Coordinate measuring machine (CMM) accuracy testing is the systematic process of verifying that a CMM performs within its specified maximum permissible error (MPE) for length measurement, probing, and scanning tasks. Performed in accordance with internationally recognized standards such as the ISO 10360 series, ASME B89.4, and GB/T 16857, these tests provide traceable evidence of dimensional measurement capability. This article explains the key standards, tested parameters, measurement artifacts, test procedures, and typical applications of CMM accuracy verification and calibration.
Technical Overview
A coordinate measuring machine (CMM) is a dimensional metrology instrument that determines the geometry of physical objects by sensing discrete points on a surface with a probing system and computing positions in a three-dimensional Cartesian coordinate system. CMMs are widely used to verify dimensions, form, orientation, and position tolerances defined by geometric dimensioning and tolerancing (GD&T) frameworks such as ISO 1101 and ASME Y14.5. Because measurement results directly support product conformity decisions, the machine's own accuracy must be quantified, verified, and documented on a routine basis.
CMM accuracy testing addresses several error sources that degrade performance over time, including mechanical wear of guideeways, scale and encoder drift, probing system degradation, structural deformation caused by temperature variation, and misalignment after collisions or relocation. Testing is generally divided into three categories: acceptance testing performed when a new machine is installed, periodic reverification testing (typically annually or semi-annually), and interim checks using calibrated check standards between formal verifications.
The dominant specification standards governing CMM accuracy testing include the ISO 10360 series (particularly ISO 10360-2 for length measurement error and ISO 10360-5 for probing systems), the ASME B89.4 family of standards in the United States, the GB/T 16857 series as the Chinese national adoption of ISO 10360, and VDI/VDE 2617 guidelines. Acceptance criteria are expressed as maximum permissible errors (MPE), commonly written in the form E0,MPE = A + L/K micrometers, where L is the measured length in millimeters, A represents the fixed error component, and L/K the length-proportional component. Supporting documents such as ISO/TS 23165 (test uncertainty) and ISO 14253-1 (decision rules for conformity) govern how results are evaluated against specifications.
Testing Samples
Step Gauges
Step gauges are rigid artifacts containing a series of precision ball or gauge block targets at known incremental distances. They are the preferred reference standard for ISO 10360-2 length measurement tests because they allow bidirectional probing along well-defined axis-parallel and diagonal directions, with calibrations traceable to a national metrology institute.
Gauge Blocks
Calibrated gauge blocks of various nominal lengths are used to construct measurement positions along body diagonals and axis directions. Pairs of gauge blocks wrung to auxiliary supports are probed in bidirectional mode to realize the required test lengths, offering high accuracy and direct traceability to the definition of the meter.
Reference Spheres and Ball Bars
A calibrated reference sphere, typically made of ceramic, steel, or zirconia with a sphericity error well below the expected probing error, is used for probing dispersion and stylus qualification tests. Ball bars and ball plates, consisting of multiple spheres mounted at known separations, provide convenient artifacts for interim checks, multi-stylus verification, and volumetric performance monitoring.
Laser Interferometer Systems
Laser interferometers serve as reference standards for axis displacement verification and legacy diagonal displacement tests. With measurement resolution at the nanometer level and traceability through stabilized laser wavelengths, they are used to evaluate axis scale errors, squareness compensation residuals, and linear positioning repeatability during maintenance and calibration procedures.
Calibrated Workpieces
For task-specific uncertainty evaluation under ISO/TS 15530-3, calibrated workpieces representative of actual parts are measured. These artifacts allow laboratories to assign realistic measurement uncertainties to production measurements rather than relying solely on machine specification data.
Testing Items
CMM accuracy testing quantifies performance through a defined set of characteristic errors, each with its own MPE specification:
Length measurement error (E0 and directional E values): the deviation between indicated and calibrated lengths measured in multiple spatial orientations, the fundamental performance indicator of any CMM.
Probing dispersion and probing form error (PFTU, PForm.MSE): the form error of a reference sphere measured with 25 distributed points, reflecting the probing system's contribution to measurement uncertainty.
Probing size error (R0): the difference between the measured and calibrated radius of the reference sphere, indicating stylus qualification and probing offset accuracy.
Multi-stylus and stylus changing form error (PForm.MLE): form errors evaluated when using multiple styli, star configurations, or stylus change systems, in accordance with ISO 10360-5.
Scanning probing errors (THP and scanning time-related values): probing form and size errors obtained in scanning mode per ISO 10360-4, critical for form evaluation of contours and freeform surfaces.
Rotary table errors (FR, FT, FA): radial, tangential, and axial errors of four-axis configurations measured per ISO 10360-3.
Repeatability and reproducibility: short-term and long-term dispersion of repeated measurements under defined conditions, evaluated for both length and form features.
