Key Technology Of High-aspect-ratio Micro Hole EDM

Micro hole with high-aspect-ratio is widely used in key parts of core equipments in fields of aerospace industry, energy power, carrying and medical appliance. Because diameter is small, aspect ratio is high and part materials are difficult to machine, the efficient and steady machining of the hole is quite difficult. EDM (electrical discharge machining) become a machining method of high-aspect-ratio micro hole since it does not has macroscopic force, is not influenced by material mechanical properties and convenient tool electrode preparation. However, because of the typical characteristics of the high-aspect-ratio micro-hole EDM, such as weak discharge energy, high pulse frequency, high wave distortion and small discharge gap, etc., there are still some difficulties in its machining process which need to be solved: technology rules are complicated, discharge states identification are inaccuracy, machining process controlling are instability, discharge debris removing is difficult, machining system precision improvement is difficult and machining efficiency is low, etc. This paper focuses on some key technical difficulties mentioned above, and works carried out are as follows: efficacious removing method of gap debris, development of high-precise and fast-response machining system, research and implementation of real-time discharge states detection method and intelligent control strategy of machining process, investigation of technology rules of high-aspect-ratio micro hole EDM and quality evaluation of high-aspect-ratio micro hole.In order to remove gap debris effectively, removing mechanism of debris is investigated by hydrodynamics method. Firstly, SEM (scanning electron microscope) is used to observe shapes of debris particles, and the results indicate that most of the debris particles generated in working fluid of deionized water is spherical. Secondly, LPSA (laser particle scatter analyzer) is used to measure the size of debris particles, and Weibull distribution of the debris particle size is obtained by fitting the histogram of the measuring results. Lastly, the model of the hydrodynamics analysis is built by debris particle function described with Weibull distribution, and debris removing states are simulated in four styles, that is electrode rotation, electrode rotation-vibration, workpiece vibration and electrode rotation. Calculation results indicate that the case of workpiece vibration and electrode rotation is the best efficacious to remove gap debris.To satisfy the high'precise and fast response of electrode feed requirements, the high-aspect-ratio micro hole EDM servo feed system of a linear motor drive is developed. Based on the results of hydrodynamics analysis, a rotation-vibration mechanism which consists of motorized spindle and piezoelectric ceramic actuator is applied to remove gap debris. Electrode wear has negative effect on machining efficiency. In order to reduce this effect, the cold drawing wire of 30cm long which is equipped with guide structure is chosen as electrode, and continuous feed is applied. To decrease the interference of stray capacitance the machine bed of marble material is applied. The high-aspect-ratio micro hole EDM machine is built by integrating all the mechanisms mentioned above. The hardware of the control system contains industrial computer, PCI1714 data sampling card and PMAC multi-axis motor control card. The software system is designed and developed on VC++on Windows operating system, which integrates the functions of data acquisition, pulse power setting, motor motion control, machine state display, etc.Based on two-stage fuzzy logic a control method of high-aspect-ratio micro hole EDM is presented to achieve the high efficiency and high stability of the machining process. Discharge states judging rules of sample points are established according to the properties of high-aspect-ratio micro hole EDM. Discharge states of all the sample points are classified by the first stage fuzzy logic with the judging rules. And discharge states of every calculating cycle are obtained by counting number of every discharge states. The servo strategy is deduced from the cycle discharge states obtained in the first stage fuzzy logic with the second stage fuzzy logic. A double-closed-loop control system of high-aspect-ratio micro hole EDM is designed by the method presented above, and a fuzzy logic controller of machining process is built. In order to improve the effects of the high-aspect-ratio micro hole EDM fuzzy control, two-stage fuzzy logic membership functions are optimized by neural network combined with BP algorithm which applies the sampling data during processing as training data.On account of the complex machining process and the unintelligible technology rules of high-aspect-ratio micro hole EDM, technology rules are investigated by dealing electrical parameters and nonelectrical parameters respectively. The nonelectrical parameters include working fluid and electrode material. The electrical parameters include voltage, current, pulse duration, pulse interval and capacitance. Technology experiments are carried out to obtain the optimal working fluid of different workpiece materials. Wear ratio between different electrode materials and different workpiece materials is obtained by chemical composition analysis of discharge products. The optimal electrode material of different workpiece materials are acquired by combining the wear ratio and machining experiments. In the case of optimal combination of workpiece materials, electrode materials and working fluid, the effects of electrical parematers on machining efficiency, electrode wear and discharge gap are studied by orthogonal experiment. High-aspect-ratio micro hole EDM experiments are carried out based on the debris removing method in the second section, the machining system in the third section, the discharge detection and machining process controlling in the fourth section and the technology rules in the fifth section. Three holes which have average aspect ratios of 10.71, 21.2 and 23.25 respectively are machined successfully in the experiments, and the machining process is stable. Meanwhile, experiment results indicate that the machined high-aspect-ratio micro holes have conicity. To control machining error and improve machining quality, methods of evaluating the roundnees and cylindricty of high-aspect-ratio micro holes are investigated. Because diameter of high-aspect-ratio micro hole is small it is difficult to measure directly. Evaluation methods of roundnees and cylindricty are presented by using digital micrograph processing technology. Error analysis of measurement results indicate that the maximum measuring relative errors of the roundnees and cylindricty are 11.98% and 11.22% respectively.In this paper, a high-aspect-ratio micro hole EDM machining system based on linear motor drive is developed, which exhibits high precision and fast response speed. The hole with diameter 0.1-0.5mm and high-aspect-ratio more than 10 can be obtained steadily by using the machining system. It is proved that the machining system based on discharge debris removing method is effectual. The machining process control method presented is applied to high-aspect-ratio micro hole EDM. Technology optimization is an effective approach to improve the machining efficiency of high-aspect-ratio micro hole EDM.