Machinery's Handbook, 31st Edition
1470 ELECTRICAL DISCHARGE MACHINING Barrel effect: In wire EDM, a condition where the center of the cut is wider than the entry and exit points of the wire, due to secondary discharges caused by particles being pushed to the center by flushing pressure from above and beneath the workpiece. Capacitor: An electrical component that stores an electric charge. In some EDM power supplies, several capacitors are connected across the machining gap and the current for the spark comes directly from the capacitors when they are discharged. Cathode: The negative terminal in an electrolytic cell or battery. In EDM incorrectly applied to the workpiece. Colloidal suspension: Particles suspended in a liquid that are too fine to settle out. In EDM, the tiny particles produced in the sparking action form a colloidal suspension in the dielectric fluid. Craters: Small cavities left on an EDM surface by the sparking action, also known as pits. Dielectric filter: A filter that removes particles from 5 μ m (0.00020 inch) down to as fine as 1 μ m (0.00004 inch) in size, from dielectric fluid. Dielectric fluid: The non-conductive fluid that circulates between the electrode and the workpiece to provide the dielectric strength across which an arc can occur, to act as a coolant to solidify particles melted by the arc, and to flush away the solidified particles. Dielectric strength: In EDM, the electrical potential (voltage) needed to break down (ionize) the dielectric fluid in the gap between the electrode and the workpiece. Discharge channel: The conductive pathway formed by ionized dielectric and vapor between the electrode and the workpiece. Dither: A slight up and down movement of the machine ram and attached electrode, used to improve cutting stability. Duty cycle: The percentage of a pulse cycle during which the current is turned on (on time), relative to the total duration of the cycle. EDG: Electrical discharge grinding using a machine that resembles a surface grinder but has a wheel made from electrode material. Metal is removed by an EDM process rather than by grinding. Electrode growth: A plating action that occurs at certain low-power settings, whereby workpiece material builds up on the electrode, causing an increase in size. Electrode wear: Amount of material removed from the electrode during the EDM pro cess. This removal can be end wear or corner wear, and is measured linearly or volumetri cally but is most often expressed as end wear percent, measured linearly. Electro-forming: An electro-plating process used to make metal EDM electrodes. Energy: Measured in joules, is the equivalent of volt-coulombs or volt-ampere-seconds. Farad: Unit of electrical capacitance, or the energy-storing capacity of a capacitor. Gap: The closest point between the electrode and the workpiece where an electrical dis charge will occur. (See Overcut ) Gap current: The average amperage flowing across the machining gap. Gap voltage: The voltage across the gap while current is flowing. The voltage across the electrode/workpiece before current flows is called the open gap voltage. Heat-affected zone: The layer below the recast layer, which has been subjected to elevated temperatures that have altered the properties of the workpiece metal. Ion: An atom or group of atoms that has lost or gained one or more electrons and is there fore carrying a positive or negative electrical charge, and is described as being ionized. Ionization: The change in the dielectric fluid that is subjected to a voltage potential whereby it becomes electrically conductive, allowing it to conduct the arc. Low-wear: An EDM process in which the volume of electrode wear is between 2 and 15 percent of the volume of workpiece wear. Normal negative polarity wear ratios are 15 to 40 percent. Negative electrode: The electrode voltage potential is negative relative to the workpiece. No-wear: An EDM process in which electrode wear is virtually eliminated and the wear ratio is usually less than 2 percent by volume.
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