CNC Electric Spark Wire Cut: How the Spark Removes Metal

• 1
  What actually happens at the spark gap
• 2
  The machine tool: wire, guides, generator and axes
• 3
  Programs: from drawing to a threaded wire
• 4
  Where wire EDM fits and where it does not
• 5
  Reading a drawing before you quote wire EDM
• 6
  Accuracy, finish and the passes that produce them
• 7
  Wire EDM against milling, laser and small-hole EDM

Wire EDM removes material by controlled electrical discharge. A thin wire, usually brass or coated brass, runs vertically between two guides. The workpiece sits in a dielectric bath, normally deionized water. A servo drives the wire toward the part until the gap is small enough for the voltage to break down the water and strike a spark.

That spark lasts microseconds. It heats a small volume of metal past its melting point, and the dielectric then implodes and flushes the molten pool away as a tiny sphere of debris. Each discharge leaves a small crater. Thousands of discharges per second, spaced by the servo and the pulse generator, walk the wire along the tool path.

Nothing touches the part. Cutting force is essentially zero, so thin walls, delicate ribs and heat-treated blanks keep their shape after the cut. That is the single biggest reason wire EDM exists alongside milling and turning. A 1 mm wide slot in 60 HRC tool steel is routine here and near-impossible on a milling machine.

The gap is small but not zero. Typical spark gap runs 0.02–0.05 mm per side depending on the generator setting. The CAM post must offset the wire path by the wire radius plus the gap, which is why the machine's offset library matters more than the drawing's nominal size.

• 1
  No cutting forceThin walls and hardened parts stay flat.
• 2
  Conductive onlyPlastics and ceramics cannot be wire cut.
• 3
  Gap is not zeroOffset = wire radius + spark gap.

A wire EDM machine is a five-axis system in most shops: X, Y, U, V and Z. The lower head moves in X and Y. The upper head adds U and V, which tilt the wire and let the machine cut a taper or a ruled surface through a die block. Z controls height, and the wire runs continuously from a supply spool to a take-up spool.

The wire is a consumable, not a cutter. Common sizes are Ø0.15 mm, Ø0.20 mm and Ø0.25 mm. Smaller wire cuts tighter internal corners and thinner slots but removes metal more slowly. A Ø0.25 mm wire can be threaded through a start hole of about 0.3 mm; a Ø0.15 mm wire needs a hole closer to 0.2 mm.

The pulse generator sets the energy per discharge. Higher peak current and longer on-time remove metal faster but leave a rougher surface and a thicker recast layer. Roughing passes can run at several hundred mm²/min on aluminium, while a finishing pass may run at a fraction of that to reach Ra 0.2–0.8 μm.

The dielectric system is not a detail. Deionized water resistivity is held in a narrow band, usually 5–15 MΩ·cm for finishing work. Filters keep debris out of the gap. If resistivity drifts, surface finish and dimensional consistency drift with it, and the operator will chase the offset all day.

• 1
  Taper cuttingU and V axes tilt the wire, often up to ±30°.
• 2
  Wire diameterØ0.15–0.30 mm covers most die and punch work.
• 3
  Water qualityResistivity and filtration drive repeatability.
• 4
  Automatic threadingKey for lights-out and multi-cavity dies.

Programming a CNC electric spark wire cut job starts with a 2D profile, not a 3D solid. The CAM system reads the profile, applies a lead-in, and offsets for wire radius and spark gap. It then splits the path into roughing and finishing passes, each with its own generator setting and offset value.

Most shops program at the machine for simple work. The operator types the contour, picks a technology table entry, and the control calculates the offsets. For dies with taper, multiple cavities or a hundred similar parts, offline CAM pays for itself quickly. The output is an NC file with X, Y, U, V, Z and generator codes on the same line.

The technology table is the heart of the program. It pairs material, thickness, wire diameter and finish requirement with a set of currents, on-times, off-times and servo voltages. A shop that runs 4140, 17-4PH and tungsten carbide will keep separate tables. Copying a table across materials is how parts come out undersized.

Verification happens twice. The CAM software simulates the path for collisions and taper interference. Then the machine runs a dry pass with the wire threaded but the generator off, so the operator can watch the path before any metal is removed. On a die block worth thousands of dollars, that dry run is not optional.

