【作者】 蔡兰蓉；

【导师】 胡德金；

【作者基本信息】 上海交通大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 针对基于光学投影或图像在线检测的精密曲线磨床一般不使用工作液的特殊要求,本文提出了以雾状介质作为工作介质的放电修整金刚石砂轮技术。雾中放电修整金刚石砂轮技术是一种新型的特种加工修整超硬磨料砂轮技术。该修整技术减少了对液体介质的依赖,符合绿色制造的潮流,容易实现自动化,可应用于精密磨床的在线修整,具有广阔的应用前景。本文采用放电蚀除砂轮材料机理研究、有限元数值模拟和实验研究相结合的方法,对放电选择性去除砂轮材料基本原理、放电修整砂轮表面瞬态温度场、砂轮表面形貌控制技术等进行了系统的研究,为开发低成本、高效率、适应性广、工业化应用程度高的超硬磨料砂轮电加工修整技术打下理论基础。对雾气中电火花放电蚀除材料机理进行了全面的研究。以气体介质电导和击穿机理为基础,详细分析了雾状介质的电导与击穿机理。雾状工作介质是连续状气体和微细工作液滴混合物,由于电场畸变,雾状介质中电火花放电的击穿电压降低为纯气体击穿电压的1/3左右。从雾状介质的击穿和放电通道形成、能量转换分布和传递、电蚀产物的抛出与极间介质消电离几个方面阐述砂轮表面材料的抛出机理,建立了单脉冲火花放电材料融熔抛出的物理模型。该模型把雾状介质中单个脉冲放电蚀除材料的过程分为三个阶段:电离准备阶段、放电热蚀阶段和消电离抛出阶段。针对传统电火花放电修整非导电和弱导电超硬磨料砂轮的局限性,提出通过辅助放电材料实现雾中放电修整非导电金刚石砂轮的技术方案。建立了放电去除砂轮表面金属结合剂和树脂结合剂以使金刚石磨粒突出的物理模型。详细分析了金刚石磨粒与结合剂两种材料热电物理性能差异,从材料学、传热学和电火花加工学三个方面,阐述了放电修整金刚石砂轮的选择性蚀除砂轮组成材料以实现砂轮整形或修锐的基本原理。并通过运动放电点热源作用于砂轮表面的温度分布解析计算,从理论上验证了放电修锐金刚石砂轮选择性去除结合剂的可行性。采用参数化编程建立了放电修整青铜结合剂金刚石砂轮三维瞬态温度场模型并对其进行了较为全面的数值模拟研究,得到了得到了青铜结合剂熔融和金刚石磨粒修整过程温度场的分布和变化规律。模拟结果表明,青铜结合剂熔融层和金刚石磨粒不同层面的最高温度均随着峰值电流的升高而升高;脉冲宽度对青铜结合剂和金刚石磨粒最高温度的影响不明显;青铜结合剂熔池的半径和深度随着峰值电流和脉冲宽度的升高而加大;青铜结合剂熔融层最高温度及达到蚀除温度的结合剂深度随着砂轮移动速度的增加而减小。利用所建立的金刚石砂轮温度场的计算机仿真系统可以对砂轮修整过程进行工艺参数调整及优化,为后续青铜结合剂和树脂结合剂金刚石砂轮的修整实验奠定了基础,有利于实现高效去除青铜和树脂结合剂且避免过高温度使金刚石颗粒石墨化丧失磨削工作能力的修锐目标。在理论分析砂轮材料选择性去除机理和放电修整砂轮温度场数值模拟基础上,在电火花成形机床上对青铜基金刚石砂轮和树脂结合剂砂轮进行了一系列不同工艺参数的放电修锐实验。实验研究了放电峰值电流、脉冲宽度、砂轮移动速度、工作介质、开路电压等工艺参数对砂轮表面形貌及修整效率的影响。利用KEYENCE三维数字显微镜观察电火花修锐前后金刚石砂轮表面的微观形貌。修锐前后金刚石砂轮表面的表面峰点高度用Vecco公司生产的Dektak6M探针表面轮廓仪来测量。不同工作介质实验结果表明,雾状乳化液修锐的砂轮表面形貌最佳,空气中修锐的砂轮表面金刚石磨粒石墨化现象最严重且修整效率低。在雾状乳化液中放电修锐青铜基金刚石砂轮实验结果表明,放电峰值电流和砂轮运动速度是影响砂轮表面形貌及金刚石磨粒性能的主要因素,放电脉冲宽度对砂轮表面金刚石磨粒形状特征的影响作用不明显,峰值电流和脉冲宽度影响是修整效率的主要因素,开路电压对砂轮表面形貌和修整效率均没有影响。放电修锐树脂结合剂砂轮实验结果表明:本文提出的树脂结合剂砂轮表面涂覆一层辅助放电材料后再进行表面放电修整的技术方案是可行的;放电修锐工艺参数对树脂结合剂砂轮表面轮廓形貌和修整效率的影响规律与对放电修锐青铜结合剂的影响规律基本一致;但由于树脂结合剂的碳化分解温度远远低于青铜结合剂的熔点,因此适合修整的放电峰值电流Ie为1～3A。通过对比不同修锐工艺参数修锐后青铜金刚石砂轮的磨削力可知,砂轮磨削能力主要取决于磨粒裸露高度、工作磨粒数量及是金刚石磨粒形状的完整性。本实验中所用金刚石砂轮磨粒的合理裸露高度为:35μm～50μm。更多还原

