In deep hole drilling, the spindle controls the rotation of the drill or workpiece. Any runout at the spindle is amplified at the drill tip — a 0.005 mm runout at the spindle nose can become 0.050 mm at the drill tip. Measuring and controlling spindle runout is essential for producing holes within diameter tolerance.
Runout Types
Runout Classification
| Runout Type | Description | Measurement Location | Effect on Drilling |
|---|
| Radial runout (spindle nose) | TIR of spindle nose outer diameter | OD of spindle nose or taper gauge line | Causes oversize hole, poor surface finish |
| Radial runout (taper) | TIR of spindle internal taper | Internal taper surface | Causes tool holder misalignment |
| Axial runout (face) | TIR of spindle face perpendicular to axis | Spindle face | Affects tool holder seating, depth accuracy |
| Runout at gauge line | TIR of test bar at a specified distance from spindle face | Test bar at 100 mm, 200 mm, 300 mm | Simulates actual tool overhang |
| Dynamic runout (at speed) | Runout measured with spindle rotating at operating speed | Same locations as static | Detects bearing issues not visible at low speed |
Runout vs Drilling Errors
| Runout at Spindle Nose | Expected Hole Diameter Increase | Expected Surface Finish Degradation | Recommended Action |
|---|
| < 0.003 mm | Negligible | None | Normal operation |
| 0.003–0.005 mm | 0.005–0.010 mm oversize | Minor | Monitor — acceptable |
| 0.005–0.010 mm | 0.010–0.020 mm oversize | Noticeable — Ra increases 0.1–0.2 µm | Plan spindle maintenance |
| 0.010–0.020 mm | 0.020–0.040 mm oversize | Significant — Ra increases 0.3–0.5 µm | Schedule spindle bearing replacement |
| > 0.020 mm | > 0.040 mm oversize | Poor — likely out of tolerance | Immediate spindle service |
Measurement Equipment
Required Equipment
| Equipment | Purpose | Specification |
|---|
| Dial indicator (lever type) | Radial and axial runout measurement | 0.001 mm (1 µm) resolution minimum |
| Magnetic base / stand | Secure indicator mounting | Rigid — no deflection under indicator load |
| Test bar (precision ground) | Runout at gauge line measurement | Precision ground — runout < 0.002 mm over 300 mm |
| Spindle taper test bar | Clean and inspect taper | Matches spindle taper (BT, CAT, HSK, etc.) |
| Solvent and lint-free cloth | Clean spindle taper and nose | No residue |
| Pry bar (for bearing check) | Axial and radial bearing clearance | Small — for gentle load application |
Equipment Verification
| Item | Check | Frequency |
|---|
| Dial indicator calibration | Calibration certificate current | Annually |
| Test bar certification | Runout certification | Annually or after any damage |
| Magnetic base condition | Magnet strength, arm rigidity | Before each use |
| Indicator mounting | No play in mounting | Before each use |
Measurement Procedures
Spindle Taper Cleaning
| Step | Action | Detail |
|---|
| 1 | Lock out spindle | Spindle cannot rotate |
| 2 | Remove any tool holder from spindle | — |
| 3 | Inspect taper visually | Look for nicks, debris, coolant residue |
| 4 | Clean taper with solvent and lint-free cloth | Rotate cloth around taper — do not push debris into spindle |
| 5 | Inspect taper again | Should be clean and dry |
| 6 | Clean spindle face | Same procedure |
| 7 | Keep spindle clean until runout measurement is complete | Do not insert any tool |
Radial Runout at Spindle Nose
| Step | Action | Detail |
|---|
| 1 | Mount dial indicator on magnetic base | Base on machine table or fixed structure — not on spindle housing |
| 2 | Position indicator plunger perpendicular to spindle nose OD | Contact at 90° to surface |
| 3 | Preload indicator plunger | 0.2–0.5 mm preload |
| 4 | Zero the indicator | Set dial to zero |
| 5 | Rotate spindle by hand | 360° rotation — slow and steady |
| 6 | Observe maximum indicator reading | Total Indicator Reading (TIR) |
| 7 | Record TIR | Maximum − minimum reading |
| 8 | Repeat 3 times | Confirm repeatability |
Radial Runout at Spindle Taper
| Step | Action | Detail |
|---|
| 1 | Clean spindle taper thoroughly | Per taper cleaning procedure |
| 2 | Insert test bar into spindle | Clean test bar shank — seat with firm push (no hammer) |
| 3 | Mount dial indicator at test bar near spindle face | At gauge line (typically 10–20 mm from spindle face) |
| 4 | Position indicator plunger perpendicular to test bar | Contact at 90° |
| 5 | Preload indicator | 0.2–0.5 mm |
| 6 | Zero the indicator | — |
| 7 | Rotate spindle by hand | 360° rotation |
| 8 | Record TIR at gauge line | — |
| 9 | Move indicator to 100 mm from spindle face | Measure along test bar |
| 10 | Record TIR at 100 mm | — |
| 11 | Move indicator to 300 mm from spindle face (if applicable) | Record TIR |
Axial Runout (Spindle Face)
| Step | Action | Detail |
|---|
| 1 | Mount dial indicator on magnetic base | Base on machine table or fixed structure |
| 2 | Position indicator plunger perpendicular to spindle face | Contact on spindle face — near outer diameter |
| 3 | Preload indicator | 0.2–0.5 mm |
| 4 | Zero the indicator | — |
| 5 | Rotate spindle by hand | 360° rotation |
| 6 | Record TIR | — |
Dynamic Runout (At Speed)
| Step | Action | Detail |
|---|
