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What is EDM? A Brief History
The acronym EDM is derived from Electrical Discharge Machining. （图片） 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
Spark Properties
Conventional machining produces high cutting forces, requiring high work holding forces, cutting fluids, high temperatures at the work
Electric 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.（图片） 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.
Wire Cutting
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.（图片） 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.
Small Hole EDM
Small hole edm is a specialized component of electrical discharge machining. A small hollow electrode spins about a spindle much like a drill and drill bit (edm drill). The electrode is electrically charged by a servo-controlled generator producing the spark. Water based dielectric flushes through and around the electrode providing a controlled environment for the spark to jump to the work piece. The electric spark erodes the surface of the work piece creating very small pockets. Eventually millions and millions of these microscopic pockets create the small hole. The size of the hole is controlled by the diameter of the electrode. The location and depth of the holes are driven by CNC ISO codes, reducing and eliminating operator error. Machines shown have automatic electrode changers for unattended machining.
Applications
Small hole edm is in many ways similar to a drilling operation but it offers the advantages of edm. Very low machining and work piece hardness are not barriers to performance.
8/26/2011
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