The electric spark is a self-excited discharge. Before the discharge, there is a high voltage between the two electrodes. When the two electrodes are close, after the medium is broken down, a spark discharge occurs, and the resistance between the two electrodes is accompanied by the breakdown process. It sharply becomes smaller, and the voltage between the two poles also becomes sharply lower. The spark passage must be extinguished in time after maintaining a short period of time (usually 10-7-10-3 s) to maintain the "cold pole" of the spark discharge. The characteristic (that is, the thermal energy of the channel energy conversion is too late to pass to the depth of the electrode), so that the channel energy acts on a very small range, and the action of the channel energy can locally corrode the electrode. The method of processing the material by the corrosion phenomenon generated by the spark discharge is called electric discharge machining, which is also called electric discharge machining or electro-erosion machining, which is referred to as EDM.
In 1943, the Soviet scholar Lazarenko discovered and invented EDM, and then developed rapidly with the improvement of pulse power and control systems. The pulse power supply originally used was a simple resistor-capacitor circuit. In the early 1950s, it was improved to a resistor-inductor-capacitor circuit. At the same time, a so-called long pulse power source such as a pulse generator is also used to improve the etching efficiency and reduce the relative loss of the tool electrode. Subsequently, high-frequency pulse power sources such as high-power tubes and thyristors appeared, which increased productivity under the same surface roughness conditions. In the mid-1960s, transistor and thyristor pulsed power supplies were introduced, which improved energy efficiency and reduced tool electrode losses, and expanded the adjustable range of rough finishing. In the 1970s, high-low voltage composite pulses, multi-loop pulses, equal-amplitude pulses, and adjustable waveform pulses were introduced, and new progress was made in processing surface roughness, machining accuracy, and reducing tool electrode losses. In terms of the control system, the discharge gap is simply maintained from the beginning, and the advance and retreat of the control tool electrode is gradually developed to use a microcomputer to timely control various factors such as electrical parameters and non-electrical parameters.
When performing EDM, the tool electrode and the workpiece are respectively connected to the two poles of the pulse power source, and immersed in the working fluid, or the working fluid is charged into the discharge gap. The tool control tool electrode is fed to the workpiece through the gap automatic control system. When the gap between the two electrodes reaches a certain distance, the pulse voltage applied on the two electrodes breaks down the working fluid to generate a spark discharge.
Instantly concentrating a large amount of thermal energy in the fine passage of the discharge, the temperature can be up to 10,000 degrees Celsius or more, and the pressure also changes drastically, so that the local trace amount of the metal material on the working surface is immediately melted, vaporized, and explosively splashed into the working fluid. Medium, rapidly condensing, forming solid metal particles, which are carried away by the working fluid. At this time, a slight pit mark is left on the surface of the workpiece, the discharge is short, and the working fluid between the two electrodes is restored to the insulation state. Immediately thereafter, the next pulse voltage is broken down at another point where the two electrodes are relatively close, generating a spark discharge, and the above process is repeated. Thus, although the amount of metal etched by each pulse discharge is extremely small, since there are tens of thousands of pulse discharges per second, more metal can be etched, which has a certain productivity. Under the condition of maintaining a constant discharge gap between the tool electrode and the workpiece, the tool electrode is continuously fed to the workpiece while the workpiece metal is being etched, and finally the shape corresponding to the shape of the tool electrode is processed. Therefore, as long as the shape of the tool electrode and the relative movement between the tool electrode and the workpiece are changed, various complicated profiles can be processed. Tool electrodes are commonly used to resist electro-erosion materials such as copper, graphite, copper-tungsten alloys and molybdenum, which have good electrical conductivity, high melting point and easy processing. During the machining process, the tool electrode also has losses, but less than the amount of metal removed from the workpiece, even close to no loss. As the discharge medium, the working fluid also plays the role of cooling and chip removal during the processing. Commonly used working fluids are mediums with low viscosity, high flash point and stable performance, such as kerosene, deionized water and emulsion. According to the form of the tool electrode and its relative motion with the workpiece, the EDM can be divided into five categories: EDM forming by means of a forming tool electrode, simple feed motion with respect to the workpiece; The wire is used as a tool electrode, and the workpiece is moved in a track shape according to a desired shape and size to cut a wire-cutting process of the conductive material; using a wire or a shaped conductive grinding wheel as a tool electrode for performing small hole grinding or forming grinding Spark grinding; EDM slewing for machining thread ring gauges, thread plug gauges, gears, etc.; other types of machining such as small hole machining, marking, surface alloying, surface strengthening, etc. EDM can process materials and complex shape workpieces that are difficult to cut by ordinary cutting methods; no cutting force during machining; no defects such as burrs and knife marks; tool electrode materials do not need to be harder than workpiece materials; Automated; the surface of the surface is deformed after processing, and must be further removed in some applications; the treatment of smog pollution caused by the purification and processing of the working fluid is troublesome.
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