The Importance Of Being Repeatable

Abstract

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. 1 The New Version of ISO 230-2 The publication in 1997 of the new version of the ISO standard ISO 230-2* for Repeatability and Accuracy has forced us to look again at what we mean by Repeatability. The old standard of 1988 had much in common with most of the other existing standards at that time: BS3800/2 (originally BS4656/16), VDI/DGQ 3441 and the old NMTBA procedure from around 1970. All these embraced the concept of Repeatability being measured as a six standard deviation spread on an assumed Normal distribution curve. Putting it very simply, this meant that we believed that nature had carefully arranged things so that we could, by measuring a few results, compute the overall Repeatability that would be expected (with 99.7% confidence) from a much larger set of data. Transactions on Engineering Sciences vol 23, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533