Electric Discharge Machining (EDM)

      EDM is a method for producing holes and slots, or other shapes, by using an electric discharge (spark) to remove unwanted material. It is also called spark erosion. Sometimes it is used to produce a part, such as producing a slot in a very hard metal, and sometimes it is used to "rescue" a part such as removing a broken tap.

      The basic idea is to move an electrode very close to the work piece, and repeatedly produce a spark between the two. This is best done while immersed in a dielectric liquid rather than in air, and it helps if the proper distance can be automatically maintained. Note that the electrode gets eaten as well as the workpiece, and some compensation must be made for this. Very good finish can be achieved, though at reduced speed. EDM is not a fast method; some jobs can take days to produce holes, so its use is limited to jobs that cannot easily be done in other ways (e.g. oblong slots or complex shapes, sometimes in very hard material). Note too the work must be conductive so it does not work on materials such as glass or ceramic, or most plastics.

      A good overview is in the Metals Handbook published by the American Society for Metals (ASM), volume 3, "Machining" (page 227 of the 8th edition, 1967).

      A wire EDM machine uses a wire (usually brass) as the electrode, which passes between guides like a bandsaw blade. The wire may be used only once. As it runs from a spool through the job, it is eroded and reduced in diameter by as much as 1/3. The old immersion system has now been largely replaced by flushing nozzles which surround the upper and lower wire guides. These blast the dielectric through the cut. This system works well, and has the tremendous advantage of being quick to shut off to remove parts, rethread broken wire, etc.

      As to the functionality of the wire EDM system, just imagine what you can do with a wire that leaves a .010" kerf through 6" thick stainless steel without heating it up. With the CNC control a taper can be imparted to the workpiece, because even the low-end machines have independently controlled upper and lower guide movement. High-end machines are mostly a way of getting more power (and therefore faster operation); more capacity; and convenience features, like automatic wire threading and a sinker-EDM head to create internal starting holes in hard material.

      EDM is well-suited for automated control. In fact, it seems almost the only way to go, which usually puts sophisticated EDM out of the reach of most home shop machinists. Luckily, the fundamentals are simple enough you can get by without computer control at the expense of constantly adjusting the gap manually.