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Hole tolerance is one of the most important specifications in deep hole drilling. Specifying a tolerance that is too tight increases cost and cycle time unnecessarily. Specifying one that is too loose risks functional failure. This reference provides comprehensive data on achievable IT grades for each deep hole drilling method, with practical guidance for designers and process engineers.
ISO Tolerance Grade System
ISO 286 defines the system of limits and fits used internationally. Tolerance grades are designated IT01 through IT18, with lower numbers indicating tighter tolerances.
IT Grade Values by Diameter Range
The tolerance value in micrometers for each IT grade depends on the nominal diameter range:
| IT Grade | 3–6 mm | 6–10 mm | 10–18 mm | 18–30 mm | 30–50 mm | 50–80 mm | 80–120 mm |
|---|---|---|---|---|---|---|---|
| IT5 | 5 | 6 | 8 | 9 | 11 | 13 | 15 |
| IT6 | 8 | 9 | 11 | 13 | 16 | 19 | 22 |
| IT7 | 12 | 15 | 18 | 21 | 25 | 30 | 35 |
| IT8 | 18 | 22 | 27 | 33 | 39 | 46 | 54 |
| IT9 | 30 | 36 | 43 | 52 | 62 | 74 | 87 |
| IT10 | 48 | 58 | 70 | 84 | 100 | 120 | 140 |
| IT11 | 75 | 90 | 110 | 130 | 160 | 190 | 220 |
| IT12 | 120 | 150 | 180 | 210 | 250 | 300 | 350 |
Values in micrometers (μm). For reference: 0.001 mm = 1 μm.
Practical Examples
For a 10 mm diameter hole:
| IT Grade | Tolerance (±) | Common Fit |
|---|---|---|
| IT7 | ±0.007 mm | H7 precision fit |
| IT8 | ±0.011 mm | H8 close fit |
| IT9 | ±0.018 mm | H9 general fit |
| IT10 | ±0.029 mm | H10 loose fit |
| IT11 | ±0.045 mm | H11 wide fit |
TIP
The difference between IT7 and IT8 for a 10 mm hole is only 0.004 mm (4 μm) per side. Yet achieving IT7 versus IT8 in deep hole drilling may require a completely different process (gun drilling with post-honing vs. gun drilling alone) and significantly higher cost. Always specify the loosest tolerance that meets functional requirements.
Achievable Tolerance by Drilling Method
Gun Drilling
Gun drilling is the standard process for precision deep holes in the 1–30 mm diameter range.
| Process Condition | IT Grade | Tolerance (10 mm hole) | Surface Finish Ra |
|---|---|---|---|
| Standard gun drilling | IT8–IT10 | ±0.011–0.029 mm | 0.8–1.6 μm |
| Precision gun drilling | IT7–IT8 | ±0.007–0.011 mm | 0.4–0.8 μm |
| Gun drilling + reaming | IT6–IT7 | ±0.005–0.007 mm | 0.4–0.8 μm |
| Gun drilling + honing | IT5–IT6 | ±0.003–0.005 mm | 0.1–0.4 μm |
Key factors affecting gun drilling tolerance:
| Factor | Impact on Tolerance |
|---|---|
| Guide bushing condition | Worn bushing increases diameter variation |
| Tip offset accuracy | ±0.01 mm offset error = ±0.02 mm diameter error |
| Coolant pressure stability | Fluctuations cause diameter variation |
| Machine alignment | Misalignment causes taper |
| Material consistency | Hardness variation affects diameter |
BTA Drilling
BTA drilling is the standard process for larger diameter deep holes (16–200 mm+).
