What Is EDM?
Introducing the Industry

The production of many consumer goods, from cell phone cases to automobile parts, relies on moulded parts. The mould cavities needed to produce these parts often are created through the application of electrical discharge machining (EDM).

EDM, simply defined, is a metal-removal process that involves using a series of generated electric sparks to erode material from a workpiece under carefully controlled conditions.

The two basic types of EDM technology are known as wire-cutting and ram, or die-sinking, processes. Each process uses its own unique machine and consumable materials, but the fundamental technologies are similar. Wire EDM machines use travelling wire to pass the spark to the workpiece, whereas sinker EDM machines use a fabricated electrode. Many shops employ both systems. Each technique has advantages and disadvantages, depending on the type of work being undertaken.

Wire EDM is a form of electrical discharge machining in which an electrically energized thin wire is used as an electrode to slice through metal. The workpiece is submerged in a tank with dielectric fluid. Then, when the voltage running through the wire reaches a certain level, a spark jumps the gap and melts a small portion of the workpiece.

This form of EDM processing offers many distinctive advantages, chief among them being its ability to cut difficult contours. Also, wire-cutting EDM can handle some of the hardest materials used in the tool and mould making industry. Because, unlike conventional machining techniques, there are no cutting forces involved, the process is ideal for machining delicate parts. And finally, toolmakers can use wire EDM to create intricate profiles that they could not achieve with other metal-cutting methods.

The basic elements of a sinker EDM machine are the power source, with leads to the electrode and workpiece; a work tank to hold the dielectric fluid and workpiece; and a servo mechanism to position the ram head that holds the electrode. Because the electrode and workpiece should never touch each other, a working gap has to be maintained by the servo mechanism, which monitors activity in the gap.

In the process of die-sinking, a machined electrode made of graphite or copper is lowered to the correct position, a spark jumps the gap and discharges into the workpiece, and a small piece of molten metal is expelled, leaving a small crater. Each spark is part of a controlled cycle of on-time and off-time that can repeat as often as 250,000 times per second.

The succession of sparks and resulting craters reproduces in the workpiece the shape of the electrode in mirror image. The finished cavity is produced by controlling the operating parameters and the electrode material through roughing and finishing operations.

The EDM process is used in many industries and in a variety of applications other than mould and die manufacturing. Depending on the application, electrical discharge machining operations may be performed with a simple manual machine or by means of sophisticated systems that include robotics and ID chips incorporated in the tooling for traceability.

Thanks to Emmanuel Ambrosetto of Poco Graphite SARL in Limonest, France, for contributing to this article. Photos courtesy of Sodick Ltd. (top, center photos) and Makino Formenbautechnologie GmbH (bottom photo).