Inspection Methods

CMM. Visual.
NDT. Optical.
Which to specify.

Q: First article inspection — what's involved?

FAI verifies first production part meets all drawing requirements. Typical content: (1) Drawing reference and revision. (2) All dimensions measured against nominal. (3) GD&T features evaluated. (4) Material certifications attached. (5) Traceability to raw material heat. (6) Process records (heat treatment, coating). (7) Inspector signature and date. Output document 10-40 pages typical. Standard customer-specific format (AS9102 for aerospace, customer-specific for automotive).

Q: What to inspect 100% vs sample?

100% inspection: critical dimensions (safety, function, mating), low-volume parts (<100), aerospace/medical/nuclear. Sampling inspection: high-volume production, stable process with proven Cpk, non-critical dimensions. Common sampling: AQL per MIL-STD-105 or ISO 2859, SPC with control charts. Specify 100% only where needed — costs more per part.

Q: CMM programming and setup?

First part measurement: 1-3 hours programming (operator), plus 30min-2hr actual measurement. For 10 parts, setup overhead amortized — per-part inspection cost modest. For 1-piece orders, programming cost dominates — often 60-80% of inspection cost. For repeat production, programming cost invested once pays back over subsequent orders.

Q: NDT — when is it required?

NDT required for: welds on pressure vessels (ASME IX), critical aerospace structural parts, safety-critical components (medical, nuclear). Penetrant or magnetic particle standard on welds. Ultrasonic or X-ray on critical forgings. For standard commercial parts, NDT not required. For fatigue-critical or safety-critical parts, specify per applicable standard.

Q: CMM accuracy limitations?

Typical production CMM: ±0.002 mm repeatability on precision features. Limitations: (1) Workpiece size vs machine volume. (2) Temperature variation affects accuracy. (3) Operator skill on complex GD&T. (4) Probe tip diameter (3mm min typical). (5) Access to internal features. For sub-micron precision, specialty metrology equipment and environmental controls needed — typically external calibration lab.

Q: Documentation retention?

Inspection records retention varies by industry: Aerospace: 10-25+ years (FAR requirement). Medical devices: full product life + 2 years per FDA. Automotive: 15 years per IATF 16949. Oil & gas: 10-25+ years per customer. General commercial: 7 years typical. Our records retained per customer requirements; standard retention 10 years for quality-controlled orders.

Different features need different inspection methods. Over-specifying inspection wastes money; under-specifying misses defects. Here's how to think about inspection per feature type.

Free DFM Review First article inspection

01 · Methods by feature

Method for each feature type.

Feature type	Best method	Tolerance	Speed
Linear dimensions	Calipers / CMM	±0.025 to ±0.002	Fast / Medium
Hole diameter	Pin gauge / CMM	±0.025 to ±0.005	Very fast / Medium
Hole position	CMM	±0.02 to ±0.005	Medium
Flatness	Granite plate / CMM	0.02 to 0.005	Fast / Medium
Parallelism	CMM / height gauge	0.02 to 0.005	Medium
Concentricity	Dial indicator / CMM	0.01 to 0.002	Fast / Slow
Surface roughness	Profilometer	Ra 0.02+ µm	Medium
Thread quality	Go/no-go gauge	Class 2B/3B	Very fast
GD&T features	CMM	Per drawing	Medium
Complex 3D surfaces	Laser scan / CMM	0.02 to 0.005	Slow
Hidden features	X-ray / CT	Per drawing	Slow / Expensive

02 · CMM details

Coordinate Measuring Machines.

CMM is workhorse for dimensional inspection of complex parts. Touch-probe measures point coordinates in 3D space.

Capability

±0.002 mm precision

Modern CMM: ±0.002 mm repeatability on precision features. Full GD&T evaluation — flatness, parallelism, position, profile tolerance.

Speed

Medium speed

Individual feature measurement 10-30 seconds. Full part first-article: 30 minutes to 4 hours depending on complexity. Production inspection: optimized fixturing for fast checks.

Cost

$50-500 per part

Inspection cost varies with complexity. Simple parts: $30-100 for first article. Complex aerospace: $200-1000. Production inspection: $5-50 per part when set up.

Limitations

Internal features hard

CMM probes can't reach some internal features. Very small holes (<3mm), deep pockets — alternative methods (optical, X-ray, destructive).

Programming

Part program upfront

First CMM program 1-3 hours depending on complexity. Once programmed, subsequent parts fast. Programming cost amortized over production run.

AS9102

Aerospace FAI standard

AS9102 first-article inspection standard. CMM reports in AS9102 format for aerospace customers. Includes all drawing dimensions, references to drawing, actual measurements.

03 · NDT methods

Non-destructive testing.

For critical parts where defects must be detected without destruction. Each method has specific capabilities.

