Appearance
A deep-hole-drilled aerospace shaft arrives at first article inspection. The bore is specified at 25.000 mm H7 (+0.021 mm). The CMM reports 25.035 mm. The supplier's air gauge measured 25.006 mm. The difference is 0.029 mm — three times the entire H7 tolerance band. Investigation reveals two root causes: the supplier calibrated their air gauge at 20°C but the shaft was measured at 35°C immediately after machining, and the CMM operator used a 4-point circle on a bore with 3-lobe spiralling from the BTA process. A properly conducted first article inspection would have caught both issues before production began.
What Is First Article Inspection and Why It Matters for Deep Hole Drilling
First article inspection (FAI) is a complete dimensional and quality verification of the first production part from a new or changed manufacturing process. For deep hole drilled components, FAI is critical because:
| Factor | Why FAI Is Essential |
|---|---|
| High workpiece value (€500–€15,000) | A setup error can scrap the first part before production begins |
| Hidden bore geometry | Bore defects are invisible without dedicated measurement equipment |
| Process sensitivity to setup | Guide bushing alignment, coolant pressure, and tool geometry all affect bore quality |
| Long cycle times | Hours of production can be wasted before an error is detected |
| Multiple interacting characteristics | Diameter, straightness, and surface finish are interdependent in deep hole drilling |
Standards Governing FAI
| Standard | Application | Key Requirement |
|---|---|---|
| AS9102B/AS9102D | Aerospace | Three-form FAI documentation, ballooned drawing, independent verification |
| PPAP (AIAG) | Automotive | 18-element submission, including dimensional results, capability studies, material certifications |
| ISO 9001 / AS9100D | General / Aerospace | Clause 8.3.4 requires verification of first production parts |
| Customer-specific | Varies | Additional requirements for defence, oil and gas, nuclear |
The AS9102 FAI Framework
AS9102 uses a three-form structure for documenting first article inspections. Each form serves a specific purpose.
Form 1: Part Number Accountability
| Field | Content |
|---|---|
| Part number and revision | Must match the drawing |
| Part name | Description of the component |
| Drawing number and revision | Current drawing revision used for inspection |
| Organisation name and supplier code | Manufacturer performing the FAI |
| FAI number | Unique identifier for this FAI |
| Material certification reference | Heat number and material specification |
| Special process certifications | Heat treatment, surface treatment, NDT |
| Change documentation | ECR/ECO numbers if applicable |
Form 2: Product Accountability — The Ballooned Drawing
The ballooned drawing is a key FAI deliverable. Every dimension, tolerance, and note on the drawing is assigned a numbered balloon, and the corresponding characteristic is listed on Form 2.
For deep hole drilled components, typical ballooned characteristics include:
| Balloon Number | Characteristic | Tolerance |
|---|---|---|
| 1 | Bore diameter — entry | Ø25.000 H7 (+0.021) |
| 2 | Bore diameter — mid-point | Ø25.000 H7 (+0.021) |
| 3 | Bore diameter — exit | Ø25.000 H7 (+0.021) |
| 4 | Bore roundness | 0.010 mm |
| 5 | Bore cylindricity | 0.025 mm |
| 6 | Bore straightness | 0.050 mm per metre |
| 7 | Bore concentricity to datum A | Ø0.050 mm |
| 8 | Bore surface finish (Ra) | 1.6 µm |
| 9 | Bore surface finish (Rz) | 6.3 µm |
| 10 | Bore position (true position to datums A-B-C) | Ø0.100 mm |
| 11 | Overall length | 500 ±1.0 mm |
| 12 | Outer diameter at journal A | Ø50.000 h6 (−0.016/−0.025) |
Form 3: Characteristic Accountability
Form 3 records the actual measured value, the inspection method, and the result for each ballooned characteristic:
| Balloon | Characteristic | Requirement | Actual | Method | Result |
|---|---|---|---|---|---|
| 1 | Bore dia. — entry | 25.000–25.021 | 25.008 | Air gauge | Pass |
| 2 | Bore dia. — mid | 25.000–25.021 | 25.012 | Air gauge | Pass |
| 3 | Bore dia. — exit | 25.000–25.021 | 25.018 | Air gauge | Pass |
| 4 | Bore roundness | 0.010 max | 0.004 | CMM | Pass |
| 5 | Bore cylindricity | 0.025 max | 0.011 | CMM | Pass |
| 6 | Bore straightness | 0.050/m max | 0.032 | Ultrasonic | Pass |
| 7 | Concentricity to A | Ø0.050 max | 0.018 | CMM | Pass |
| 8 | Ra surface finish | 1.6 µm max | 0.85 | Profilometer | Pass |
Note: AS9102 §4.7.3(b) requires that inspection results be expressed in quantitative (variable) terms. Attribute data (pass/fail) is only permitted when no variable measurement technique is feasible. For deep hole drilling applications, variable measurement is always feasible — air gauges, CMMs, and profilometers all produce quantitative data.
