【作者】 张学成；

【导师】 李圣怡；

【作者基本信息】 国防科学技术大学 ， 机械工程， 2007， 博士

【摘要】 随着现代光学系统对成像质量要求的提高,越来越多的非球面被采用,而且对光学零件的形状精度和表面粗糙度的要求也越来越高。由于非球面曲率变化的影响,光学零件偏离球面越多就越难加工,小磨头确定性抛光是目前加工非球面较可行的方法。但是对于高陡度非球面、大长径比内腔、小尺寸非球面光学零件等由于机械干涉和陡峭的局部倾斜度的限制,还没有有效的抛光方法。本文研究的磁射流抛光技术结合了射流抛光技术和磁流变抛光技术的特点,是一种小磨头确定性抛光技术。该技术利用喷嘴出口附近的局部轴向磁场对磁流变液射流束产生集束稳定作用,保持束径在长距离内基本不变,形成稳定的细长射流束,大大提高了确定性抛光去除函数的稳定性,且由于射流束径较小,适应上述特殊非球面光学零件的修形和抛光加工。本文的主要内容包括:1.根据磁射流抛光技术的特点,设计了一台磁射流抛光样机。对样机中的磁射流抛光所需要的磁场进行了分析,并结合有限元方法优化了喷嘴末端形状,设计出了具有明显集束效果的磁场结构形式。2.应用铁磁流体力学原理和射流理论研究了磁场稳定射流束的机理,分析了磁射流的聚束稳定性问题。从外磁场作用下磁流变液微观结构的演化出发,基于磁流变液的流变力学特性和基本的连续介质力学方程,数值模拟了磁射流运动的特征。最后通过实验研究了磁场对磁射流结构的稳定作用,实验结果与仿真结果表明,轴向磁场可以稳定磁射流,使得它能以没有显著的扩散和结构破坏的准直状态运行几十厘米远的距离。3.应用计算流体力学(CFD)方法分析了磁射流抛光技术的材料去除机理。利用CFD方法,分析了垂直冲击和倾斜冲击情况下磁射流与工件表面相互作用的流场,对比实验研究得到的磁射流抛光材料去除分布特征,证明了磁射流在工件表面径向扩展流动产生的径向剪切作用导致材料去除的机理。4.进行了磁射流抛光技术的修形工艺研究。通过工艺实验总结出各主要工艺参数(磁场强度、抛光距离、射流速度、抛光粉粒径等)对去除效率和表面粗糙度的影响规律。基于离散迭代方法求解驻留时间,对一个口径55mm的球形K9玻璃零件进行面形修正,面形精度从0.566μm PV提高到0.176μm PV。为了提高磁射流的抛光能力,进行了去除函数优化研究。最后进行了磁射流去除波纹度实验。更多还原

【Abstract】 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.更多还原