Research On Key Techniques Of Micro Machining Of Micro-Structure On Hard And Brittle Non-Metallic Materials

Micro machining of hard and brittle non-metallic material has been a challenging issue in manufacturing areas. Microcrystalline-mica-ceramics (MCMC) and polycrystalline diamond (PCD) are two typical hard and brittle non-metallic materials and have broad application prospects. As a kind of ceramics, MCMC provides excellent insulation, high temperature resistance, low thermal conductivity and other good properties. The thermal conductivity of MCMC is less than one-tenth of that of silicon. The heat loss in a thrust system will be greatly reduced, if MCMC is used as combustion chamber and nozzle materials in a micro-thruster. As a superhard material, polycrystalline diamond (PCD), is of uniform high hardness, superior wear and corrosion resistance and many other excellent properties, and it is an ideal material for micro-hole parts and has wide potential applications in aerospace, automotive and electronics industries. The micromachining performance of MCMC and PCD were studied in this dissertation by using micro ultrasonic machining (micro-USM) and micro electrical discharge machining (micro-EDM), respectively.For generating the tools for micro-EDM and micro-USM, block electrode discharge grinding (BEDG) and edge electrode discharge grinding (EEDG) methods were investigated. In the study of BEDG, an irregular shaped block with a slope was used as a sacrificial electrode that copied its shape onto the workpiece. This method was used in rough machining of conical electrode to improve the machining efficiency. The flexibility of EEDG process was investigated using different tool paths designed for CNC. Different methods of tool electrode wear compensation for fabricating micro electrodes were carried out for forming various electrode shapes.MCMC was chosen as a case study to resolve the major problems of micro Laval-nozzle machining techniques. Micro-USM tools were prepared online quickly and accurately by a method combined with BEDG and EEDG. The experiments designed by orthogonal-experiment method were adopted to investigate the influences of machining conditions on tool wear ratio in the micro-USM of MCMC. Base on the experiments, a group of optimized micro-USM parameters were chosen. The behavior of abrasives in micro-USM slurry flow was analyzed. The necessity of pre-drilled hole was emphasized in the machining of micro Laval-nozzle. Furthermore, a micro Laval-nozzle was made on a MCMC piece with a throat diameter of 170?m.Base on the analysis of the relationship between physical properties of a PCD and micro-EDM performance, a series of experiments were carried out to investigate the micro-hole micro-EDM machining performance of PCD. Experimental results indicate that positive polarity machining, in which the workpiece is connected to the cathode, is suitable for micro-EDM of PCD because of the protection brought over by the adhesion on the electrode. In positive polarity machining, an appropriate volume of material adhesion onto the tool electrode can help to increase the material removal rate (MRR) and reduce the relative tool wear ratio (TWR). In contrast, an excessive volume of material adhesion can lead the machining into a vicious circle in which MRR is low, TWR is high and micro-holes are drilled with oversized diameters. An optimal machining conditions were chosen, and a through-hole with a diameter of 145?m and a depth of 650?m on PCD machined under the chosen optimal machining conditions shows satisfactory machining results.On the basis of the investigation into the micro-hole machining performance of PCD by micro-EDM, an approach of micro-EDM combined with ultrasonic vibration act upon workpiece was proposed to improve micro-hole machining performances of PCD. Mechanical calculation of the cavitation bubble collapsing process and the particle motion in the dielectric fluid was carried out. The drop-off effect of diamond particles caused by ultrasonic vibration was analysed with the consideration of the special allographic structure of PCD. Under different machining conditions, a series of experiments were carried out to investigate the micro-hole machining performance of PCD by using ultrasonic enhanced micro-EDM. The experimental result indicates that the discharge status is improved,the arcing duration is reduced and the discharge frequency is increased by the effect of ultrasonic vibration. The adhesion onto the electrode is reduced due to the ultrasonic vibration act upon workpiece, which brings both positive and negative impacts in the machining performance. Therefore, in ultrasonic enhanced micro-EDM, the optimal amplitude chosen as 2?m can expand the optimal ranges of EDM machining conditions, hence improving both machining performance and surface quality.