Errors in form, position, orientation, and stroke. Part 1

Errors in the shape and relative position of the geometric elements that make up a mechanical part are caused during their manufacture by: imperfections in the production tools that generate them, cutting forces, stresses induced by gripping devices, heating, etc.

Examining defects on the actual surface of a mechanical part allows you to:

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

• to identify weaknesses in its development process.

Current measuring methods, combined with electronic and computerized measuring chains, make it possible to check actual shapes against highly accurate, virtually perfect reference shapes and even mathematical definitions.

Thanks to the growing availability of sophisticated measuring devices, such as three-dimensional measuring machines with integrated mathematical software, it is possible to quickly determine, through digitization, errors in the shape, orientation, and position of the components of a mechanical part.

The standards setting tolerances for shape, orientation, and position defects (NF E 04-552, 553, 554/ISO 1101) provide a purely geometric definition of the problem, disregarding the inspection process. However, the GPS concept (see article "Geometric specification of products" by J. H. Marchèse), which is at the heart of the geometric specification of products, brings together the functional specification of the product (carried out by the design office) and the qualification of the product (metrology laboratory).

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

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

This document presents errors in shape and relative position of elements in accordance with standard NF E 04-552. This standard allows the tolerance of shape and relative position to be expressed by defining a geometrically defined area within which the toleranced element must be located. The tolerance zone defines the acceptable limits of imperfections in actual surfaces, compatible with strictly functional conditions.

This text incorporates certain elements from the dossier Erreurs de forme et de position (Errors of Form and Position), written by Louis-Paul GAZAL and Roger RECORDIER.