The STEP Standard - ISO 10303

STEP for Geometric Dimensioning and Tolerancing

Manufacturing needs more than a geometric model to make a part. There are many additional specifications required but most important is a description of the required tolerances because this will drive the selection of the manufacturing process and the manufacturing tools used to manufacture the part.

A model of Geometric Tolerance and Dimension (GD&T) information was added to STEP as during the second edition of AP203, and further refined in AP242. Several iterations have already been made with significant feedback about functionality from the CAD vendors.

The new model is highly integrated with the existing model for geometry because the GD&T data qualifies geometric items already in the geometry model with tolerances and datums that may also be in the geometry model. The necessary data sharing has been achieved by developing a highly intricate data model with many inter-twined data definitions. This makes implementing the GD&T model almost as challenging as implementing the original geometry models that were very object oriented with many inheritance relationships.

The model includes both semantic tolerances (computer interpretable linked data with explicit values and modifiers) and presentation tolerances (drawings in space that can be interpreted by a person). Both can be attached to a part and associated with each other.

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