Wire electrical discharge machining (EDM) is a process of metal machining in which a tool discharges thousands of sparks to a metal workpiece. A non-conventional process, though hardly a new one, wire EDM works on parts resistant to conventional machining processes, but only if these parts are electrically conductive; usually, they are non-ferrous, and include steel, titanium, superalloys, brass, and many other metals. Instead of cutting the material, EDM melts or vaporizes it, producing comparatively small chips and providing a very accurate cut line. Industry-wide acceptance has led to a wide variety of EDM applications, as it is highly versatile, can cut hard metals, and utilizes a relatively compact amount of workspace.
There are two main types of EDM: conventional, or sinker, and wire, or wire cut. Conventional EDM, as described above, uses a tool to disperse the electric current. This tool, the cathode, runs along the metal piece, the anode, and the electrical current reacts to melt or vaporize the metal. As a result of the dielectric fluid, usually a hydrocarbon oil into which both the cathode and the workpiece are immersed, the minute chips produced by the process wash away from the piece. Wire cut EDM (or WCEDM) discharges the electrified current by means of a taut thin wire, which acts as the cathode and is guided alongside the desired cutting path, or kerf. The dielectric fluid in this case—usually deionized water—is flushed through the cut as it proceeds, again serving to carry off particles and control the sparks. The thin wire allows precision cuts, with narrow kerfs (~0.015 in. routinely, with finer kerfs available) and tolerances of +/- 0.0001 in. possible. This heightened precision allows for complex, three-dimensional cuts, and produces highly accurate punches, dies, and stripper plates.
Wire cut EDM equipment is run by computer numerically controlled (CNC) instruments, which can control the wire on a three-dimensional axis to provide greater flexibility. Simple cuts are done by varying the x-y coordinates of the cutter with more complex cuts achieved by adding axes of motion to the wire guides. Both four- and five-axis wire EDM machines and services are available. Whereas conventional EDM cannot always produce tight corners or very intricate patterns, wire EDM’s increased precision allows for intricate patterns and cuts. Additionally, wire EDM is able to cut metals as thin as 0.004 in. and thicker materials upwards of 16 in. routinely, with thicker sections possible. At a certain material thinness, wire EDM will simply cause the metal to evaporate, thereby eliminating potential debris. The wire of a WCEDM unit emits sparks on all sides, which means the cut must be thicker than the wire itself. In other words, because the wire is surrounded by a ring of current, the smallest and most precise cutting path possible is the added diameter of the ring and wire; technicians easily account for this added dimension. Manufacturers continue to produce thinner and thinner wire to allow for smaller kerfs and even finer precision.
Because of its versatility, manufacturers use EDM wire cutting machine operation for an extensive range of applications. Because the process can cut very small pieces, it is often an ideal choice for the production of small, highly detailed items that would normally be too delicate for other machining options. Additionally, the process is cost-effective for low quantity projects and can prove to be beneficial in prototype manufacturing, even if the actual project is carried out by different means.
It is important to remember that the wire in the process is constantly moving, and not to be reused. As a result, the copper, brass, or other metallic wire can be miles long, adding cost to the process. And, while the process uses no force and thus does not cause burrs and can be used on delicate items, the possibility of thermal stress is certainly present.
Most machining operations using wire EDM begin with a rough pass with a fairly fast feed rate and high dielectric flow. Successive skim passes take smaller cuts with a reduced dielectric flow to bring finished surfaces into tolerance. The reduced dielectric flow avoids distorting the wire during these skim passes.
Cuts that do not begin along an edge of a part (such as holes) require predrilling to allow the wire to thread through. Many wire EDM machines are fitted with small hole drilling electrodes for this purpose, permitting starting holes to be made in hardened steels without the use of conventional drills. Small hole drills usually employ an EDM electrode mounted in a spinning mandrel with dielectric pumped through the electrode to flush the hole. The process of making small holes in hardened material using standalone EDM drills is sometimes referred to as hole popping.
A major advantage of wire EDM over conventional EDM is that the continuous supply of wire minimizes the wear that occurs with a fixed electrode. A wire EDM machine can consume a lot of wire, however, adding expense. Many wire EDM machines are self-threading so that if a wire breaks the process can be continued almost without interruption.
Another major advantage is that parts may be cut after heat treatment which eliminates the possibility of distortion arising from post-machining treatments. Also, because the wire EDM method exerts no tooling pressure upon the workpiece, small, delicate parts are easily machined. Fixturing requirements are minimal compared to conventional machining methods. Very fine surfaces are possible.