• 1
  2D profile firstWire EDM is a contour process, not a 3D surfacing one.
• 2
  Offsets per passEach skim pass has its own gap compensation.
• 3
  Material-specific tablesNever reuse a table across alloys without testing.

Wire EDM earns its cost on features that are hard to mill. Sharp internal corners, deep narrow slots, hardened punches, and contours through 60 HRC material all point to wire. So do parts where cutter force would deflect the workpiece, such as thin webs or long slender sections.

It is slow for bulk removal. A block that could be milled in twenty minutes may take hours on the wire, especially if most of the volume has to be cut away. The practical rule is to mill or turn the bulk first, heat treat if needed, then wire cut only the critical profile. That sequence also avoids distortion from machining after hardening.

The process needs a conductive material. Aluminium, brass, copper, steel, titanium and most superalloys work. Plastics, ceramics, glass and unfired carbides do not. Some materials bring their own problems: aluminium forms a tough oxide that can slow the cut, and beryllium copper needs filtration attention because the debris is hazardous.

Start holes are the other limit. Every internal cut needs a hole the wire can be threaded through, typically drilled or, in hardened material, cut by small-hole EDM. If a part has no room for a start hole, wire EDM may not be the right process regardless of the geometry.

• 1
  Good fitHardened steel, sharp corners, thin walls, die profiles.
• 2
  Poor fitBulk removal, blind pockets, non-conductive materials.
• 3
  Start hole neededPlan it in the drawing, not on the shop floor.

Tolerance callouts matter more than the outline. A general ±0.1 mm note with a few tight dimensions is normal for wire work. Demanding ±0.005 mm across every feature of a large plate is expensive and often unnecessary, because the machine can hold it but the setup and temperature control cost more.

Corner radii decide whether wire can reach the feature at all. The smallest internal radius equals the wire radius plus the spark gap. A 0.1 mm internal corner cannot be cut with Ø0.25 mm wire. Either the design opens the radius or the shop moves to a smaller wire and accepts a slower cut.

Surface finish should be specified per face and per function. A sealing face may need Ra 0.2–0.8 μm, while a clearance contour is fine at Ra 1.6–3.2 μm. Asking for a fine finish everywhere adds skim passes and time without adding function. Note whether the finish applies after heat treatment, since that changes the pass count.

Finally, state the material and its condition. The same profile in 6061 aluminium and in hardened D2 tool steel behaves very differently at the generator. If the part will be hardened after cutting, say so, because the shop may need to leave stock for grinding or plan a stress-relief step.

• 1
  Tolerance per featureNot one blanket callout across the plate.
• 2
  Smallest radiusWire radius plus spark gap sets the floor.
• 3
  Finish per faceSpecify only where the function needs it.
• 4
  Material conditionAnnealed, pre-hardened or hardened after cutting.

A wire cut is not one pass. The first pass roughs the profile and leaves a recast layer plus a rough surface. Subsequent skim passes remove that layer and step the finish down. A two-pass cut reaches roughly Ra 1.6–3.2 μm. Four or more passes are needed for Ra 0.2–0.8 μm on steel.

Each pass shifts the offset by a few micrometres. The control stores these as an offset table, and the operator tunes them per material and thickness. That is why a shop with a settled process can hold ±0.005 mm on a 50 mm thick hardened block, while a shop copying parameters from a manual will struggle to hold ±0.05 mm.

Thermal effects are small but real. The recast layer on a single-pass cut can be 5–20 μm deep and is brittle. For a die that will see impact, the layer should be removed by skimming or by a light stress-relief. Leaving it on a fatigue-loaded part is a common and avoidable failure source.

Measurement closes the loop. Wire EDM parts are usually checked on a CMM or with a micrometre and pin gauges, at a controlled temperature. A part measured hot off the machine can read several micrometres different from the same part after it stabilizes.

• 1
  Pass count drives finishTwo passes for clearance, four or more for seals.
• 2
  Recast layerSkim it off on fatigue or impact parts.
• 3
  Measure settledLet the part reach room temperature first.