<title>Magnetorheological finishing of IR materials</title>

Abstract

Magnetorheological finishing (MRF) is a subaperture lap, deterministic process developed at the Center for Optics Manufacturing (COM). MRF can remove subsurface damage from an optical component while correcting figure errors and smoothing small scale microroughness. The "standard" magnetorheological (MR) fluid for finishing of optical glasses consists of magnetic carbonyl iron and nonmagnetic cerium oxide particles in water. This composition works well for a variety of soft and hard glass types, but it does not perform adequately for certain single crystal materials and polycrystalline compounds used in IR applications. In this paper, we describe modifications to MRF and finishing experiments for LW, ZnSe, CaF2, AMTIR-l, ZnS, MgF2 sapphire, and CVD diamond. MRF OF OPTICAL GLASSESIn MRF, a magnetic-field-stiffened ribbon offluid is used to polish out a workpiece. Removal occurs through the shear stress created as the ribbon is dragged into the converging gap between the part and some reference or carrier surface. The zone of contact is restricted to a spot which is smaller than the part diameter. The removal spot constitutes a subaperture lap that conforms perfectly to the local topography ofthe part. Deterministic finishing offlats, spheres, and aspheres is accomplished by mounting the part on a rotating spindle and sweeping it through the spot under computer control, such that dwell time determines the amount ofmaterial removed. The MR fluid lap is unique, because 1) its compliance is adjustable through the magnetic field, 2) it carries heat and debris away from the polishing zone, 3) it does not load up, and 4) it does not loseits shape. A preliminary theory and an extensive review ofthe MRF process are given elsewhere.1'2 1.1 "Standard" MR fluid Most MRF work done to date utilizes an MR fluid consisting ofmagnetic particles of carbonyl iron (CI) in water, with small concentrations of stabilizers added to inhibit oxidation. Although this formulation will polish glass, removal rates are accelerated with the addition of cerium oxide (CeO2). A "standard" formulation that has been used in many ofthe experiments at the COM consists of(given in vol. %) 36/C!, 6/CeO2, 55/water, and 3/stabilizers. This high solids content fluid exhibits an apparent viscosity3 of 0.5 Pasec outside of a magnetic field. In a ragnetic field of 160-240 kA/m (2-3 kG), the apparent viscosity exceeds 10 000 Pasec at a shear rate of 8/sec4, making it stiff enough to support loads for finishing. Figure 1 gives initial size histograms and median particle sizes for the solid particles5. The CI (left top) @ 4.5 tm is in the middle size range for commercially available magnetic powders6, whereas the CeO2 (left bottom) 3.5 im is a coarse, impure, and aggressive commercial polishing abrasive7. The scanning electron micrograph (SEM) of the MR fluid shown on the right in Fig. 1 was taken after a week of use. The round CI particles are essentially unaltered from their initial median particle size, but the irregularly shaped CeO2 particles have been broke...

Research and Prospect of Automobile Magneto-rheological Fluid Dual Clutch Transmission

“…In recent several years , the research about MRF and its relative devices has made great strides, not only that we have got the high-quality MRF with stable performance and the yield strength can reach to 100kPa [3] , and the related applications also extended to the valve, sealing, instrumentation, sensors, polishing and automobile, machinery and other engineering field [4] . At present, the development of magneto-rheological device research mainly focus on the magnetic fluid active damping shock absorber [5] , magneto-rheological valve [6] , magneto-rheological polishing [7] , engine fan clutch [8] , etc. But the research in the automotive clutch which needs to transmit big torque is less and the application technology is still not mature.…”

A multi-scale approach of roughness measurements: Evaluation of the relevant scale

“…[10]. Jacobs et al have found characteristic texture called "orange peel" in the surface of a ZnSe part after MRF (MagnetoRheological Finishing) [11]. A roughness analysis shows that the surface features causing this microroughness are not pits or scratches but are rather hills and valleys on the part surface.…”

Experimental investigation into Ball end Magnetorheological Finishing of silicon

“…Hence, this process requires determinism in controlling the finishing action. MRF process uses the magnetically stiffened magnetorheological polishing A c c e p t e d M a n u s c r i p t 4 (MRP) fluid to deterministically finish optically flat, spherical, and aspherical surfaces down to nanometres level [9,10]. The finishing forces in the MRF process mainly depend on the behaviour of MRP fluid as well as machining parameters such as rotational speed of the tool, working gap between workpiece surface and the tip of the tool, and applied magnetic field strength.…”