| Process Condition | IT Grade | Tolerance (50 mm hole) | Surface Finish Ra |
|---|---|---|---|
| Standard BTA drilling | IT9–IT10 | ±0.031–0.050 mm | 1.6–3.2 μm |
| Fine BTA drilling | IT8–IT9 | ±0.020–0.031 mm | 0.8–1.6 μm |
| Precision BTA | IT7–IT8 | ±0.013–0.020 mm | 0.8–1.6 μm |
| BTA + roller burnishing | IT8–IT9 | ±0.020–0.031 mm | 0.2–0.4 μm |
| BTA + honing | IT6–IT7 | ±0.008–0.013 mm | 0.1–0.4 μm |
Key factors affecting BTA tolerance:
| Factor | Impact |
|---|---|
| Guide pad condition | Worn pads cause oversize holes |
| Cutting edge symmetry | Unequal edge height causes imbalance |
| Coolant flow stability | Flow variation affects chip evacuation |
| Machine rigidity | Flex causes diameter variation |
| Drill tube condition | Bent tube causes oversize and taper |
Ejector Drilling
| Process Condition | IT Grade | Tolerance (30 mm hole) | Surface Finish Ra |
|---|---|---|---|
| Standard ejector | IT9–IT11 | ±0.026–0.065 mm | 1.6–3.2 μm |
| Optimized ejector | IT8–IT9 | ±0.016–0.026 mm | 0.8–1.6 μm |
Comparison Across Methods
| Method | Typical IT Grade | Diameter Range | Relative Cost per Hole |
|---|---|---|---|
| Twist drilling | IT10–IT13 | 1–50 mm | 1× (lowest) |
| Gun drilling | IT7–IT10 | 1–30 mm | 3–5× |
| BTA drilling | IT7–IT10 | 16–200 mm+ | 2–4× |
| Ejector drilling | IT8–IT11 | 18–65 mm | 2–3× |
| Reaming (post-drill) | IT5–IT8 | Any | +1–2× to drilling |
| Boring (post-drill) | IT6–IT10 | Any | +1–3× to drilling |
| Honing (post-drill) | IT3–IT6 | Any | +2–5× to drilling |
Depth Effect on Achievable Tolerance
The achievable tolerance degrades as hole depth increases. This is one of the most important considerations in deep hole drilling tolerance specification.
Depth Factor for Gun Drilling
| Depth-to-Diameter Ratio (L/D) | IT Grade Shift | Tolerance Multiplier |
|---|---|---|
| < 10:1 | Baseline | 1.0× |
| 10:1 – 30:1 | +0 to +1 IT grade | 1.0–1.6× |
| 30:1 – 60:1 | +1 IT grade | 1.6× |
| 60:1 – 100:1 | +1 to +2 IT grades | 1.6–2.5× |
| > 100:1 | +2 IT grades | 2.5× |
Example: A 10 mm diameter hole drilled to 50 mm depth (L/D = 5:1) may achieve IT7. The same hole drilled to 500 mm (L/D = 50:1) may only achieve IT8. At 1,000 mm depth (L/D = 100:1), IT9 may be the realistic limit.
