The Basics of EDM and a Guide to Using It

Electrical discharge machining (EDM), sometimes referred to as “spark machining,” is one of the most useful and unique tools in a machinist’s arsenal. Instead of using a sharp cutting tool, the process uses sparks to cut through material, opening up new manufacturing possibilities.

But it’s not always easy to determine when EDM should be used. In fact, some engineers may be so accustomed to CNC machining technology that they haven’t even considered the possibility of using EDM to machine a part or prototype—even though EDM can greatly improve the quality of the final product or reduce overall project costs. This article attempts to quickly introduce EDM by discussing seven specific situations where EDM may be superior to traditional machining.

1.What is EDM?

EDM is a metal part manufacturing and prototyping process. The structure of an EDM machine is similar to that of a CNC machine, but the important difference is that EDM uses electrical discharges (sparks) to cut through metal materials instead of using sharp cutting tools.

So how do you control the spark so that it cuts through metal? During EDM, a soft metal or graphite tool acts as one of two electrodes, while a conductive metal workpiece acts as the other. The machine applies a voltage while both the tool and the workpiece are immersed in a dielectric fluid. This setup causes the liquid dielectric to break down, creating an arc powerful enough to remove material from the workpiece. The debris is then flushed away by replacing the liquid dielectric.

2.There are two main types of EDM:

Sinking EDM uses a metal die as an electrode and is suitable for creating complex shapes because the die is pre-made into a specific shape.

Wire EDM uses a taut wire and is suitable for cutting thick plates and particularly hard metals.

3.Advantages of EDM over conventional machining include:

Ability to penetrate very hard metals

Ability to machine complex shapes

Reduced distortion of fine features

However, it is not suitable for all jobs. Not only is EDM not suitable for plastics, it is also much slower than conventional machining, and CNC is superior in most cases.

4.Here are some engineering situations where EDM can produce better results than conventional CNC machines:

①When sharp internal angles are required

As long as the correct wire type, tension and feed conditions are selected, EDM (especially wire EDM) will show its advantages when machining sharp internal angles.

With conventional CNC machines, it is difficult to achieve sharp or square inside corners because the cutting tools are usually round, leaving fillets (radii) on the workpiece. There are ways to get around this, such as using square end mills and single-sided undercuts, but machinists generally accept the necessity of rounded inside corners.

Although the wire used for wire EDM is also round, it is much thinner than the CNC cutting tool, so almost square inside corners can be machined. With wire EDM, corner radii can be reduced to as little as 0.005 inches.

②When Very Deep Cuts Are Required

With conventional CNC machines, milling deep cuts and cavities requires long cutting tools. However, tools with a high length-to-diameter ratio can cause chatter (excessive tool vibration), resulting in a poor quality finish.

EDM offers a solution for very deep cuts because it is not prone to chatter. In most cases, EDM can easily machine holes with length:diameter ratios up to 20:1, but in some cases, the ratio can be as high as 100:1.

③When the workpiece material is very hard

One of the main uses of EDM is for machining items such as molds and dies, or even the machine tool cutting tools themselves. These items are often made of extremely hard materials such as hardened steel or tungsten carbide, which are difficult for the cutting tool to penetrate. One advantage of EDM is that the hardness of these metals does not greatly affect the machine’s ability to meet tight tolerances. Other metals that can be cut with EDM include Hastelloy (a nickel-molybdenum alloy), titanium, and Inconel.

④When a part requires a mirror finish

EDM typically produces a part with a surface finish consisting of pits or dimples. However, by closely controlling certain parameters, the process can produce an excellent mirror finish without the need for polishing.

Importantly, EDM does not produce a directional surface with “layers” like conventional machining. The size of the pits or dimples can be greatly reduced by using slow EDM at low power. The final result is a very smooth surface finish of about 5 RMS (4 micro-inch Ra).

Another way to create a mirror effect is to introduce fine abrasive powder into a dielectric fluid, which creates an effect similar to sandblasting, but during the process rather than after.

⑤24/7 Production

Although this is more of a concern for manufacturers than engineers, EDM offers important possibilities in automation and unattended or “lights-out” manufacturing. Because EDM is more predictable than traditional machining and less susceptible to erroneous interruptions, EDM machines can be automated and run around the clock with minimal supervision. If manufacturers can automate the loading and unloading of workpieces (and electrodes), they can create an efficient, automated manufacturing workflow.

⑥When Making Molds

EDM is sometimes preferred over CNC machining in mold making, in part because of its ability to precisely cut hard metals such as tool steel. Sometimes a CNC mill or EDM machine is used first, followed by a wire EDM machine for details or the deepest cuts. Many injection molding companies use a combination of CNC machining and EDM machining, choosing one based on the job requirements.

⑦When Tight Tolerances Are Required

Precision machining becomes easier because very tight tolerances can be achieved with EDM. Importantly, since there is no physical contact, the workpiece is less likely to be deformed by EDM. Through multiple passes, EDM can achieve tolerances as low as +/- 0.0002”. However, cutting speeds need to be significantly reduced to ensure the highest accuracy.