Spark erosion (EDM) in the tool trade

Some tool parts simply cannot be milled or turned finished – either because the steel is already hardened to over 60 HRC or because the geometry has sharp internal corners that no milling tool can reach. Here spark machining, also called EDM (Electrical Discharge Machining) takes over, which shapes steel using electrical discharges instead of mechanical cutting.

§The principle: electrical discharges

Between an electrode and the workpiece, sparks occur when the voltage is increased sufficiently. Each discharge creates intense local heat that melts and evaporates a microscopic amount of material from the workpiece's surface. Because the electrode and workpiece never touch mechanically, the process can shape even the hardest, hardened steel without wearing down the tool the way a mill would.

§Sinking sparking (sinker-EDM)

With spark erosion the electrode — typically made of graphite or copper — is shaped as a negative of the cavity to be cut. The electrode is gradually lowered into the workpiece while the spark discharges remove material until the workpiece's cavity is a mirror-inverted copy of the electrode's shape. The method is used especially to make complex blind holes in injection molds.

§Wire EDM

Wire EDM instead uses a thin, current-conducting wire that is continuously fed through the workpiece while it cuts a contour, like a very precise, electric fretsaw. The method is suitable for through-cuts, complex profiles and workpieces with very tight tolerances — for example, stamps and dies for stamping tools.

• 01Sinker-EDM: shaped electrode cuts blind cavities and 3D surface shapes
• 02Wire EDM: thin wire, cuts through contours and profiles
• 03Both methods can machine hardened steel without mechanical wear on the tool
• 04Both take place in a dielectric liquid that rinses away the melted particles

§When do you choose EDM (electrical discharge machining)?

Spark erosion is slower and more expensive per hour than conventional milling, so it is typically chosen when the task cannot be solved otherwise: hardened steel that must be shaped after heat treatment, sharp internal corners without radius, or extremely tight tolerances in the die's punch and matrix.