Appearance
IT7 in deep hole drilling means something fundamentally different than IT7 in turning — the depth ratio, tool guidance method, and chip evacuation constraints make every IT grade a negotiation between geometry and process capability.
Overview
Tolerance in deep hole drilling is multidimensional. Diameter tolerance (IT grade) is the most commonly specified requirement, but straightness, roundness, cylindricity, and surface finish are equally critical for components that must seal, slide, or withstand pressure.
Three processes dominate deep hole drilling tolerance capability:
| Process | Diameter IT Range | Typical Standard | Ra Range (µm) |
|---|---|---|---|
| Gun drilling | IT7 – IT10 | IT8 | 0.4 – 3.2 |
| BTA / STS drilling | IT7 – IT10 | IT8 – IT9 | 1.6 – 6.3 |
| Ejector / DTS drilling | IT9 – IT11 | IT10 | 1.6 – 6.3 |
The values within each range depend on workpiece material, depth-to-diameter ratio, machine condition, coolant parameters, and tool wear state.
ISO 286 Tolerance System
ISO 286 defines 20 tolerance grades (IT01 through IT18). For deep hole drilling, IT7 through IT11 are the relevant range.
Tolerance Values by Diameter Range
| Nominal Diameter (mm) | IT7 (mm) | IT8 (mm) | IT9 (mm) | IT10 (mm) | IT11 (mm) |
|---|---|---|---|---|---|
| 3 – 6 | 0.012 | 0.018 | 0.030 | 0.048 | 0.075 |
| 6 – 10 | 0.015 | 0.022 | 0.036 | 0.058 | 0.090 |
| 10 – 18 | 0.018 | 0.027 | 0.043 | 0.070 | 0.110 |
| 18 – 30 | 0.021 | 0.033 | 0.052 | 0.084 | 0.130 |
| 30 – 50 | 0.025 | 0.039 | 0.062 | 0.100 | 0.160 |
| 50 – 80 | 0.030 | 0.046 | 0.074 | 0.120 | 0.190 |
| 80 – 120 | 0.035 | 0.054 | 0.087 | 0.140 | 0.220 |
| 120 – 180 | 0.040 | 0.063 | 0.100 | 0.160 | 0.250 |
Interpreting the table
For a 50 mm diameter hole specified at IT8, the tolerance band is 0.046 mm (0.039 if using the 30–50 row). If specified as H8 (hole basis), the bore diameter is 50.000 mm at minimum and 50.046 mm at maximum. This is the total allowable variation in diameter — not the allowable variation in position, straightness, or roundness.
Process Capability by Method
Gun Drilling
Gun drilling is the most precise deep hole drilling method for small to medium diameters (1–50 mm).
| Parameter | Standard Capability | Best Practice |
|---|---|---|
| Diameter tolerance | IT8 – IT9 | IT7 (IT6 in exceptional cases) |
| Surface finish (Ra) | 0.8 – 3.2 µm | 0.4 – 0.8 µm |
| Straightness | 0.10 – 0.30 mm/m | 0.05 – 0.10 mm/m |
| Roundness | 0.005 – 0.015 mm | 0.003 – 0.010 mm |
| Cylindricity | 0.010 – 0.025 mm | 0.005 – 0.015 mm |
Gun drilling achieves tighter diameter tolerances than BTA at small diameters because the single-lip cutting edge produces a stable, well-defined cutting action. The V-shaped flute provides consistent chip evacuation, and the guide pads burnish the bore surface during cutting.
BTA / STS Drilling
BTA drilling covers larger diameters (16–500 mm) with high material removal rates.
| Parameter | Standard Capability | Best Practice |
|---|---|---|
| Diameter tolerance | IT8 – IT10 | IT7 – IT8 |
| Surface finish (Ra) | 1.6 – 6.3 µm | 0.8 – 1.6 µm |
| Straightness | 0.10 – 0.30 mm/m | 0.05 – 0.15 mm/m |
| Roundness | 0.010 – 0.030 mm | 0.005 – 0.015 mm |
| Cylindricity | 0.015 – 0.035 mm | 0.010 – 0.020 mm |
BTA drilling uses multiple cutting edges with indexable carbide inserts. The tolerance capability is influenced by insert quality, head geometry, and the stability of the chip evacuation through the drill tube.
Ejector / DTS Drilling
Ejector drilling uses a double-tube system and does not require a pressure head seal, making it suitable for retrofitting onto conventional machines.
| Parameter | Standard Capability | Best Practice |
|---|---|---|
| Diameter tolerance | IT9 – IT11 | IT8 – IT9 |
| Surface finish (Ra) | 1.6 – 6.3 µm | 0.8 – 3.2 µm |
| Straightness | 0.15 – 0.40 mm/m | 0.10 – 0.25 mm/m |
| Roundness | 0.015 – 0.040 mm | 0.010 – 0.025 mm |
| Cylindricity | 0.020 – 0.050 mm | 0.015 – 0.030 mm |
Ejector drilling generally has wider tolerance variation than BTA because the double-tube system reduces the available space for chip evacuation, and the Venturi-effect coolant return creates different pressure dynamics at the cutting zone.
