Planarization of Gratings using Magnetorheological Finishing
- PDF / 2,140,997 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 3 Downloads / 267 Views
ABSTRACT Surface planarization of BK7 glass gratings with periods of 130, 30, 16, 8 and 5 gm was performed with the magnetorheological finishing (MRF) process at the Center for Optics Manufacturing (COM). Approximately 0.5 gm of material was removed in the experiments. Grating height decreased as a function of grating period, going from -0.44 gm to -0.044 gm for a 130 gm period, and -0.44 jim to -30 A for grating periods of 30, 16, 8 and 5 gm. The microroughness on ridges and in valleys of the grating structures also decreased with material removal from -100 A to -10 A for gratings with 30 and 16 jim periods. INTRODUCTION Interest in field-assisted finishing has increased greatly during the past decade [1-10]. Among the various approaches, the most recently developed process is called magnetorheological finishing (MRF) [5, 6]. The MRF process is best understood by thinking of the magnetorheological (MR) fluid as a compliant replacement for the conventional rigid lap and polishing slurry in loose abrasive and chemomechanical polishing processes [11-131. The MR fluid is an aqueous suspension of magnetic carbonyl iron particles and polishing abrasives (typically -36% CI, -55% H20, -6% CeO 2, and 3% stabilizers in volume percent). Application of a magnetic field causes an increase in the flow resistance (such as apparent viscosity and yield stress) of the magnetorheological suspension and enhances the chemomechanical interaction between the polishing agents and workpiece. This causes material removal and polishing of the workpiece surface. The technical importance is that, unlike a rigid lap, the fluid's shape and stiffness can be manipulated and controlled in real time. This allows for the deterministic production of spheres, cylinders, toroids, and aspheres using minimal specialized tooling.
Fig. 1 The preprototype MRF machine 11
01997 Materials Research Society
MRF REMOVAL FUNCTION The MRF removal function in the zone of high pressure is specific to the machine platform, the magnetic field strength, the workpiece geometry, and the properties of the material being finished. All experimental results reported in this paper were obtained on the preprototype MRF machine as shown in Fig. 1.Fig. 2 shows the fluid flow direction and removal "spot" for a 40 mm diameter, 84 mm convex radius of curvature BK7 glass lens, immersed in the MR fluid for 5 seconds. The spindle arm was oriented at an angle of 0=20 and it was locked to prevent workpiece rotation. Interferometrically derived depth profiles show that the removal function has a backwards "D" shape, with region of peak removal at the point of deepest penetration of the lens surface into the suspension. The peak removal rate is 4.6 irm/min, and the volumetric 3 GOOD removal rate is 0.48 mrn /min. ---------I Fig. 3 shows the relative volumetric t"•" removal rates (normalized to BK7 glass) as a I function of hardness for different materials, a2 4 e0 measured under identical MRF process Dwt W* conditions. Removal rates are seen to increase with decreasing hardness for a v
Data Loading...