Deep-Hole-Specific Inspection Characteristics
Deep hole bores require additional verification points beyond those of conventional machined features:
| Characteristic | Why It Matters | Where to Measure |
|---|---|---|
| Bore diameter at multiple depths | BTA drilling produces taper along the bore length | Entry, 25%, 50%, 75%, exit |
| Roundness (2-lobe and 3-lobe) | Spiralling creates lobing that two-point measurement misses | 3+ cross-sections along bore |
| Cylindricity | Combined measure of roundness, taper, and straightness | Full bore length |
| Straightness | Determines shaft balance and bearing load distribution | Full bore centreline |
| Concentricity | Bore must be concentric to OD to avoid imbalance | Multiple cross-sections |
| Surface finish inside bore | Affects seal life, fluid flow, fatigue initiation | Bore interior wall |
| Borescope inspection | Detects scoring, chatter marks, chip entrapment | Full bore length |
Measurement Method Selection
| Characteristic | Recommended Method | Alternative Method | Notes |
|---|---|---|---|
| Bore diameter | Air gauge (±1 µm) | 3-point bore gauge (±3 µm) | Measure at minimum 3 axial positions |
| Roundness | CMM (12+ points) | Roundness tester | 4-point CMM may miss 3-lobe error |
| Cylindricity | CMM scanning | — | Requires scanning probe, not touch-trigger |
| Straightness | Ultrasonic wall thickness | CMM with long stylus | Ultrasonic method preferred for L/D > 10 |
| Concentricity | CMM (bore to OD) | Ultrasonic wall thickness | Requires both bore and OD measurement |
| Surface finish | Stylus profilometer | Optical profilometer | Ra and Rz per ISO 4287 |
| Internal visual | Borescope/videoscope | — | Full-length inspection for scoring, chatter |
| Hardness | Rockwell or Brinell | — | Per material specification |
| Material verification | PMI (positive material ID) | Chemical analysis | XRF or OES |
FAI Workflow for Deep Hole Components
Phase 1: Pre-Inspection Review
| Step | Activity |
|---|---|
| 1 | Review the drawing and all specification requirements |
| 2 | Identify all characteristics requiring verification (ballooning) |
| 3 | Select measurement methods for each characteristic |
| 4 | Verify that all inspection equipment is calibrated and within validity |
| 5 | Establish temperature control (workpiece, gauges, and environment) |
| 6 | Prepare inspection plan (characteristic-by-characteristic) |
Phase 2: Temperature Conditioning
Deep hole drilled components are typically long (1–20 metres) and sensitive to thermal expansion:
| Material | Coefficient of Expansion | Expansion of 1 m shaft per °C |
|---|---|---|
| Steel | 11.5 × 10⁻⁶ /°C | 0.0115 mm |
| Aluminium | 23.0 × 10⁻⁶ /°C | 0.0230 mm |
| Stainless steel | 17.0 × 10⁻⁶ /°C | 0.0170 mm |
For H7 bores (tolerance typically 0.015–0.035 mm), a temperature difference of 2–5°C between measurement and calibration can consume the entire tolerance band.
Recommended practice: Soak the workpiece in the measurement environment for a minimum of 4 hours before FAI measurement. Record the workpiece temperature and the reference temperature of all master rings and gauges.