Straightness
Straightness in deep hole drilling is fundamentally different from diameter tolerance. It measures how much the hole axis deviates from a straight line over the hole length.
| Depth Ratio (L/D) | Gun Drilling (mm/m) | BTA Drilling (mm/m) | Counter-Rotating (mm/m) |
|---|---|---|---|
| < 20:1 | < 0.10 | < 0.10 | < 0.05 |
| 20:1 – 50:1 | 0.10 – 0.20 | 0.10 – 0.20 | 0.05 – 0.10 |
| 50:1 – 100:1 | 0.20 – 0.30 | 0.20 – 0.30 | 0.10 – 0.15 |
| > 100:1 | 0.30 – 0.50 | N/A (not typical) | 0.15 – 0.25 |
Counter-rotation — rotating the workpiece opposite the drill direction — approximately halves straightness deviation by canceling the rotational component of drill drift.
Straightness is not the same as diameter tolerance
A hole can have perfect IT7 diameter tolerance but be bent 0.5 mm per meter. Straightness is independently specified and independently measured. For deep holes above 20:1 L/D, straightness is often the more demanding specification. Always verify both values on the print.
Roundness and Cylindricity
Roundness
Roundness measures how close the bore cross-section is to a true circle. Deep hole drilling typically produces slightly lobed cross-sections from the rotation of the tool and the pressure of the guide pads.
| Process | Typical Roundness | Best Practice |
|---|---|---|
| Gun drilling | 0.005 – 0.015 mm | 0.003 – 0.008 mm |
| BTA drilling | 0.010 – 0.030 mm | 0.005 – 0.015 mm |
| Ejector drilling | 0.015 – 0.040 mm | 0.010 – 0.020 mm |
Three-lobed roundness patterns are common in BTA drilling and indicate the influence of the three guide pads. Two-pad tools generally produce two-lobed patterns with smaller amplitude.
Cylindricity
Cylindricity combines roundness, straightness, and diameter variation along the bore length.
| Process | Typical Cylindricity | Best Practice |
|---|---|---|
| Gun drilling | 0.010 – 0.025 mm | 0.005 – 0.015 mm |
| BTA drilling | 0.015 – 0.035 mm | 0.010 – 0.020 mm |
| Ejector drilling | 0.020 – 0.050 mm | 0.015 – 0.030 mm |
Cylindricity is the most comprehensive and difficult tolerance to hold in deep hole drilling because it accumulates errors from tool wear along the bore length, material hardness variations, and coolant pressure fluctuations.
Surface Finish
| Process | Ra Standard (µm) | Ra Best Practice (µm) | Rz Typical (µm) |
|---|---|---|---|
| Gun drilling | 0.8 – 3.2 | 0.4 – 0.8 | 4 – 12 |
| BTA drilling | 1.6 – 6.3 | 0.8 – 1.6 | 8 – 20 |
| BTA fine boring | 1.6 – 3.2 | 0.8 – 1.6 | 6 – 15 |
| Ejector drilling | 1.6 – 6.3 | 0.8 – 3.2 | 8 – 25 |
Surface finish in gun drilling is generally better than in BTA because:
- The single-lip cutting edge produces a thinner, more consistent chip
- The V-flute evacuates chips without dragging them across the finished surface
- The guide pads burnish the bore surface during the cut
Factors Affecting Tolerance Capability
Depth-to-Diameter Ratio
Tolerance capability degrades as depth ratio increases. A process that achieves IT8 at 20:1 L/D may produce only IT10 at 100:1 L/D in the same material.
