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EDM.  Abbreviation for electrical discharge machine; the technique makes exact size metal replicas – or their dies – by the process of spark erosion. The process is commonly called "burning a die." Under moderate magnification any surface created by spark erosion exhibits a typical scale of microscopic cups (tiny explosion craters). The scaly surface has become very hard that requires stoning to smooth down the tiny craters. Special abrasive stones are available for this stoning of EDM surfaces. A piece struck by a die made by EDM, produces a surface with a microscopic pebbly effect, in contrast to smooth surface of dies made by conventional methods by machining or hubbing, particularly in recessed areas that cannot be stoned.

EDM requires a pattern for each copy made as the tracers destroys the surface of the original and the type of relief – positive or negative – is immaterial. To make a die of coin size by EDM requires less than an hour; to EDM a die of a medal of, say 2 1/2 to 3 inches requires two hours or more. Two absolutely identical copies cannot be made by EDM (thus such copies are so obvious, counterfeits made by this process are always easy to detect). A set of electrodes are required for each "burning" by EDM and anomalies occur because of electrode burnoff. The life of EDM dies are about half of hubbed dies because the grain structure has been so adversely affected by erosion into the steel. The process is similar to electrochemical machining (ECM).  See also electrochemical metal removal.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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