Wire EDM (Wire Electro Discharge Machining, or WEDM) involves a continuously spooling conductive wire (the most widely used is brass). A power supply generates rapid electric pulses that create a discharge between the workpiece and electrode (the wire). The discharge causes the melting, and probably the vaporisation, of a minute piece of material, slowly eating into the work piece. Any electrical conductive material can be machined irrespective of hardness. The position of the wire with respect to the workpiece is controlled in the x and y planes usually by CNC. On some machines the wire can be tilted to create tapered parts. An advantage of this process is that no mechanical stresses are created in the workpiece because the wire does not make contact with it.
Typical Uses for Wire EDM
- 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.
Design guidelines for Wire EDM
Wie EDM is 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.
When the wire turns a corner, a sharp edge is created on the outside corner. A radius will feature on the inside corner. This is unavoidable, but can be reduced by using a thinner wire. The radius of the corner is governed by the diameter of the wire and the spark gap.
To achieve a high quality surface finish or high precision, it is necessary to perform successive lower power “skims” which gradually remove less material on each cut. It is possible to achieve mirror finishes on some machines using up to four or more skims.
A local heat affected zone is created. The heat generated by the sparks causes this.
Process variations
On some machines, it is possible to adjust the angle of the wire to create angled faces or tapered parts. Some machines have an adjustable bed, which also allows the workpiece to be angled, creating an angled cut.
Tradenames/alternative names for Wire EDM
- Spark erosion
- Electro Discharge Wire Cutting (EDWC)
- EDM Electro-Discharge Machining (see separate process record).
- Wire cutting
- Wire erosion
The environment
- The process poses no great environmental problems.
- The wire is not reusable and requires disposal.
The economics
There are no tooling costs as the wire creates the required profile. However, 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/hour. Typical machining times are 1 to 6 hours.
CNC is common.
Technical notes
Wire EDM works the same way standard Electro Discharge Machining (EDM). It 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.
Other
Factors influencing speed or cost:
- Wear and material type of electrode
- Current
- 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.
