STEP Standard FAQ (Frequently Asked Questions)

Answers to common questions from STEP beginners and implementers, covering 30+ items.

📑 Table of Contents

Basic Concepts
File Operations
AP Selection
Persistence and Simulation (IDs/Names)
Implementation
Troubleshooting
Geometry and Topology
T-Splines
Tools and Resources

Basic Concepts

Q1: What is STEP?

A: STEP (Standard for the Exchange of Product model data) is an international standard (ISO 10303) for exchanging 3D CAD data between different CAD systems. It can store not only geometry but also colors, assemblies, and PMI (Product and Manufacturing Information/tolerances).

Q2: What is the difference between STEP and STL?

	STEP	STL
Representation	B-rep (Mathematically precise surfaces)	Triangular mesh (approximate)
Precision	Theoretically infinite	Approximate (mesh size dependent)
Colors/Assemblies	Supported	Not Supported
PMI	Supported (AP242)	Not Supported
File Size	Larger	Smaller
Primary Use	CAD data exchange	3D printing

Guideline: Use STEP for precise data exchange between CAD systems; use STL for 3D printing or visualization.

Q3: What is the difference between STEP and IGES/OBJ?

IGES (Initial Graphics Exchange Specification):

Predecessor to STEP.
Supports 2D/3D.
Now deprecated (replaced by STEP).

OBJ:

Simple 3D mesh format.
Supports colors/textures.
For CG and visualization (not suitable for CAD data exchange).

→ For modern CAD data exchange, STEP is the standard choice.

Q4: What is the difference between Part 21 and Part 28?

Part 21 (ISO 10303-21):

ASCII format (.stp, .step).
Most common.
Human-readable.

Part 28 (ISO 10303-28):

XML format.
High affinity with XML technologies.
Redundant and large.

In Practice: 99% of files use Part 21. Part 28 is rarely used.

Q5: What is B-rep?

A: Boundary Representation. A method for defining 3D shapes by their boundaries: faces, edges, and vertices. It is the primary geometry representation in STEP.

Hierarchy:

Solid
  └─ Shell
      └─ Face
          └─ Edge
              └─ Vertex

Pros: Mathematically precise, parametrically editable.
Cons: Complex, high computational cost.

File Operations

Q6: Is it okay to open it in a text editor?

A: Yes. Since STEP files are plain text, you can open them with Notepad or VS Code.

Considerations:

Large files (tens of MBs+) take time to open.
Be careful when editing (syntax errors can corrupt the file).
Use a STEP viewer to view the actual geometry.

Recommended uses: Header inspection, entity searching, debugging.

Q7: Why are the file sizes so large?

A: STEP tends to be large due to:

Text Format: More redundant than binary.
Complete Geometry Description: Mathematically precise surfaces and curves.
Management Data: Metadata for products, assemblies, PMI, etc.

Mitigation:

Use tessellation (polygonal approximation) (AP242).
Compression (.zip or .step.gz).
Remove unnecessary information.

Q8: Is there a binary format?

A: Officially, no. STEP is fundamentally a text format.

However:

Some CAD vendors offer proprietary binary compression versions.
Not standard, leading to compatibility issues.

In Practice: Zip compression is the standard mitigation.

Q9: What is the encoding?

A: The STEP Part 21 specification is based on ISO 8859-1 (Latin-1), but in practice:

It is recommended to stay within the ASCII range.
Non-ASCII characters are represented using \X2\...\X0\ escape sequences (Unicode).

Implementation Note:

Reading as UTF-8 may cause issues.
Handling of escape sequences is necessary.

Q10: What is a typical file size?

Geometry Complexity	Estimated File Size
Simple Part (cube, etc.)	A few KB to hundreds of KB
Typical Part	1MB to 10MB
Complex Assembly	10MB to 100MB
Ultra-Large Assembly	100MB to several GB

Handling Large Files:

Use streaming parsers.
Use partial loading.
Split assemblies.

AP Selection

Q11: Which AP should I use?

