Electro-Discharge Machining (EDM) is machining with sparks. The work-piece is held in a jig submerged in a dielectric fluid such as kerosene. A power supply generates rapid electric pulses that create a discharge between the work piece and an electrode (a continuous wire or a shaped graphite form) at the point at which the two are closest
The discharge creates a plasma causing the melting, and probably the vaporisation, of a minute bite of material, slowly eating into the work piece; the debris is swept away by the dielectric fluid. EDM is remarkable for its ability to shape difficult materials (provided they are conductors) and do so with great precision. Any electrical conductive material can be machined irrespective of hardness.
An advantage of EDM is that any conductive material can be machined regardless of its hardness. A further advantage is that no mechanical stresses are created in the workpiece because the wire does not make contact with it.
- Cutting and shaping of metals and conducting ceramics that are difficult to shape in any other way: dies for moulding, stamping, extrusion and forging; making tool fixtures; aircraft and medical parts.
- Prototype parts.
- Burr-free parts.
Examples of uses
Design guidelines for EDM
- EDM is very versatile and very precise, but it is slow. It is particularly attractive when limited numbers of parts are to be made from very hard, conducting materials: moulds, dies etc. But it has other uses such as the machining of metal foams (fast by EDM and difficult to cut without surface damage by other methods) and low volume production of thin-walled parts in light metals and steels.
- Undercuts are possible using specialist have hinged electrodes or retractable features.
- Care must be taken in the design of the electrode to ensure that there are adequate channels for flushing the dielectric fluid/debris.
- A local heat affected zone is created. The heat generated by the sparks causes this.
- Small hole drilling is possible, with diameters as small as 0.13mm to 0.2mm
- EDWC (Electro-Discharge Wire Cutting). See separate process record.
- Electro-discharge sawing (EDS). This combines the motion of either a bandsaw or circular disc saw with the electrical erosion of the workpiece.
- EDM milling uses a rotating electrode which follows a path similar to mechanical milling. This allows complex shapes to be created from simple electrodes, and promotes the flushing of the dielectric fluid (this helps remove debris). Wear of the electrode is optimised by the rotational and contouring motion of the electrode.
- EDG (Electro-Discharge Grinding). Similar to EDM except the electrode is a rotating graphite wheel.
- Spark erosion.
- Electrical Discharge Texturing.
- Ram EDM
- Die-sinking EDM.
- The process poses no great environmental problems.
- Dielectric fluid is recycled through a filter system and can last up to 2 years before it is required to be changed.
- Die and tool costs are low but the slow rate of cutting (measured in a few mm/min) limits its use to small batch sizes. The process can be fully automated with low labour costs, giving an overall cost of about £20/hr. Typical machining times are 1-6 hours.
- EDM works by electric breakdown: the enormous potential gradient between tool and work-piece pulls electrons free from molecules of the dielectric, and accelerates them into projectiles, kicking more electrons from molecules as they pass, thereby creating a cascade. The resulting plasma, with temperatures of around 10,000C blasts a tiny nugget of metal from the surface. When the pulse ends, the blast stops. EDM is particularly used to cut Stellite, Inconel, Hastelloy, Nitralloy, Waspaloy, Nimonic, Udimet, tool steels, tungsten carbide and titanium alloys.
- High machining rates are proportional to high currents and a rougher surface finish will ensue.
Factors influencing speed or cost:
- Wear and material type of electrode
- Workpiece material
- Pulse duration
- Surface finish affected by spark intensity
- HAZ or recast material surfaces may have to be treated if surfaces have to withstand fatigue.