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| ELECTRICAL DISCHARGE MACHINING BERYLLIUM COPPER |
 Electrical Discharge Machining
(EDM) is commonly used to produce beryllium copper molds and dies, to drill small, burr-free holes and to make prototype quantities of contacts for the aerospace and electronic markets. EDM is not dependent on the strength or hardness of the work piece and is used to machine beryllium copper in its age hardened state with no effect on the alloy’s strength and no further heat treatment required.
Machining speeds are determined by the area of the work piece, the work piece material and the machining conditions. Since beryllium copper exhibits high electrical conductivity, machining rates are typically 20% lower than that of tool steels. When EDM beryllium copper, it is suggested that the equipment parameters be set at the machine manufacturer’s recommendations for copper and then adjusted accordingly to produce the desired results.
Compared to steel, beryllium copper must be EDM’d with low amperage and high voltage to produce acceptable results. The polarity of a solid-state power supply can be either electrode positive or negative. Electrode negative polarity produces the highest metal removal rates and a rougher surface. Recently, it has become more common to use electrode positive polarity to increase the work-to-electrode wear ratio, while providing a smoother surface. A dielectric fluid is required in all EDM operations. The dielectric acts as a spark conductor, a coolant, and a flushing medium that carries away swarf. For conventional EDM, the most common dielectric fluids used are light petroleum-based oil for ram EDM and deionized water for wire EDM.
The surface texture of EDM'd beryllium copper resembles overlapping, small craters that exhibit no directionality. The surface roughness can range from 8 microinch Ra (0.2 micrometer) for finishing operations, to 500 microinch Ra (13 micrometer) for roughing operations. Recast and heat-affected layers occur on the order of 0.0001 to 0.005 inch (0.0025 to 0.127 mm) and should be removed for fatigue-sensitive applications. Shot peening provides a smoother surface and improves fatigue life, but abrasive and electrochemical methods are required to remove the recast and underlying heat-affected layer. For most applications, removal of these layers is not necessary. |
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