Depth Factor for BTA Drilling
| Depth-to-Diameter Ratio (L/D) | IT Grade Shift | Tolerance Multiplier |
|---|---|---|
| < 20:1 | Baseline | 1.0× |
| 20:1 – 50:1 | +0 to +1 IT grade | 1.0–1.6× |
| 50:1 – 100:1 | +1 IT grade | 1.6× |
| > 100:1 | +1 to +2 IT grades | 1.6–2.5× |
Why Depth Affects Tolerance
| Factor | Effect |
|---|---|
| Tool deflection | Longer tool = more deflection under cutting force |
| Vibration | Longer tool = lower natural frequency, more vibration |
| Coolant pressure drop | Longer passage = lower pressure at cutting edge |
| Chip evacuation difficulty | Longer path = chip packing risk |
| Thermal expansion | Longer tool = more thermal growth |
Tolerance by Diameter Range
Gun Drilling Tolerance by Diameter
| Diameter Range | Standard (IT8) Tolerance | Precision (IT7) Tolerance |
|---|---|---|
| 3–6 mm | ±0.009 mm | ±0.006 mm |
| 6–10 mm | ±0.011 mm | ±0.007 mm |
| 10–18 mm | ±0.013 mm | ±0.009 mm |
| 18–30 mm | ±0.016 mm | ±0.010 mm |
BTA Drilling Tolerance by Diameter
| Diameter Range | Standard (IT9) Tolerance | Precision (IT8) Tolerance |
|---|---|---|
| 18–30 mm | ±0.026 mm | ±0.016 mm |
| 30–50 mm | ±0.031 mm | ±0.020 mm |
| 50–80 mm | ±0.037 mm | ±0.023 mm |
| 80–120 mm | ±0.043 mm | ±0.027 mm |
| 120–180 mm | ±0.050 mm | ±0.032 mm |
Common Fit Specifications for Deep Drilled Holes
| Fit | IT Grade | Application | Typical Deep Hole Use |
|---|---|---|---|
| H6 | IT5–IT6 | Precision bearings, metering | Requires honing after drilling |
| H7 | IT7 | Precision sliding fits, bearing seats | Achievable with precision gun drilling |
| H8 | IT8 | Close running fits | Standard precision gun drilling |
| H9 | IT9 | General engineering fits | Standard BTA drilling |
| H10 | IT10 | Loose running fits | Non-critical bores |
| H11 | IT11 | Wide clearance | Rough drilling, secondary finishing planned |
Tolerance Cost Relationship
Relative Cost by IT Grade
| IT Grade | Relative Cost (IT9 = 1.0×) | Deep Hole Drilling Process Required |
|---|---|---|
| IT11+ | 0.7× | Standard twist drilling or rough deep drilling |
| IT10 | 0.8× | Standard deep hole drilling |
| IT9 | 1.0× (baseline) | Standard gun drilling or BTA |
| IT8 | 1.3–1.5× | Precision gun drilling or fine BTA |
| IT7 | 2–3× | Precision gun drilling with optimized parameters |
| IT6 | 3–6× | Gun drilling + reaming or honing |
| IT5 | 6–12× | Gun drilling + precision honing |
Cost Drivers for Tight Tolerances
| Cost Driver | Impact |
|---|---|
| Slower feed rates | Direct cycle time increase |
| More frequent tool changes | Higher tool cost + downtime |
| Secondary operations | Additional machine time + handling |
| Increased inspection | More measurement time + equipment |
| Higher scrap rate | Tighter tolerances = more out-of-spec parts |
| Machine capability requirements | Premium machines needed for IT6+ |
WARNING
A common mistake in deep hole drilling specifications is calling out H7 tolerance for a hole with L/D > 50:1. The depth alone adds one IT grade to the achievable tolerance, meaning the realistic expectation should be H8 or H9. Specifying H7 on a 500 mm deep hole forces the shop to use secondary operations (honing) that may not have been quoted, leading to disputes, delays, and cost overruns. Designers should specify tolerance for the finished hole condition, accounting for depth effects.
Straightness Relationship to Tolerance
Hole straightness and diameter tolerance are related but independent characteristics.
Typical Straightness by Process
| Process | Straightness (per 100 mm) | Straightness (per 1,000 mm) |
|---|---|---|
| Gun drilling, precision | 0.01–0.03 mm | 0.05–0.15 mm |
| Gun drilling, standard | 0.03–0.08 mm | 0.15–0.40 mm |
| BTA drilling, precision | 0.02–0.05 mm | 0.10–0.25 mm |
| BTA drilling, standard | 0.05–0.10 mm | 0.25–0.50 mm |
| Ejector drilling | 0.05–0.10 mm | 0.25–0.50 mm |
Interaction Between Tolerance and Straightness
A hole that meets diameter tolerance at every measured point may still be rejected for excessive straightness deviation. Conversely, a straight hole may have diameter variation within the tolerance band. Both characteristics must be specified and measured independently.