Phase 3: Dimensional Measurement
Perform measurements in the following sequence:
- External dimensions — OD, length, step positions (least sensitive to bore-specific thermal effects)
- Bore diameter at multiple depths — Air gauge, starting at entry, progressing to full depth
- Bore geometry — CMM for roundness, cylindricity, concentricity
- Bore straightness — Ultrasonic wall thickness scan
- Surface finish — Profilometer inside the bore
- Borescope inspection — Visual inspection of the entire bore surface
Phase 4: Documentation
Complete AS9102 Forms 1, 2, and 3 (or equivalent):
| Form | Content | Sign-off |
|---|---|---|
| Form 1 | Part number, revision, material, special processes | Quality manager |
| Form 2 | Ballooned drawing with all characteristics listed | Inspection planner |
| Form 3 | Actual measurements with methods and results | Inspector |
| Supporting data | CMM reports, air gauge readings, material certs, NDT reports | Attached to FAI |
Phase 5: Review and Approval
| Step | Activity |
|---|---|
| 1 | Review all measurements against drawing requirements |
| 2 | Investigate any non-conformances (disposition per engineering) |
| 3 | Resolve measurement disputes (correlation between methods) |
| 4 | Submit FAI package to customer (if required) |
| 5 | Approve process for production |
| 6 | Retain FAI documentation per record retention requirements |
Warning: A common FAI pitfall in deep hole drilling is using a 4-point CMM circle on a bore with 3-lobe spiralling. The CMM will fit a circle through points that do not capture the lobing, producing an incorrect diameter and roundness result. Always verify lobing by comparing 2-point and 3-point measurements, or by using a CMM scanning probe with 12+ points per cross-section.
Common FAI Issues in Deep Hole Drilling
| Issue | Root Cause | Prevention |
|---|---|---|
| CMM and air gauge disagree on diameter | Different measurement definitions (clearance average vs. circle fit); or 3-lobe bore with 2-point measurement | Use master artefact correlation; verify lobe count before CMM programming |
| Bore passes at entry, fails at exit | Drill wander or whiplash in deep bore | Increase sampling positions; measure at minimum 5 depths |
| Surface finish fails specification | Roughness from worn guide pads detected only inside bore | Include borescope and profilometer in FAI plan |
| Temperature-related measurement error | Workpiece not stabilised to reference temperature | Soak workpiece; record and report temperatures |
| Straightness fails on CMM but passes ultrasonic | CMM stylus deflection error in deep bore | Use ultrasonic method for L/D > 10; verify CMM stylus calibration |
| Material certification missing | Raw material traceability not established | Verify material certs at incoming inspection, before machining |
PPAP for Deep Hole Components (Non-Aerospace)
For non-aerospace applications, the Production Part Approval Process (PPAP) is used instead of AS9102. PPAP requires:
| PPAP Element | Deep Hole Drilling Application |
|---|---|
| Dimensional results | Complete FAI dimensional report including bore measurements |
| Material certification | Steel grade verification (e.g., 34CrNiMo6, EA4T1) |
| Process capability study | Cp and Cpk for bore diameter (minimum 30 parts, 25+ subgroups) |
| Control plan | SPC sampling frequency, corrective action plan |
| Process flow diagram | All manufacturing steps from raw material to final inspection |
| Gauge R&R study | Repeatability and reproducibility of bore measurement system |
| Initial sample | First article part or production run sample |
Complete FAI Checklist for Deep Hole Drilled Components
| Check | Completed |
|---|---|
| Drawing and revision verified current | ☐ |
| All characteristics ballooned on drawing | ☐ |
| Measurement methods selected for each characteristic | ☐ |
| Inspection equipment calibrated and current | ☐ |
| Temperature conditioning completed (min 4-hour soak) | ☐ |
| Workpiece temperature recorded at time of measurement | ☐ |
| Bore diameter measured at minimum 3 depths (entry, mid, exit) | ☐ |
| Bore measured in minimum 2 axes at each depth | ☐ |
| Roundness verified with sufficient points (12+ for CMM, or air gauge jet comparison) | ☐ |
| Straightness measured (ultrasonic preferred for L/D > 10) | ☐ |
| Concentricity between bore and external datums verified | ☐ |
| Surface finish measured inside bore (Ra and Rz) | ☐ |
| Borescope inspection completed for full bore length | ☐ |
| Material certification reviewed and attached | ☐ |
| Special process certifications attached (heat treat, NDT, surface treatment) | ☐ |
| All measurements recorded as variable data (actual values) | ☐ |
| Non-conformances identified and dispositioned | ☐ |
| FAI package completed and approved | ☐ |
Troubleshooting
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Bore diameter passes air gauge but fails CMM | 3-lobe bore geometry | Check lobe count; use 3-jet air plug; increase CMM point count |
| CMM results non-repeatable on deep bore | Long stylus deflection | Use shorter stylus; apply stylus compensation; verify with air gauge |
| Straightness fails specification | Drill wander or misalignment | Check guide bushing alignment; reduce feed rate; verify pilot hole |
| Surface finish fails inside bore | Worn guide pads or chatter | Inspect and replace guide pads; check for chatter marks |
| Temperature affecting measurements | Inadequate soak time | Extend soak to 4+ hours; measure and record temperature |
FAQ
What is a first article inspection for deep hole drilling?
