A gun drill change looks simple — loosen the holder, swap the drill, tighten it back. But the difference between a correctly installed gun drill and a poorly installed one is 0.01 mm of runout that determines whether the hole is straight and round or rejects. Every step in the change procedure exists for a reason.
| Tool | Purpose |
|---|
| Torque wrench (tool holder specific) | Tighten holder to specification |
| Dial indicator with magnetic base | Measure drill runout |
| Clean lint-free cloths | Clean holder bore, drill shank |
| Soft brass or plastic hammer | Tap drill to adjust runout (if applicable) |
| Collet pick or removal tool | Remove collet from holder |
| Coolant passage cleaning wire | Clear coolant holes in drill |
| Magnifying glass or microscope (10×) | Inspect cutting edge condition |
| Calibrated micrometer | Measure drill diameter |
| Safety glasses and gloves | Personal protection |
Tip: Keep a dedicated tool change kit at each machine. When operators have to search for tools, they skip steps. A complete kit at the machine makes the procedure faster and more consistent.
Drill Removal Procedure
Step 1: Safe Removal
| Step | Action | Detail |
|---|
| 1.1 | Stop spindle | Confirm spindle is stopped and locked out |
| 1.2 | Depressurize coolant | Bleed high-pressure coolant line |
| 1.3 | Retract drill to fully withdrawn position | Clear of workpiece and bushing |
| 1.4 | Clean exposed drill shank | Remove coolant and chips to avoid contamination |
Step 2: Remove from Holder
| Step | Action | Detail |
|---|
| 2.1 | Loosen holder clamping mechanism | Follow holder manufacturer procedure |
| 2.2 | Slide drill out of holder | Pull straight — do not twist or bend |
| 2.3 | Place drill in designated location | Protect cutting edge from damage |
| 2.4 | Inspect used drill | Note wear pattern for tool life tracking |
Step 3: Clean Holder
| Step | Action | Detail |
|---|
| 3.1 | Clean holder bore | Lint-free cloth, ensure no chips remain |
| 3.2 | Clean collet (if applicable) | Inspect for damage, replace if worn |
| 3.3 | Clean holder coolant seal | Check O-ring condition |
| 3.4 | Inspect holder taper | Wipe clean, check for nicks |
New Drill Inspection
Step 4: Pre-Installation Checks
| Check | Tool | Acceptance Criteria | If Out of Spec |
|---|
| Drill diameter | Micrometer | Within +0.000/-0.005 mm of nominal | Reject — do not install |
| Shank diameter | Micrometer | Within +0.000/-0.005 mm of holder bore | Reject or change collet |
| Shank straightness | V-blocks + dial indicator | < 0.01 mm runout over shank length | Reject drill |
| Coolant hole clear | Compressed air | Air flows freely | Clean with wire, recheck |
| Cutting edge condition | Magnifying glass (10×) | No chips, cracks, or dull edges | Return to regrind |
| Overall length | Tape measure or ruler | Matches setup sheet | Adjust setup or reject |
Warning: Installing a drill with a blocked coolant hole is a guaranteed chip jam and tool breakage. Always verify coolant flow through the drill before installation. Compressed air through the coolant passage is a quick check — visible airflow at the tip confirms the passage is clear.
Drill Installation Procedure
Step 5: Install in Holder
| Step | Action | Detail |
|---|
| 5.1 | Insert drill through bushing (if applicable) | Guide tip through bushing carefully |
| 5.2 | Insert drill shank into holder | Push fully to seat against stop |
| 5.3 | Align coolant flats (if applicable) | Match holder orientation |
| 5.4 | Tighten holder | Follow torque specification for holder type |
| 5.5 | Verify drill is fully seated | Mark shank position, check after tightening |
Step 6: Tightening Torque by Holder Type
| Holder Type | Torque Specification | Tool | Notes |
|---|
| Hydraulic chuck | Per manufacturer (typically 30–60 Nm) | Torque wrench | Do not overtighten — damages hydraulic mechanism |
| ER collet chuck | Per collet size (typically 40–90 Nm) | Torque wrench with ER nut socket | Use torque, not feel |
| TG collet | Per manufacturer (typically 70–120 Nm) | Torque wrench | Higher torque than ER |
| Shrink-fit | N/A (heating process) | Induction heater | Follow heating/cooling cycle |
| Side-lock | Per screw size (typically 20–40 Nm) | Torque wrench | Tighten setscrew evenly |
Tip: Torque matters. Under-tightened holders allow the drill to slip or pull out. Over-tightened holders distort the collet and increase runout. Use a torque wrench every time — the feel method is not reliable enough for deep hole drilling tolerances.
