Advanced ISO Tolerancing

As Lavenir is definitely in the 3D CAD context, specifications must be completely defined in this digital model, without attaching any significance to the graphic representation, such as the presence or absence of a framed dimension, the view in which the specification is drawn, or the direction of a reference line. Specified surfaces and references are perfectly identified in the 3D model. This enables the specification to be fully expressed in a database or other language.

Conventional ISO dimensioning assumes that a mechanism is composed of rigid parts assembled without deformation or adjustment, and that the references are planes, cylinders, spheres, revolutions, prismatics or complex surfaces of sufficient extent to block the corresponding degrees of freedom. Companies' needs are much broader. A plane can be very narrow in width, a left-hand surface can be almost flat, a surface can have a strong influence on a requirement in one direction only, the bending of parts or settings can compensate for certain defects, the part to be manufactured can be painted at a later date... To reduce production costs by maximizing tolerances, dimensioning must be optimized to just what is needed, even if this means having a slightly heavier handwriting.

To meet these multiple needs, the ISO dimensioning standards NF EN ISO 1101 : 2017, and NF EN ISO 5459 : 2011 describe, often very succinctly, new modifiers that offer multiple possibilities. To use them, it is sometimes necessary to define additional geometry in the nominal model (location elements, contact elements, direction elements, etc.).

These advances are still not very detailed in the standards, which leaves a great deal of scope for exploitation. The real difficulty is that some proposals have undoubtedly been published prematurely, with numerous limitations that we need to be able to get round by making additions to meet industrial needs. The paradox is that these needs have existed for a long time, and that each company has already developed its own script with varying degrees of success. To enable companies to work on real, sometimes complex cases, we need to put these different uses into the context of the new standards.

The first aim of this article is to describe the areas of use of the new tools defined by the standards, their limitations and the rules for writing them, and to supplement this with useful extensions that are not described in the standard, but are a priori understandable and currently exploitable....