CNC spark processing explains: how sparks cut hardened metal

• 1
  What a CNC spark processing explains at the gap
• 2
  Wire EDM, sinker EDM, and hole drilling
• 3
  Tolerance, surface finish, and the recast layer
• 4
  When EDM beats milling, and when it does not
• 5
  EDM variants compared for part selection

A CNC spark processing explains a simple idea: two conductive objects at different voltages jump a spark across a small gap, and that spark melts and vaporizes a tiny amount of metal. In EDM the electrode never touches the workpiece. The cut happens in a dielectric fluid, usually deionized water or hydrocarbon oil, which insulates until the voltage rises and then flushes debris out of the gap.

The gap is small. On a finishing pass it can sit between 0.01 mm and 0.05 mm, and the servo rides that gap thousands of times per second. If the gap closes, you get a short and a burn mark. If it opens too far, the spark stops and cutting stalls. Control systems are judged on how fast they recover from both.

Each discharge lasts a few microseconds. On-time and off-time are set per pass, and they decide the crater size, the recast layer, and the surface finish. Short on-time with high frequency gives a fine finish and a thin recast layer. Long on-time removes more metal per spark but leaves a heat-affected zone you may have to remove later.

Because there is no cutting force, hardness does not matter much. A 60 HRC tool steel block cuts at roughly the same rate as a 30 HRC block. That is the core engineering meaning of the process: it removes material by thermal energy, not by mechanical pressure.

Wire EDM uses a thin brass or coated wire, typically 0.1 mm to 0.3 mm, running through the workpiece. The wire never touches the part, so you can stack plates, cut a 0.2 mm slot, or slice a hardened die insert after heat treatment. Taper cuts up to about 30° are routine on a five-axis wire machine.

Sinker EDM, also called ram EDM, burns a shaped electrode into the part. You need an electrode for every cavity shape, usually graphite or copper. That tooling cost is the main reason sinker work is chosen for deep ribs, sharp internal corners, and blind pockets that a rotating cutter cannot reach.

Small-hole EDM drills start holes from Ø0.3 mm upward, including angled entries and holes in curved surfaces. It is common before wire EDM, because the wire needs a start hole. It also handles injector nozzles and cooling passages where a twist drill would wander or break.

All three variants share the same physics and the same limits. They cut conductive material only, they are slower than milling in bulk, and they leave a recast layer that needs a finishing pass if the part sees fatigue or high cycle stress.

A well-run EDM process holds ±0.005 mm on position and a fine finish of Ra 0.2–0.8 μm after the finishing passes. A single roughing pass lands closer to Ra 1.6–3.2 μm, which is fine for a clearance surface but not for a sealing face or a sliding bore.

The recast layer is the part engineers forget. Each spark quenches molten metal almost instantly, leaving a thin layer of resolidified material that is harder and more brittle than the base metal. On a die it can help wear resistance. On a fatigue-loaded aerospace bracket it is a crack starter.

To control it, plan multiple passes. A typical sequence is one roughing pass, two semi-finishing passes, and two or three finishing passes, each removing a few micrometres. The final passes reduce the recast layer to 1–5 μm and bring the surface into the fine range.

Inspection matters here. We check the first article on a CMM and confirm surface finish with a profilometer, then monitor in process. Without that, a part can measure to size and still fail on surface integrity.

Choose EDM when the material is too hard for a cutter, when the feature is too deep or too narrow for a tool to reach, or when the part is too thin to survive cutting force. Hardened dies, turbine blade roots, surgical instruments, and micro-holes all fit that description.

Milling still wins on bulk removal and on cost. A 4,000 mm aluminum frame with pockets and bolt holes is a milling job from start to finish. Putting it on a wire machine would take days and gain nothing, because aluminum cuts fast with a carbide tool and the geometry is open.

A practical split is common: mill the shape while the steel is soft, heat treat, then EDM only the critical features that moved or that a cutter cannot reach. This keeps the hard cutting where it belongs and avoids re-cutting a whole part after hardening.

Rule of thumb: if you can reach the feature with a rigid cutter and the material is under about 45 HRC, mill it. If you cannot reach it, or the material is hard, or the wall is thin, EDM it. Contact us before you commit if the feature is between those cases.