PMI Support

Handling Product and Manufacturing Information (PMI) in STEP is one of the most significant implementation challenges.

1. Types of PMI

Graphical PMI (Representation)

• "Visual lines and characters for human consumption."
• It looks correct in CAD, but it does not contain "tolerance values" as data (it's simply a collection of POLYLINEs).

Semantic PMI (Data)

• "Attribute data that a computer can understand."
• Downstream software (CAM, inspection, etc.) can directly read the tolerance types and values. This is the core of AP242.

2. PMI Support by AP

AP	Graphical	Semantic	Primary Entities
AP203	❌	❌	-
AP214	✅	❌	DRAUGHTING_CALLOUT
AP242	✅	✅	GEOMETRIC_TOLERANCE, SHAPE_ASPECT

3. Implementation Structure (PMI Linkage)

This diagram shows how PMI "Location/Appearance (Presentation)" and "Meaning (Semantic)" are linked together.

Important Links

• SHAPE_ASPECT_RELATIONSHIP: Used when defining a tolerance between two faces (e.g., perpendicularity between Datum A and Datum B).
• REPRESENTATION_ITEM: Stores the actual text data or polylines.

4. Implementation Hurdles

The most common reason why "PMI disappeared even though I used AP242" is either insufficient implementation on the receiving CAD side or the sender only outputting "Graphical" PMI.

• CAx-IF Recommendation: To exchange semantic information, you must strictly follow the CAx-IF Recommended Practices for PMI and correctly configure the SHAPE_ASPECT structure.

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📚 Next Steps

• CAD Support Matrix - Check PMI support status across different CAD systems.

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