The basics of ISO Tolerancing with functional approach

Today, ISO dimensioning standards are recognized and indispensable for establishing contractual definition drawings between companies, especially in the context of international trade. The general format for writing these dimensioning specifications using a tolerance frame has not changed, but the new modifiers and the new criteria for associating references and specified surfaces bring a real revolution to the dimensioning approach, now allowing the expression of assembly behavior and functional requirements in the right way to maximize tolerances and reduce inspection costs.

Since the 2000s, designers, manufacturers and metrologists have learned to handle specifications with symbols defined in standards valid at the time of writing, including NF EN ISO 1101 : 2017, NF EN ISO1660 : 2017, NF EN ISO 2692 : 2015 and NF EN ISO 5459 : 2011 and NF EN ISO 5458 : 1999.

Since 2011, standards have added a huge number of modifiers and changed the meaning of some widely-used specifications. Draft standards DIS 5459.2 : 2017, WD 2692 : 2018 and DIS 5458 : 2017 impose further changes. As a specification must be read according to the standards in force at the time the drawing is created, there will inevitably be problems, especially during the transitional phase of installing these new standards in companies.

This summary document is very incomplete and cannot replace the thousands of pages contained in the thirty or so standards dedicated to quotation. Only the documents published by ISO provide the official definitions needed to settle any disputes.

The standards are very complicated, still incomplete, sometimes unnecessarily restrictive and riddled with errors and inconsistencies, due in particular to the time lag between publications. Despite these shortcomings, the ISO dimensioning language is a formidable tool that should be widely used.

To explain this language in simple terms, this article presents the fundamental principles and the latest developments, focusing on the most useful and robust scripts in the face of recent or foreseeable changes in meaning.

Definitions are given in the context of the 3D CAD digital chain, based on functional requirements, which makes it possible to clearly identify the areas of use of specifications and modifiers. In particular, standard definitions consider a continuous set of points on all surfaces. In practice, however, only a limited number of points are used for identification. It is therefore imperative to adapt the definitions so that they can be used in real life.

This article rigorously respects the standardized meaning of the symbols, even if the expression is sometimes significantly different or simplified...