Machinery's Handbook, 31st Edition
ELECTRICAL DISCHARGE MACHINING 1479 Wire EDM.— In the wire EDM process, with deionized water as the dielectric fluid, car bon is extracted from the recast layer, rather than added to it. When copper-base wire is used, copper atoms migrate into the recast layer, softening the surface slightly so that wire-cut surfaces are sometimes softer than the parent metal. On wire EDM machines, very high amperages are used with very short on times, so that the heat-affected zone (HAZ) is quite shallow. With proper adjustment of the on and off times, the depth of the HAZ can be held below 1 micron (0.00004 inch). The cutting wire is used only once, so that the portion in the cut is always cylindrical and has no spark-eroded sections that might affect the cut accuracy. The power source controls the electrical supply to the wire and to the drive motors on the table to maintain the preset arc gap within 0.l micron (0.000004 inch) of the programmed position. On wire EDM machines, the water used as a dielectric fluid is deionized by a deionizer included in the cooling system, to improve its properties as an insulator. Chemical balance of the water is also important for good dielectric properties. Drilling Holes for Wire EDM: Before an aperture can be cut in a die plate, a hole must be provided in the workpiece. Such holes are often “drilled” by EDM, and the wire threaded through the workpiece before starting the cut. The “EDM drill” does not need to be rotated, but rotation will help in flushing and reduce electrode wear. The EDM process can drill a hole 0.04 inch (1 mm) in diameter through 4-inch (101 mm) thick steel in about 3 minutes, using an electrode made from brass or copper tubing. Holes of smaller diameter can be drilled, but the practical limit is 0.012 inch (0.3 mm) because of the overcut, the lack of rigidity of tubing in small sizes, and the excessive wear on such small electrodes. The practical upper size limit on holes is about 0.12 inch (3 mm) because of the comparatively large amounts of material that must be eroded away for larger sizes. However, EDM is commonly used for making large or deep holes in such hard materials as tungsten carbide. For instance, a 0.2-inch (5 mm) hole has been made in carbide 2.9 inch (74 mm) thick in 49 minutes by EDM. Blind holes are difficult to produce with accuracy, and must often be made with cut-and-try methods. Deionized water is usually used for drilling and is directed through the axial hole in the tubular electrode to flush away the debris created by the sparking sequence. Because of the need to keep the extremely small cutting area clear of metal particles, the dielectric fluid is often not filtered but is replaced continuously by clean fluid that is pumped from a supply tank to a disposal tank on the machine. Wire Electrodes: Wire for EDM generally is made from yellow brass containing copper 63 and zinc 37 percent, with a tensile strength of 50,000–145,000 lb f /in 2 (345–1000 MPa), and may be from 0.002–0.012 inch (0.05–0.30 mm) diameter. In addition to yellow brass, electrode wires are also made from brass alloyed with alumi num or titanium for tensile strengths of 140,000–160,000 lb f /in 2 (965–1103 MPa). Wires with homogeneous, uniform electrolytic coatings of alloys such as brass or zinc are also used. Zinc is favored as a coating on brass wires because it gives faster cutting and reduced wire breakage due to its low melting temperature of 419 ° C, and vaporization temperature of 906 ° C. The layer of zinc can boil off while the brass core, which melts at 930 ° C, contin ues to deliver current. Some wires for EDM are made from steel for strength, with a coating of brass, copper, or other metal. Most wire machines use wire negative polarity (the wire is negative) because the wire is constantly renewed and is used only once, so wear is not important. Important qualities of wire for EDM include smooth surfaces, free from nicks, scratches and cracks, precise diameters to ± 0.00004 inch( ± 1 micron) for drawn and ± 0.00006 inch( ±2 μ m) for plated, high tensile strength, consistently good ductility, uniform spooling, and good protective packaging.
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