GD&T: Introduction & Scope

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Introduction

This document describes the recommended practices for implementing dimensions, dimensional and geometric tolerances. This document incorporates the content of “Recommended Practices for Dimensions and Dimensional Tolerances� written by Markus Hauser, Mike Strub and Tom Hendrix, dated April 18, 2000 and “Recommended Practices Guide for Geometric Tolerances� written by David Briggs and Tom Hendrix dated March 14, 2003. These two documents were combined to ensure a consistent approach to dimensioning and tolerancing. This document conforms to the agreements reached by the Tolerance Harmonization team representing AP203, AP214, AP224, AP238 and AP240.

Tolerances treat the uncertainty with which the realized shape or measurements of a real manufactured object compare to their design ideals. If all parts could be manufactured perfectly as designed, there would be no need for tolerancing practices. However, it is certain that this cannot be done for finite cost in any but the most trivial cases. In the drawing world, tolerances are noted on the drawing per standard notations such as ANSI Y14.5 or ISO 1101.

This document is not intended as a primer on geometric tolerancing. The explanations included are only provided to relate common tolerancing techniques to the STEP entity structures. This is not a comprehensive coverage of any existing draughting standard but does provide a capability to exchange a variety of typical models. Future versions of this document will address additional capabilities, in particular the STEP Structures required to convey the Presentation of Geometric and Dimensional Tolerances, as well as the application of Tolerances at the Assembly level.

Scope

There are two main methods of tolerancing:

• dimensional tolerancing and
• geometric tolerancing.

Dimensional tolerancing is the less complex of the two methods of applying tolerances. It is also called “direct tolerancing of dimensions� because a tolerance can be specified only where a dimension is defined. Direct dimensioning and tolerancing address the acceptable range of values of an individual dimension of a manufactured object. Direct tolerancing amounts to generalizing the single value of a dimension to be a range.

Geometric tolerances are the more complex of these two types. A geometric tolerance specifies a geometric region, such as an area or a volume, in which the realized feature must lie in order to meet the design criteria. Geometric tolerancing separates the specification of tolerance from the dimensioning, thus allowing more flexibility and allowing more precise controls that relate more directly to the form, fit and function of the part. This document covers the recommended usage and implementation of geometric tolerances defined in Application Integrated Construct AIC 519, Application interpreted construct: Geometric tolerances.

This document covers the application of tolerances to boundary representation solid models. The application of tolerances to wireframe or other geometric models is not covered here.