The Changing Face of Model Annotation

3D GD&T provides higher-quality parts and greater productivity, but speed bumps slow its full acceptance into the future.

Today, industry stands at the crossroads of the old world of 2D drawings, the world of annotated 3D models, and the future where 3D engineering data can be consumed and understood directly by software. Today, the two international geometric and tolerancing (GD&T) standards that cover annotation largely define engineering specifications in terms of how they are graphically presented on a 2D drawing or annotated 3D model. This “presentation data” is only useful for human consumption. For computer systems to consume data without human intervention, the data must be explicitly modeled in a manner that captures its underlying meaning. This underlying meaning is called “representation (semantic) data.”

Fortunately, several groups are working to develop semantic models of GD&T and other annotation in STEP and other standards that will help move industry away from the world of 2D drawings to that of 3D model-based product definition (MBD). MBD uses product definition data (datasets) to provide complete specifications for components and assemblies. Both ISO and the Long Term Archival and Retrieval of product definition data (LOTAR) group are currently working on new standards.

No doubt, modern 3D CAD software lets engineers model, visualize, and test accurate representations of complex geometries much more quickly than in the past. But CAD geometry alone cannot define acceptable as-produced parts and assemblies.

At best, a CAD model depicts a theoretically perfect part or assembly. Mechanical components work in the physical world, and perfect parts have never been made. So, allowable variation (tolerances) must be part of the product definition.

However, because a CAD model represents a theoretically perfect entity, simply applying plus and minus tolerances to it in a general note or tolerance block proves insufficient. A CAD model provided as all or part of the design deliverable makes it legally binding, similar to an engineering drawing. The only way to clearly and unambiguously define the acceptable geometry of as-produced parts — through the use of GD&T.

Moving into the future, that there are two international GD&T standards poses problems. ASME Y14.5-2009 is defined by the American Society of Mechanical Engineers (ASME). ISO 1101 is defined by the International Standards Organization (ISO). At first glance, the two standards look almost identical, but there are significant differences. This makes it difficult for multinational corporations whose staff must understand both standards.