Why CMM inspection matters for precision parts
Calipers and micrometers measure single dimensions in isolation. A coordinate measuring machine (CMM) maps the entire part geometry in three-dimensional space, capturing the relationships between features that actually determine whether an assembly works. A bore that measures 10.000 mm on a bore gauge can still fail if its position relative to the datum scheme is 0.05 mm off.
A bridge CMM uses a tactile probe with a ruby stylus to collect discrete points on part surfaces. The machine references these points against a calibrated coordinate system established by the part datums. Modern CMMs like the Zeiss Contura or Hexagon Global achieve measurement uncertainty down to ±0.001 mm in a temperature-controlled environment. That level of accuracy requires the inspection room held at 20 ±1 °C, because aluminum grows roughly 0.023 mm per meter per degree Celsius.
What a first article inspection (FAI) actually covers
A first article inspection report documents that the production process can make parts to drawing. It is performed on the first article (or the first several articles) produced using the intended production tooling, fixtures, program, and material. The FAI is not a random sample check; it is a complete dimensional validation of every characteristic on the drawing.
- Ballooned drawing: every dimension and GD&T callout numbered for traceability
- Dimensional results: actual measured values for every numbered characteristic, not just pass/fail
- Material certification: mill test report (MTR) showing alloy, heat number, and chemical composition
- Process records: machine used, operator, program revision, fixture ID, inspection date
- Non-conformance handling: any out-of-tolerance characteristics documented with corrective action
Understanding measurement uncertainty
Every measurement carries uncertainty. A CMM reporting 10.002 mm is not saying the part is exactly 10.002 mm; it is saying the true dimension lies within a confidence interval around that reading. The expanded measurement uncertainty (typically k=2, 95% confidence) for a Zeiss CMM in a controlled lab is around ±0.001 to ±0.003 mm depending on feature size and probe configuration.
This matters because a dimension at 10.005 mm with a ±0.005 mm tolerance should not be accepted on a CMM with ±0.003 mm uncertainty if the reading is right at the limit. The measurement uncertainty ratio (tolerance divided by measurement uncertainty) should be at least 4:1 for critical dimensions per ISO 14253-1. A good shop will tell you their uncertainty budget; a shop that cannot explain measurement uncertainty is guessing.
The difference between CMM reports and production inspection
CMM inspection on a first article is a slow, meticulous process in a temperature-controlled room. It is not how production parts are checked long-term. For ongoing production, shops use a mix of approaches tailored to feature criticality.
| Inspection Method | Best For | Typical Uncertainty | When to Use |
|---|---|---|---|
| CMM (bridge type) | Complex 3D geometry, GD&T position/profile | ±0.001–0.003 mm | First article, PPAP, critical feature validation |
| Optical comparator | 2D profiles, radii, angles, thread forms | ±0.005 mm | High-volume production checks, visual profiles |
| Air gage | Bores, outer diameters on turned parts | ±0.001 mm | High-volume tight-tolerance diameters |
| Hand tools (micrometer/caliper) | Quick checks, non-critical dimensions | ±0.01–0.02 mm | In-process spot checks, low-criticality features |
| Surface roughness tester | Ra/Rz surface finish verification | ±5% of reading | Sealing surfaces, bearing journals, cosmetic finishes |
How to review an FAI report in five minutes
You do not need to check every line on a 200-balloon report. Focus on the critical characteristics: true position callouts with datums, profile tolerances on functional surfaces, tight-tolerance bores, and any dimension related to fit or seal. Cross-reference three to five of those values against the CMM raw data if available. If the measurements show actuals clustered near the nominal with adequate margin from the limits, the process is centered. If actuals are piled against one tolerance limit, the process is drifting.
TruPart provides complimentary FAI reports on all production orders, with actual measured values and CMM traceability. That includes material certifications and process documentation at no additional charge. Some shops charge 200-500 USD for an FAI; others bury the cost in the piece price. Either way, an FAI without measurement data is not an FAI.
When to request additional inspection beyond FAI
FAI validates the initial setup. For ongoing production, consider requesting in-process inspection reports, statistical process control (SPC) data on critical dimensions, or periodic layout inspections. For high-volume automotive work, PPAP submission includes all of this plus capability studies showing Cpk ≥ 1.33 on significant characteristics. Medical device work may require 100% inspection on critical dimensions with documented traceability to each part serial number.
Frequently Asked Questions
What is the difference between a CMM report and an FAI report?
A CMM report shows dimensional data measured on a coordinate measuring machine for a specific set of features. An FAI report is a comprehensive document that covers every drawing characteristic (measured by whatever method is appropriate) and includes material certs, process records, and ballooned drawing reference. CMM data is often the primary input to an FAI, but they are not the same thing.
How many parts are measured in a first article inspection?
Traditionally one part from the first production run, but AS9102 (aerospace) recommends measuring 3-5 pieces to account for within-run variation. The key requirement is that parts are made with production tooling, production settings, and production material — not hand-tweaked prototypes.
Do I need an FAI for prototype parts?
Not typically. FAI is a production validation step. For prototypes, a CMM inspection report on critical dimensions is usually sufficient. CMM reports on prototype parts at no charge are standard when critical tolerances are specified.
What temperature is required for accurate CMM inspection?
ISO 1 standards specify 20 °C as the reference temperature for dimensional metrology. CMM inspection labs should be held at 20 ±1 °C for precision work. Parts must soak at this temperature long enough to reach thermal equilibrium before measurement, especially aluminum parts which have a high coefficient of thermal expansion.
Can a shop provide FAI without a CMM?
For simple parts with only 2D dimensions and no GD&T position/profile callouts, hand tools and an optical comparator can technically produce an FAI. However, any part with true position, profile of a surface, or complex 3D geometry requires CMM inspection to measure those characteristics properly.