Machinery's Handbook, 31st Edition
ELECTRICAL DISCHARGE MACHINING 1471 Orbit: A programmable motion between the electrode and the workpiece, produced by a feature built in to the machine, or an accessory, that produces a cavity or hole larger than the electrode. The path can be planetary (circular), vectorial, or polygonal (trace). These motions can often be performed in sequence, and combined with x-axis movement of the electrode. Overcut: The distance between one side of an electrode and the adjacent wall of the workpiece cavity. Overcut taper: The difference between the overcut dimensions at the top (entrance) and at the bottom of the cavity. Plasma: A superheated, highly ionized gas that forms in the discharge channel due to the applied voltage. Positive electrode: The electrode voltage potential is positive with respect to the work piece. Negative electrode is the opposite of this condition. Power parameters: A set of power supply, servo, electrode material, workpiece mate rial, and flushing settings that are selected to produce a desired metal removal rate and surface finish. Quench: The rapid cooling of the EDM surface by the dielectric fluid, which is partially responsible for metallurgical changes in the recast layer and in the heat- affected zone. Recast layer: A layer created by the solidification of molten metal on the workpiece sur face after it has been melted by the EDM process. Secondary discharge: A discharge that occurs as conductive particles are carried out along the side of the electrode by the dielectric fluid. Spark in: A method of locating an electrode with respect to the workpiece, using high frequency, low amperage settings so that there is no cutting action. The electrode is advanced toward the workpiece until contact is indicated and this point is used as the basis for setting up the job. Spark out: A technique used in orbiting, which moves the electrode in the same path until sparking ceases. Square wave: An electrical wave shape generated by a solid state power supply. Stroke: The distance the ram travels under servo control. UV axis: A mechanism that provides for movement of the upper head of a wire EDM machine to allow inclined surfaces to be generated. White layer: The surface layer of an EDM cut that is affected by the heat generated during the process. The characteristics of the layer depend on the material, and may be extremely hard martensite or an annealed layer. Wire EDM: An EDM machine or process in which the electrode is a continuously unspooling, conducting wire that moves in preset patterns in relation to the workpiece. Wire guide: A replaceable precision round diamond insert, sized to match the wire, that guides the wire at the entrance and exit points of a wire cut. Wire speed: The rate at which the wire is fed axially through the workpiece (not the rate at which cutting takes place), adjusted so that clean wire is maintained in the cut slowly enough to minimize waste. The EDM Process.— During the EDM process, energy from the sparks created between the electrode and the workpiece is dissipated by the melting and vaporizing of the work piece material preferentially, only small amounts of material being lost from the elec- trode. When current starts to flow between the electrode and the work, the dielectric fluid in the small area in which the gap is smallest, and in which the spark will occur, is transformed into a plasma of hydrogen, carbon, and various oxides. This plasma forms a conducting passageway, consisting of ionized or electrically charged particles, through which the spark can form between the electrode and the workpiece. After current starts to flow, to heat and vaporize a tiny area, the striking voltage is reached, the voltage drops, and the field of ionized particles loses its energy, so that the spark can no longer be sustained. As the voltage then begins to rise again with the increase in resistance, the electrical
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