Planar oscillation-enabled deterministic magnetorheological micro-polishing

Micro-optics are attracting ever-increasing attention in advanced optical systems owing to their merits of high optical performance and high structure compactness. Although deterministic polishing is crucial for obtaining optically qualified components, the decreased micro-optic feature size imposes significant challenges on the polishing process. To proceed the post-processing technique for micro-optics, we developed a novel magnetorheological micro-polishing (μMRP) method, which features a non-contact and deterministic removal of the workpiece materials. An electromagnetic polishing tool for stiffening the MR slurry was specially designed to tightly focus the high-gradient magnetic field within a small region around the tip, thereby enabling the material removal to occur at the micro-scale. The pulse-width modulation (PWM) control strategy for the excitation currents could continuously refresh the MR slurry in the polishing, as well as precisely control the material removal. Moreover, a planar dual-axial oscillation was employed to move the tool to generate relative motions between the abrasives and workpieces for removing the materials. In practice, smooth Gaussian-shape footprints with full width at half-maximum (FWHM) around hundreds of micrometers were achieved, and the influence of the working gap, oscillation frequency, and duty ratio on the material removal was further investigated. Finally, the deterministic material removal capability was demonstrated by generating a sinusoidal micro-grid surface with an amplitude of 80 nm and a spatial period of 600 μm. The surface roughness and form error were about Sa = 1.027 nm and 6.5 nm (RMS), demonstrating that the developed μMRP is promising for figuring micro-optics.