PPAP and FAI for Deep Hole Drilling: Documentation & Quality

In conventional machining, quality documentation proves that parts meet print. In deep hole drilling, quality documentation must also prove that the process was controlled — because many critical characteristics of a deep hole (straightness, surface finish at the bore midpoint, subsurface integrity) cannot be measured on the finished part. PPAP and first article inspection documentation are how deep hole drilling suppliers demonstrate process control to their customers.

Overview ​

PPAP (Production Part Approval Process) and FAI (First Article Inspection) are the two dominant quality frameworks for manufacturing documentation. PPAP is primarily used in automotive (governed by AIAG standards) and an increasing number of industrial sectors. FAI per AS9102 is required for aerospace and defence.

Both frameworks share the same goal: prove that the manufacturing process can produce conforming parts reproducibly. For deep hole drilling, this means demonstrating control over parameters that standard machining does not require: coolant pressure and flow, straightness deviation per metre, and surface finish at depth.

PPAP for Deep Hole Drilling ​

The AIAG PPAP 4th Edition defines 18 elements required for part approval. For deep hole drilling operations, several of these elements require specific attention.

The 18 PPAP Elements ​

Submission Levels ​

PPAP defines five submission levels. Level 3 is the default for most production parts:

For deep hole drilling applications, Level 3 is typical because the process risk is higher than conventional machining and the customer needs visibility into process parameters.

AS9102 First Article Inspection ​

AS9102 is the aerospace standard for first article inspection. It applies to deep hole drilling suppliers serving aerospace and defence customers.

When FAI Is Required ​

An AS9102 FAI must be performed for:

The Three AS9102 Forms ​

A complete FAI report (FAIR) comprises three forms:

Form 1 — Part Number Accountability:

• Part number, revision level, and drawing number
• Bill of materials for assemblies
• Overall FAI result (pass/fail with conditions noted)

Form 2 — Product Accountability:

• Raw material certification (steel grade, heat number, mill test report)
• Special process certifications (heat treatment, surface coating, non-destructive testing)
• Functional test results if applicable

Form 3 — Characteristic Accountability:

• Every ballooned design characteristic with actual measurement results
• Critical for deep hole drilling — records diameter at specified depths, straightness deviation, surface finish, true position of cross-holes

Independence Requirement ​

A critical requirement of AS9102 is that the person performing the FAI measurement cannot be the same person who manufactured the part. For deep hole drilling, this means the inspection should be performed by a quality technician using independent measurement equipment — not the machine operator verifying with the setup tools.

Variable Data vs Attribute Data ​

AS9102 requires measurement results in quantitative (variable) terms whenever possible. Attribute data (pass/fail) is only acceptable when no quantitative method is feasible. For deep hole characteristics:

Process Flow and PFMEA ​

The process flow diagram and PFMEA form the foundation of the quality planning documentation.

Process Flow Diagram ​

For a deep hole drilling operation, the process flow must include every step from raw material receipt to final inspection. A typical deep hole drilling process flow:

1. Raw material receipt and inspection — material certification, hardness check, ultrasonic testing
2. Facing and centring — preparing the workpiece ends for drilling
3. Pilot hole drilling — pre-drilling for BTA or gun drilling start
4. Deep hole drilling — the primary operation (gun drilling, BTA, or ejector drilling)
5. Intermediate inspection — bore diameter, straightness check
6. Secondary operations — cross-hole drilling, port drilling
7. Deburring — internal and external burr removal
8. Heat treatment (if required) — stress relief after rough drilling
9. Finish boring or honing — final size and surface finish
10. Final inspection — all characteristics verified
11. Cleaning and preservation — FOD prevention, rust protection

PFMEA for Deep Hole Drilling ​

The PFMEA identifies potential failure modes, their effects and causes, and assigns risk priority numbers (RPN) or action priority (AP) per the AIAG-VDA FMEA Handbook.

Risk Priority for Deep Hole Drilling ​

The severity ratings for deep hole drilling failures tend to be higher than for conventional machining because:

• The bore is often a functional surface (bearing surface, oil passage, hydraulic bore)
• Rework is difficult or impossible (straightness deviation cannot be corrected)
• Part value is typically high (large shafts, manifolds, engine components)
• Hidden defects cannot be detected by visual inspection

Control Plan ​

The control plan translates PFMEA outputs into specific controls for each process step. For deep hole drilling, the control plan must specify:

Tip: For critical deep hole characteristics (straightness, bore diameter at depth), consider 100% inspection at initial production run-down to establish capability, transitioning to statistical sampling only after demonstrating Cpk > 1.67.

Initial Process Capability Studies ​

PPAP requires initial process capability studies for all special characteristics identified on the drawing or control plan.

