Title: On the choice of a capability index with an application in carbon fibre manufacturing

Authors: Daniel Grau

Addresses: Laboratory of Applied Mathematics, CNRS UMR 5142, IUT de Bayonne, Universite de Pau et des Pays de l'Adour, 17 Place Paul Bert, 64100 Bayonne, France

Abstract: Process capability indices Cp, Cpk, Cpm and Cpmk have been proposed to the manufacture industry to provide measures of process performance. The family C″p(u, v) depending on two parameters allows to generalise the previous indices for any position of the target in the tolerance interval. Grau (2010) studied the links between the value of the C″p(u, v) index and the maximum deviation of the process mean, as well as the maximum percentage of non-conforming items manufactured under the assumption of normality. From these results, we provide tables which allow the practitioner to choose a pair (u, v), so that the resulting index C″p(u, v) will meet his objectives best. We develop a decision making rule based on the natural estimator which can be used to test whether the process is capable or not. We then present a real study made on a carbon fibre manufacturing process to illustrate how this reasoning can be applied.

Keywords: quality technology; process capability indices; process performance; process centring; departure ratio; non-conforming items; symmetric tolerances; asymmetric tolerances; carbon fibre manufacturing; decision making rules; natural estimators.

DOI: 10.1504/IJQET.2010.035584

International Journal of Quality Engineering and Technology, 2010 Vol.1 No.4, pp.373 - 394

Available online: 30 Sep 2010 *

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