Study On The Key Techniques Of Deterministic Magnetorheological Finishing

Ultra-high processing accuracy of optical parts and components has been required with the development of science and technology. Not only the high form accuracy but also the low surface roughness has been required. Especially aspherical parts are the most efficient and elegant means of increasing the number of system parameters. The conventional aspherical optics machining method is hard to satisfy the need either in the machining efficiency or the stability of machining accuracy and machining quality. To overcome it, A relatively newfinishing technology-Magnetorheological Finishing(MRP) is researched in this thesis. Itutilizes micro-sized magnetic particle, nonmagnetic polishing abrasives, carrier fluid and hydrodynamic flow in a magnetic field to remove material from a part surface. It is a kind of deterministic process. Base of it, Aspherical parts can be surfaced by computer controlled with high form accuracy and the low surface roughness and the high efficiency. The main aspect of the research is as following:1. A MRF prototype machine tool is developed. Base on the mechanism of MRF, The overall layout of the prototype machine tool is determined. The gradient magnetic field, which is the key of MRF, is analyzed by means of scalar magnetic potential. The magnetic force acting on the carbonyl iron in magnetorheological fluid also is analyzed. The magnetic condition is deduced to meet the MRF requirement. The prototype machine tool can be used to surfacing optical parts. It is verified by the experiments.2. MR fluid that can be applied in polishing of optical parts is researched. Based of analyzing the property of MR fluid and the influence of the constituent to the polished optical surface, the components of the MR fluid are determined. The "standard" MR fluid consists of 30 vol% carbonyl iron as the magnetic component, 58 vol% water as the carrier fluid, 8 vol % cerium oxide as the abrasive and 4 vol% fluid stabilizers. A new type of magnetorheometer is developed to measure the yield stress of an MR fluid in the same field orientation used in polishing. It is shown that the MR fluid that is suitable for finishing of optical parts.3. The mechanism of the MRF is studied. It is shown that the removal in MRF is not due to normal loading of the abrasive but the shear stress. Mechanism of material removal also involves hydration of the glass surface. Deduced from the hydrodynamic lubrication theory of the Bingham plastic flow, the material removal model of MRF is established. The reliability of the model is verified by the experiment.4. The effect on magnetorheological finishing by several parameters is researched by experiments on the MRF prototype machine tool. Those parameters include intensity of magnetic field, the density of polishing abrasive, polishing wheel rotation speed, size of the gap between the optical parts and the polishing wheel. By the use of orthogonal experiment, the influence tothe removal efficiency and the surface quality by those parameters is analyzed. The principle of the optimized parameters selecting is deduced. At last, we get an optical part which RMS roughness is 0.66nm.