Electrical discharge machining

Electrical discharge machining (EDM) is a technique involving the fusion, vaporization and ejection of material. Energy is supplied by electrical discharges passing between two electrodes, the workpiece and the tool.

This technique modifies the material's surface and underlay characteristics (increased hardness, presence of residual thermal tensile stresses, presence of microcracks). The result is improved resistance to corrosion and wear, but a significant reduction in fatigue life. The machined surface is cratered, with coarse roughness in roughing (Ra = 10 to 30 µm) and good roughness in finishing (Ra = 0.4 to 1.6 µm). Material throughput is rather low: on the order of cubic centimetres for roughing, and cubic millimetres or less for finishing.

This cannot be a mass-production technique. As the discharges pass between the two electrodes, material is also removed from the tool, which wears out (volumetric wear ranging from 0.5% in roughing to 30% or more in finishing).

Its greatest quality is its precision, which can be better than 0.01 mm, provided that tool wear is kept under control.

EDM is used in a variety of industrial sectors, to machine conductive or semi-conductive materials: hardened steels, refractory metal alloys, certain composites, etc. The biggest user is the tooling sector: glassmaking and plastic injection molds, dies, punches, dies. The process is also used in aeronautics, for drilling or machining on turbine blades or disks. Other users include the nuclear industry, the medical sector (prostheses, needles), general mechanics and the automotive industry for special applications (drilling of diesel injectors, cutting of small series in Formula 1 racing).

EDM is indispensable for producing complex shapes in materials with high mechanical properties. Its potential competitors are high-speed machining (which does not, however, produce such fine detail) and rapid prototyping, which is more flexible, but not yet precise enough.