Minimal STEP Analysis

A guide to deciphering a real STEP file (.stp). Here, we break down the minimal configuration required to define a "single part." You can download the actual file here: minimal_part.step

1. Header Section

ISO-10303-21;
HEADER;
FILE_DESCRIPTION(('STEP AP242 ed2'),'21;1');
FILE_NAME('minimal_part.stp','2023-10-27T10:00:00',('Author Name'),('Company'),'Processor Name','CAD Version','');
FILE_SCHEMA(('AP242_MANAGED_MODEL_BASED_3D_ENGINEERING_MIM_LF { 1 0 10303 442 1 1 4 }'));
ENDSEC;

• FILE_SCHEMA: Defines which AP (standard version) this file complies with.

2. Data Section: Admin Data

DATA;
#10 = PRODUCT('Part_A','Part_A','Part_A description',(#20));
#20 = PRODUCT_CONTEXT('',#30,'design');
#30 = APPLICATION_CONTEXT('managed model based 3d engineering');
#40 = PRODUCT_DEFINITION_FORMATION('1','first version',#10);
#50 = PRODUCT_DEFINITION('design','',#40,#60);
#60 = PRODUCT_DEFINITION_CONTEXT('part definition',#30,'design');

• #10: The "Part" itself.
• #50: Defines the "Design State (Definition)" of the part. Geometry data is attached here.

3. Data Section: Links to Shape (Shape Header)

#70 = PRODUCT_DEFINITION_SHAPE('','',#50);
#80 = SHAPE_DEFINITION_REPRESENTATION(#70,#90);
#90 = SHAPE_REPRESENTATION('',(#100),#110);

• #70: The "junction point" between management data and geometry data.
• #90: Acts like a "catalog" for the geometry.

4. Data Section: Geometry

#100 = ADVANCED_FACE('',(#120),#130,.T.);
#110 = ( GEOMETRIC_REPRESENTATION_CONTEXT(3) 
         GLOBAL_UNCERTAINTY_ASSIGNMENT((#140))
         GLOBAL_UNIT_ASSIGNMENT((#150,#160,#170))
         REPRESENTATION_CONTEXT('Context #1','3D Context') );

• #100: A Face. From here, it is further detailed into Surfaces, Edges, and Vertices.
• #110: An important context defining units and precision (Uncertainty).

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Implementation Hint

When reading a file, first find the PRODUCT_DEFINITION (#50). From there, follow the path PRODUCT_DEFINITION_SHAPE (#70) -> SHAPE_REPRESENTATION (#90) to identify the "main geometry" of that part.