This is due to a combination of abruptly imposed longitudinal and lateral pressure gradients, surface tension forces, and aerodynamic disturbance. This results in instability of the flow over the impact zone and consequently polishing spot instability. To be utilized in deterministic high precision finishing of remote objects, a stable, relatively high-speed, low viscosity fluid jet, which remains collimated and coherent before it impinges the surface to be polished, is required. A method of jet stabilization has been proposed, developed, and demonstrated whereby the round jet of magnetorheological fluid is magnetized by an axial magnetic field when it flows out of the nozzle. It has been experimentally shown that a magnetically stabilized round jet of magnetorheological (MR) polishing fluid generates a reproducible material removal function (polishing spot) at a distance of several tens of centimeters from the nozzle. The interferometrically derived distribution of material removal for an axisymmetric MR Jet™ , which impinges normal to a plane glass surface, coincides well with the radial distribution of rate of work calculated using computational fluid dynamics (CFD) modeling. Polishing results support the assertion that the MR Jet finishing process may produce high precision surfaces on glass and single crystals. The technology is most attractive for the finishing of complex shapes like freeform optics, steep concaves, and cavities.

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Figures

Figure 1

Jet snapshot images (velocity=30m∕s, nozzle diameter=2mm)

Figure 2

Experimental setup

Figure 3

MR Jet process flow diagram

Figure 4

Example of spots taken on fused silica glass at different jet velocities. The dashed white line gives the orientation of the profile shown in Fig. 5.

Figure 5

MR Jet spot profiles of spots taken with a stand-off distance of 50mm (filled makers) and 150mm (open markers)

Figure 6

Subtraction of the full material removal maps (spots) from Fig. 5 showing insensitivity to stand-off distances

Figure 7

(a) Image showing four 100nm deep spots taken in succession; (b) standard deviation map of the four spots with a peak-to-valley of 4.2nm

Figure 8

Demonstration of ability to correct figure to extremely high precision with good convergence rate using MR Jet

Figure 9

Roughness map of a fused silica surface polished with MR Jet (NewView 5000 White Light Interferometer, 20× magnification)

Figure 10

(a) Photograph of a concave ogive, showing the glass insert at the center of an aluminum body, and (b) schematic of the ogive during polishing—the internal surface is kept normal to the impinging jet. The diameter of the aluminum shell is 58mm and the total sag of the ogive surface is 39mm.

Figure 11

Polishing results using the MR Jet system for the ogive shown in Fig. 9. Before MR Jet polishing, the p-v was 210nm and the rms was 50nm; after, the p-v was reduced by 5× and the rms by 8× to 44nm and 6nm, respectively.

Figure 12

Flow curve of an MR fluid outside of the magnetic field

Figure 13

Theoretical and experimental removal rate and rate of work profiles

Figure 14

Theoretical and experimental removal rate and rate of work profiles; v=jet velocity

Figure 15

Peak material removal rate versus jet velocity; d=diameter (mm), η=dynamic viscosity (Pa∙s)

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