GD&T Basics - Geometric Dimensioning and Tolerancing Concepts

Key Concept: Dimensioning and Tolerancing are different concepts for GD&T.

1 - Dimensioning: How to dimension a part design

Dimensioning controls part design.

Dimensioning is represented by standard dimensions to "features" around the part.

2 - Tolerancing: How to create a tolerance for a part

However, dimensions in GD&T do not control tolerance values. Tolerances are set separately by referring to geometric characteristics called "features."  A feature tolerance can be set as a form ("Is the shape good?"), orientation ("How parallel?"), position ("Where is it in space?"), or runout (a combination of features referencing multiple tolerance types simultaneously).

For example, a common geometric characteristic or feature is position.  

This is the position symbol.

The position feature lets designers set the tolerance for deviation from the true position of a feature in the part.  The tolerance for a position is called a "position tolerance."

Key Concept: FEATURES are Toleranced in GD&T - not Dimensions

A key concept is that GD&T always tolerances to features, rather than tolerancing the dimensions.

Key Concept: What is a FEATURE?

From page 4 of ASME Y14.5.1-2019: "Per ASME Y14.5-2009 feature: a physical portion of a part, such as a surface, pin hole, or slot, or its representation on drawings, models, or digital data files."

Key Concept: FEATURE CONTROL FRAME 

A feature control frame is the ASME-approved rectangular frame with symbols that control the tolerancing for geometric features.  The ISO term is "Tolerance Indicator."  In the USA, the most common term is "Feature Control Frame."

This is a feature control frame.