A: It depends on your use case:

Need PMI (Tolerances) → AP242 is mandatory.
Aerospace/Defense/MBD → AP242 is recommended.
Automotive/Need Colors & Layers → AP214 (though migration to AP242 is ongoing).
Simple Geometry only/Legacy Systems → AP203 (though AP214+ is more common now).

If in doubt: AP242 ed2 is a safe choice.

Detail: Which AP should I use?

Q12: Is there compatibility between APs?

Backward Compatibility: Generally yes.

AP242 → AP214: PMI and other data will be lost.
AP214 → AP203: Colors and layers will be lost.

Forward Compatibility: None.

Reading AP203 data as AP214 is fine (it just provides more features).

In Practice: Use the latest version supported by both the sender and receiver.

Q13: What is the difference between AP242 ed2 and ed3?

AP242 ed3 (ISO 10303-242:2022) Key Additions:

Enhanced Electrical Harness support.
Additive Manufacturing (3D printing) support.
MBSE (Model Based Systems Engineering) integration.

Compatibility: High compatibility with ed2.

Adoption: CAD support for ed3 is increasing, but Electrical and AM features are still being adopted.

Persistence and Simulation (IDs/Names)

Q14: Can I assign persistent IDs or names to faces?

A: Yes, but not using the #123 instance IDs.

In STEP, “persistence” is achieved by using semantic constructs (AP242 is the best option), but the exact linkage to faces/edges and what downstream tools can consume depends on the CAD exporter and the receiver.

AP242: Best available support (semantic labeling + CAx-IF recommended practices). In practice, exporters may use SHAPE_ASPECT and/or other identification/attribute mechanisms.
AP214: Vendor-dependent (some exporters preserve names/attributes, but it is not standardized for robust CAE automation).
AP203: Generally not supported for user-defined face/edge labels.

Important: Even with AP242, the actual implementation depends on your CAD software's export capabilities.

Detailed guide: Persistent IDs and Face Naming

Q15: Does Ansys Workbench read face names?

A: Yes, it can import them as Named Selections, but the exact method varies by Ansys version.

Ensure the STEP file is exported as AP242 (best compatibility).
In Ansys Geometry import settings (DesignModeler or SpaceClaim), enable "Import Named Selections" or similar option.
In many workflows, Ansys creates Named Selections via Named Selection Processing + Attribute Processing, driven by a key such as Layer or Color (i.e., it is an attribute-to-named-selection mapping, not a guaranteed “read any SHAPE_ASPECT name” behavior).
If names don't appear, verify: (1) AP242 export, (2) Face names were exported, (3) Import settings are correct.

Reference (Ansys Help, STEP import preferences):

https://ansyshelp.ansys.com/public/Views/Secured/corp/v252/en/ref_cad/cadSTEP.html

Q16: Why do my face names disappear in simulation?

A: Common reasons:

Topology Change: If you modify the geometry significantly, the CAD system might "lose" the link between the name and the new face.
AP Version: Exporting as AP203 will strip all name labels.
Export Settings: Many CAD tools have "Export names" disabled by default.

Implementation

Q17: What is the recommended parser library?

Language	Library	Characteristics
C++	OpenCascade	Commercial-grade, feature-rich
	STEP Tools SDK	Commercial, high performance
Python	pythonOCC	Wrapper for OpenCascade
	ifcopenshell	Targeted for IFC but a useful reference
JS/TS	Custom Implementation	Part 21 parsers are relatively simple
C#	Xbim	Targeted for IFC but applicable

Recommendation: Use OpenCascade for full-scale C++ implementation; use a custom parser for lightweight requirements.

Q18: How do I traverse entities?

A: Basic pattern:

python

# 1. Find the PRODUCT
product = find_entity_by_type(step_file, 'PRODUCT')

# 2. Traverse references
product_def_formation = traverse(product, 'PRODUCT_DEFINITION_FORMATION')
product_def = traverse(product_def_formation, 'PRODUCT_DEFINITION')

# 3. Access geometry data
shape = traverse(product_def, 'PRODUCT_DEFINITION_SHAPE')
shape_rep = traverse(shape, 'SHAPE_REPRESENTATION')

# 4. Get geometry items
faces = filter_items(shape_rep.items, 'ADVANCED_FACE')

Detail: Data Model Map

Q19: How are units handled?

