Errors of shape, position, orientation and beat. Part 1

Errors in the shape and relative position of the geometric elements that make up a mechanical part are due to imperfections in the production tools that generate them, to cutting forces, to stresses induced by gripping devices, to overheating, etc.

The examination of defects on a real surface belonging to a mechanical part, allows :

• check the suitability of the part for the function(s) it is to perform within the mechanism in which it is installed;

• identify weaknesses in its development process.

Today's measuring equipment, combined with electronic and computerized measurement chains, enable real shapes to be checked against very precise, near-perfect reference shapes, and even mathematical definitions.

Thanks to the growing availability of sophisticated measuring equipment, such as coordinate measuring machines with integrated mathematical software, it is now possible to quickly determine errors in the shape, orientation and position of mechanical components by digitizing them.

The standards governing the tolerancing of defects in shape, orientation and position (NF E 04-552, 553, 554/ISO 1101) give a purely geometric definition of the problem, ignoring the inspection process. However, the GPS concept (see article "Spécification géométrique des produits" by J. H. Marchèse), which is at the heart of the geometric product specification activity, brings together the functional product specification approach (carried out by the design office) and the product qualification approach (metrology laboratory).

The rigorous use of geometric specification standards enables the design office to produce a clear and coherent product definition during the design phase.

This definition can then be unambiguously interpreted by those involved in the product realization and qualification phases.

The present file presents errors in the shape and relative position of elements in accordance with standard NF E 04-552. This standard allows form and relative position tolerance to be expressed by defining a geometrically defined zone of space, within which the tolerated element must lie. The tolerance zone defines the acceptable limits of imperfections on real surfaces, compatible with strictly functional conditions.

This text reproduces certain elements of the dossier Erreurs de forme et de position, written by Louis-Paul GAZAL and Roger RECORDIER.