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MMM LECTURE NOTES FINAL.pdf

Modern manufacturing methods modern manufacturing

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Modern Manufacturing Methods
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Modern Manufacturing Methods Fresh dielectric fluid rushes in, flushing the debris away and quenching the surface of the workpiece. Unexpelled molten metal solidifies to form what is known as the recast layer. The expelled metal solidifies into tiny spheres dispersed in the dielectric liquid along with bits from the electrode. The remaining vapor rises to the surface. Without a sufficient off time, debris would collect making the spark unstable. This situation creates an arc, which damages the electrode and the workpiece. The relation between the amount of material removed from the anode and cathode depends on the respective contribution of the electrons and positive ions to the total current flow. The electron current predominates in the early stages of the discharge. Since the positive ions are roughly 104 times more massive than electrons, they are less easily mobilized than the electrons. Consequently the erosion of the anode workpiece should be greater than that of the cathode. At the end of the EDM action, the plasma channel increases in width, and the current density across the interelectrode gap decreases. With the fraction of the current due to the electrons diminishing, the contributions from the positive ions rise, and proportionally more metal is then eroded from the cathode. The high frequency of voltage pulses supplied, together with the forward servo- controlled tool motion, toward the workpiece, enables sparking to be achieved along the entire length of the electrodes. Figure 5.4 shows the voltage and current waveforms during EDM. Figure 5.6 shows the periodic discharges occurring when using an RC generator in EDM.
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Modern Manufacturing Methods Working principle of EDM process Electrical discharge machining (EDM) is one of the most widely used non-traditional machining processes. The main attraction of EDM over traditional machining processes such as metal cutting using different tools and grinding is that this technique utilises thermoelectric process to erode undesired materials from the work piece by a series of discrete electrical sparks between the work piece and the electrode. A picture of EDM machine in operation is shown in Figure 1. The traditional machining processes rely on harder tool or abrasive material to remove the softer material whereas non-traditional machining processes such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired shape. So,the hardness of the material is no longer a dominating factor for EDM process. A schematic of an EDM process is shown in Figure 2, where the tool and the workpiece are immersed in a dielectric fluid. EDM removes material by discharging an electrical current, normally stored in a capacitor bank, across a small gap between the tool (cathode) and the workpiece (anode) typically in the order of 50 volts/10amps.
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  • Fall '12
  • JeraldBrevick
  • EDM, conventional machining processes, Jet Machining

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