Runout Measurement and Adjustment
Step 7: Measure Runout
| Measurement Point | Target Runout | Method |
|---|
| Holder nose | < 0.005 mm | Dial indicator at holder face |
| Drill shank (near holder) | < 0.010 mm | Dial indicator 10 mm from holder |
| Drill tip | < 0.020 mm (100 mm overhang) | Dial indicator at drill tip |
| Drill tip (300 mm overhang) | < 0.050 mm | Dial indicator at drill tip |
| Guide bushing bore | < 0.010 mm TIR | Dial indicator inside bushing |
Step 8: Adjust Runout
| Adjustment Method | Holder Type | Procedure |
|---|
| Tap adjustment | Collet chucks | Tap drill tip gently with brass hammer in direction needed |
| Index collet | ER, TG | Rotate collet 90° in nut, retighten, remeasure |
| Index drill | All | Rotate drill 180° in holder, retighten, remeasure |
| Shrink-fit reposition | Shrink-fit | Heat, rotate, cool, remeasure |
| Hydraulic reposition | Hydraulic | Depressurize, rotate drill, repressurize, remeasure |
Runout Troubleshooting
| Problem | Likely Cause | Fix |
|---|
| High runout at tip, low at holder | Drill shank bent | Replace drill |
| High runout at both tip and holder | Debris in holder bore | Clean holder thoroughly |
| Inconsistent runout readings | Debris on drill shank | Clean shank, remeasure |
| Runout changes when tightened | Collet damaged | Replace collet |
| Cannot achieve target runout | Holder damaged or worn | Replace or repair holder |
Coolant Connection Verification
Step 9: Coolant Check
| Check | Method | Pass Criteria |
|---|
| Coolant flow at drill tip | Position drill over container, jog coolant | Steady stream from coolant hole |
| Coolant pressure at holder | Observe pressure gauge | Within 5% of target pressure |
| Seal integrity | Check for leaks at holder joint | No visible leak |
| Coolant timing | Verify coolant-on signal in cycle | Coolant flows before feed starts |
| Flow rate | Flow meter or bucket timing | Within 10% of specification |
Pre-Cut Checklist
Step 10: Final Verification
| Item | Check | Verified |
|---|
| Drill diameter matches setup sheet | Micrometer | ☐ |
| Runout within specification | Dial indicator | ☐ |
| Coolant flow verified | Visual | ☐ |
| No coolant leaks | Visual | ☐ |
| Guide bushing correct size | Label check | ☐ |
| Guide bushing condition OK | Visual | ☐ |
| Part loaded and centered | Check | ☐ |
| Feed and speed correct in program | CNC screen | ☐ |
| Entry feed override set | CNC screen | ☐ |
| Chip tray empty | Visual | ☐ |
| Tool life counter reset | CNC screen | ☐ |
Documentation
| Field | Value |
|---|
| Date and time | |
| Machine number | |
| Operator name | |
| Drill part number | |
| Drill serial number (if tracked) | |
| Previous drill life (meters) | |
| Runout at tip | |
| Coolant pressure verified | |
| Cycle start time | |
Tip: A tool change log that records runout and drill life data is the foundation of process improvement. After 20–30 tool changes, patterns emerge — certain holders consistently show higher runout, certain drill lots have shorter life, certain operators get better results. Use the data to improve the process.
Common Installation Mistakes
| Mistake | Consequence | Prevention |
|---|
| Installing without cleaning holder bore | Debris causes runout and tip misalignment | Clean holder before every installation |
| Over-tightening collet nut | Collet distortion, runout increase | Use torque wrench |
| Skipping coolant flow check | Blocked coolant hole causes chip jam | Verify flow before first cut |
| Not checking bushing condition | Worn bushing causes wandering | Inspect bushing at every tool change |
| Misaligning coolant flats | Reduced coolant flow to drill | Align flats per manufacturer spec |
| Installing damaged drill | Immediate failure or poor finish | Inspect cutting edge before installation |
| Forgetting to reset tool life counter | Missed tool change alarm, overused tool | Reset counter as part of procedure |
FAQ
How often should I change a gun drill?
Change a gun drill when it has reached its expected tool life (measured in meters drilled), when surface finish degrades, when spindle load increases by 10–15%, or when chip shape changes from ideal C-chips to longer, stringier chips. Do not wait for breakage — proactive changes are safer and more economical.
What runout is acceptable for a newly installed gun drill?
Maximum 0.010 mm runout at the holder nose and 0.020 mm at the drill tip for a 100 mm overhang. For precision work, target 0.005 mm at the holder and 0.010 mm at the tip. Higher runout causes oversize holes, poor surface finish, and accelerated guide pad wear.
Do I need to check coolant flow every time I change a drill?
Yes. Every time. A coolant hole can be blocked by debris from the previous run, from handling, or from storage. The 30-second check of blowing compressed air through the coolant passage or jogging the coolant pump prevents a tool breakage that costs hours of downtime.
Can I reuse a collet indefinitely?
No. Collets wear over time. The spring segments lose elasticity, and the bore wears from repeated tool insertion. Replace collets every 6–12 months in production use, or sooner if runout increases beyond specification. A worn collet is a common hidden cause of runout problems.
What should I do if runout is still high after adjusting?
If runout remains high after cleaning, indexing the collet, and rotating the drill, the likely cause is either a damaged holder or a bent drill shank. Try a known-good drill in the same holder to isolate the problem. If runout improves with a different drill, the original drill has a bent shank. If runout stays high, the holder needs repair or replacement.
A gun drill change done correctly takes 10 minutes and prevents hours of trouble. Do not rush it, do not skip steps, and always verify runout before cutting. This article reflects industry practice as of 2026.
