| Micro electrical discharge machining (micro EDM) is a nontraditional machining method to realize the manufacturing of micro scale parts and features by miniaturizing conventional electrical discharge machining (EDM) technology. Because of the advantages of noncontact machining and almost no cutting force within the processing, micro EDM can be applied in fabricating smaller parts and micro features with high precision and no distortion. For micro EDM processing is not limited by the strength and hardness of materials, it is very fit for the micro fabrication of hard and brittle materials. Additionally, micro EDM has been regards as one of the most potential methods to produce three-dimensional (3D) microstructures with complex structure owing to its outstanding compatibility with numerical control techniques. Therefore, micro EDM has become an important direction in micro manufacturing field, and has been more and more widely used to machining key parts in the fields of aerospace, electrommunication, mould, optical and medical instruments.However, the research on machining mechanism and technology of micro EDM is still not enough. In the term of mechanism research, as the EDM itself is a complex, transient and random process, and the decrease of machining scale, changes of machining conditions, especially the effect of high frequency pulses, add more uncertainty to the physical process of micro EDM; at the respect of technology research, the tool wears and the influences of discharge machining on surface properties seriously affect the machining accuracy and quality of micro EDM. These existing problems limit the further development of micro EDM technology. Thereby, by starting from the machining condition changes in miniaturization of EDM, it has an important significance to study unique discharge mechanism of micro EDM, discuss the influence of high frequency pulses on machining process, analyze the causes of tool wear and methods to reduce wear, and summarize the law of surface characteristics and methods to improve surface quality in micro EDM.The above problems in micro EDM are focused on in this thesis, and four main research aspects have been carried out as follows:(1) Study on the discharge mechanism of micro EDM. The machining condition changes in the process of EDM miniaturization and the influences of these changes on micro EDM processing are analyzed. Though the characteristic analysis of kerosene dielectric, a series of state changes of microscopic particles caused by the impact of external energy are discussed before the discharge breakdown, and the microscopic situations that various micro materials and energy interacts in discharge breakdown, spark maintenance and deionization processes are proposed; the model of positive ion field electron emission is present forward, this complement the streamer theory in the lack of ionized particle number; the electrode material removal process and energy conversion problem are conducted.(2) Study on the influences of high frequency pulses on micro EDM. The skin effect in high frequency electromagnetic field is introduced to analyze the special impact of high frequency pulses, and the influences of skin effect on micro EDM is demonstrated with electromagnetic field theory and experiments; nonuniform distribution laws of electric field intensity and current density under high frequency pulses are summarized, and based on this, the spark location change, form of material removal, tool shape change are analyzed; the online electrode modification method to obtain different tool end shape from changing machining pulse frequency is experimental studied, and the method through adopting the smooth surface of different tool shapes after electrode modification is tried to manufacture freeform surface.(3) Study on relative technology of tool wear. Composite plating is used to plating electrical erosion resistant material on tool surface for the fabrication of wear-resist electrodes. The experimental study proves that this method effectively reduces the edge wear in deep hole machining and improves the processing precision of micro holes; a way to study tool wear, tool end shape change based on image processing is put forward, through the contour extraction of worn tool from electrode image after machining and curve fitting of contour data, a new path to digital computation and analysis tool wear in micro EDM is opened; the reasons causing unpredictable failure of tungsten tool in micro electrical discharge milling are conducted, by simulation study of micro flow field, the influencing factors of alternating pressure of dielectric fluid causing the failure are revealed, and the harm of this unpredictable failure and its preventive measures are discussed.(4) Study on surface properties of electrode in micro EDM. The electrode surface topography after micro EDM is experimentally observed, the forming processes of surface discharge craters, globules of debris and micro spherical holes are focused on; the crack phenomena on electrode surface are investigated, different crack formations of various cracks are analyzed, and based on this, cracks are divided into groups, the restraining measures of every type of crack are discussed; surface exfoliation phenomenon is observed, the causing reasons of surface exfoliation are analyzed through the interlacing and expanding of cracks and surface forces, some methods to avoid surface exfoliation are proposed.These studies not only further reveal the physical essence of discharge process in micro EDM, but also provide a basis for widespread applications of micro EDM technology. |
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