Meaning
Precision polishing processes utilize a smart fluid that changes viscosity in the presence of a magnetic field to remove sub-micron amounts of material from optical surfaces. In high-end optics manufacturing, magnetorheological finishing is employed to eliminate figure errors without inducing subsurface damage.
Fluid Dynamics
Magnetizable carbonyl iron particles and polishing abrasives are suspended in a carrier liquid to form the polishing slurry. When this slurry is pumped onto a rotating wheel and enters a magnetic field, the iron particles align to form a rigid, compliant polishing pad. This localized pad conforms perfectly to the surface of the optic as the glass is swept through the contact zone.
Form Correction
Computer-controlled dwell times allow the machine to remove more material from high spots than from low spots. By varying the speed of the workpiece across the polishing zone, magnetorheological finishing corrects asymmetric errors that conventional pitch polishing cannot resolve. High precision lenses for lithography depend on this method to achieve sub-nanometer wave front accuracy.
Boundary Limit
The viscosity change is temporary and reverses immediately when the slurry leaves the magnetic field. High material costs and the need for continuous slurry maintenance limit the process to high-value optics.
