Machinery's Handbook, 31st Edition
1476 ELECTRICAL DISCHARGE MACHINING The fine grain sizes and high densities of graphite materials that are specially made for high-quality EDM finishing provide high wear resistance, better finish, and good repro duction of fine details, but these fine grades cost more than graphite of larger grain sizes and lower densities. Premium grades of graphite cost up to five times as much as the least expensive and about three times as much as copper, but the extra cost often can be justified by savings during machining or shaping of the electrode. Graphite has a high resistance to heat and wear at lower frequencies, but will wear more rapidly when used with high frequencies or with negative polarity. Infiltrated graphites for EDM electrodes are also available as a mixture of copper particles in a graphite matrix, for applications where good machinability of the electrode is required. This material presents a trade-off between lower arcing and greater wear with a slower metal-removal rate, but costs more than plain graphite. EDM electrodes are also made from copper, tungsten, silver-tungsten, brass, and zinc, all of which have good electrical and thermal conductivity. However, because these metals have melting points below those encountered in the spark gap, they wear rapidly. Copper with 5 percent tellurium, added for better machining properties, is the most commonly used metal alloy. Tungsten resists wear better than brass or copper and is more rigid when used for thin electrodes but is expensive and difficult to machine. Metal electrodes, with their more even surfaces and slower wear rates, are often preferred for finishing operations on work that requires a smooth finish. In fine-finishing operations, the arc gap between the surfaces of the electrode and the workpiece is very small, and there is a danger of dc arcs being struck, causing pitting of the surface when particles dislodged from a graphite electrode during fine-finishing cuts are not flushed from the gap. If struck by a spark, such a particle may provide a path for a continuous discharge of current that will mar the almost completed work surface. Some combinations of electrode and workpiece material, electrode polarity, and likely amounts of corner wear are listed in Table 3. Corner wear rates indicate the ability of the electrode to maintain its shape and reproduce fine detail. The column headed Capacitance refers to the use of capacitors in the control circuits to increase the impact of the spark without increasing the amperage. Such circuits can accomplish more work in a given time, at the expense of surface-finish quality and increased electrode wear. Table 3. Types of Electrodes Used for Various Workpiece Materials Electrode Electrode Polarity Workpiece Material Corner Wear (%) Capacitance Copper + Steel 2–10 No Copper + Inconel 2–10 No Copper + Aluminum <3 No Copper − Titanium 20–40 Yes Copper − Carbide 35–60 Yes Copper − Copper 34–45 Yes Copper − Copper-tungsten 40–60 Yes Copper-tungsten + Steel 1–10 No Copper-tungsten − Copper 20–40 Yes Copper-tungsten − Copper-tungsten 30–50 Yes Copper-tungsten − Titanium 15–25 Yes Copper-tungsten − Carbide 35–50 Yes Graphite + Steel <1 No Graphite − Steel 30–40 No Graphite + Inconel <1 No Graphite − Inconel 30–40 No Graphite + Aluminum <1 No Graphite − Aluminum 10–20 No Graphite − Titanium 40–70 No Graphite − Copper N/A Yes
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