CMM Inspection
Also: coordinate measuring machine, CMM, CMM measurement, dimensional inspection, first article inspection, FAI, FAIR, AS9102, GD&T inspection, PC-DMIS, Calypso, coordinate metrology, layout inspection
CMM inspection probes a part on a coordinate measuring machine to verify dimensions and GD&T, and documents it in first article (AS9102) and PPAP reports.
127 shops tagged CMM inspection among 344 inspection and testing companies in the Noramark directory
A coordinate measuring machine moves a probe in three axes over a granite table and records the X, Y and Z position of each point it touches. Most job-shop CMMs are bridge machines with air bearings and glass scales; gantry machines measure car bodies and large structures, horizontal-arm machines measure sheet-metal assemblies, and portable arms and laser trackers go to the part. A touch-trigger probe records single points and suits holes, planes and positions. A scanning probe stays in contact and collects thousands of points along a path, which form characteristics such as flatness, cylindricity and profile need. Laser-line and structured-light sensors capture whole surfaces for comparison with CAD, and CT scanning measures internal features no probe can reach.
The software (PC-DMIS, Zeiss Calypso, Mitutoyo MCOSMOS, PolyWorks and others) runs a program built from the CAD model: it aligns the part to its datum reference frame as the drawing defines it, measures each feature, and evaluates size, position, profile, runout and form per ASME Y14.5 or ISO GPS. How it fits the points matters: least-squares, minimum-zone and maximum-inscribed fits give different answers from the same data, and too few points under-report form error. A good report states the fitting method, the point count and the alignment.
The paperwork is often what the customer is buying. A first article inspection (FAI) measures every characteristic on the drawing on the first parts from a new process, revision or source, and records each against a ballooned drawing. AS9102 is the aerospace format: Form 1 for part number accountability, Form 2 for materials, special processes and functional tests, and Form 3 for every characteristic with its measured result. Automotive customers ask for dimensional results in a PPAP. Production inspection samples the critical characteristics at a planned frequency, often feeding SPC.
Every measurement carries uncertainty. CMM accuracy is stated per ISO 10360 as a length-measurement error that grows with length, and the rule of thumb is that measurement uncertainty should be no more than a tenth, and never more than a quarter, of the tolerance. Temperature is the biggest error on the shop floor: ASME Y14.5 dimensions apply at 20 °C (68 °F), and a 300 mm aluminum part 5 °C warm measures about 35 µm (0.0014") long. Good labs hold 20 ± 1 °C and let parts soak before measuring.
At a glance
| Typical tolerances | A good bridge CMM in a lab held at 20 ± 1 °C has a stated length-measurement error of about 2 µm plus 2-4 µm per metre of measured length (ISO 10360-2), which verifies tolerances down to about ±0.01 mm (±0.0004"). Shop-floor CMMs are somewhat looser; portable arms run about ±0.025-0.05 mm (±0.001-0.002"), and laser and structured-light scanners about 0.01-0.05 mm (0.0004-0.002"). Tighter tolerances need high-accuracy CMMs, tighter temperature control or dedicated gauges. |
|---|---|
| Size limits | Job-shop and contract-lab bridge CMMs commonly measure envelopes from about 500 x 400 x 400 mm (20 x 16 x 16") up to about 1,000 x 2,000 x 1,000 mm (40 x 80 x 40"), with table loads from several hundred kilograms to over a tonne. Gantry CMMs measure several metres. Portable arms reach 1.2-4.5 m (4-15 ft) in diameter and can be leapfrogged along a large part; laser trackers cover tens of metres. At the small end, stylus balls down to about 0.3 mm (0.012") reach small features; micro-features go to vision or optical systems. |
| Surface finish | A CMM does not measure surface roughness; a stylus profilometer does, reporting Ra, Rz and other parameters per ASME B46.1. CMM probing leaves no mark on metal parts. Soft plastics and polished optical or cosmetic surfaces may call for low-force probes or non-contact sensors. Rough cast, blasted or coated surfaces add scatter to the measurement, so datums on castings are usually machined or defined by datum targets. |
| Lead time | Routine production inspection the same day. A new program takes a few hours to a day; a full first article with an AS9102 package typically takes 2-5 business days after the parts reach a contract lab, less when the CAD model carries PMI. Rush service within 24 hours is common for simple parts. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Any rigid part: metal, plastic, ceramic or composite, machined, cast, molded, printed or formed. Soft or thin plastic parts deflect under probe force and need low-force probes or non-contact sensors. Sheet metal and other non-rigid parts must be held in a fixture that applies the restraint the drawing specifies, or they are measured in their free state. Parts fresh from a machine, a washer or an oven must cool to lab temperature first.