Capability Requirements ​

Deep Hole Capability Study Considerations ​

Capability studies for deep hole drilling require special attention because:

• Sample size: Minimum 30 parts recommended (PPAP minimum is typically 25 for variable data)
• Within-part variation: Measure diameter at multiple depths — the bore may be within tolerance at the entrance but out at depth
• Time-order sampling: Collect parts in production sequence to detect drift from tool wear
• Stratification: If multiple spindles or tool positions are used, collect capability data for each position

Measurement Systems Analysis ​

Gage R&R studies for deep hole measurement must account for:

Acceptance criteria per AIAG MSA 4th Edition:

• GR&R < 10%: Excellent measurement system
• GR&R 10–30%: Acceptable for most applications
• GR&R > 30%: Unacceptable — system must be improved

Documentation Hierarchy ​

The quality documentation for a deep hole drilling PPAP or FAI follows a hierarchical structure:

Level 1: Part Submission Warrant (PSW) or FAI Form 1
    └── Summary document declaring overall result
    
Level 2: Supporting Plans
    ├── Process Flow Diagram
    ├── PFMEA (with RPN/AP ratings)
    └── Control Plan
    
Level 3: Evidence Documents
    ├── Dimensional Results (ballooned drawing with measurements)
    ├── Material Certifications and Test Results
    ├── Initial Process Capability Study (Cpk/Ppk charts)
    ├── Measurement Systems Analysis (GR&R)
    ├── Laboratory Qualifications
    └── Sample Parts and Master Sample Records

For deep hole drilling, the dimensional results section must include measurements that are specific to deep hole geometry:

• Bore diameter at minimum three depths (entrance, midpoint, exit)
• Straightness deviation per unit length and total
• Roundness at multiple cross-sections
• Surface finish (Ra, Rz) at specified locations
• Wall thickness uniformity (for tubular parts)
• True position of cross-holes relative to the main bore axis
• Burr condition at entry, exit, and cross-hole intersections

Summary ​

FAQ ​

What is the difference between PPAP and FAI? ​

PPAP (Production Part Approval Process) is an automotive-industry framework governed by AIAG standards that includes 18 elements covering the complete quality system — from design records through process capability to part submission. FAI (First Article Inspection per AS9102) is an aerospace standard focused on verifying that the first production part meets all design requirements through three specific forms. PPAP is broader in scope; FAI is deeper in dimensional verification.

What PPAP submission level is typical for deep hole drilling? ​

Level 3 is the most common for deep hole drilling — it requires the part submission warrant, product samples, and complete supporting data including dimensional results, capability studies, PFMEA, control plan, and MSA. The comprehensive documentation is appropriate given the higher process risk and difficulty of rework for deep hole features.

How do I measure deep hole straightness for PPAP dimensional results? ​

Deep hole straightness is measured using laser displacement systems (non-contact, on-machine), coordinate measuring machines with extended probes (for holes up to ~300 mm depth), or air gauge systems that measure concentricity along the bore. For PPAP, report straightness deviation per unit length (typically mm/m or in/ft) and total deviation over the full bore length.

What capability index (Cpk) is required for deep hole characteristics? ​

The minimum acceptable Cpk is 1.33 for most automotive applications. For safety-critical or special characteristics, Cpk > 1.67 is typically required. For deep hole drilling, achieving Cpk > 1.33 for straightness and bore diameter at depth may require tighter process controls (coolant pressure monitoring, tool change intervals, feed rate optimisation) than for conventional machining.

What PFMEA failure modes are specific to deep hole drilling? ​

Key deep-hole-specific failure modes include: hole straightness deviation (caused by guide pad wear or coolant pressure fluctuation), surface tearing at guide pad contact area (inadequate lubricity), chip evacuation failure (coolant pressure below minimum), burr formation at cross-hole intersections, and diameter taper along the bore length. Each of these has higher severity than equivalent failures in conventional drilling because deep hole features are typically functional surfaces.

Is a control plan required for deep hole drilling PPAP? ​

Yes. The control plan (PPAP element 12) is mandatory for all PPAP submission levels above Level 1. For deep hole drilling, the control plan must specify parameters unique to the process: coolant pressure, flow rate, concentration, filtration level, peck cycle depth and retract distance, tool change frequency, and inspection methods for bore diameter and straightness at specified depths.

How many parts are needed for an initial process capability study? ​

The AIAG PPAP manual recommends a minimum of 25 parts for variable data capability studies (Cpk/Ppk) and 20 subgroups for attribute data. For deep hole drilling, a minimum of 30 parts is recommended to account for the higher process variability from tool wear and coolant system drift. Parts should be collected in production sequence to detect trends.

What measurement systems are needed for deep hole FAI? ​

Deep hole FAI requires: bore gauges or air probes for diameter measurement at multiple depths (with appropriate reach), surface finish profilometer, laser or mechanical straightness measurement system, CMM for true position and geometric tolerances, and borescope for internal visual inspection. All gauges must have GR&R < 30% (preferably < 10%) and current calibration traceable to national standards.

Can standard PPAP documentation cover deep hole drilling? ​

Standard PPAP templates designed for conventional machining do not adequately cover deep hole drilling. The control plan must include coolant parameters (pressure, flow, concentration, filtration) that are irrelevant to conventional machining. The PFMEA must address failure modes unique to deep hole drilling (straightness deviation, guide pad wear, chip evacuation). Customised templates are required.

What triggers a PPAP re-submission for deep hole drilling? ​

PPAP must be re-submitted when any of the following change: manufacturing process (e.g., gun drilling to BTA), tooling source or geometry, CNC program (affecting feed, speed, or peck cycle), coolant type or concentration range, inspection method, or production location. A change in drill manufacturer or drill geometry — even if the hole print remains the same — triggers a PPAP re-submission because the change affects the process capability.