For functional purposes:
| Application | Critical Parameter |
|---|---|
| Hydraulic cylinder (seal) | Diameter tolerance + surface finish |
| Guide bushing bore | Straightness |
| Cooling channel | Diameter (flow) |
| Fastener hole | Position (not diameter or straightness) |
| Bearing seat | Diameter tolerance + roundness |
Process Selection by Tolerance Requirement
Decision Matrix
| Required Tolerance | L/D < 20:1 | L/D 20:1–50:1 | L/D 50:1–100:1 | L/D > 100:1 |
|---|---|---|---|---|
| IT5–IT6 | Drill + hone | Drill + hone | Drill + hone | Drill + hone |
| IT7 | Gun drill (precision) | Gun drill (precision) | Gun drill + ream | Gun drill + ream |
| IT8 | Gun drill (standard) | Gun drill (precision) | Gun drill (precision) | Gun drill + ream |
| IT9 | BTA or gun drill | BTA or gun drill | Gun drill (standard) | Gun drill (precision) |
| IT10 | BTA or gun drill | BTA or gun drill | BTA or gun drill | Gun drill (standard) |
| IT11+ | Twist drill | BTA | BTA | BTA |
FAQ
Q: What IT grade can gun drilling achieve? Standard gun drilling achieves IT8–IT10. Precision gun drilling with optimized parameters can achieve IT7. Achieving IT6 or better requires secondary operations such as reaming or honing.
Q: What IT grade can BTA drilling achieve? Standard BTA drilling achieves IT9–IT10. Fine BTA with optimized parameters achieves IT8. Precision BTA can achieve IT7. As with gun drilling, IT6+ requires secondary operations.
Q: How does hole depth affect achievable tolerance? Tolerance degrades with increasing depth. Each 30–50× diameter of depth typically adds one IT grade. A 10 mm hole at 100 mm depth may achieve IT7; at 500 mm depth, IT8; at 1,000 mm depth, IT9.
Q: What is the difference between H7 and H8 tolerance? For a 10 mm hole, H7 = +0.015/0 mm (tolerance band 15 μm), H8 = +0.022/0 mm (tolerance band 22 μm). The difference is 7 μm (0.007 mm) — about one-tenth of a human hair diameter. Yet achieving H8 is significantly more economical in deep hole drilling.
Q: Is tolerance affected by workpiece material? Yes. Materials with consistent machinability (alloy steel, brass) produce more consistent hole sizes than materials with variable hardness (cast iron, stainless steel). Materials that work harden (stainless, Inconel) can cause diameter variation if feed is not maintained.
Q: What is the most economical IT grade for deep hole drilling? IT9 is the most economical for most deep hole drilling applications. It is reliably achievable with both gun drilling and BTA drilling without requiring secondary operations or premium process controls.
Q: Can a deep hole have tighter tolerance at the entry than at the exit? Yes. Diameter variation along the hole length is normal. Entry diameter is typically closest to nominal. The exit may be slightly larger (gun drilling) or have more variation (BTA). The tolerance specification should apply to the entire hole length unless otherwise noted.
Q: What tolerance can be achieved with gun drilling plus roller burnishing? Roller burnishing improves surface finish (Ra 0.2–0.4 μm) but does not significantly change diameter tolerance. The tolerance is established by the drilling process; burnishing only smooths the surface profile within the existing tolerance band.
Q: How should straightness be specified relative to diameter tolerance? Straightness and diameter tolerance are independent. A common practice is to specify straightness as a percentage of the diameter tolerance band. For precision applications, straightness should be 0.01–0.03 mm per 100 mm of depth regardless of diameter tolerance.
Q: What is the relationship between feed rate and hole tolerance? Feed rate directly affects diameter tolerance through its effect on cutting forces and tool deflection. Higher feed rates increase radial forces, which can cause the tool to deflect and produce oversize holes. Consistent feed rate is essential for consistent hole diameter.