Articulated-arm measuring machines

Choose the arm length, contact probe or scanner, mounting and alignment together. FARO’s Quantum X supports contact and optical acquisition. FARO, Quantum X brochure, revised October 10, 2024

For the 2.0 m Quantum X.M, FARO reports ±0.027 mm EUNI for 7-axis contact measurements and 0.033 mm LDIA with the FAROBlu Max xR. The xR also lists 15 μm point spacing. EUNI and LDIA describe different tests, and point spacing describes sampling.

Choose a configuration for the features and tolerances being inspected. Portability and access are useful, but they do not make all arm lengths, probes, scanners, or mounting arrangements metrologically interchangeable.

Contact probing and scanning

ISO 10360-12:2016 covers acceptance and reverification tests for articulated-arm CMMs using tactile probes and optionally optical distance sensors, including laser line scanners. Its public abstract identifies calibrated test lengths and gives scanner-accessory testing in Annex E. The standard was published in October 2016 and its catalog records confirmation in 2021. ISO 10360-12:2016

Match acquisition to the measurement task. For a practical inspection plan, distinguish discrete datum/feature points from dense surface coverage, and specify the probe or scanner used for each. Define access, occluded areas, point distribution, and how measured data become fitted features or a comparison surface. More points do not automatically correct an unsuitable measurement strategy.

FARO's Quantum Max accuracy documentation illustrates distinct tests: single-point articulation repeatability uses a hard probe in a reference hole, while volumetric unilateral length performance uses a hard probe and traceable length artifact distributed through the working volume. A repeated-point result is therefore different from performance across the measurement volume. FARO, Accuracy

Read performance terms precisely

A bounded example comes from the October 10, 2024 Quantum X brochure, for a 2.0 m Quantum X.M:

The brochure defines non-contact LDIA as a sphere-location diameter error based on ISO 10360-8 Annex D; it is not the same characteristic as contact EUNI. It separately lists minimum point spacing, including 15 μm for xR. These values must not be collapsed into one generic “accuracy” or treated as a tolerance guarantee. FARO specification table and footnotes

For equipment selection, obtain the complete specification and certificate for the exact model, arm size, sensor, and accessories. A headline number cannot supply missing test conditions or establish comparability with another manufacturer's differently named characteristic.

Alignment and the reported coordinate frame

Coordinates need a defined relationship to the part. FARO's CAM2 guidance requires measured and nominal information for datum features used in datum alignments. Its multiple-coordinate-system guidance warns that displayed feature values depend on the active coordinate system and alignment. FARO, Applying Datums to Features, updated February 20, 2025, Creating Multiple Coordinate Systems

For a usable inspection procedure, identify the drawing revision, datum features, alignment method, nominal geometry, and reported coordinate frame. Distinguish an alignment intended to realize drawing datums from a best-fit surface comparison. Record registration or device repositioning when used. Check that reported results use the intended frame before interpreting a color map or feature position.

Illustrative consequence: if an otherwise unchanged measured point is reported after translating the origin by 1 mm along X, its X coordinate changes by 1 mm in the opposite direction. The point has not physically moved. This simple coordinate transformation shows why a small numerical deviation or visually attractive comparison alone cannot validate the chosen alignment.

Environment, mounting, and technique

FARO's accuracy guidance identifies mounting instability, surrounding vibration, rapid temperature changes, and poor point-collection technique as contributors to degraded results. It specifically cautions that an arm mounted on a granite table or cart should measure objects mounted to that same support. Treat the relative stability of arm and part as part of the measurement arrangement. FARO, Understanding Accuracy with the FaroArm and ScanArm

Thermal compensation is not permission to ignore temperature history. FARO's November 2, 2021 overview describes monitoring arm temperature and its rate of change, with stabilization indication, and directs users to their model's guide. Follow the applicable model procedure instead of applying another arm's warm-up or temperature limits. FARO, Internal Temperature Compensation Monitoring

For a local plan, consider workpiece temperature, restraint, surface condition, probe qualification, access, sampling, and operator method. Confirm the configuration after probe changes or repositioning according to the equipment procedure. An operating-environment rating and the conditions supporting a performance claim are different information.

Task-specific uncertainty and acceptance

NIST explains that the actual CMM measurand and conditions often differ from those used in calibration. Calibration information must therefore be combined with measurement-specific factors to form a task-specific uncertainty statement. A calibration certificate establishes valuable performance information; it does not supply uncertainty for every hole, plane, or profile measured afterward. Steven Phillips, NIST, Traceability, Calibration, & Measurement Uncertainty Issues, June 1, 2000

NPL's Good Practice Guide 130, September 2013, discusses task-specific uncertainty methods, including calibrated-workpiece comparison and uncertainty-evaluating software, and the role of uncertainty in decisions. Its general CMM guidance must be applied with a method appropriate to the articulated arm and task. David Flack, NPL GPG 130

Define the measurand, acquisition and fitting strategy, uncertainty approach, and acceptance rule before making conformity claims. Retain the actual instrument configuration, relevant verification results, environment, alignment, and software settings with the inspection result. Neither point-cloud density nor repeatability alone resolves the risk of accepting a part close to a specification limit.