What drives the cost
- Number of characteristics: every ballooned dimension and GD&T callout is a line in the report
- Programming and fixturing for a new part, higher without a CAD model or with a complex datum scheme
- GD&T complexity: profile, position with material condition modifiers and composite callouts take more points and analysis
- Report format: AS9102 forms, PPAP dimensional results, customer templates, colour maps
- Sampling plan: 100% inspection versus a first article and periodic samples
- Accuracy demands: temperature-controlled labs, high-accuracy machines, ISO/IEC 17025 accredited reports
- Part size: large parts need a gantry CMM, portable arm or laser tracker, often on site
When to use it
- First article inspection on new parts, new revisions, new processes or a new supplier (AS9102 or customer format)
- Verifying GD&T: position, profile, runout and datum relationships that hand tools cannot evaluate
- Complex prismatic and contoured parts: housings, manifolds, castings, 5-axis parts, molds
- PPAP dimensional results and capability studies on critical characteristics
- Comparing a part with its CAD model as a colour map, or reverse engineering a part without a drawing
- Settling a dimensional dispute between supplier and customer
When not to
- Simple dimensions a caliper, micrometer, height gauge or go/no-go gauge checks faster on the shop floor
- Surface roughness, which needs a profilometer, and thread function, which needs thread gauges
- 100% inspection at high volume: use fixture gauges, air gauges or vision systems
- Internal features the probe cannot reach: use CT scanning or sectioning
- Finding cracks, porosity or internal flaws: that is nondestructive testing
Design tips
- Dimension critical features with GD&T to a clear datum reference frame per ASME Y14.5; an ambiguous datum scheme gives ambiguous results.
- Make datum features large enough to probe with several points, and reachable when the part is fixtured.
- Send a STEP or native model with PMI and a ballooned drawing; they cut programming time and remove interpretation.
- Put inspection requirements on the PO: FAI per AS9102, sample size, report format, and whether raw data or colour maps are needed.
- Add restraint notes for non-rigid parts: where, and with what force, to clamp for measurement.
- Tolerance only what the function needs; every tight GD&T callout adds measurement time and uncertainty.
- Ask for the measurement uncertainty on characteristics toleranced tighter than about ±0.025 mm (±0.001").
Frequently asked questions
- What is CMM inspection?
- Dimensional inspection on a coordinate measuring machine: a probe touches or scans the part, the software fits the points to features, and the result is compared with the drawing, including GD&T. The output is a report showing each characteristic, its measured value and pass or fail.
- What is a first article inspection report?
- A record that every characteristic on the drawing was measured on a part from the production process and meets the requirement. It is done on the first parts from a new process, revision or supplier. In aerospace it follows AS9102 and also accounts for materials and special processes.
- What is AS9102?
- The aerospace standard for first article inspection. It defines three forms: Form 1 for part number accountability, Form 2 for materials, special processes and functional testing, and Form 3 for each drawing characteristic and its measured result against a ballooned drawing.
- How accurate is a CMM?
- A good bridge CMM in a temperature-controlled lab measures within a few microns over a few hundred millimetres, per its ISO 10360 rating. Shop-floor CMMs are looser, and portable arms run about ±0.025-0.05 mm. Part temperature, fixturing and the fitting method often matter more than the machine.
- How much does CMM inspection cost?
- Contract metrology labs in the U.S. commonly bill about $100-200 per hour, with programming charged once. A simple part report can be a few hundred dollars; a full AS9102 first article on a complex part with hundreds of characteristics can run into the thousands.
