30-Second Summary: The achievable accuracy of deep hole machining varies by method and conditions. Understanding the relationship between tolerance grade (IT), surface roughness (Ra), and straightness is essential for developing a sound process plan. Tolerance degrades with increasing L/D ratio — this relationship should be considered at the design stage.
Achievable Accuracy by Method
| Method | Tolerance Grade | Surface Roughness Ra | Straightness | Typical Roundness |
|---|---|---|---|---|
| Gundrilling (tool rotates) | IT6–IT11 | 0.4–6.3 μm | ~0.001 in/in (0.03 mm/m) | 0.005–0.020 mm |
| Gundrilling (workpiece rotates) | IT7–IT9 | 0.4–3.2 μm | 0.001–0.003 in/ft (0.08–0.25 mm/m) | 0.003–0.015 mm |
| Gundrilling (counter-rotation) | IT6–IT8 | 0.4–1.6 μm | Best (0.0005 in/ft or 0.04 mm/m) | 0.003–0.010 mm |
| BTA Drilling | IT8–IT10 | 0.4–3.2 μm | 0.05–0.5 mm/m | 0.010–0.050 mm |
| Ejector Drilling | IT9–IT11 | 0.8–3.2 μm | 0.08–0.5 mm/m | 0.015–0.060 mm |
| Pull Boring | IT7–IT9 | 0.4–1.6 μm | ~0.001 in/ft (0.08 mm/m) | 0.003–0.015 mm |
| Skive & Roller Burnishing | IT7–IT8 | < 0.2 μm (to 0.05) | — | 0.005–0.020 mm |
| Honing | IT5–IT7 | 0.1–0.8 μm | Can improve existing hole | 0.002–0.010 mm |
| Reaming | IT7–IT9 | 0.8–3.2 μm | Follows existing hole | 0.005–0.025 mm |
IT Grades in Microns — Quick Reference
The table below shows the tolerance band in microns (μm) for selected diameters at common IT grades.
| Nominal Diameter (mm) | IT6 (μm) | IT7 (μm) | IT8 (μm) | IT9 (μm) | IT10 (μm) | IT11 (μm) |
|---|---|---|---|---|---|---|
| 3–6 | 8 | 12 | 18 | 30 | 48 | 75 |
| 6–10 | 9 | 15 | 22 | 36 | 58 | 90 |
| 10–18 | 11 | 18 | 27 | 43 | 70 | 110 |
| 18–30 | 13 | 21 | 33 | 52 | 84 | 130 |
| 30–50 | 16 | 25 | 39 | 62 | 100 | 160 |
| 50–80 | 19 | 30 | 46 | 74 | 120 | 190 |
| 80–120 | 22 | 35 | 54 | 87 | 140 | 220 |
| 120–180 | 25 | 40 | 63 | 100 | 160 | 250 |
| 180–250 | 29 | 46 | 72 | 115 | 185 | 290 |
Example: A 40 mm hole at IT8 has a tolerance band of 39 μm (e.g., 40.000 to 40.039 mm).
Tolerance vs. L/D Ratio
Achievable tolerance degrades as hole depth increases. Use the following as a rough guide when planning your process:
| L/D Range | Gundrilling (optimal setup) | Gundrilling (typical) | BTA Drilling |
|---|---|---|---|
| 10:1–30:1 | IT6–IT8 | IT7–IT9 | IT8–IT9 |
| 30:1–60:1 | IT7–IT9 | IT8–IT10 | IT8–IT10 |
| 60:1–100:1 | IT8–IT10 | IT9–IT11 | IT9–IT11 |
| > 100:1 | IT9–IT11 | IT10–IT11 | Not recommended |
These ranges assume proper setup (aligned guide bushings, adequate coolant pressure, appropriate feeds and speeds). Tighter tolerances are achievable with workpiece-rotating configurations and counter-rotation setups.
