A single-lip gun drill cannot steer itself. With one cutting edge and no way to self-center, it must be handed a start — a pilot hole on machining centers and lathes, a starting bushing on dedicated gun-drilling machines. The first 1.5 to 3 diameters decide the straightness, the size and the tool life of the entire bore. Get the pilot geometry, the bushing clearance and the entry feed right and the rest of the hole follows; get them wrong and you get bell mouth, wander and snapped drills.
In deep hole drilling the tool is supported at the spindle and, once cutting, by its own guide pads riding the hole wall. At the instant of entry it has nothing to hold it — and that is the instant the hole is decided. A deflection here becomes a bell mouth, an oversize bore, a wandering axis or a snapped tool. The pilot hole (or starting bushing) is the fixed reference that constrains the tool within thousandths of a millimetre of the spindle axis during entry.
| Failure without a pilot or bushing | Mechanism | Result |
|---|---|---|
| Walking / wandering start | The drill slips over the entrance face instead of cutting | Off-location, bell-mouthed bore |
| Oversize entry | The gun drill follows the pilot it is handed | Hole takes the pilot diameter, not the drill |
| Chipped cutting edge | Improper pilot bottom geometry on hard alloys | Edge chipping on engagement (ER < 0.6) |
| Tool breakage at entry | Full speed, full feed, no guidance | Snapped tool, costly recovery |
| Source | Pilot diameter rule |
|---|---|
| Guhring | Pilot-hole tolerance G8; nominal tool tolerance always Ø m7 |
| Korloy | Pilot 0.01–0.02 mm (H7) larger than the gun drill |
| Carbide & Diamond Tooling | Drill, ream or bore pilot to drill dia +0.0005 in (+0.001 in max), −0 |
| Protool / TIMTOS | Pilot-hole diameter tolerance to ISO F7 |
Guhring recommends a minimum pilot depth of 1.5×D to 3×D, increasing for small diameters and for very deep final holes:
| Final drilling depth | Ø 0.9–1.799 mm | Ø 1.8–3.999 mm | Ø 4.0–7.999 mm | Ø 8.0–11.999 mm | Ø 12.0–52 mm |
|---|---|---|---|---|---|
| Up to 20×D | 3.0×D | 2.5×D | 2.0×D | 1.5×D | — |
| Up to 30×D | 3.0×D | 3.0×D | 2.5×D | 2.0×D | 1.5×D |
| Up to 40×D | 4.0×D | 3.0×D | 2.5×D | — | — |
Other manufacturers land in the same band: Walter recommends ~2×D for holes of 16×D and deeper, Korloy ~2.5×D, and a 1×D pilot suffices for shallow holes. Whatever the multiple, the pilot must be deep enough to fully engage the gun drill's guide pads before the drill takes full feed.
Dedicated gun-drilling machines always use a starting bushing. When a gun drill runs on a machining center or lathe, a pilot hole is usually cheaper and simpler than mounting a bushing — bushing setup is time-consuming, and small-part bushing tolerances are hard to control.
Machine the pilot with a carbide drill held to m7 tolerance (Guhring series 6400/5510) with a sharp ~140° point, or use a 180° flat-bottom drill for a true seating surface. For strict position and concentricity requirements, mill or lathe-drill the pilot.
| Pilot tool | Point angle | When to use |
|---|---|---|
| Carbide twist drill, m7 | ~140° | Standard quick piloting |
| Flat-bottom drill (180°) | 180° | True flat seating; oblique or curved entry |
| Reamer / bore | — | Tight diameter control; strict position |
Spot-face or face the entry square to the spindle — an off-square start is the #1 cause of drift in long bores.
On uneven or angled surfaces, spot-drill first so the pilot begins from a true, centered start.
Carbide drill to 1.5–3×D at ~140°, or a flat-bottom drill for a seating surface.
Ream or bore where diameter control matters; add an entry chamfer where advantageous.
The pilot must fully engage the gun drill’s guide pads before full feed is taken.
Clear swarf from the pilot and deburr the entry before the gun drill enters.
Entry is a two-stage event. The gun drill approaches at reduced speed and reduced feed, is allowed to seat fully inside the pilot, and only then ramps to production parameters. Gundrilling then runs continuously — no pecking.
| Phase | Spindle RPM | Feed | Coolant |
|---|---|---|---|
| Approach into pilot | 200–500 RPM | ~20 in/min (0.2–0.3 mm/rev) | Off (internal-coolant drills) |
| Seated in pilot | Ramp to operating RPM | Ramp up as pads engage | On, high pressure |
| Cutting to depth | Operating | Production feed, continuous | On, high pressure |
| Retract | 200–300 RPM | Fast retract feed | Off |
Bell mouth — a tapered, oversized entrance — is the unavoidable tendency of self-piloting tools, produced during the brief entrance instability of the first 0.3–0.8 s of engagement. Its severity is driven by starting-bush clearance, guide-pad design, pilot geometry and alignment.
| Factor | Effect | Prevention |
|---|---|---|
| Starting-bush clearance | Dominant factor in entrance stability | Minimize; change the bush as it wears |
| Pilot bottom geometry | Poor engagement on hard alloys | Conical bottom, ER > 0.6 |
| Bush-to-workpiece alignment | Grows the bell mouth and moves the hole | Check at least weekly; hold tight TIR |
| Guide-pad design | Two separate pads give 30–50% smaller force fluctuation | Use properly designed two-pad drills |
| Off-square entry face | Bell mouth plus drift at depth | Spot-face before drilling |
| Method | Starting method | Seal | Notes |
|---|---|---|---|
| Gundrill (single lip) | Starting bushing or pilot hole | Coolant through the tool | Pilot 1.5–3×D for 16×D+ holes |
| BTA / STS (single tube) | Bushing in the pressure head | Pressure head seals against the workpiece | Bushing mandatory; no pilot needed on dedicated machines |
| Ejector / DTS (twin tube) | Pilot hole very common; bushing sometimes | No face seal needed (Venturi suction) | Pilot typical on conventional machines |
| Mistake | Consequence |
|---|---|
| Running full speed with no pilot or bushing | Drill snaps on engagement |
| Pilot too shallow for the guide pads | Pads unsupported; wander and oversize |
| Pilot drilled oversize | Walking start, bell mouth, oversize bore |
| Pilot undersize (below the drill) | Interference, edge damage, breakage |
| Fast feed at entry | Chipped cutting edge, drift |
| Coolant on before the drill seats (internal) | Whip and broken drill |
| Pecking the gun drill | Rubbing, work hardening, breakage |
| Ignoring pilot bottom shape on superalloys | Edge chipping (ER < 0.6) |
| Symptom | Likely cause | Fix |
|---|---|---|
| Bell-mouthed entry | Loose starting bush or misalignment | Tighten clearance, realign, replace bush |
| Oversize first few diameters | Pilot drilled oversize | Pilot to ~+0.02 mm over drill |
| Chipped edge at start | Poor pilot bottom / fast entry | Conical bottom, slow entry feed |
| Hole not straight | Pilot too shallow | Extend pilot to 1.5–3×D |
| Breakage at entry | No support; coolant on too early | Use bushing / pilot; follow entry sequence |
| Short tool life | Pilot diameter mismatch | Match pilot to drill diameter |