The introduction of the latest (1997) version of ISO 230-2-the ISO Standard for Repeatability and Accuracy of Positioning of Machine Tools-has changed the way we calculate Repeatability. 'Standard Uncertainty' has replaced 'Standard Deviation', and the band width has been reduced This new concept of Repeatability is examined from both a theoretical and a practical standpoint, and evidence is presented of the way results vary and what sorts of distributions of Repeatability occur in practice. The scope of accuracy measurements and the time scale are then expanded, since positioning is not the only factor controlling a machine's overall accuracy: thermal problems, in particular, are discussed Problems of maintaining: Repeatability in the longer term are discussed with particular reference to installing the machine in a customer's premises.