Visual inspection (VT)

• Most common, first-line inspection
• Detects surface cracks, porosity, dimensional errors
• Eye + 10× magnifier typical
• Liquid penetrant enhancement common
• No volume inspection (surface only)

Liquid penetrant (PT)

• Detects surface cracks in any material
• Dye penetrates crack, developer reveals
• Fast, cheap, widely used
• Surface only — cannot detect subsurface
• Aerospace and structural welding standard

Magnetic particle (MT)

• Detects surface and near-surface defects in ferrous material
• Iron particles collect at flux leakage points
• Faster than penetrant for ferrous parts
• Only works on ferromagnetic materials (steel)
• Parts must be demagnetized after test

Ultrasonic (UT)

• Detects internal defects via sound wave reflection
• Can measure thickness, locate voids, cracks
• Works on most materials
• Requires skilled operator
• Widely used for thick sections, welds, forgings

Radiographic (RT / X-ray)

• X-ray image shows internal structure
• Detects porosity, voids, dimensional internal features
• Expensive, safety controls required
• Excellent for weld inspection
• Film or digital detection

CT (computed tomography)

• 3D X-ray scanning of complete part
• Non-destructive dimensional inspection of internal features
• Very expensive, slow
• Used for critical aerospace and medical
• Complements CMM for features CMM cannot reach

04 · Documentation

Inspection documentation.

First Article Inspection (FAI): Comprehensive inspection of first production part against all drawing requirements. Documents initial process capability. Standard formats: AS9102 (aerospace), PPAP (automotive). Typically 10-40 pages for complex parts. Required for aerospace, automotive launch, medical device validation.

In-process inspection: Sampling inspection during production. Control plan specifies: which features to inspect, frequency (every 10 parts, every hour, statistical sampling), method, acceptance criteria. SPC (Statistical Process Control) monitors process capability with Cpk targets.

Final inspection: 100% inspection or sampling of finished parts before shipment. Critical dimensions typically 100%; standard dimensions typically AQL sampling. Certificate of conformance accompanies shipment for qualified customers.

Material certification: Per EN 10204 3.1 — certificate of analysis with material chemistry, mechanical properties, heat number. Required for aerospace, medical, oil & gas. Our standard documentation for quality-critical orders.

Our inspection capability: Zeiss CMM for precision dimensional, profilometer for surface roughness, optical comparator for 2D features, digital calipers and micrometers for standard dimensions. For NDT (penetrant, magnetic particle, ultrasonic), we partner with certified NDT suppliers. For X-ray and CT, partner specialty labs. Full inspection documentation per customer specification.

FAQ

First article inspection — what's involved?

FAI verifies first production part meets all drawing requirements. Typical content: (1) Drawing reference and revision. (2) All dimensions measured against nominal. (3) GD&T features evaluated. (4) Material certifications attached. (5) Traceability to raw material heat. (6) Process records (heat treatment, coating). (7) Inspector signature and date. Output document 10-40 pages typical. Standard customer-specific format (AS9102 for aerospace, customer-specific for automotive).

What to inspect 100% vs sample?

100% inspection: critical dimensions (safety, function, mating), low-volume parts (<100), aerospace/medical/nuclear. Sampling inspection: high-volume production, stable process with proven Cpk, non-critical dimensions. Common sampling: AQL per MIL-STD-105 or ISO 2859, SPC with control charts. Specify 100% only where needed — costs more per part.

CMM programming and setup?

First part measurement: 1-3 hours programming (operator), plus 30min-2hr actual measurement. For 10 parts, setup overhead amortized — per-part inspection cost modest. For 1-piece orders, programming cost dominates — often 60-80% of inspection cost. For repeat production, programming cost invested once pays back over subsequent orders.

NDT — when is it required?

NDT required for: welds on pressure vessels (ASME IX), critical aerospace structural parts, safety-critical components (medical, nuclear). Penetrant or magnetic particle standard on welds. Ultrasonic or X-ray on critical forgings. For standard commercial parts, NDT not required. For fatigue-critical or safety-critical parts, specify per applicable standard.

CMM accuracy limitations?

Typical production CMM: ±0.002 mm repeatability on precision features. Limitations: (1) Workpiece size vs machine volume. (2) Temperature variation affects accuracy. (3) Operator skill on complex GD&T. (4) Probe tip diameter (3mm min typical). (5) Access to internal features. For sub-micron precision, specialty metrology equipment and environmental controls needed — typically external calibration lab.

Documentation retention?

Inspection records retention varies by industry: Aerospace: 10-25+ years (FAR requirement). Medical devices: full product life + 2 years per FDA. Automotive: 15 years per IATF 16949. Oil & gas: 10-25+ years per customer. General commercial: 7 years typical. Our records retained per customer requirements; standard retention 10 years for quality-controlled orders.