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

Errors in the shape and relative position of the geometric features that make up a mechanical part are caused, during their manufacture, by imperfections in the production tools used to create them, cutting forces, stresses induced by gripping devices, heat generation, etc.

Examining defects on the actual surface of a mechanical part makes it possible to:

• to verify that this part is capable of performing the function(s) it is intended to perform within the mechanism in which it is installed;

• to identify weaknesses in its development process.

Current measuring methods, combined with electronic and computer-based measurement systems, make it possible to compare actual shapes against highly precise, nearly perfect reference shapes—and even against mathematical definitions.

Thanks to the growing use of sophisticated measuring equipment, such as coordinate measuring machines with built-in 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 establishing tolerances for defects in shape, orientation, and position (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 lies at the heart of the field of geometric product specification, bridges the gap between the functional product specification process (carried out by the design office) and the product qualification process (metrology laboratory).

The strict application of geometric specification standards enables the engineering department to develop a clear and consistent product definition during the design phase.

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

This document presents the shape and relative position tolerances of components in accordance with standard NF E 04-552. This standard allows shape and relative position tolerances to be expressed by defining a geometrically defined region of space within which the component must lie. The tolerance zone defines the acceptable limits of imperfections in actual surfaces, consistent with strictly functional requirements.

This text draws on certain elements from the report *Errors of Form and Position*, written by Louis-Paul Gazal and Roger Recordier.