The tolerancing process for precise mechanical systems in a context of short or long run in industrial production requires a rational method from the specification of the functional requirements until the final products are checked. Nowadays the optimization of the tolerances is generally carried out empirically. Compromises must be made between the fiinctional requirements in the design process and the manufacturing step. The limits of accuracy imposed by the process must be taken into account. The need for a rational method is particularly necessary for new products in the field of traditional mechanisms as well as of micromechanics or even of micro-systems. In the design process, in the case of an assembly, the ftinctional requirements must be defined in geometrical terms, in order to satisfy the customer requirements. Then, these geometrical requirements must be translated into specifications on the various parts so that on the one hand the assembly can be carried out under well defined conditions and on the other hand, after assembly, the functional requirements are strictly respected. This transfer of specifications with creations of new specifications of the assembly requires a three-dimensional geometrical analysis taking into account the geometrical deviations-form defects, position and orientation-and size deviations. Clearances in the assemblies also will intervene. They are necessary to ensure the mechanical motions but also to compensate for variations of geometry in the case of hyper constrained assemblies. In the phase of industrialisation, the geometrical and dimensional tolerances will be necessary for the choice of machines, for manufacturing planning process and for the measurement processes during the production but also for the final quality control of the parts and of the system. The methods of assemblies are also strongly conditioned by the functional requirements.

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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