The MRF process is best understood by thinking of the magnetorheological fluid as a compliant replacement for a conventional rigid sub-aperture polishing lap. The fluid's viscosity is magnetically manipulated, while in contact with a portion of the workpiece surface, to create a "subaperture polishing lap" that conforms to the optical surface. MRF has distinct advantages that eliminate the problems of classical polishing:

• The interferometrically characterized MRF "polishing tool" never dulls or changes.
  
• The MRF "polishing tool " adapts to complex shapes because it is a compliant fluid.
  
• MRF removal rates are very high resulting in short processing times.

About one liter of magnetorheological fluid is loaded into the closed-loop fluid delivery system, where fluid properties such as temperature and viscosity are continually monitored and controlled. The fluid is drawn out of the conditioner and extruded onto a rotating spherical wheel in a thin ribbon that will contact the optical surface. The ribbon is then removed via suction and fed back into the conditioner. An electromagnet, located below the polishing wheel, has specially designed pole pieces that extend up to the underside of the apex of the wheel rim. These pole pieces exert a strong local magnetic field gradient over the upper side of the wheel. When the magnetorheological fluid passes through the magnetic field, it stiffens in milliseconds, then returns to its original fluid state as it leaves the field, again in milliseconds. This precisely controlled zone of magnetized fluid becomes the polishing tool. When an optical surface is placed into the fluid in this zone, the stiffened fluid ribbon is "squeezed" from its original thickness of about 2 mm, to about l mm. The "squeezing" results in significant shear stress and subsequent polishing pressure over that section of the optical surface. At the same instant, the MR fluid conforms to the local curvature of the part being polished.

MR Fluid Delivery System

The Q22 MRF process requires two input functions: one to describe the shape distribution and removal rate of the MRF "polishing tool," and another to describe the initial surface error of the optic to be polished and corrected. A PC driven software algorithm performs a deconvolution of the two functions to (l) generate a prediction of how well the MRF "polishing tool" can fix the error, and (2) provide a set of instructions for the Q22's four axis CNC controller. The machine controller positions the lens through the fluid while maintaining a constant gap above the wheel surface, and maintains surface normal for any shape (including plano, spherical or aspheric). Deterministic removal is achieved by varying the dwell time (the time any one point on the lens remains in the stiffened zone) as a function of position of the rotating lens in contact with the MR fluid.