Draughting model and MDGPR

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This page belongs to the Recommended Practices for the Presentation of GD&T.

The concept of a shape_representation is widely known by STEP developers and implementers. Many STEP implementations already exist that support subtypes of shape_representation that contain locally defined constraints defining the type of geometry the shape_representation can represent (e.g., advanced_brep_shape_representation, manifold_surface_shape_representation).

AP202 and AP214 introduce another representation concept, known as a draughting_model. Conceptually, a draughting_model is a representation that should contain styled annotations which are not part of the 3D model. Best way to relate it to geometrical model is to reuse the same coordinate space as the 3D geometrical shape is using (in the case of Figure 1 - as advanced_brep_shape_representation). So 3 shape_representations are reusing the same context: draughting_model, advanced_brep_shape_representation) and mechanical_design_geometric_presentation_representation. In addition, draughting_model and mechanical_design_geometric_presentation_representation are related by representation_relationship. Mechanical_design_geometric_presentation_representation is styling only elements of 3D model. In the case of Figure 1 example - it is styling manifold_solid_brep (via styled_item) from advanced_brep_shape_representation.

Figure 1. Relationship between draughing_model, MDGPR and A-BREP

#10159=MANIFOLD_SOLID_BREP(,#10158);
#10167=(GEOMETRIC_REPRESENTATION_CONTEXT(3)GLOBAL_UNCERTAINTY_ASSIGNED_CONTEXT((#10166))
 GLOBAL_UNIT_ASSIGNED_CONTEXT((#4,#5,#3))REPRESENTATION_CONTEXT('MASTER','3D'));
#10168=ADVANCED_BREP_SHAPE_REPRESENTATION(,(#10159),#10167);
#10200=MECHANICAL_DESIGN_GEOMETRIC_PRESENTATION_REPRESENTATION(,(#10201),#10167);
#10201=STYLED_ITEM(,(#10202),#10159); 
#10300=DRAUGHTING_MODEL('Leader curve callout',(#200, #10311),#10167);
#10301=REPRESENTATION_RELATIONSHIP(, $, #10300, #10200);

Complete Part21 example can be found here callouts.stp.

All items of a draughting_model are mapped_item entity data types, styled_item entity data types, camera_model entity data types or an axis2_placement. According to newly introduced changes in EXPRESS schemas (in particular AIC-520), draughting_callout entity data type is also allowed as an item in the draughting_model. This is important change as previously only lower level items (like various subtypes of styled_item) could be directly placed in draughting_model. Draughting_callout is a kind of aggregation of curves, text, etc. So placing it draughting_model is semantically richer than placing styled curves, text, etc. as separate items. For more information about callouts please refer to Callouts section. In the example provided in Figure 2 draughting_model consists of 2 elements:

• leader_directed_callout (subtype of draughting_callout), collecting annotation elements like leader curve, text, etc.
• camera_model_d3_with_hlhsr (subtype of camera_model). This item is placed in draughting_model just in order to get 3D context.

Complete Part21 example can be found here callouts.stp.

Figure 2. The content of draughting model

#200=LEADER_DIRECTED_CALLOUT('callout',(#205,#173,#400));
#10300=DRAUGHTING_MODEL('Leader curve callout',(#200, #10311),#10167);
#10310=CAMERA_USAGE(#10311,#10300);
#10311=CAMERA_MODEL_D3_WITH_HLHSR(,#10312,#10320,.T.);

Complete Part21 example can be found here callouts.stp.

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