Interest of the Inertial Tolerancing Method in the Case of Watch Making Micro Assembly

International Federation for Information Processing Digital Library; Micro-Assembly Technologies and Applications; 03/2008; DOI:10.1007/978-0-387-77405-3_18 In book: Micro-Assembly Technologies and Applications, Publisher: Springer US, pp.189-197
Source: OAI

ABSTRACT A mechanical part to tolerance is traditionally expressed as a [Min Max] interval which allows the definition of the conformity
of the characteristic. Inertial tolerancing offers a new point of view of the conformity based on the mean square deviation
to the target. This article demonstrates the efficiency of inertial tolerancing and proposes a comparison with the traditional
tolerancing method in the case of watch making micro assembly.

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    ABSTRACT: Traditionally, tolerances are defined by an interval [LSL; USL] which can lead to several ambiguous interpretations of conformity. This paper examines an alternative method for setting specifications: “inertial tolerancing”. Inertial tolerancing consists of tolerancing the mean square deviation from the target rather than the distance. This alternative has numerous advantages over the traditional approach, particularly in the case of product assembly, mixed batches and conformity analysis. Coupled with a capability index Cpi, this alternative method leads to minimizing production costs for a specified level of quality. We propose to compare both approaches: traditional and inertial tolerancing.
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    ABSTRACT: Production yield is usually defined as a function of the variability and tolerance interval of processes, and off-centering (bias) is often neglected. When working in precision assembly, where tolerance intervals are specified in the range of some micrometers, off-centering effects can no longer be neglected. Indeed, the smaller the interval, the more sensitive the yield becomes, inertial tolerancing is a methodology derived from Taguchi loss function, to specify part dimensions and assemblies so that off-centerings are taken into account. We discuss different inertia allocation methods proposed by Pillet and propose a new method where inertia are simply allocated according to possible process performances. Concerning production indicators, due to this methodology, we suggest to abandon traditional capability indices and use sigmadelta-tolerancing with square error. We benefit from sigmadelta-graphs to monitor processes and to help to visually quantify the lack of production and assembly processes. Finally, we describe an entire design procedure, that leads from product functional requirements to production specification, and apply it to an industrial case study. A key element that we provide in this procedure is a novel formulation of assembly yield in the presence of off-centering. The procedure is not "single-shot" but rather iterative, starting from both opposite ends : customer needs and break-even production costs
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