GlossaryKind of term: Manufacturing termHole EDM (start-hole drilling)
What is hole EDM?
Hole EDM erodes small, deep holes with a rotating tube electrode, often as the start hole for wire EDM in hardened material.
Hole EDM is spark erosion with a thin brass or copper tube electrode that rotates while dielectric is pumped through it at high pressure. Discharges remove the material ahead of the tip; the flushing carries the debris away and keeps the gap stable. It makes holes from a few tenths of a millimetre to several millimetres across, deep relative to their diameter, in any conductive material, including hardened steel and carbide. The classic job is the start hole for wire EDM; others are cooling holes, small spray holes and removing a broken tap or drill.
What decides the quality of an EDM-drilled hole?
No cutting force acts, so a thin electrode can go deep and wanders less readily than a long twist drill. The electrode does wear along its length, often heavily, and the hole comes out slightly larger than the electrode because of the spark gap. Straightness, diameter and the quality of entry and exit depend on electrode diameter, depth, flushing, material and settings. The wall carries a thin recast layer. For a start hole that does not matter; for a hole with a tight size or position tolerance, hole EDM is usually a preparatory operation, not the final one.
How does hole EDM work in practice?
Eroding one hole often takes a few minutes, but every hole needs positioning and alignment, and sometimes its own setup on a machine other than the mill or the wire EDM. On a one-off that setup can outweigh the erosion itself. Electrodes and guides are consumables. Start holes are often drilled before hardening; where that is not possible, or in carbide, hole EDM comes after hardening and before wire EDM. It is priced as an operation with its own time and rate, or as a subcontract operation at an EDM shop.
What is hole EDM often confused with?
Hole EDM is not drilling: without cutting force hardness does not matter, but the hole comes out slightly larger than the electrode and the wall has a recast layer. Sinker EDM uses a shaped electrode that forms a cavity; hole EDM uses a rotating tube electrode that makes a round hole.
Frequently asked questions
Why do I need a start hole for wire EDM?
Because the wire has to pass through the workpiece before it can cut a closed profile. It is often drilled before hardening; in hardened material or carbide it is usually eroded, at or near the start point of the profile.
Is an EDM-drilled hole accurate enough as a finished size?
For a cooling hole or a start hole, often yes. For a fit or a tight position tolerance, usually not: the hole is slightly larger than the electrode, can taper slightly and has a recast wall. A finishing operation then follows.
Does hole EDM belong as its own line in the estimate?
That is the clearest choice: its own time and rate, or a subcontract operation. It runs on a different machine from the milling and needs its own setup; folded into the milling rate it would disappear from view.