Not Your Standard Metal Fabrication Machine

The employee at the used machine tool company witnessed a new arrival being brought in. It looked very different than any of the other machines they had in stock, like lathes, leaf brakes, or plasma tables. The machine was largely white, almost pristine looking when compared to most of the machines they carried. It was boxy in shape but had a tall extension going up from the main body. Was this some sort of a mill, with the tall thing being its headstock?

While the employee had been on the job a couple of years, this was completely new to him. Since it was his job to log in all the machines into inventory and list them in their advertising, he asked what it was. “An EDM,” his boss responded, seeming a little annoyed.

“Okay—does that stand for something?”

“Electrical discharge machine,” came the terse reply. The employee felt even more confused.

After another tentative question, he learned that somehow this bizarre machine could cut through metal with a wire, of all things.

“Okay, what category should I list it under?” he asked the ever more exasperated employer.

“EDM,” said the boss.

“Um, yes, but does it go under shears? Saws? Lasers, maybe?”

“No,” the employee was told, in a voice straining to be patient, “EDM is its own category.”

The employee shrugged and a short time later added a new category to their equipment listings: “EDM.” He wondered what strange type of machine they’d bring in next.

A Spark of Genius

The process of using “electrical discharge machining” to remove metal from a workpiece in a controlled way may sound more science fiction than science fact, but the concept is a very real principle. Microscopic pieces of a workpiece can be removed or “eroded” by use of pulsating electrical discharges—in other words, sparks. 

Electric spark erosion is observable in everyday life in an uncontrolled way whenever an electrical short results in part of the metal being burned away. This non-conventional machining process involves two pieces of electrically conductive material being brought close together—without touching—so that a white-hot electrical spark jumps the gap between them, rapidly melting away dust-sized chunks of material.

Both the EDM’s tool and the workpiece are considered electrodes, with the tool an anode and the workpiece a cathode in a circuit. A servo control mechanism automatically maintains a hair’s breadth gap constantly between them—close enough so they spark, but far enough that they don’t touch, as that would cause dangerous arcing that could damage both pieces. The gap between both electrodes is filled with a dielectric fluid—a liquid that is non-conductive to electricity.

No matter how good an insulating material is, given a high enough voltage it will conduct electricity. The fluid between the two electrodes insulates them until the voltage is increased sufficiently to cause a dielectric breakdown in it, allowing an electric arc to be produced between the tool and the workpiece, resulting in the erosion of material. Material erosion is controlled in electrical discharge machining due to the insulating nature of the fluid at lower voltages and the precise movements of the servo.

The EDM process works very well for machining hard metals that otherwise would be difficult to work with, like pre-hardened steel, titanium, or high-performance nickel alloys. The material doesn’t need to be heat-treated before or after the procedure, eliminating the deformation that accompanies that activity. While a slow-moving technique—making it problematic for use in mass production situations— electrical discharge machining can essentially work on any conductive material, no matter its mechanical properties.

Types of Electrical Discharge Machines

Any machine that uses the electric spark erosion principle is considered an electrical discharge machine, though the two most common types are the conventional and wire EDM machines.

• Conventional EDM Machines are popular because of their ability to create complex profiles in metal. A previously machined shape, often made of graphite or copper, becomes the tool (the top electrode). It is moved vertically down towards the workpiece (the bottom electrode) through a dielectric fluid and a negative impression in the shape of the tool is then eroded away from the workpiece by the several hundred thousand sparks that are generated per second. Other names for conventional type EDM machines are Cavity-type, Die Sinker, Ram, Sinker, and Volume EDMs.
• Wire EDM Machines use a vertically positioned needle-thin wire, often made of brass, as the tool electrode. This wire, which is kept under tension, continuously moves on a programmed route, eroding a narrow path as it travels through a workpiece that has been submerged in a tank of dielectric fluid. Since erosion takes place on the tool as well as the workpiece, the wire is constantly being unspooled and automatically fed throughout the operation. Other names for wire electrical discharge machining are Spark Eroding, Spark Machining, Wire Burning, and Wire Erosion.
• Hole Drilling EDM Machines use a rotating tube as the electrode which sprays a pressurized jet of dielectric fluid as it goes. It is ideal for making holes in materials too hard for standard drilling and is regularly used in the aerospace industry. It can also be used to prep a workpiece so a wire EDM can cut a shape inside of it, where normally it would have to start cutting on the outside and work its way in.

History of Electrical Discharge Machining

Joseph Priestley, an 18^th century English philosopher and chemist, made several important contributions to science during his lifetime, such as discovering oxygen through the decomposition of heated mercuric oxide and inventing carbonated water. He studied electricity extensively and discussed it with many noted experimenters in the field, including Benjamin Franklin during a trip he made to Great Britain. As early as 1766, Priestley described ringlike marks forming in metal due to electrical discharges from a Leyden jar, an early type of capacitor used to store a charge. This erosive effect became the principle behind the cutting processes of the electrical discharge machine.

Conventional electrical discharge machining was discovered by accident in 1943 by Boris and Natalya Lazarenko, a husband-and-wife team of Russian scientists. Failing to find a way to keep sparking from eroding electrodes made of tungsten, they observed that the erosion could be controlled if the electrodes were submerged in a dielectric fluid. This led them to invent the first true EDM machine. Americans Harold Stark, Victor Harding, and Jack Beaver independently developed their own EDM machine at about the same time. Wire EDMs were later introduced during the 1960s.

Watch the Sparks Fly

As strange as electrical discharge machining sounds to those unfamiliar with it, it really is an amazing technology. As a contactless cutting method, it is perfect for machining very small or delicate workpieces and for creating very fine holes. Really hard material can be machined to extremely close tolerances by use of an EDM machine.

As EDM technology is perfected in the coming years, its drawbacks—such as significant power consumption—will be greatly mitigated, allowing these science-fiction-sounding electric spark erosion machines to become even more prominent in shops of all sizes in the future.