Main Factors Affecting Accuracy
| Factor | Typical Impact | Mitigation |
|---|---|---|
| Spindle-to-bushing misalignment | 0.01 mm misalignment produces 0.1 mm deviation over 500 mm depth | Align to within 0.01 mm (0.0004 in) using laser alignment |
| Guide bushing clearance | Excess clearance (>0.03 mm) causes bell-mouth entry and axis deviation | Maintain drill diameter +0.02 mm clearance |
| Material non-uniformity | Hardness variation of ±20 HB causes measurable force fluctuation | Pre-drill or spot-face; consider pre-annealing for critical parts |
| Coolant temperature | 10°C rise in coolant = 0.12 mm expansion per meter of workpiece steel | Install chiller; maintain 20–35°C range |
| Spindle runout | 0.005 mm TIR runout produces oversize hole by 0.008–0.015 mm | Maintain spindle runout < 0.003 mm |
| Machine guideway wear | Progressive deviation with depth; more pronounced on older machines | Annual laser calibration; use counter-rotation to cancel effects |
| Coolant filtration | >30 μm particles cause guide pad scoring and diameter variation | Maintain 15–20 μm filtration for gundrilling |
| Whip guide support spacing | Insufficient support causes drill tube vibration and hole lobing | Support every 40 × D maximum |
Geometric Tolerances
| Characteristic | Typical Achievable Range | Measurement Method |
|---|---|---|
| Roundness | 0.003–0.050 mm (method and L/D dependent) | CMM, roundness tester, air gaging |
| Cylindricity | 0.010–0.100 mm (increases with depth) | CMM with helical scanning |
| Concentricity (to external datum) | 0.02–0.10 mm (workpiece-rotating: 0.01–0.05 mm) | Dial indicator, CMM |
| Perpendicularity (to face) | 0.01–0.05 mm per 100 mm diameter | Dial indicator, CMM |
| Surface roughness (as-drilled) | Ra 0.4–6.3 μm | Profilometer, optical profiler |
Inspection Methods
| Method | Application | Accuracy | Notes |
|---|---|---|---|
| Air gaging | Diameter, roundness | ±0.5–2 μm | Fastest production method; requires master rings |
| Plug gages (Go/No-Go) | Diameter verification | Depends on class | Simple pass/fail; no size data |
| Three-point bore micrometer | Diameter | ±2–5 μm | Good for field measurement; workpiece must be stationary |
| CMM (Coordinate Measuring Machine) | Diameter, roundness, cylindricity, position | ±1–5 μm | Comprehensive but slow for deep holes; limited depth in standard CMMs |
| Laser straightness system | Straightness | ±2–10 μm/m | On-machine or post-process; measures axis deviation directly |
| Precision mandrel + indicator | Straightness, concentricity | ±5–15 μm | Traditional method; mandrel must fit bore closely |
| Contact profilometer | Surface roughness | ±0.01 μm Ra | Standard for Ra measurement; stylus may not reach deep bores |
| On-machine optical/laser probe | Diameter, roundness, straightness | ±7 μm (straightness) | Emerging technology; measures during or immediately after machining |
Thermal Effects on Tolerance
Deep hole machining generates significant heat. Post-cooling dimensional changes can be substantial:
| Material | Thermal Expansion Coefficient | Hole Diameter Change (40 mm bore, 30°C rise) |
|---|---|---|
| Steel (carbon/alloy) | 11.5 × 10-6 /°C | ~0.014 mm shrinkage on cooling |
| Stainless steel (austenitic) | 17.3 × 10-6 /°C | ~0.021 mm shrinkage on cooling |
| Aluminum (6061) | 23.6 × 10-6 /°C | ~0.028 mm shrinkage on cooling |
| Titanium (Ti-6Al-4V) | 8.6 × 10-6 /°C | ~0.010 mm shrinkage on cooling |
| Cast iron | 10.5 × 10-6 /°C | ~0.013 mm shrinkage on cooling |
💡 Design tip: Specify tolerance requirements as "post-machining, at 20°C" conditions during the design phase. Deep holes experience thermal expansion from cutting heat during machining, and dimensions may shrink by 0.005–0.025 mm after cooling (depending on material and bore diameter). Measure the bore after allowing the workpiece to return to room temperature for final acceptance.
Process capability: For Cpk ≥ 1.33 (industry standard for production), the process spread (6σ) must be less than 75% of the tolerance band. If the print calls for IT8 (± half the band), the process must hold ±~75% of that range in production.
Straightness Standards
- ISO 1101: Defines straightness tolerance as the diameter of the smallest cylinder that can contain the actual axis
- ISO 12780: Defines straightness measurement procedures and evaluation methods
- Typical callout: "Straightness 0.05 mm per 1000 mm" or "0.001 in/ft"
- Evaluation: Least squares method (LSM) or minimum zone method (MZM) per ISO standards