Non-contact drilling by controlled electrical spark erosion. No cutting force, no burrs, no broken drills — just a rotating tubular electrode and a pressurized dielectric that erodes the hole a micron at a time. The only practical way to drill deep micro-holes in hardened steel, carbide, titanium, and superalloys.
Thermal erosion, not mechanical cutting — the electrode never touches the workpiece.
A tubular electrode rotates and approaches the conductive workpiece through a dielectric. Each controlled spark melts and vaporizes a microscopic crater from the workpiece (and a little from the electrode tip), eroding the hole profile thousands of times per second.
Deionized water or dielectric is pumped through the hollow electrode center at 3–12 MPa (30–120 bar), flushing eroded debris back up the annulus and keeping the spark gap clean — the key to maintaining hole straightness at depth.
Servo drives the rotating electrode toward the workpiece; at the narrow gap the dielectric breaks down and a spark ignites.
The spark melts a small volume of material, which is ejected as the plasma collapses.
Pressurized dielectric through the electrode bore flushes the debris out of the deep hole.
The servo feeds the electrode forward, repeating the cycle thousands of times per second until breakthrough.
The hollow tube electrode is both tool and flush conduit. Most are brass or copper; the bore carries dielectric to the spark gap.
| Electrode Material | Typical Diameters | Best For | Notes |
|---|---|---|---|
| Brass tube (single & multi-hole) | Ø0.10–4.0mm single; Ø1.2–6.0mm multi-hole | Mould & die steels (e.g. SKD-11) | Best balance of conductivity, machinability, cost; the default for fast hole EDM |
| Copper tube | Ø0.15–6.0mm | Cemented carbide (e.g. KG5) | Superior electrical/thermal conductivity for fine detail and hard materials |
| Tungsten / tungsten carbide | down to Ø0.1mm | Micro-holes, very hard materials | Highest rigidity; used when tube diameters shrink below practical brass sizes |
| Multi-hole tubes | Ø1.2–6.0mm | Larger-diameter fast drilling | Multiple flush channels give better verticality and faster slag removal |
| Parameter | Typical Range | Effect / Note |
|---|---|---|
| Peak current | 15–25 A (0.3–1.0mm electrodes) | Most influential parameter; higher current erodes faster but increases electrode wear and hole diameter |
| Pulse on-time | 5–90 µs | For 1.0mm electrodes, fastest drilling is near 40–60 µs on-time |
| Pulse off-time | ~10 µs | Kept short for fast erosion while letting the plasma de-ionize |
| Electrode rotation | Continuous, tubular electrode | Improves roundness and evens electrode wear |
| Dielectric pressure | 3–12 MPa (30–120 bar) | Deionized water pumped through the electrode bore flushes the deep hole |
| Servo feed | Closed-loop gap control | Too slow → short circuits; too fast → oscillation. Keeps the spark gap constant |
| Water conductance | <10 µS/cm | Precision holes need maintained deionized water; conductivity drift degrades finish |
EDM is slow per hole, but it works where nothing else will. Published cycle times for a fast hole EDM machine (deionized water dielectric, brass tube electrode):
| Workpiece Material | Electrode Diameter | Hole Depth | Drilling Time |
|---|---|---|---|
| Alloy tool steel | Ø0.5mm | 12.5mm | 25–35 seconds |
| Alloy tool steel | Ø0.5mm | 25mm | 50–60 seconds |
| Alloy tool steel | Ø1.0mm | 25mm | 55–65 seconds |
| Tungsten carbide | Ø0.5mm | 12.5mm | 4–5 minutes |
| Aluminum | Ø1.0mm | 70mm | ~3 minutes |
| Criteria | EDM Drilling | Gundrilling |
|---|---|---|
| Diameter range | 0.05–6mm (typical 0.3–6mm) | 1–50mm |
| Aspect ratio (L/D) | 30:1+ typical; >50:1 specialized; up to 100:1 | Up to 300:1 |
| Material removal | Thermal (spark erosion) | Mechanical (shear) |
| Speed | Slow | Moderate |
| Surface finish Ra | 0.4–3.2 µm (recast layer present) | 0.4–1.6 µm as-drilled |
| Burr formation | None | Entry/exit burrs |
| Cutting force | None (non-contact) | Mechanical contact |
| Material hardness | No limit (any conductive) | Limited by tool wear |
| Hole straightness | Good on curved & angled surfaces | Can deviate in hard alloys |
| Tool breakage risk | None (electrode erodes) | Real in micro diameters |
| Machine / tooling cost | High (machine + electrode consumables) | Moderate |
| Secondary operations | Recast layer removal often needed | Deburring, sometimes honing |
| Decision Factor | Choose EDM | Choose Conventional / Gundrill |
|---|---|---|
| Hole diameter | Below ~0.5–1mm | Above 1mm |
| Material hardness | >45 HRC, carbide, superalloys | <35 HRC, standard steels |
| Aspect ratio | Moderate-to-deep (≤30:1) | Very deep (>50:1) |
| Surface integrity | Post-EDM recast removal acceptable | No thermal damage allowed |
| Production volume | Low to medium | Medium to high |
| Thin-walled part | Preferred (no cutting force) | Risk of distortion |
| Burr-free requirement | Preferred (no burrs) | Needs deburring |
The price of spark erosion is a thermally altered surface. Know it, plan for it.
| Layer | Typical Thickness | Characteristics | Risk |
|---|---|---|---|
| Recast layer (“white layer”) | 0.005–0.050mm | Resolidified molten material; micro-cracks, residual tensile stress, segregation, porosity | Micro-cracks & stress are the origin of fatigue cracks |
| Heat-affected zone (HAZ) | 0.01–0.10mm | Altered metallurgy beneath the recast layer | Reduced local properties in critical alloys |
| Underlying material | — | Unaffected base metal | Fine if recast is managed |
Recent research pushes EDM hole drilling beyond its speed and recast limits.
EDM needs an electrically conductive workpiece — everything conductive is fair game, hardness is irrelevant.
| Material Family | Examples | Why EDM Works Here |
|---|---|---|
| Tool & die steels | H13, D2, A2, hardened mold steels | Hardness >45 HRC stops conventional drills; EDM is unaffected |
| Stainless & alloy steels | 304, 316, 17-4PH, 4140, 4340 | Clean burr-free holes without work-hardening issues |
| Titanium | Grade 2, Grade 5 (Ti-6Al-4V) | Conventional drilling work-hardens titanium; EDM cuts it without tool contact |
| Cemented carbide | WC grades (e.g. KG5) | Often the only practical method for small holes in carbide |
| Nickel superalloys | Inconel 718 / 625, Waspaloy, Hastelloy | Heat-resisting alloys that resist conventional cutting |
| “Gummy” metals | Copper, aluminum, brass | No chip sticking or tearing as with mechanical drilling |