| 1 | Complete static runout measurement first | Establish baseline |
| 2 | Mount indicator at test bar (100 mm from face) | Proximity probe or non-contact preferred for high speed |
| 3 | Set spindle to low speed (500–1,000 RPM) | Observe indicator reading — compare to static |
| 4 | Increase speed to operating speed (2,000–5,000 RPM) | Observe reading at speed |
| 5 | Record any increase in runout | Dynamic runout should not exceed 200% of static |
| 6 | Reduce speed and stop | — |
Acceptance Criteria
Runout Limits by Machine Type
| Machine Type | Radial at Nose | Radial at Gauge Line | Radial at 100 mm | Radial at 300 mm | Axial Face |
|---|
| New spindle (any) | < 0.003 mm | < 0.003 mm | < 0.005 mm | < 0.010 mm | < 0.003 mm |
| Gun drilling (precision) | < 0.005 mm | < 0.005 mm | < 0.008 mm | < 0.015 mm | < 0.005 mm |
| Gun drilling (standard) | < 0.008 mm | < 0.008 mm | < 0.015 mm | < 0.025 mm | < 0.008 mm |
| BTA drilling | < 0.010 mm | < 0.010 mm | < 0.020 mm | < 0.030 mm | < 0.010 mm |
Action Limits
| Measurement | Warning Level | Service Required | Immediate Stop |
|---|
| Radial at gauge line | 0.005–0.008 mm | 0.008–0.015 mm | > 0.015 mm |
| Radial at 100 mm | 0.010–0.015 mm | 0.015–0.025 mm | > 0.025 mm |
| Axial face | 0.005–0.008 mm | 0.008–0.015 mm | > 0.015 mm |
Corrective Actions
| Condition | Likely Cause | Corrective Action |
|---|
| High radial runout at nose only | Taper contamination, spindle nose damage | Clean taper, inspect for nicks, dress if possible |
| High radial runout at test bar, increasing with distance | Spindle bearing wear | Replace spindle bearings |
| High radial runout at test bar, consistent at all distances | Test bar or holder issue | Check test bar for damage, check tool holder |
| High axial runout | Spindle bearing axial play or face damage | Tighten bearing preload, replace bearings |
| Dynamic runout > 2× static runout | Bearing preload insufficient, bearing damage | Adjust preload, replace bearings |
| Intermittent runout (varies with rotation) | Bearing race damage, debris in bearing | Replace bearings |
Measurement Frequency
| Machine Usage | Static Runout (Monthly) | Static Runout (Quarterly) | Dynamic Runout | After Event |
|---|
| Continuous (24/7) | ✓ | — | ✓ (monthly) | After spindle impact, after coolant ingress incident |
| Standard (8–16 hours) | — | ✓ | — | After spindle impact, after any crash |
| Low usage | — | ✓ (every 6 months) | — | After spindle service |
| New machine | — | ✓ (run-in period) | ✓ (after run-in) | — |
FAQ
How do I measure spindle runout on a deep hole drilling machine?
Clean the spindle taper thoroughly. Mount a dial indicator (0.001 mm resolution) on a magnetic base attached to the machine table or a fixed structure — not to the spindle housing. Position the indicator plunger perpendicular to the test bar at the gauge line (10–20 mm from the spindle face). Preload the indicator 0.2–0.5 mm, zero it, and rotate the spindle by hand. Record the total indicator reading (TIR). Repeat at 100 mm and 300 mm from the spindle face.
What is acceptable spindle runout for deep hole drilling?
For precision gun drilling: radial runout at the gauge line should be < 0.005 mm. For standard gun drilling: < 0.008 mm. For BTA drilling: < 0.010 mm. Axial runout should be < 0.005 mm for precision and < 0.010 mm for standard. If runout exceeds these values, the spindle bearings may need replacement or the spindle taper may need cleaning or dressing.
What causes spindle runout to increase?
The most common causes are: spindle bearing wear (normal — develops gradually), spindle taper contamination (debris or coolant residue prevents the tool holder from seating properly), spindle nose damage (a crash or dropped tool dings the nose or taper), temperature changes (the machine must be at operating temperature for accurate measurement), and tool holder wear or damage (runout measured at the tool may be caused by the holder, not the spindle).
How does spindle runout affect hole quality?
Spindle runout causes the drill tip to rotate off-center, producing an oversize hole. The hole diameter increases approximately 2× the radial runout at the drill tip. Runout also causes uneven chip load on the drill cutting edges — one edge cuts more material and wears faster. Surface finish degrades because the cutting edge traces a non-circular path. For tight-tolerance holes, controlling spindle runout is essential.
How often should spindle runout be measured?
Measure runout quarterly for standard production machines, monthly for continuous operation, and after any event that could affect the spindle (crash, dropped tool, coolant ingress incident). Also measure after spindle bearing replacement to verify the repair. Establish a trend — a gradual increase over several measurements indicates normal bearing wear; a sudden increase indicates damage or contamination.
Spindle runout is a fundamental machine condition that directly controls hole quality. A clean spindle with good bearings produces holes within tolerance. A contaminated taper or worn bearings produces scrap. Measure runout regularly, clean the taper carefully, and address bearing wear before it affects production. This article reflects industry practice as of 2026.
