Overview of STEP AP 242 functionalities for CAD 3D Geometry, assembly  and PMI interoperability

The objective of this page is to provide an overview, from the users perspective, of the STEP AP 242 functionalities for 3D CAD interoperability. It is summed up according to the following paragraphs:
            - Overview and illustration of capabilities
            - Examples of associated use cases
            - Dependencies / related standards
            - Status of availability of COTS solutions
            - Status of recommended practices / implementers forums

Overview and illustration of capabilities

o        Geometry

  • Exact shape representation, which is use in the current CAD tool implementing in STEP AP214 and STEP AP203
  • Tessellated shape representation. This representation is new in STEP. It allows display efficiently the 3D shape by using a simplified representation (example: triangulated faces). The STEP tessellated representation allows to have topological information (example: face is a collection of triangle)
  • Parametric representation: Representation allowing  the preservation of the parameter and the construction history 

o        PMI

§         Graphic presentation
The advantages of graphical Presentation are that since it is rendered, it is presented in the STEP file exactly as it was in the CAD system. Since the instantiation is done using basic geometric entities, basically every viewer or CAD system which finds the link in the file structure can display it.
The PMI is stored in STEP using geometric entities:
- polylines (points, curves,…) , already use in STEP AP214
- or tessellated , new in AP242 

§         Semantic representation
The PMI is stored in the file with the PMI semantic entities. It captures the semantic of the Geometric Dimension and Tolerance. After exchange, it allows the interpretation and the modification like in the native CAD system (ISO and ASME PMIs).

o        Assembly

External element reference: this mechanism allows a reference to a specific geometric element thought an assembly in several files. Example: constraint between 2 faces of different models.

Example of associated use cases

-          Interoperability between different systems

CAD-CAECAD 3D design geometry to/from Engineering Analysis


CAD 3D geometry with PMI and assemblies exchange with the supply chain

CAD-CAMCAD 3D with PMI "semantic" exchange for down stream reuse


 -          Long Term Archiving and Retrieval (LTA&R):

o        LTA&R of CAD 3D  PMI "graphic" for certification
LTA&R of CAD 3D with PMI "semantic" design for manufacturing and repairs

-          Visualization of archived CAD 3D tessellated geometry

Note: the list is not exhaustive. For more information, refer to the CAX IF use cases and the STEP AP 242 use cases described by the respective industry associations.

Dependancies / related standards

STEP AP 242 CAD 3D geometry data model is based on the convergence of STEP AP 203 and STEP AP 214, and is also shared with STEP AP 209, AP 210, AP 238 standards (list not exhaustive) 

Status of availability of COTS solutions

For more details, reference to the CAx IF web site, page “implementation coverage”: http://www.cax-if.org/vendor_info.php 

Related recommended practices

STEP recommended practices for CAD 3D geometry: http://www.cax-if.org/joint_testing_info.html#recpracs

Associated CAx Implementer Forum: http://www.cax-if.org/index.html