Machinery's Handbook, 31st Edition
ELECTRICAL DISCHARGE MACHINING 1475 The depth of the HAZ depends on the amperage and the length of the on time, increasing as these values increase, to about 0.012-0.015 inch (0.30–0.38 mm) deep. Residual stress in the HAZ can range up to 650 N/mm 2 . As the HAZ cannot be removed easily, it is best avoided by programming the series of cuts taken on the machine so that most of the HAZ produced by one cut is removed by the following cut. If time is available, cut depth can be reduced gradually until the finishing cuts produce an HAZ having a thickness of less than 0.0001 inch (2.54 mm). Workpiece Materials.— Most homogeneous materials used in metalworking can be shaped by the EDM process. Some data on typical workpiece materials are given in Table 2 . Sintered materials present some difficulties caused by the use of a cobalt or other binder used to hold the carbide or other particles in the matrix. The binder usually melts at a lower temperature than the tungsten, molybdenum, titanium, or other carbides, so it is preferentially removed by the sparking sequence, and the carbide particles are thus loos- ened and freed from the matrix. The structures of sintered materials based on tungsten, cobalt, and molybdenum require higher EDM frequencies with very short on times, so that there is less danger of excessive heat buildup, leading to melting. Copper-tungsten electrodes are recommended for EDM of tungsten carbides. When used with high fre- quencies for powdered metals, graphite electrodes often suffer from excessive wear. Workpieces of aluminum, brass, and copper should be processed with metallic elec trodes of low melting points such as copper or copper-tungsten. Workpieces of carbon and stainless steel that have high melting points should be processed with graphite electrodes. The melting points and specific gravities of the electrode material and of the workpiece should preferably be similar. Table 2. Characteristics of Common Workpiece Materials for EDM
Vaporization Temperature
Melting Point
Specific Gravity
Conductivity (Silver = 100)
Material
° F
° C
° F
° C
Aluminum Brass Cobalt Copper Graphite
2.70 8.40 8.71 8.89 2.07
1220 1710 2696 1980 1202 2300 4748 2651 2500
660 930 1480 1082 1285 650 1260 2620 1455 1371 1500 1510 1700 3370 420
4442
2450
63.00
…
…
5520 4710 6330
2900 2595 3500 1110 2150 5560 2730
16.93 97.61 70.00 39.40 15.75 17.60 12.89 … 12.00
N/A
Inconel Magnesium Manganese Molybdenum Carbon Steel Tool Steel Stainless Steel Nickel
… 2350
…
1.83 7.30 10.20 8.80 7.80
2025 3870 10,040 4900
… … …
… 2730 … 2750
… …
Titanium Tungsten
4.50 18.85 6.40
3200 6098 790
5900 10,670 1663
3260 5930 906
13.73 14.00
Zinc 26.00 Electrode Materials.— Most EDM electrodes are made from graphite, which provides a much superior rate of metal removal than copper because of the ability of graphite to resist thermal damage. Graphite has a density of 1.55 to 1.85 g/cm 3 , lower than most metals. Instead of melting when heated, graphite sublimates, that is, it changes directly from a solid to a gas without passing through the liquid stage. Sublimation of graphite occurs at a temperature of 3350 ° C (6062 ° F). EDM graphite is made by sintering a com- pressed mixture of fine graphite powder (1 to 100 micron particle size) and coal tar pitch in a furnace. The open structure of graphite means that it is eroded more rapidly than metal in the EDM process. The electrode surface is also reproduced on the surface of the work- piece. The sizes of individual surface recesses may be reduced during sparking when the work is moved under numerical control of workpiece table movements.
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