Repeatability describes how closely a CNC axis returns to the same measured position during repeated moves under defined test conditions. It is about the spread of the repeated results, not merely whether one move landed on the commanded coordinate.
Positioning accuracy and repeatability answer different questions. Accuracy compares achieved position with commanded position; repeatability describes consistency across repeated approaches. A machine can repeat a consistent offset and still require calibration or compensation.
A meaningful test documents the measurement system, axis positions, direction of approach, warm-up and environmental conditions, compensation state and statistical treatment. ISO 230-2 provides methods for determining accuracy and repeatability of positioning for numerically controlled axes.
A brochure value describes a model specification under stated conditions. It does not establish the present condition of a particular used machine.
ISO 230-2:2014 — Determination of accuracy and repeatability of positioning of numerically controlled axes. ISO lists the edition as current following its 2025 confirmation. See the Machine Blue Book methodology for inspection and evidence boundaries.
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