A: Defined in the GEOMETRIC_REPRESENTATION_CONTEXT within the STEP file:

step

#500 = ( LENGTH_UNIT() NAMED_UNIT(*) SI_UNIT(.MILLI.,.METRE.) );

→ .MILLI.,.METRE. = mm

Prefixes:

.MILLI. = 10^-3
.CENTI. = 10^-2
.KILO. = 10^3
None = 1

Implementation: Pre-calculate and cache unit conversion factors.

Detail: Common Pitfalls - Units

Q20: How is precision handled?

A: Defined by UNCERTAINTY_MEASURE_WITH_UNIT:

step

#600 = UNCERTAINTY_MEASURE_WITH_UNIT(1.0E-6,(#500),'distance_accuracy_value','...');

→ Precision is 10^-6 (unit depends on #500; if #500 is mm, then 10^-6 mm = 1 μm).

Implementation Note:

Use as the tolerance for geometric operations.
Differences in precision between sender and receiver can lead to "gaps" in geometry.
Recommended values: mm-based systems use 1.0E-3 (0.001 mm = 1 μm), m-based systems use 1.0E-6 (1 μm).

Troubleshooting

Q21: How do I check if a file is corrupted?

Quick Check:

Do HEADER and DATA sections exist?
Does it start with ISO-10303-21;?
Does it end with END-ISO-10303-21;?
Can all #number references be resolved?

Tools:

STEP Validators: STEP Tools, CAx-IF tools.
FreeCAD: Check if it opens without errors.

Common Errors:

Reference inconsistency (missing #number).
Syntax errors (mismatched parentheses, etc.).
Encoding issues.

Q22: Why are colors disappearing?

A: Common causes:

AP Limitation: AP203 does not support color; AP214 or later is required.
Style Definition Issue: STYLED_ITEM is not directly linked to the geometry.
Color Definition Layer: Different CAD systems behave differently depending on whether the color is attached to a Shell or a Face.

Mitigation:

Use AP214 or AP242.
Verify the link: STYLED_ITEM → PRESENTATION_STYLE_ASSIGNMENT → SURFACE_STYLE_RENDERING → COLOUR_RGB.

Q23: Why is the assembly structure breaking?

Missing NAUO (NEXT_ASSEMBLY_USAGE_OCCURRENCE).
Placement Matrix Inconsistency: Issues with CONTEXT_DEPENDENT_SHAPE_REPRESENTATION.
Broken Parent-Child Links: Reference errors between PRODUCT_DEFINITIONs.

Debugging:

List all PRODUCT entities.
Display parent-child relationships via NAUO as a tree.
Verify validity of placement matrices (e.g., checking if they are not identity matrices by mistake).

Detail: Common Pitfalls - Assembly

Q24: Why is PMI missing or unreadable?

AP Incompatibility: Semantic PMI is exclusive to AP242.
Dangling PMI: GEOMETRIC_TOLERANCE is not linked to the target face (SHAPE_ASPECT).
CAD Implementation Differences: Each CAD system interprets PMI differently.

Mitigation:

Use AP242.
Follow CAx-IF Recommended Practices.
Validate with test cases.

Geometry and Topology

Q25: How do I check if a model is "water-tight" (closed)?

A: A model is "water-tight" (a solid) if it uses MANIFOLD_SOLID_BREP and its CLOSED_SHELL is topologically sound.

Every Edge must be shared by exactly two Faces.
There are no "naked edges" (edges with only one face).
The orientation of faces must consistently point "out" from the volume.

Q26: How are fillets and complex blends represented?

