What is Wire EDM - A Short History
The acronym EDM is derived from Electrical Discharge Machining.
The EDM process we know today started with the observations of Joseph Preistly in 1770. He noticed that electrical discharges had removed material from the electrodes in his experiments. This is also known as electro-discharge erosion.
In the 1940's Soviet researchers developed a machining process that formed the foundation for modern EDM.
Charmilles introduced their spark erosion machine at the 1955 European Machine Tool Exhibition in Milan.
Features of the Wire EDM Process
- Low work holding forces
- Low cutting forces
- Very accurate process tolerances held +/- 0.0001"
- Complex profile capability
- No tool wear (the wire is continually replenished)
- Environmentally friendly (by products are easily recycled)
- Hardened materials are easily machined
Conventional machining produces high cutting forces, requiring high work holding forces, cutting fluids, and high temperatures.
- Spark temperatures are between 8,000 and 12,000 degrees C (plasma zone)
- The range of the sparks varies from a few microns to 1 mm
- Controlled by the generator and are adjustable for different material types and desired surface finishes
Electrical Discharge Machining
The basic EDM process is really quite simple. An electrical spark is created between an electrode and a work piece. The spark is visible evidence of the flow of electricity. This electric spark produces intense heat with temperatures reaching 8000 to 12000 degrees Celsius, melting almost anything. The spark is very carefully controlled and localized so that it only affects the surface of the material. The EDM process usually does not affect the heat treat below the surface. With wire EDM the spark always takes place in the dielectric of deionized water. The conductivity of the water is carefully controlled making an excellent environment for the EDM process. The water acts as a coolant and flushes away the eroded metal particles.
EDM wire cutting uses a metallic wire to cut a programmed contour in a workpiece. Extrusion dies and blanking punches are very often machined by wire cutting. Cutting is always through the entire workpiece. To start machining it is first necessary to drill a hole in the workpiece or start from the edge. On the machining area, each discharge creates a crater in the workpiece and an impact on the tool. The wire can be inclined, thus making it possible to make parts with taper or with different profiles at the top and bottom. There is never any mechanical contact between the electrode and workpiece (see above). The wire is usually made of brass or stratified copper, and is between 0.1 and 0.3 mm diameter.
Depending on the accuracy and surface finish needed a part will either be one cut or it will be roughed and skimmed. On a one cut the wire ideally passes through a solid part and drops a slug or scrap piece when it is done. This will give adequate accuracy for some jobs but most of the time skimming is necessary. A skim cut is where the wire is passed back over the roughed surface again with a lower power setting and low pressure flush. There can be from one to nine skim passes depending on the accuracy and surface finish required. Usually there are just two skim passes. A skim pass can remove as much as 0.002" of material or a as little as 0.0001". During roughing ( i.e. the first cut) the water is forced into the cut at high pressure in order to provide plenty of cooling and eliminate eroded particles as fast as possible. During skimming (accuracy / finish cuts) the water is gently flowed over the burn so as not to deflect the wire.
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