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 is being added to STEP as part of an upgrade to AP-203 called Edition 2. Several iterations have already been made with significant feedback about functionality from the CAD vendors. The initial GD&T models were developed for several different AP's with different scopes so each was slightly different. A new harmonized model for all the known GD&T requirements was produced in September 2004.

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.

Consequently, there are questions as to whether or not the new model can be implemented so early implementation projects are being formed to show that such implementation is feasible and valuable. The history of the original AP-203 implementation project is relevant here. It can be predicted that if the first experiments are successful then an industry segment that really needs the new efficiency will work with the vendors to pilot the new capability to a level where it can be used for complex products, and then implementation forums will take it to a level where all users including small job shops can take advantage of the new capability.

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