A first article inspection (FAI) is a complete dimensional and quality verification of the first production part from a new deep hole drilling setup. It verifies that the bore diameter, roundness, straightness, concentricity, surface finish, and all other specified characteristics meet the drawing requirements before production begins.
What standard governs first article inspection for aerospace deep hole components?
AS9102B/AS9102D is the governing standard. It requires three forms of documentation: Form 1 (part number accountability), Form 2 (product accountability with ballooned drawing), and Form 3 (characteristic accountability with actual measurement values). All inspection results must be expressed in quantitative (variable) terms.
What bore measurements are required in a deep hole FAI?
Bore diameter must be measured at multiple depths (minimum 3, recommended 5 positions along the bore). Roundness, cylindricity, straightness, and concentricity to external datums must also be verified. Surface finish (Ra and Rz) must be measured inside the bore. Borescope inspection is recommended for the full bore length.
Why do CMM and air gauge give different bore diameter results?
CMM and air gauge measure different geometric properties. An air gauge averages the clearance over the air jet area and reports an average diameter. A CMM samples discrete points and fits a circle. On a perfectly round bore, both methods agree. On a bore with lobing or ovality, they produce different results. Correlation through a calibrated master artefact is recommended.
How does temperature affect deep hole FAI measurements?
A steel shaft expands by approximately 0.0115 mm per metre per °C. For an H7 bore with a tolerance of 0.021 mm, a 2°C temperature difference between calibration and measurement can consume the entire tolerance band. Workpieces must be temperature-conditioned (soaked) for a minimum of 4 hours before measurement.
What is a ballooned drawing?
A ballooned drawing is a copy of the engineering drawing on which every dimension, tolerance, and note requirement has been assigned a numbered balloon (circle with number). The balloon numbers correspond to the characteristic numbers on AS9102 Form 2 and Form 3, providing traceability from the drawing to the inspection report.
How many points should a CMM measure on a deep hole bore?
A minimum of 8–12 points per cross-section is recommended for diameter and roundness measurement. A 4-point measurement can miss 3-lobe spiralling patterns common in BTA drilling. For cylindricity, measure at minimum 3 cross-sections with 12 points each, or use a scanning probe for continuous data.
What is the difference between AS9102 and PPAP?
AS9102 is the aerospace first article inspection standard, using a three-form structure (Forms 1, 2, 3). PPAP is the automotive Production Part Approval Process, requiring 18 elements including dimensional results, material certifications, process capability studies, control plans, and gauge R&R studies. Both serve the same purpose — verifying that the production process produces conforming parts — but use different documentation frameworks.
What measurement method is best for bore straightness in deep holes?
Ultrasonic wall thickness measurement is the preferred method for straightness verification in deep holes (L/D > 10). The workpiece outer diameter is measured, wall thickness is scanned at multiple positions and angles, and the bore centreline is calculated. CMM with a long stylus is an alternative for shallower bores but suffers from stylus deflection error.
What should be done when a first article fails?
When a first article fails, the non-conformance must be documented, the root cause identified, and corrective action implemented. The process setup must be adjusted (tool change, parameter adjustment, fixture alignment) and a new first article produced and inspected. The failed part must be clearly identified and segregated. A deviation or waiver may be requested if the non-conformance does not affect fit, form, or function.
Conclusion
First article inspection for deep hole drilled components requires a comprehensive protocol that addresses the unique challenges of bore measurement: temperature sensitivity of long components, the multiple geometric characteristics that define bore quality, the selection of appropriate measurement methods for each characteristic, and the independent verification requirement of AS9102. The three engineering priorities for FAI in deep hole drilling are: establishing proper temperature conditioning and control before any measurement is taken, using measurement methods that correctly characterise the specific geometry errors relevant to deep hole drilling (lobing, taper, straightness, concentricity), and documenting all results quantitatively according to the AS9102 framework to ensure traceability and reproducibility.