Q&A: What is MRF technology? How is it different from other polishing techniques?

NASA Tech Briefs, May 2004 by Tariff, Matthew

MRF stands for magnetorheological finishing, a deterministic polishing technique that utilizes surface information from an interferometer and a polishing tool whose shape and removal rate is fully characterized and does not change. The technology depends on using a magnetorheological fluid that contains iron and which has a viscosity subject to the effects of magnetic fields.

When the MR fluid is delivered to the moving workpiece, an electromagnet generates a field that causes the fluid to stiffen and becomes a subaperture polishing tool. When the stiffened fluid spins out of the magnetic field, it reverts to a liquid and is recycled through the system. The removal rate of the polishing tool is kept constant by monitoring the flow rate and the pressure within the delivery system and adding deionized water to keep the fluid's viscosity constant, and thus, keeping the polishing tool constant.

The MRF machine's computer uses the known removal rate of the polishing tool and the surface topography of the workpiece to determine a dwell schedule - how much time the tool should spend on any given spot on the workpiece. In regions where there are high spots on the surface, the machine holds the workpiece over the polishing tool for a greater amount of time. When the polishing run is over, the workpiece is removed and tested on an interferometer, the before and after metrology can then be compared to evaluate the final surface accuracy.

MRF technology routinely offers surface accuracies of 1/20-[lambda] P-V and, with more care, can achieve 1/50-[lambda] P-V or better surfaces. The limit of how well a surface may be polished is largely determined by the quality of the metrology, specifically, the accuracy of interferometric data characterizing the workpiece's surface and the characterization of the removal rate of the polishing tool.

The actual run times on an MRF machine can vary from minutes to days depending on the size and difficulty of the optic. Larger volume runs require some tooling and fixturing. Larger optics or polishing runs that must remove large amounts of material require longer process times. The MRF requires a polished surface to begin with, and any subsequent operations (such as centering or coating) add to turnaround times.

Alternatively, traditional polishing techniques - using adjustable stroke spindle machines - are capable of routinely producing commercially available optical flats with surface accuracies of l/4-[lambda] or 1/10-[lambda] peak-to-valley. Accuracies of 1/20-[lambda] or better are possible, but processing may climb to several days for a single optic. Many iterations between polishing the piece on a spindle, then removing and cleaning the part to check progress on an interferometer, then returning the piece to the spindle for additional corrections may be required. The number of iterations required to achieve precision flats is largely determined by the skill and experience of the optician.

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Matthew Tariff, an optical manufacturing engineer at Edmund Industrial Optics, contributed this month's Q&A. He graduated from the University of Rochester with a B. S. in Optics in 2002. For more information email techsup@edmundoptics.com. Visit Edmund online at www.edmundoptics.com.