Electrode cutting technology for CNC processing

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  How electrode cutting technology removes metal
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  Wire EDM and sinker EDM: two different tools
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  Pulse parameters that set speed and finish
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  Where electrode cutting technology fits in a CNC shop
• 5
  Accuracy limits and failure modes
• 6
  Electrode cutting technology compared with milling

Electrode cutting technology is a thermal erosion process, not a mechanical cut. A shaped electrode and the workpiece sit in a dielectric bath, usually deionized water for wire work and hydrocarbon oil for sinker work. The power supply charges the gap until the dielectric breaks down and a spark bridges the two surfaces.

Each spark lasts a few microseconds and reaches roughly 8,000 to 12,000 °C at the plasma channel. That heat melts and partly vaporizes a tiny crater of material. The dielectric then quenches the crater and flushes the debris away before the next pulse fires.

Nothing touches the part. There is no cutting force, no tool pressure and no work hardening at the cut edge. That is the whole reason this process exists: it cuts steel that a carbide end mill cannot touch without losing its edge in seconds.

The gap between electrode and workpiece never closes. The control keeps it at 0.02 to 0.5 mm depending on the pulse setting, and a servo axis moves the head to hold that distance while the part erodes.

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  No mechanical forceThin walls and slender ribs hold their shape.
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  Any conductive materialSteel, carbide, copper, titanium, Inconel.
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  Hardness is irrelevantA 60 HRC die block erodes at the same rate as annealed stock.

Wire EDM uses a brass or coated brass wire as a continuous electrode, 0.1 to 0.3 mm in diameter. The wire runs once and is discarded, so the electrode never wears into the part profile. This makes wire the default choice for through-holes, punch and die profiles, and any straight-walled cut.

Sinker EDM, also called ram EDM, uses a machined graphite or copper electrode with the negative of the cavity shape. It burns a three-dimensional pocket, which is what you need for a ribbed mold core, a deep square corner or a coined detail that no end mill can reach.

The electrode wears as it burns. A graphite electrode might lose 0.1 to 0.5 percent of its length per roughing pass, so we cut several electrodes or use orbital motion to compensate. Orbiting the electrode while it sinks lets one undersized electrode open a cavity to final size.

Wire is faster and more predictable on prismatic work. Sinker is the only option once the feature is blind and three-dimensional.

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  Choose wire forThrough profiles, sharp internal corners, hardened punches.
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  Choose sinker forBlind cavities, ribs, fine engraving, tapered details.

Four settings control the outcome: on-time, off-time, peak current and servo voltage. On-time is how long each spark burns. Longer on-time removes more material per pulse but leaves a wider, rougher crater. Off-time lets the dielectric flush debris; cut it too short and the gap shorts out.

Roughing runs high current and long on-time to move metal fast, and leaves a recast layer 5 to 25 μm thick. That layer is harder than the base metal and can carry micro-cracks. Semi-finish passes cut it back, and two or three finish passes bring the surface to Ra 0.8–1.6 μm or better.

For a finish below Ra 0.4 μm we add a low-current pass with very short on-time. Material removal rate drops sharply at that point. On a typical die insert, the last pass may take as long as all the roughing combined.

Debris is the limiting factor, not power. If chips are not flushed from the gap, the next spark fires through them and the burn turns unstable. Good flushing often buys more speed than a bigger generator.

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  RoughingHigh current, long on-time, Ra 3.2 μm or coarser.
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  Semi-finishMedium current, removes most of the recast layer.
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  FinishingLow current, short on-time, Ra 0.2–0.8 μm achievable.

Most of the work we run on EDM arrives from a milling or turning operation. A mold core is milled to within 0.1 mm, heat treated to 52–60 HRC, then finished on the sinker because a cutter would chatter or break in the hardened corner. The two processes are a sequence, not a competition.

Copper and graphite are the two common electrode materials. Graphite machines faster and wears less on roughing, but it cannot hold a razor edge, so it is a poor fit for sharp internal corners. Copper holds detail better and gives a cleaner finish, at roughly half the removal rate.

We hold ±0.005 mm on EDM work and inspect 100 percent of parts before shipment, with reports on request. That tolerance is achievable because there is no tool deflection to fight, but it depends on stable dielectric temperature and a clean gap.

Parts that should not go on EDM: large flat faces, open pockets, anything you can reach with a 3 mm or larger end mill. Burning those features costs three to five times what milling costs and gains nothing.

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  Good fitHardened dies, deep ribs, sharp internal corners, thin slots.
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  Poor fitOpen geometry, soft aluminum, high-volume simple parts.
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  Batch sizeOne prototype to 10,000+ part runs, no minimum order quantity.

Recast layer is the first thing to watch. If a die fails early in service, the crack usually starts in an unremoved white layer rather than in the base steel. Finishing passes exist to remove it, and skipping them to save an hour is a false economy on a tool that has to survive a million cycles.

Electrode wear is the second limit. On deep, narrow ribs the electrode corner rounds off and the cavity loses its sharpness. We compensate by undersizing the electrode and orbiting, or by switching to a second electrode for the finish pass.

Taper and flushing also matter. Wire EDM can cut a draft angle by tilting the head, but the achievable taper shrinks as the part gets thicker. On a 150 mm tall block, expect far less angle than on a 20 mm plate.

Thermal drift is the quiet one. A dielectric tank that warms by 2 °C over a long burn moves the part. For tight work we let the machine and tank stabilize before the finish passes start.

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  Recast layerRemove with finish passes, then verify on a cross-section.
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  Electrode wearUndersize and orbit, or plan a separate finishing electrode.
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  FlushingWeak flushing causes arcs, taper and scrapped cavities.