Deep Hole Drilling GuidePractical technical reference
🔵 RING CUTTING · SOLID CORE RECOVERED

Trepanning Deep Holes

Trepanning removes the bore as a ring annulus, leaving the center intact as a solid slug. When the core has value — or when a solid drill would need unrealistically high power — trepanning is the method that gets a big deep hole out of a bar.

Ø50–2000+mmDiameter range
Up to 10 m+depthSingle pass
Solid corerecoveredThe center slug
Lowerpowervs solid BTA drilling
PRINCIPLE

How Trepanning Works

A trepanning head carries cutting inserts arranged around a ring, with a hollow boring bar behind it. The tool cuts only the annulus at the bore wall; the center column stays in place until the cut is finished, then is withdrawn as a solid core (slug). Coolant is delivered through the boring bar (internal delivery) or externally, and chips return through the tube or along the annulus depending on the system.

💡 The economics: trepanning removes only the material that actually becomes the hole. Power, time and coolant load all scale with the ring area, not the full bore area — that is why very large deep bores are trepanned rather than drilled from solid. On a Ø300 mm bore, a solid drill removes ~70× more metal than trepanning leaves.
MethodBore diameterChip returnCore
GundrillØ0.5–50 mmExternal V-fluteNo (drilled from solid)
BTA (STS)Ø12–250+ mmInternal tubeNo
TrepanningØ50–2000+ mmInternal tube / annulusYes — recovered
WHEN TO TREPAN

Trepan vs Drill From Solid

✅ Trepan when…

  • Bore diameter is large (> ~Ø100 mm) and L/D is high
  • The center core has value (billet recovery, gun barrels, test coupons)
  • Machine power or thrust is the limiting factor
  • Through-hole with open ends for core removal

⚠️ Avoid when…

  • The hole is blind — no way to extract the core
  • Diameter is small enough that solid drilling is efficient
  • Core straightness matters to the customer — cores can wander
  • You need maximum metal removal rate on medium diameters
⚠️ Core handling: the slug stays attached at the bottom of the cut until the tool breaks through. Catch it on a catcher or guide, or let it drop onto a receiving trough. A core that drops and skids can damage the freshly cut bore wall.
TOOLING

Trepanning Heads & Boring Bars

Trepanning heads carry multiple indexable inserts (carbide) arranged around the ring, with chip-breaker geometry sized to the insert width. The boring bar is a hollow tube — stiffer and more economical than a solid drill body of the same diameter. Balance and rigidity dominate because the head spans a large diameter.

💡 Vibration: the wide ring of inserts is prone to chatter, especially at entry. Reduce insert count, increase feed stability and use tuned boring bars on L/D > 20:1. Vibration dampers pay for themselves fast on trepanning bars.
PROCEDURE

Typical Trepanning Sequence

1
Pilot the start

Cut a short pilot bore or mount a starting bushing at the entry face.

2
Enter slowly

Feed the head in at ~50% of target feed until the full ring is engaged.

3
Cut to depth

Continuous feed with steady coolant flow. Monitor spindle load — a jump means insert loss or chip blockage.

4
Break through & catch core

Reduce feed at breakout; catch the core on the receiving side.

5
Withdraw

Retract with coolant flowing to keep chips clear.

APPLICATIONS

Where Trepanning Is Used

🚗 Gun BarrelsLarge-caliber tubes trepanned to save billet material.
🔧 Pressure VesselsNozzle and port bores in thick-wall forgings.
🛡 Oil & GasDownhole tool bodies where the core becomes another part.
⚙️ Rolls & ShaftsPaper-mill and extruder rolls cored out to reduce weight.
🔧 Cylinder TubesVery large hydraulic tubes pre-cored before finishing.
✈ AerospaceLarge structural spars and landing-gear components.
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