Saw a video the other day about drilling with EDM (electrical discharge machining) and it’s just amazing (the process, not the video). Then I found some videos about cutting with EDM.
How does cutting with EDM work?
AIUI EDM works by causing a tiny arc that vaporizes away the material, and repeating this many times a second. I can understand how this works for a single contact point, but when you’re cutting with the wire (kind of like a band saw) your area of contact is many times that size.** How do you ensure it cuts evenly along the whole length? ** From what I know of electricity, it’s the first place to arc is going to ionize the intervening material (the dielectric, in this case) and later sparks will be drawn towards this ionized area due to the lowered resistance. You have to wait for it the ionized area to settle down or be flushed away.
It’s inherently self-correcting.
The arc is attracted to the highest spot, so that will get eroded first. Once it gets eroded enough, another spot attracts the arc. This process is repeated many times/second.
The wire is continuously moved, so it always presents an reasonably straight surface.
Okay, that explains a lot. I was thinking it was somehow cutting along it’s entire length with each pulse which is not how electrical arcs work (in my understanding).
What separates this from MIG welding which uses the same principle to add material? Is it a different type of material in the wire/rod, or just a higher current (or voltage)?
I suspect it’s a matter of “tuning” - EDM is designed for high-intensity discharges, of very short duration. These pulses blast small amounts of the surface off into the dielectric fluid. With MIG, the current is more-or-less continuous, although the magnitude changes as the arc oscillates in length (creating the typical “frying” sound). The wire is also fed into the molten pool, so there is material being added to the surface. With EDM, the wire never touches the base metal, and is not supposed to erode very much, so there’s no metal available to be deposited.