How accurate are CMMs?

Typical CMM accuracy is within a few microns (1 µm = 1 micron = 0.001 metre), though submicron precision can be achieved with specialist machines. The accuracy depends on the machine and the features being measured, but a CMM’s baseline accuracy is generally determined during calibration and expressed as a Maximum Permissible Error (MPE) over a given distance. For example, an accuracy specification might be given as MPE = ±(2.3 + L/333)µm (with L in mm). In this case, the maximum error over a distance of 1 metre would be 2.3 + 1000/333 = 5.3µm.

The accuracy of a CMM is primarily determined through calibration, typically performed in accordance with ISO 10360 and carried out by a service engineer with traceable certification, such as UKAS in the UK. This quantifies the machine’s specifications and its ability to accurately measure linear, point-to-point distances. To evaluate how reliably a CMM can measure specific features, such as hole positions or surface profiles, a Gauge R&R (Repeatability and Reproducibility) study is recommended.

An ISO 10360 CMM calibration involves verifying the machine’s accuracy using gauges certified to traceable international standards by national organisations, such as UKAS in the UK. The calibration is typically performed by OEM service engineers accredited by these national bodies. The process determines the various Maximum Permissible Errors (MPEs) for the CMM. The checks include measuring various length bar gauges at multiple positions within the working volume to assess how accurately the CMM can measure the distance between any two points. This process ensures the machine meets its specified accuracy requirements.

MPE(E₀) represents the Maximum Permissible Error for length measurements with the CMM’s stylus tip aligned as close as possible to the ram axis (typically the Z axis in bridge systems). This test evaluates the machine’s baseline accuracy without a lateral offset of the stylus tip. It is assessed by measuring 5 different lengths in 7 locations, repeated three times each, to give a total of 105 measurements, all of which must be within the specification.

MPE(E₁₅₀) refers to the Maximum Permissible Error with a stylus tip offset of 150 mm from the ram axis. This test introduces additional factors not assessed in E₀, such as twist in the machine’s axes. If a 150 mm offset is not feasible, an alternative distance (denoted as EL, with L indicating the nominal offset) can be specified, such as MPE(E₂₅₀) for a 250 mm offset as typically used by the Renishaw REVO probing system. It is assessed by measuring 5 different lengths along two 2D diagonals, repeated three times each, to give a total of 30 measurements, all of which must be within the specification.

MPE(P_FTU) represents the Maximum Permissible Error for form measurement of a CMM using a single stylus, tactile probe. The designation “P_FTU” is defined in ISO 10360-5, where each letter specifies aspects of the measurement:
P = Probing error being evaluated
F = Form used for the evaluation (alternatively, L = Location or S = Size)
T = Tactile probe used
U = Unique, single stylus probe (alternatively M = fixed Multi stylus e.g. star, or N = Numerous fixed probes)
This parameter measures form error by calculating the Gaussian least-squares sphere fit from 125 individual probing points on a calibrated sphere, providing a metric for the CMM’s probing accuracy for form features.

MPE(R₀) denotes the Maximum Permissible Error for measurement repeatability. During an ISO 10360 CMM calibration, it is calculated as the range between the maximum and minimum errors for each of the 5 lengths measured at 7 positions in the MPE(E₀) assessment, resulting in 35 ranges.
Unlike other MPE tests, MPL(R₀) does not require calibrated artefacts, as it focuses on measurement consistency rather than absolute error, assessing the stability and repeatability of results.