Optical and multi-sensor system errors: performance parameters for imaging probing systems per ISO 10360-7, optical distance sensors per ISO 10360-8, and combined probing systems per ISO 10360-9.
Testing Methods
ISO 10360-2 Length Measurement Test
The core verification procedure measures calibrated test lengths in seven spatial positions: three directions parallel to each machine axis and four body diagonals. Five different test lengths are selected within the range, and each length is measured three times with bidirectional probing, producing 105 individual results. All indicated errors must remain within the specified E0,MPE, and the procedure is repeated for different stylus configurations where applicable. Testing is conducted with the artifact positioned off-center and at different heights within the measuring volume to expose volumetric error components.
Probing System Verification
Probing tests per ISO 10360-5 require 25 points distributed on a calibrated reference sphere within a defined measurement path sequence. The radial range of the measured points yields the probing form error, while the deviation of the mean measured radius from the calibrated value yields the probing size error. Additional test sequences apply to fixed multi-stylus systems, articulated stylus holders, and stylus changing systems to demonstrate equivalence of measurement results across configurations.
Scanning Mode Tests
For scanning CMMs, ISO 10360-4 defines tests in which predefined scanning paths, including four lines on the equatorial plane and one line on the polar plane of a reference sphere, are executed at a specified scanning speed. The resulting probing form error (THP) and timing are compared against specification. Scanning tests on standard rings or spheres verify the machine's ability to capture dense point clouds with controlled dynamic behavior.
Environmental Control and Uncertainty Evaluation
All accuracy tests are performed at a reference temperature of 20 °C per ISO 1, with tight limits on temperature stability and gradients (typically ±0.5 °C per hour and ±1.0 °C per day, depending on machine accuracy class). Temperature sensors monitor the machine structure and artifact, and thermal compensation is documented. The test uncertainty ratio is evaluated according to ISO/TS 23165, and conformity decisions follow the decision rules of ISO 14253-1, with guard bands applied where required.
Verification Intervals and Interim Checks
Accredited third-party laboratories and metrology service providers typically recommend full reverification every 12 months, with more frequent interim checks using ball bars or check standards after collisions, relocations, software updates, or major repairs. Measurement management systems aligned with ISO/IEC 17025 and ISO 9001 require documented calibration schedules and traceability records for all reference artifacts.
Applications
CMM accuracy testing underpins measurement reliability across a broad range of industries and regulatory contexts:
Automotive manufacturing: verification of powertrain components, body-in-white fixtures, and stampings under IATF 16949 quality management requirements.
Aerospace: validation of turbine blades, structural castings, and composite assemblies where tight tolerances and full traceability are mandated.
Medical devices: dimensional verification of implants, surgical instruments, and orthopedic components in regulated production environments.
Precision machining and tooling: qualification of dies, molds, and high-accuracy machined parts against GD&T drawings.
Electronics and semiconductors: measurement of connectors, lead frames, and miniaturized components with sub-micrometer requirements.
Calibration and testing laboratories: ISO/IEC 17025-accredited facilities offering CMM verification services and uncertainty-evaluated measurements.
Supplier audits and capability studies: demonstration of gauge capability through measurement systems analysis (MSA) and Gage R&R studies.
FAQ
What does E0,MPE mean? E0,MPE is the maximum permissible error of bidirectional length measurement in any orientation within the measuring volume. It is the headline accuracy specification of a CMM and is expressed as a formula such as ±(1.9 + L/300) µm, where L is the length in millimeters.
How often should a CMM undergo accuracy testing? A full ISO 10360 reverification is typically performed every 12 months, although high-usage machines, strict contractual requirements, or unstable environments may justify semi-annual testing. Interim checks with calibrated artifacts are recommended monthly or after any collision, repair, or relocation.
What is the difference between an acceptance test and a reverification test? An acceptance test is conducted when a machine is newly installed or upgraded, using test lengths covering at least 66% of each axis and the longest diagonal, jointly agreed between supplier and user. A reverification test confirms ongoing compliance during use and may cover a reduced set of positions agreed between the parties.
Is accuracy testing the same as calibration? Verification determines whether the machine meets its specifications and results in a pass or fail statement. Calibration additionally quantifies the relationship between indicated and true values and provides measured deviations with associated uncertainties, which can be used to correct or compensate systematic errors.
Which standards apply to non-cartesian CMMs? Articulated arm CMMs are tested under ISO 10360-12, laser trackers under ISO 10360-10, and imaging and optical distance sensor systems under ISO 10360-7 and ISO 10360-8 respectively, with corresponding ASME B89.4 documents applicable in North America.
What happens if a CMM fails the accuracy test? If any measured error exceeds the MPE after accounting for test uncertainty and decision rules, the machine requires corrective action such as geometric error compensation, scale recalibration, probing service, or structural repair, followed by complete retesting before returning to production use.
