Deep Hole Drilling First Article Inspection (FAI) Guide

Deep Hole Drilling First Article Inspection (FAI)

First Article Inspection (FAI) for deep hole drilling is complicated by the fact that many standard measurement methods cannot reach the features being inspected. A deep, small-diameter bore cannot be measured with conventional micrometers or calipers — specialized equipment and methods are required.

This guide covers FAI planning specifically for deep hole features, measurement method selection, sampling strategies at depth, and AS9102 form preparation.

FAI Planning for Deep Hole Features

Features to Verify

Feature	Typical Tolerance	Measurement Challenge
Diameter (at entry)	±0.025 mm	Easy — accessible
Diameter (at 50% depth)	±0.025 mm	Moderate — reach required
Diameter (at full depth)	±0.025 mm	Difficult — small bore, extreme depth
Straightness	0.1 mm/m	Moderate — mandrel or CMM required
Surface finish (Ra)	0.8 µm	Difficult — profilometer reach limited
Roundness	0.02 mm	Moderate — CMM with multiple points
Hole depth	±0.5 mm	Easy — depth gauge
Position / location	±0.1 mm (typical)	Easy — CMM reference

Sampling Strategy at Depth

Because deep holes cannot be measured continuously, a sampling strategy is essential:

Hole Length	Recommended Measurement Depths	Rationale
< 50 mm	Entry + midpoint + full depth	Three points sufficient
50–200 mm	Entry, 25%, 50%, 75%, full depth	Five points captures variation
200–500 mm	Every 50 mm	Consistent coverage
> 500 mm	Every 100 mm, plus entry and exit	Scales with length

Measurement Method Selection

By Feature

Feature	Preferred Method	Alternative Method	Notes
Diameter — entry	Air gauge (plunge type)	Bore gauge (digital)	Air gauge is fastest
Diameter — mid-depth	Air gauge (two-jet with extension)	CMM with extension bar	Air gauge requires 5–30 mm gap
Diameter — deep	Three-point bore gauge with long extension	Ultrasonic (indirect)	Bore gauge accuracy decreases with extension length
Straightness	Mandrel + indicator	CMM with alignment	Mandrel is simplest for 5–20 mm dia
Surface finish	Profilometer with extension	Replica method	Profilometer limited to ~200 mm max
Roundness	CMM (8+ points per cross-section)	Three-point bore gauge at multiple angles	CMM provides more data
Concentricity	CMM with datum reference	Mandrel between centers	Use appropriate datum

Measurement Depth Capabilities

Method	Maximum Practical Depth	Notes
Air gauge — standard	300 mm	With 300 mm extension bar
Air gauge — custom	2,000 mm	Custom extension; calibration critical
Bore gauge (digital)	500 mm	Accuracy degrades beyond 300 mm
CMM (star probe)	300 mm	Limited by probe reach
CMM (articulating head)	500 mm	Flexible head extends reach
CMM (extension bar)	1,000 mm	Sag compensation required
Profilometer (extension)	200 mm	Surface finish only
Mandrel (straightness)	2,000 mm	Straightness measurement only
Ultrasonic	Any	Diameter indirect — wall thickness measurement
Borescope	Any	Visual inspection — no dimensional data

AS9102 FAI Form for Deep Hole Features

Form 2 (Product Accountability)

List each deep hole feature as a separate line item:

Item	Feature ID	Characteristic	Tolerance	Measured Value	Method	Result
1	Hole A — diameter entry	12.00 mm	±0.025 mm	12.012 mm	Air gauge	Pass
2	Hole A — diameter mid	12.00 mm	±0.025 mm	12.008 mm	Air gauge	Pass
3	Hole A — diameter full depth	12.00 mm	±0.025 mm	12.015 mm	Bore gauge	Pass
4	Hole A — depth	200 mm	±0.5 mm	200.2 mm	Depth gauge	Pass
5	Hole A — position X	0.0 mm	±0.1 mm	0.03 mm	CMM	Pass
6	Hole A — position Y	0.0 mm	±0.1 mm	0.02 mm	CMM	Pass
7	Hole A — surface finish	0.8 µm Ra	±0.3 µm	0.62 µm Ra	Profilometer	Pass

Form 3 (Characteristic Accountability)

For complex deep holes with multiple characteristics:

Characteristic	Sample Size	Frequency	Cpk Requirement
Diameter	3 cross-sections per hole	Every 25th hole	≥ 1.33
Straightness	1 per 10 holes	Per production run	≥ 1.33
Surface finish	1 per 10 holes	Per production run	≥ 1.33
Position	1 per hole	Per production run	≥ 1.67

CMM Probe Selection

For Deep Hole Measurement

Probe Type	Best For	Limitations
Star probe	Multiple points at same depth	Limited depth (~3× probe shank length)
Straight extension (+ spherical tip)	Deep, large diameter holes	Sag at > 200 mm — calibrate
Articulating head (PH10 type)	Angled holes, multi-direction	Slower; needs pre-calibration
Custom multi-tip	Production inspection	Dedicated to one feature

Depth-Specific Inspection Challenges

At Entry (First 10 mm)

Easy access — all methods work
May be affected by bellmouth or chamfer
Measure 5 mm into the hole (past chamfer)

At Mid-Depth (25–75% of hole)

Most methods need extension
Air gauge with extension is preferred
CMM extension bars require compensation
Straightness measurement begins here

At Full Depth

Most difficult to measure
Bore gauge may be the only option
Air gauge custom extension may be impractical if gap is small
Consider ultrasonic measurement as alternative

Reporting Requirements

Minimum Report Content

Information	Required For
Customer PO / part number	All industries
Hole feature identification	All drilling jobs
Measured values at each depth	Aerospace, defense, nuclear
Measurement method and equipment ID	All certified industries
Operator / inspector identification	All industries
Date of inspection	All industries
Calibration status of measurement equipment	ISO 9001, AS9100

Summary

First Article Inspection for deep hole drilling requires careful method selection based on feature type and measurement depth. Diameter is best measured by air gauge (up to 300 mm) or bore gauge with extension (up to 500 mm). Straightness is best measured by mandrel + indicator (up to 2 m). Surface finish is the most challenging — profilometer extensions reach only ~200 mm. AS9102 FAI forms should list each hole feature as a separate line item with measured values at multiple depths (minimum 3: entry, mid, full depth). For quality certification requirements, see deep hole drilling quality certification standards. For measurement methods overview, see deep hole measurement methods guide.