Electrical Discharge Machining (EDM) is a metal removal process accomplished by the creation of thousands of electrical discharges per second that flow between an electrode and the workpiece in a dielectric fluid. It has the effect of vaporizing the metal in a very small controlled area. The EDM process can be used on any material that is electrically conductive, including exotic materials such as Waspaloy or Hastaloy, which are very difficult to machine using conventional methods.

There are two major types of EDM, Wire EDM and Sinker EDM (sometimes called Ram or Conventional EDM). Sinker EDM is the older of the two. Although the EDM principle was discovered during the early 1940s, it was not until the early 1950s that the first practical Sinker EDM machine was designed. Wire EDM was introduced as a machining process in 1969.

The main difference between the two is the type of electrode used. Wire EDM, as its name suggests, uses wire as the electrode. The wire burns a predetermined contour through the part leaving a path of 0.001" to 0.003" larger than the wire diameter. The wire diameter ranges from 0.0005" to 0.0130". Brass is the most common wire used along with zinc-coated wire. Smaller wires are generally made of molybdenum or tungsten.

Sinker EDM, as its name suggests, sinks the shape that is on the electrode part way into the workpiece. The common types of electrode materials used are graphite (for ease of machining and wearability) and copper tungsten (for fine finish requirements).

EDM, particularly Wire EDM, is competitive with other more standard machining methods. There are many reasons for this; however, they all point to the fact that you must be careful in who you select to do our EDM work.

Important Considerations

• Older Wire EDM machines would cut at 3 in²/hr. Newer machines provide cutting rates up to 25 in²/hr. In order to keep up with current technology, your EDM source should have a generous annual machine replacement budget.

• Tight tolerances have become commonplace in EDM with accuracies held to 0.000005". To hold this tolerance requires many years of experience. Do you want to place your work with a shop that has been in the Wire EDM business for 20 years or 1 year?

• EDM is now not only acceptable for machining aerospace, aircraft and many exotic materials, it is the preferred method in many instances. Do you want to do business with a shop that has a full quality system approved to do business with the major aircraft and aerospace manufacturers or do you want to do business with a shop who has little or no quality system in place?

• The EDM process can be used for prototype or production runs. For timely deliveries, you need a shop with several Wire and Sinker EDM machines.

Although Wire and Sinker EDM machining has many advantages, it is sometimes more economical to machine a part partially by EDM and partially by other means. You should ask your EDM source if they have other machining capabilities such as full toolroom capabilities or machining centers or laser cutting machines to supplement their EDM processes.