It’s not every day that industrial machining sends you into a fit of joy. But today could be that day. If you spend time on the internet, you might have come across GIFs of metal parts fitting together so precisely that the boundaries between them seem to disappear . My lord, the joy that sparks. Like, literally sparks: This is the mystical world of electrical discharge machining, or EDM, by far the most oddly satisfying field in all of engineering.
This may be the first you’ve heard of EDM, but it’s probably touched your life in some way. The tiny, precise components of an implantable medical device? EDM. Jet engine components made of hardy nickel alloy? That’ll be EDM too. If a manufacturer needs parts that fit as tightly together as humanly possible, or parts made of superhard materials that traditional machining techniques can’t touch, EDM is the way to go.
Traditional milling involves the mechanical shaping of a material—“applying a mechanical force against the workpiece to physically make a chip or remove material,” says Brian Pfluger, EDM product line manager at Makino, a machine tool maker. “Whereas with EDM we're not physically touching the part—we're machining with lightning bolts.”
First of all, that’s metal as hell. But more specifically, we’re talking lots and lots of little sparks. The “blade” in an EDM machine is actually a superfine brass wire through which electricity courses. Even though the machine is cutting through extremely tough materials, such as carbide (which is so tough that traditional milling techniques use it to drill through other materials), the blasts of electricity it uses are relatively weak. But the blasts come with extremely high frequency, something like 20,000 sparks per second along the length of the brass wire.
"It almost looks like a laser line, but if you really slowed it down it would be sparks all the way up and down that line," says Steve Sommer, vice president of Reliable EDM, an EDM shop. "Each spark is almost like a little miniature explosion."
The wire itself never actually touches the material. The rapid-fire sparks vaporize teeny tiny bits of the metal being cut, on the order of 5 microns wide. (A micron is a millionth of a meter. For context, a red blood cell in your body is between 6 and 8 microns wide.) This is known as sublimation.
“It's just like dry ice,” says Pfluger. “You go directly from a solid to a gas.” These so-small-they’re-almost-nonexistent particles then get caught up in a dielectric fluid running over the EDM and are flushed away. “That's kind of like washing your hair—rinse and repeat, rinse and repeat.” The fluid also helps keep the machinery from overheating.
Looking at these GIFs, you might assume the two pieces of metal are cut from the same piece of metal, but that’s not necessarily the case—each comes from its own slab. Part of what makes the gaps between them so imperceptibly small is that a manufacturer can take multiple passes at each of the two parts to hone them. (The gaps are so small, in fact, that air has a hard time escaping as the snowflakes above drop into place, thus they move super slowly.)
“When you get those nice interlocking sliding parts, typically they're high accuracy and fine finish,” Pfluger says. “From an accuracy standpoint, that's probably 5 microns or under in terms of total clearance between the two parts.” Pfluger says they can even get down to 2 microns or fewer. With more traditional forms of machining, you can only manage about 20 microns of clearance.
A shortcoming of EDM, though, is that it’s a slower method than mechanically machining parts. With typical methods, “you're going to set the cruise control at 60 miles an hour to just cruise on down the highway,” Pfluger says. “With EDM, you're going to hit a little bit more traffic.” Because it’s the sparks, not the wire, making contact with the material, the machine has to constantly adjust its alignment. If the wire hits the workpiece, it could snap. “What you're doing is you're stabilizing that machining spark gap and that very small distance between the electrode and the workpiece.”
Still, with that deliberateness comes a way to cut the hardest of materials with both astonishing accuracy and astonishing power—essentially weaponized lightning. Gaze upon these works and despair that nothing else may ever fit together with such mesmerizing joy.
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