【Abstract】 A novel truing and dressing method on superabrasive grinding wheel, namely mist-jetting electrical discharge truing and dressing (MEDD), has been proposed in this paper. MEDD process eliminates vast dielectric liquid and produces less corrosion and pollution to function units in contrast with conventional EDD. It is a green manufacturing technology and has an extensive application prospects on truing/dressing system on-line of precision grinding machine. In this paper, the mechanism of the selective removal of the bond, the transient temperature fields as well as wheel surface profiles and topography control technologies of MEDD process have been systematically studied combining methods of theoretical analysis, FEM numerical simulation and experimental study .It will have an important theoretical value and practical significance not only for the cost reducing and increasing efficiency but also for promoting the development and application of MEDD technology on preparation of superabrasive grinding wheels.The author proposed a complete study on the material removal mechanism of EDM with misted dielectric. The conductance and disruption of misted dielectric are analyzed systematically on the basis of gas medium. Electric field distortion is caused within misted dielectric for the reason that misted dielectric is composed of continuous gas and tiny liquid drops. Consequently, the disruption voltage is about a factor of three than that for gas medium. The material removal mechanism of EMDD process is analyzed on four aspects, which are the disruption of misted dielectric and formation of discharge channel, the conversion of discharge energy, the materials’ shoot off and deionization process and a model of single pulse discharge is established. The model shows that the process of material removal for a single pulse discharge includes three stages, ionizing, electrical discharge, and de-ionizing.Conventional EDD technology shows important limitations that it is only applicable to metal-bonded wheels. This paper proposed novel MEDD technology which could truing/dressing non-electrical superabrasive grinding wheels such as resin-bonded diamond wheel and vitrified CBN wheel by means of covering electrical material on the surface of wheel. The principle of removing metal-bond and resin bond during MEDD process is illustrated. The thermal and electrical physical properties differences between bonds and diamond abrasives are analyzed. At the same time, the mechanism of selective removal of the bond and revealing diamond grains is completely clarified on three aspects, which are material science, conduction of heat and electrical discharge machining theory. The calculattion results of temperature distribution indicate that MEDD technology is feasible for metal-bonded and resin-bond superabrasive grinding wheel.Three-dimensional finite element model of transient temperature fields in MEDD process has been built up and relatively comprehensive numerical simulation has been carried out with parametric programming methods. The temperature distribution and variation rules of bronze-bond and diamond grain have been obtained. Simulation results indicated that the maximal temperature of bronze-bond and diamond grains at different layers increase with discharge current. It also reveals that the depth and width of bronze-bond crater increase with the discharge current and pulse duration. However, the maximal temperature of bronze-bond and diamond grains does not increase too much with the growth of pulse duration. At the same time, the maximal temperature of bronze-bond and the depth of molten bronze-bond go down as the speed of grinding wheel increases. The simulation model is benefit to quality forecast, parameter regulation and optimization in the process of truing and dressing bronze-bond diamond grinding wheel. It also helps to avoid that high temperature make diamond grain graphitization or the temperature is not high enough to remove the metal bond well. The simulation model can keep from the blindness to true and dress diamond wheel directly.A series of dressing experiments were carried out on a TROOP434 Die-Sinking Electrical Discharge Machine according to bond selective removal mechanism and temperature fields numerical simulation results. The influence of process parameters including discharge current, pulse duration, speed of wheel, discharge voltage and different dielectric fluid on dressing quality and efficiency was systematically studied with experiments. The condition of the wheel before and after MEDD was analyzed by studying the wheel surface topography. Wheel surface topographies before and after dressing were observed by optical microscope (model: KEYENCE VH-8000 Digital HD); a Dektak 6M profilometer has been used in this work to quantify the grit protrusion before and after dressing. Different dielectric experiments results showed that wheel surface topography with emulsion mist is the best, and that the dressing quality and efficiency of experiments with gas were the worst. Dressing bronze-bond diamond wheel experiments results indicated that discharge current and speed of grinding wheel had direct influence on the wheel surface topography and the performance of diamond grains. As well as, pulse duration had less influence on the wheel surface topography. The experiments results also revealed that discharge current and pulse duration were chief parameters which can affect dressing efficiency, and that discharge voltage had no influence on dressing quality and efficiency.Dressing resin-bond diamond wheel experiments results indicated that the EMDD method proposed in this paper has been applicable to non-electrical bond wheels. Simultaneity, dressing process parameters have the same influence on dressing quality and efficiency as dressing bronze-bond diamond wheel experiments. However, the appropriative discharge current in dressing resin-bond diamond wheel process was much less than one in dressing bronze-bond diamond wheel for the reason that the decomposing point of resin-bond is 200-400℃is less than melting point of bronze-bond. The results of grinding forces measurement showed that the performance of wheel depended on the protrusion height of diamond grain and the number of cutting grains on the wheel surface. The appropriate protrusion height of diamond wheel used in experiments was 35μm～50μm.更多还原