Most shops already own the best gun-drilling spindle they will ever use — the tailstock of a CNC lathe. With through-tool coolant and a steady rest, a lathe can hold deep, straight gundrilled bores without a dedicated machine, especially for round bar work.
Gundrilling needs three things: a spindle that holds the tool (or part), a rigid axis to feed against, and high-pressure coolant through the drill. A lathe already has a rigid tailstock axis and a spindle — add through-tool coolant and it is a functional gun drilling machine for rotating-workpiece setups, which is exactly the configuration that gives the best straightness on round bars.
| Setup | How it works | Best for |
|---|---|---|
| Tailstock gundrill | Gundrill mounted in a tailstock adapter; part rotates in the chuck | Round bars, shafts, center holes |
| Live-tool turret | Gundrill in a driven (live) turret station; part stationary | Eccentric holes, irregular parts, hex bar |
| Counter-rotation | Part and tool rotate opposite | Highest precision, critical straightness |
Gundrilling without high-pressure through-tool coolant is not gundrilling. On a lathe you need a rotary union or a coolant-thru tailstock to get oil from the machine’s pump (or an external booster) into the drill.
Use a steady rest near the tailstock for L/D > 10:1. Whip in the unsupported span is the main source of bore drift.
The gundrill must enter dead true — shim the tailstock or adapter until runout is < 0.02 mm at the tip.
Start at ~50% feed for the first 3–5×D, then ramp to full feed. Never peck — gundrills cut continuously.
Watch spindle load and coolant pressure; both should be steady. A climb means a dull drill or a packed flute.
| Factor | Lathe setup | Dedicated gundrill machine |
|---|---|---|
| L/D capability | ~40:1 practical | 100:1+ routine, 300:1+ possible |
| Straightness | ~0.001 in/ft with part rotation | 0.03–0.08 mm/m with counter-rotation |
| Capital cost | Low (retrofit existing machine) | High |
| Whip-guide spacing | Hard to add along the axis | Built-in steady rests & whip guides |
| Production rate | Good for small batches | Best for high volume |