Electro-Discharge Machining E.D.M.

Sometimes, this process is called spark machining or spark erosion because rather than using electrolysis, the technique involves the removal of metal by the energy of electrical sparks, which travel from the electrically negative tool electrode through a dielectric fluid and explosively strike the electrically positive workpiece.

The intense heat of the strike causes local particles of metal to instantaneously vaporise without a molten metal phase (a process known as ‘sublimation’). Though, away from the actual centre of the explosion, molten fragments of metal are washed away with the vapour by the dielectric fluid.

A suitable fluid (usually kerosene) is fed under pressure between the electrode and the workpiece, to maintain a uniform electrical resistance. The spark rate is around 10,000 per second and the gap between the tool and the workpiece is critical and must be maintained throughout the operation, at approximately 0.025 mm - 0.075 mm.

The real advantage of EDM is that, not only is it suitable on materials which are difficult to machine conventionally, but it also excels in its ability to produce high aspect ratio, very small holes of any cross-sectional in very hard metals. Typical holes achievable by this method are in the regions of 0.025 mm diameter x 750 mm deep.

A novel variation of EDM is a technique sometimes referred to as ‘wire-cutting’, which uses a moving fine piece of copper or nickel wire as the electrode. The wire, 0.05 mm - 0.25 mm in diameter is positioned by, and fed over two pulleys and resembles a simple band-saw operation. The workpiece is mounted on a table, which can be moved in two axes and when the table is computer controlled, the wire-cutting process can cut accurate, complex shapes in metals (e.g. dovetails, fir-trees etc.) which are difficult to machine with conventional tools.