How to Use This Card
I designed this quick reference card to be posted on every machine in the shop. It covers the six most common gun drilling problems and their solutions. Each entry follows the same format: symptom, most likely cause, first action.
Operators who use this card solve most problems without waiting for a setup technician or engineer. I’ve seen operators resolve coolant pressure issues in under two minutes by following the card instead of waiting 20 minutes for a technician to arrive. The card does not cover every possible problem, but it covers the 90% that happen regularly.
Chips Not Clearing
Most likely cause: Low coolant pressure at the drill tip.
Action: Check the pressure gauge at the tool holder, not the pump. If pressure is below 40 bar for diameters under 12 mm, or below 20 bar for larger diameters, increase the pump pressure setting. If the pump is already at maximum, check the coolant filter pressure differential and the rotating union for leaks.
If pressure checks out, look at the chip shape. Long stringy chips will not clear through the flute. Increase feed rate by 10-15% to break the chips shorter.
Tool Breaking
Most likely cause: Worn guide bushing causing drill vibration.
Action: Check the guide bushing ID with a bore gauge. Clearance between the bushing and the drill should be 0.005-0.015 mm for gun drilling. If clearance exceeds 0.05 mm, replace the bushing immediately. A worn bushing allows the drill to vibrate at the entry point, which creates fatigue stresses that break the drill a few millimeters into the bore.
Secondary cause: Chip clogging from low coolant pressure (see above). A chip pack seizes the drill in the bore and the torque spikes to the breaking point.
Surface Finish Poor
Most likely cause: Dull or chipped cutting edge.
Action: Remove the drill and inspect the cutting edge under 10x magnification. If the edge shows any rounding, chipping, or built-up material, replace or regrind the drill. A gun drill with a sharp edge cuts cleanly and produces Ra 0.8-1.6 um surface finish. A dull edge burnishes the bore wall and produces a rough or glazed finish.
Secondary cause: Coolant concentration too low. Check with refractometer. Below 8% concentration, the coolant loses lubricity and surface finish degrades.
Hole Off-Center
Most likely cause: Machine misalignment.
Action: Mount a test bar in the spindle and a dial indicator on the table. Run the indicator along the test bar over 100 mm of travel. Runout should be within 0.02 mm per 100 mm. If alignment is out, adjust the headstock or workpiece fixture.
Secondary cause: Uneven material hardness. Check the material hardness at multiple points around the bore entry. A hardness variation above 5 HRC will cause the drill to deflect toward the softer side.
Vibration or Chatter
Most likely cause: Inadequate workpiece support.
Action: Add a steady rest at the vibration node point. The vibration node is located approximately at the midpoint of the unsupported workpiece length. Rubber rollers on the steady rest dampen vibration better than steel rollers.
Secondary cause: Spindle speed at resonance with the workpiece natural frequency. Change the spindle speed by 10-15% to move away from the resonant frequency.
Hole Oversized
Most likely cause: Worn guide bushing clearance.
Action: Same as tool breaking check above. Measure the bushing ID. Excess clearance allows the drill to wobble at the entry and cut an oversize bore. Replace bushing if clearance exceeds 0.05 mm.
Secondary cause: Drill diameter undersized from excessive regrinds. Check the drill tip diameter with a micrometer. If the drill is more than 0.10 mm undersized from the nominal diameter, retire the drill.
Problem-Symptom Matrix
| Symptom | Primary Cause | Secondary Cause |
|---|---|---|
| Chips not clearing | Low coolant pressure | Stringy chips from low feed |
| Tool breaking | Worn guide bushing | Chip clog / pressure low |
| Poor surface finish | Dull tool edge | Coolant concentration low |
| Hole off-center | Machine misalignment | Uneven material hardness |
| Vibration / chatter | Poor workpiece support | Speed at resonance |
| Hole oversized | Worn guide bushing | Drill undersized |
When the Card Does Not Solve It
If the quick reference card does not solve the problem, follow the five-step troubleshooting sequence: coolant pressure, chip formation, guide bushing, tool condition, alignment. If that still does not work, call for engineering support.
Key Takeaways
- Check coolant pressure at the tool tip first for any chip clearing problem.
- A worn guide bushing causes both tool breakage and oversized holes.
- Replace the drill at the first sign of edge wear for consistent surface finish.
- Use a test bar to verify alignment within 0.02 mm per 100 mm.
- Change spindle speed by 10-15% to avoid resonant vibration.