I have had this conversation a dozen times. A shop calls and says they tried gun drilling on a manual lathe, snapped the drill in the first hole, and want to know what went wrong. The answer is almost always the same: coolant pressure, pilot hole, or feed rate. Sometimes all three.
Here is what I see most often.
Failure 1: Not Enough Coolant Pressure
This is the #1 first-time killer. A standard lathe coolant pump puts out 20-50 psi. Gun drilling needs 500-3000 psi depending on diameter. The difference is not subtle — at 50 psi, chips do not move. They pack behind the drill and the torque spikes until something breaks.
I have a rough rule of thumb for minimum pressure by diameter:
| Drill Diameter | Minimum Pressure | Minimum Flow |
|---|---|---|
| Under 3mm | 2000 psi | 1 GPM |
| 3-6mm | 1500 psi | 2 GPM |
| 6-12mm | 1000 psi | 5 GPM |
| 12-20mm | 600 psi | 10 GPM |
| Over 20mm | 400 psi | 15 GPM |
These are minimums. Running higher is always better. If your machine cannot deliver these pressures, you cannot gun drill reliably on it. I cover this in detail in Gun Drilling Coolant Pressure.
Failure 2: Wrong Pilot Hole
The most common pilot hole mistake is drilling it with a twist drill and leaving a tapered bottom. A gun drill tip needs a flat-bottomed pilot hole. If the bottom is tapered, the tip contacts off-center and deflects.
The pilot hole should be:
- 0.5-1.0 drill diameters deep
- 0.01-0.02mm larger than the drill diameter
- Flat-bottomed (bored, not twist-drilled)
- Deburred at the entry
A bad pilot hole causes the drill to walk on entry. Once it has walked 0.05mm, it will not recover. I cover entry preparation in Drill Walking Systematic Diagnosis.
Failure 3: Feeding Before the Drill Is Cutting
The procedure matters. I see operators start the spindle, then feed the drill into the work. The drill tip skids across the surface instead of cutting, and the cutting edge chips.
The correct sequence is:
- Bring the drill tip into the pilot hole with the spindle stopped
- Start coolant flow
- Start spindle rotation
- Start feed
If the drill tip is not fully inside the pilot hole before the spindle starts, the tip will skid and chip.
Failure 4: Pecking
Gun drills are designed for continuous feed. Pecking — retracting during the cut — interrupts chip flow. Chips that were moving back along the flute settle, pack, and block the next pass. On the second peck, the drill drives into packed chips and breaks.
I have seen operators who come from twist drilling try to peck a gun drill. Every time, it ends with a broken tool. Gun drilling is continuous. Start at the top and do not stop until you reach depth.
Failure 5: Wrong Coolant Type
Water-based coolant does not lubricate a gun drill well enough. Gun drilling needs oil — specifically, oil with extreme pressure (EP) additives. Water-soluble coolants can work at very high concentrations (10%+), but oil is better.
I have seen shops use straight water-based coolant at 5% concentration and wonder why their tool life is measured in minutes. The coolant provides both lubrication and chip evacuation. If the lubricity is wrong, the drill edge wears fast and the chips do not slide.
Failure 6: Starting Without a Guide Bushing
A gun drill without a guide bushing will wander. The first engagement determines the hole direction. Without a bushing to constrain the tip, the drill will follow the path of least resistance — usually downhill from the center.
I have seen operators try to gun drill without a bushing by using a center drill to start. This works for the first few mm, but as soon as the tip clears the center drill divot, it walks. The bushing is not optional. I cover selection in Gun Drill Entry Bushings Guide.
The Pattern Behind All Six Failures
Every one of these failures comes from treating gun drilling like conventional drilling. Gun drilling is a different process. It needs high-pressure coolant, continuous feed, a proper pilot or bushing, and the right lubricant. Respect those requirements and it works reliably. Ignore any one of them and it breaks.
Key Takeaways
- Coolant pressure under 500 psi will not reliably evacuate chips for most diameters. Check your pump before buying a drill.
- Pilot holes must be flat-bottomed and 0.01-0.02mm oversized. A twist-drilled pilot with a tapered bottom will cause deflection.
- Never peck a gun drill. Continuous feed only.
- The starting sequence matters: position in pilot, coolant on, spindle on, feed on — in that order.
- Use oil-based coolant with EP additives. Water-soluble at standard concentrations will not lubricate adequately.
- A guide bushing is not optional. Without it, the drill will wander on entry.