Material removal by controlled electrical sparks between an electrode and the workpiece.
EDM cuts any electrically conductive material regardless of hardness, which makes it the standard process for hardened tool steel, complex internal profiles and sharp internal corners that no milling cutter can reach.
It leaves a recast layer — a thin surface of re-solidified metal with different properties from the bulk, often containing microcracks. Where fatigue life matters the recast layer must be removed by a finishing pass or by polishing.
Using an EDM'd surface as a finished surface on a fatigue-critical part. The recast layer is a crack initiation site.
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