| Depth Ratio | IT Grade Degradation |
|---|---|
| < 20:1 | Baseline capability |
| 20:1 – 50:1 | +0.5 – 1 IT grade |
| 50:1 – 100:1 | +1 – 2 IT grades |
| > 100:1 | +2 – 3 IT grades |
Material
| Material | Effect on Tolerance | Recommended Process Adjustment |
|---|---|---|
| Carbon steel (1045, 1026) | Best tolerance capability | Standard parameters |
| Alloy steel (4140, 4340) | Good capability | Reduce speed 10–20% |
| Stainless steel (304, 316) | Moderate capability | Lower feed, higher pressure |
| Aluminum | Good capability | Higher speeds, risk of built-up edge |
| Titanium alloys | Reduced capability | Low speeds, high pressure |
| Superalloys (Inconel) | Reduced capability | Low speeds, coated tools, high pressure |
Machine Condition
- Spindle run-out (TIR) should be ≤ 0.02 mm for IT8, ≤ 0.01 mm for IT7
- Guide bush alignment within 0.01 mm coaxiality
- Coolant pressure stability ±5% of set point
- Feed system backlash ≤ 0.01 mm for consistent diameter control
Tolerance Selection Guide
| Required IT Grade | Suitable Processes | Notes |
|---|---|---|
| IT7 | Gun drilling (≤ 30 mm), BTA with counter-rotation | Requires best practice conditions |
| IT8 | Gun drilling, BTA drilling | Standard capability for both |
| IT9 | Gun drilling, BTA, ejector | Most economical range |
| IT10 | BTA, ejector, gun drilling | Wide process choice |
| IT11 | Ejector, conventional drilling | Acceptable for non-critical bores |
When IT7 is required, specify H7, not just IT7
"IT7" defines the total tolerance band width, but not its position relative to the nominal diameter. "H7" specifies a hole basis fit where the minimum diameter equals the nominal diameter (all tolerance is positive). This is important for deep hole drilling because the process naturally produces holes on the plus side of nominal — drill deflection and run-out tend to enlarge rather than reduce the bore. Specifying H7 aligns process capability with inspection expectations.
Summary
| Parameter | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Diameter range | 1 – 50 mm | 16 – 500 mm | 18 – 150 mm |
| Diameter IT grade | IT7 – IT9 | IT7 – IT10 | IT9 – IT11 |
| Straightness (mm/m) | 0.05 – 0.30 | 0.05 – 0.30 | 0.10 – 0.40 |
| Roundness (mm) | 0.003 – 0.015 | 0.005 – 0.030 | 0.010 – 0.040 |
| Cylindricity (mm) | 0.005 – 0.025 | 0.010 – 0.035 | 0.015 – 0.050 |
| Surface finish (Ra, µm) | 0.4 – 3.2 | 0.8 – 6.3 | 0.8 – 6.3 |
| Max practical L/D | 300:1 | 100:1 | 100:1 |
FAQ
What IT grade does gun drilling typically achieve?
Gun drilling typically achieves IT8 as standard capability. Under best practice conditions — sharp tool, proper coolant pressure (60–120 bar), good machine condition, and favorable material — IT7 is achievable. At extreme depth ratios (> 100:1) or in difficult materials, IT9–IT10 is more realistic.
What is the difference between IT7 and H7 in deep hole drilling?
IT7 defines the total tolerance band width (e.g., 0.025 mm for a 50 mm diameter). H7 defines both the width AND the position — the hole's minimum diameter equals the nominal diameter, and the maximum is nominal + IT7 value. Deep hole drilling naturally produces holes on the plus side, so H7 is the appropriate specification. Specifying just "IT7" without a fit designation is ambiguous.
How straight is a typical gun-drilled hole?
A typical gun-drilled hole achieves 0.10–0.30 mm straightness deviation per meter of depth. With counter-rotation (workpiece rotates opposite the drill), this improves to 0.05–0.15 mm per meter. Straightness degrades with increasing depth ratio — expect 0.30–0.50 mm per meter at L/D > 100:1.
Can BTA drilling achieve the same tolerance as gun drilling?
In the diameter range where both processes can operate (16–50 mm), gun drilling generally achieves tighter diameter tolerances (IT7–IT8 vs IT8–IT9 for BTA). However, BTA drilling achieves better surface finish consistency because chips are evacuated through the drill tube interior and do not contact the finished bore surface. BTA also achieves higher material removal rates at equivalent tolerance.
What tolerance can ejector drilling (DTS) achieve?
Ejector drilling typically achieves IT9–IT11 diameter tolerance with Ra 1.6–6.3 µm surface finish. This is 1–2 IT grades wider than BTA because the double-tube system limits chip evacuation space and creates different coolant flow dynamics. Ejector drilling is best suited for applications where IT9 or wider tolerance is acceptable and the ability to retrofit onto existing machines is the primary consideration.
How does depth ratio affect achievable tolerance?
Tolerance degrades as depth ratio increases. As a rule of thumb: at L/D < 20:1, a process achieves its baseline IT grade. From 20:1 to 50:1, expect 0.5–1 IT grade degradation. From 50:1 to 100:1, expect 1–2 IT grades. Beyond 100:1, expect 2–3 IT grades degradation from baseline. This applies to all deep hole drilling methods and should be factored into tolerance stack-up calculations during design.
Process capabilities depend on machine configuration, tooling, material, and operating parameters. The values in this article represent typical production ranges. Consult process engineers and machine builders for application-specific capability studies. This article reflects industry knowledge as of 2026.