| The use of aspherical surfaces in modern optical systems is growing due to higher demands on image quality. And the demands on the accuracy of their shape and on the quality of their surfaces also continue to grow. The aspherical optics are difficult to manufacture if they deviate from sphere too large for the varying curvature. Therefore deterministic polishing by sub-aperture is the rather feasible method to manufacture asphircal surfaces. Especially there are no effective methods for the finishing of steep aspherics, cavities with big ratio of length-diameter and small-scale aspherical optics, which are difficult to finish due to mechanical interferences and steep local slopes. Magnetorheological Jet Polishing (MJP) researched in this thesis is a sub-aperture deterministic polishing technology, which combines the characteristics of both Fluid Jet Polishing and Magnetorheological Finshing technology. In MJP, the jet of magnetorheological (MR) fluid is magnetized by an axial magnetic field when it flows out of the nozzle. And the MR jet is concentrated, and collimated into a stable slender jet, which may travel a long distance without loss of structure. Therefore the stable jet results in a stable removal function. MJP is advantageous for shape corrections and roughness reduction of the above-mentioned special aspherical surfaces. The main content includes:1. A MJP prototype machine tool is designed. The overall layout of the prototype is determined based on the mechanism of MJP. And the magnetic field for MJP is analyzed. The shape of the nozzle tip is optimized by Finite Element Method. And the magnetic field is concentrated, collimated, and shaped in the vicinity of the tip of the nozzle.2. The stabilization of MR jet is analyzed based on the theory of ferrohydrodynamics and hydrodynamics of jet. First the micro-structure of MR fluid under the external magnetic field is analyzed. Then the motion of MR jet is numerically simulated based on the basic equations of continuous media and characteristics of MR fluid. At last experiments are conducted to research the stabilization of magnetic field on the jet. The numerical and experimental results demonstrate that the axial magnetic field can stable the MR jet. As a result, the stable MR jet may travel up to tens of centimeters without significant divergence or structural breakup.3. The mechanism of material removal in MJP is studied by the method of Computational Fluid Dynamics (CFD). The characteristics of the flow field of MR jet under conditions of both normal and oblique impingements are simulated. Compared with experimental results, the conclusion is gained that the material is removed by shearing actions caused by the radial spread flow of the MR jet over the workpiece surface. 4. The shape corrections and roughness reduction of MJP are studied. The effect of several main parameters on the removal efficiency and surface quality is researched through experiments. Those parameters include intensity of magnetic field, off-set distance, jet velocity, diameter of polishing particles, and so on. A spherical part is polished employing the discrete iterative method to compute the dwell time. And the figure accuracy is improved form 0.566μm PV to 0.176μm PV. To improve the performance, the removal function of MJP is optimized by rotating the nozzle around an eccentric axis. At last the experiment is conducted to remove the waveness. |
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