Size tolerancing revisited: A basic notion and its evolution in standards

Abstract

Size is a fundamental descriptor of objects—it allows us to quantify “how big” objects are and to compare and classify objects based on this notion. In the world of International Organization for Standardization Geometrical Product Specification and Verification, size is defined much more narrowly: it is restricted to features of size, and the methods of inducing size values from an actual workpiece are strictly controlled. The release of ISO 14405-1:2010 has introduced a rich new set of size specification modifiers, which includes two-point and spherical local sizes, least squares, maximum inscribed and minimum circumscribed associations, as well as calculated diameters (inferred from the circumference, area, or volume of the feature of interest). Further modifiers allow the specification of statistics of local size measurements, such as maximum, minimum, range, average, and others. This article will present “size” as a fundamental engineering notion from several viewpoints and trace its evolution in engineering drawings. It will then discuss the implications of the use of the recently standardized size modifiers in engineering design and investigate the issues that may arise in the application and interpretation of these extensions to size.