Advanced ISO Tolerancing

As the future is undoubtedly in the context of 3D CAD, 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 allows 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 only one direction, bending of parts or adjustments can compensate for certain defects, the part to be manufactured can be painted later....

To reduce production costs by maximizing tolerances, we need to optimize the dimensioning to the extent necessary, even if this means having a slightly heavier handwriting.

To meet these multiple business 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 a large number of limitations that we need to be able to overcome by making additions to meet industrial needs.

The paradox is that these needs have been around for a long time, and 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 shape these different uses in 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...