Deep Hole Drilling Tolerances and Surface Finish Guide

Deep hole drilling is capable of precision that often eliminates the need for secondary operations. However, the achievable tolerance and surface finish depend on the method, diameter, depth ratio, material, and process condition.

This guide provides a comprehensive reference for tolerance and surface finish capabilities across all deep hole drilling methods.

Tolerance Capability by Method

Diameter Tolerance

Method	Standard Production	Precision	High Precision	ISO Grade Equivalent
Gun drilling	±0.050 mm	±0.025 mm	±0.013 mm	IT7–IT11
BTA drilling	±0.075 mm	±0.050 mm	±0.025 mm	IT8–IT11
Ejector drilling	±0.075 mm	±0.050 mm	±0.025 mm	IT8–IT11
Trepanning	±0.150 mm	±0.100 mm	—	IT10–IT12
EDM drilling	±0.025 mm	±0.013 mm	±0.005 mm	IT6–IT9

Factors Affecting Diameter Tolerance

Factor	Impact	How to Optimize
Tool condition	Most significant	Regrind at 0.25 mm wear land
Guide pad wear	High	Replace at 0.15 mm wear
Spindle runout	Direct	Maintain < 0.005 mm TIR
Coolant temperature	Moderate (±0.005 mm/10°C)	Use chiller; maintain 30–40°C
Depth ratio	Significant	Reduce parameters at depth
Material hardness	Moderate	Harder materials hold tighter tolerances
Machine rigidity	Significant	Contra-rotation improves consistency

Tolerance by Diameter (Gun Drilling, Standard Conditions)

Diameter	Typical Tolerance (mm)	Variation at Depth
3–6 mm	±0.013 mm	Increases ~0.002 mm per 100 mm depth
6–12 mm	±0.020 mm	Increases ~0.003 mm per 100 mm depth
12–20 mm	±0.025 mm	Increases ~0.004 mm per 100 mm depth
20–30 mm	±0.030 mm	Increases ~0.005 mm per 100 mm depth

Surface Finish Capability

As-Drilled Surface Finish

Method	Typical Ra Range	Best Achievable Ra	RMS Equivalent
Gun drilling	0.4–1.6 µm	0.2 µm	8–63 µin
BTA drilling	0.8–3.2 µm	0.4 µm	16–125 µin
Ejector drilling	0.8–3.2 µm	0.4 µm	16–125 µin
Trepanning	1.6–6.3 µm	0.8 µm	63–250 µin
EDM drilling	0.2–1.6 µm	0.1 µm	8–63 µin

Surface Finish vs. Feed Rate

Surface finish is directly related to feed rate. The relationship follows approximately:

Ra ≈ K × f² / r

Where:

Ra = surface finish (µm)
f = feed rate (mm/rev)
r = tool nose radius (mm)
K = material constant

Feed Rate Reduction	Ra Improvement	Cycle Time Cost
Reduce 25%	~20% better Ra	+33% longer
Reduce 50%	~45% better Ra	+100% longer
Reduce 75%	~75% better Ra	+300% longer

Factors Affecting Surface Finish

Factor	Impact	Fix
Feed rate	Strong — higher feed = rougher finish	Reduce feed; accept longer cycle
Guide pad condition	Strong — worn pads cannot burnish	Replace at 0.15 mm wear
Tool nose radius	Moderate — larger radius = better finish	Use largest radius that avoids chatter
Vibration	Severe — chatter ruins finish	Add whip guide; reduce speed
Coolant lubricity	Moderate — poor lube causes galling	Use neat oil with EP additives
Built-up edge	Moderate — irregular cutting edge	Increase speed; check coating

Straightness Capability

Method	Standard (mm/m)	Precision (mm/m)	Best (mm/m)
Gun drilling (single rotation)	0.40	0.25	0.12
Gun drilling (contra-rotation)	0.12	0.08	0.04
BTA drilling	0.40	0.25	0.12
Ejector drilling	0.50	0.30	0.15

Hole Quality Characteristics

Roundness

Method	Typical Roundness (mm)
Gun drilling	0.005–0.015 mm
BTA drilling	0.010–0.030 mm
Ejector drilling	0.010–0.030 mm

Hole Shape

Deep-drilled holes typically exhibit:

Gun drilling: Slightly three-lobed shape (from three-point contact of cutting edge + two guide pads)
BTA/ejector: More uniform roundness (multiple cutting edges create more balanced forces)
Entry bellmouth: 0.01–0.03 mm oversize at the first 1–2 mm of entry (normal)
Exit bellmouth (through-holes): 0.02–0.05 mm oversize at exit (reduced with reduced breakthrough feed)

Acceptance Criteria by Application

Application	Diameter Tolerance	Surface Finish	Straightness
Fuel injector bore	±0.005 mm	Ra 0.2 µm	0.02 mm per 100 mm
Hydraulic spool bore	±0.013 mm	Ra 0.4 µm	0.04 mm per 300 mm
Medical bone screw	±0.025 mm	Ra 0.4 µm	0.08 mm per 100 mm
Mold cooling channel	±0.050 mm	Ra 1.6 µm	0.12 mm per 300 mm
Automotive oil gallery	±0.050 mm	Ra 0.8 µm	0.12 mm per 300 mm
Aerospace actuator bore	±0.013 mm	Ra 0.4 µm	0.04 mm per 300 mm
Structural bolt hole	±0.100 mm	Ra 3.2 µm	0.20 mm per 300 mm

Improving Hole Quality

To Improve Tolerance

Technique	Improvement	Cost
Regrind tool earlier	±0.025 → ±0.013 mm	More tool changes
Add contra-rotation	±0.050 → ±0.025 mm	Machine modification
Reduce feed	±0.050 → ±0.030 mm	Longer cycle time
Stabilize coolant temperature	Reduce variation by 50%	Add chiller
Use smaller depth ratio	Significant	May require multiple setups

To Improve Surface Finish

Technique	Ra Before	Ra After	Trade-off
Reduce feed 50%	1.6 µm	0.8 µm	2× cycle time
Index inserts earlier	1.6 µm	0.8 µm	More tool changes
Replace guide pads	1.6 µm	0.6 µm	Pad cost
Add secondary operation	1.6 µm	0.2 µm	Additional setup

Summary

Deep hole drilling can achieve IT7–IT11 tolerances and Ra 0.4–3.2 µm surface finish depending on the method and conditions. Gun drilling offers the best precision (up to ±0.013 mm) and surface finish (Ra 0.2–0.8 µm). BTA and ejector drilling offer good precision (±0.050 mm) with higher productivity. Factors that most affect quality are tool condition, guide pad wear, feed rate, coolant temperature, and depth ratio. For critical applications requiring better than as-drilled quality, secondary operations like reaming, honing, or skiving can improve tolerance and finish.

For measurement methods, see deep hole measurement methods. For straightness optimization, see hole straightness guide. For complete process capability, see SPC and capability guide. For a complete overview, visit the precision and quality guide.