CNC Machining Quality Control Explained: CMM, FAI, SPC, and Inspection Reports (2026)
CNC Machining Quality Control Explained: CMM, FAI, SPC & Inspection Reports (2026)
Why QC matters: A $150 CMM inspection report can prevent a $5,000 batch rejection. Understanding the four levels of CNC quality control — from basic dimensional checks to full statistical process control — helps you specify the right level for your project without overpaying.
4 Levels of CNC Inspection: What You Get at Each Level
| Level | Method | Accuracy | Cost per Order | Best For |
|---|---|---|---|---|
| 1. Basic | Caliper, micrometer, pin gauges | ±0.01–0.02mm | $0 (included) | Non-critical parts, internal components |
| 2. Dimensional Report | CMM or vision system, key dimensions | ±0.002–0.005mm | $30–80 | Functional parts with 5–15 critical dims |
| 3. FAI (First Article) | Full CMM, all dimensions, AS9102 format | ±0.001–0.003mm | $100–250 | Aerospace, medical, first production batch |
| 4. SPC + Cpk | Statistical sampling, control charts | Process-controlled | $200–500+ | High-volume production, automotive |
Inspection Methods Comparison
| Method | Accuracy | Speed | Cost | Best For |
|---|---|---|---|---|
| Caliper / Micrometer | ±0.01mm | Fast | $ | Quick checks, OD/ID, lengths |
| CMM (Touch Probe) | ±0.001–0.003mm | Medium | $$ | Complex geometry, GD&T, FAI reports |
| Vision System | ±0.002–0.005mm | Fast | $$ | 2D profiles, small features, high volume |
| Surface Roughness Tester | Ra 0.01–10μm | Slow | $ | Surface finish verification |
| Hardness Tester | ±1 HRC | Fast | $ | Heat treat verification |
What’s in Each Report Type
| Report | Contents | When to Request |
|---|---|---|
| Dimensional Inspection | Measured vs nominal for key dimensions, pass/fail per tolerance | Every order — should be standard |
| FAI (AS9102) | Every dimension on drawing, material certs, process certs, ballooned drawing | First production run, new supplier, design change |
| Material Certificate | Mill test report, chemical composition, mechanical properties, heat number | Aerospace, medical, structural parts |
| CMM Report | Full 3D point cloud comparison to CAD model, color deviation map | Complex contours, tight GD&T, legacy part reverse engineering |
| SPC Report (Cpk) | Control charts, Cpk values (target ≥1.33), process capability histogram | Production runs over 200 pcs, automotive PPAP |
How to Specify QC in Your RFQ
Add these three lines to your RFQ to get the right inspection without overpaying:
- “Standard dimensional inspection report included with every shipment” — ensures basic QC at no extra cost
- “FAI per AS9102 required on first article, CMM report on 5 critical dimensions” — for production qualification
- “Material certificates required for [specific material]” — for traceability requirements
More Quality & Precision Guides
- CNC Machining Tolerances Explained: IT Grades, GD&T Basics, and What ±0.01mm Really Means (2026)
- CNC Machining Surface Finish Guide: Ra Values, Methods, and Cost Impact Compared (2026)
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Industry Quality Control Examples
Quality control priorities differ by application. These related guides show how part function changes inspection, finishing, packing, and document needs.
- Semiconductor Equipment CNC Parts – Precision plates, fixtures, clean surfaces, PEEK parts, inspection documents, and supplier review.
- Battery and Energy Equipment CNC Parts – Cooling plates, insulating spacers, fixtures, mixed metal and plastic RFQs, and no-burr areas.
- Marine Hardware CNC Machining – Stainless fittings, brackets, bushings, aluminum housings, corrosion risk, and export packing.
- Optical and Camera CNC Components – Lens mounts, camera housings, adapter rings, black anodizing, burr control, and alignment datums.