A: It depends on the complexity of the fillet and the CAD system's export settings:

Elementary Surfaces: Simple, constant-radius fillets are often represented as CYLINDRICAL_SURFACE (for straight edges) or TOROIDAL_SURFACE (for circular edges).
NURBS: Complex blends, variable-radius fillets, or corner transitions are represented as NURBS patches (B_SPLINE_SURFACE).
Standardization: Many modern exporters convert all complex transitions to NURBS to ensure maximum compatibility, even if they could technically be represented as elementary surfaces.

Q27: Does STEP support CSG (primitives like Blocks and Spheres)?

A: Yes. STEP supports CSG (Constructive Solid Geometry) via entities like BLOCK, CYLINDER, SPHERE, and TORUS.

However, most modern CAD systems export everything as B-rep (faces/edges) for maximum compatibility.
If you need CSG, check your CAD system's export settings for "Keep primitives".

Q28: How do I handle doughnuts or "inverted" shapes?

A: These use TOROIDAL_SURFACE.

A Doughnut is a torus where the major radius is larger than the minor radius.
Inverted/Spindle Torus: If the minor radius is larger, the shape self-intersects.
The orientation (normal) of the face determines which side is "solid".

Q29: What is G2 continuity and does STEP support it?

A: G2 (Curvature) Continuity means that at the junction of two surfaces, not only the tangent but also the curvature is identical.

STEP supports this by explicitly declaring continuity in B_SPLINE_CURVE or B_SPLINE_SURFACE entities.
Maintaining G2 is critical for high-end styling (Class-A surfaces) in automotive design.

T-Splines

Q30: Does STEP support T-Splines?

A: No, not natively. The STEP standard (ISO 10303) is built on NURBS and B-Splines.

When you export a T-Spline model to STEP, the CAD software converts the T-Spline mesh into multiple standard NURBS patches (B_SPLINE_SURFACE).
This conversion ensures that the file can be opened in any CAD system, but you lose the "T-junction" editability of the original T-Spline model.

Tools and Resources

Q31: Are there free STEP viewers?

Tool	Platforms	Features
FreeCAD	Windows/Mac/Linux	Open-source, editable
3D-Tool Free Viewer	Windows	Lightweight, fast
OpenCascade CAD Assistant	Windows/Mac/Linux	Based on OpenCascade

Online:

Various web services exist, but quality is limited.

Q32: Where can I get a validator?

Free:

CAx-IF Recommended Practice checkers: https://www.cax-if.org/

Commercial:

STEP Tools: High-performance validator (Paid).

Quick Check:

Open in FreeCAD and check for warnings/errors.

Q33: Where is the official documentation?

Official Standards (Paid):

ISO 10303 series: Purchase from ISO or national standards organizations.

Free Resources:

CAx-IF: https://www.cax-if.org/ (Recommended Practices)
STEP modularization: https://www.stepmod.org/ (Schema reference)
WikiSTEP: http://www.wikistep.org/

Q34: What is CAx-IF?

A: CAD-CAx Implementor Forum: An international group that establishes STEP implementation guidelines between CAD vendors.

What they provide:

Recommended Practices.
Test Cases.
Benchmark Files.
Interoperability validation results.

Importance for Implementers:

Following CAx-IF guidelines significantly improves interoperability.
It is the only "de-facto standard" for correct implementation.

Website: https://www.cax-if.org/

Q35: What is LOTAR?

A: Long Term Archiving and Retrieval: A standard for long-term preservation and retrieval of digital STEP data.

Purpose:

Ensure data remains readable for decades (30-50 years).
Essential for aerospace and defense industries with long product lifecycles.

Relationship with AP242: AP242 supports LOTAR.

Q36: What is MBD?

A: Model Based Definition: A practice where the 3D model is the only "source of truth" (Master), containing all design and manufacturing information.

Difference from Traditional Methods:

Traditional: 2D drawings are the master; 3D is a reference.
MBD: 3D model is the master; 2D drawings are unnecessary or subordinate.

Relationship with STEP: Semantic PMI in AP242 enables MBD.

Q37: Should I build my own parser or use a library?

Recommended for Custom Implementation:

Only a basic Part 21 parser is needed (relatively simple).
Lightweight and high performance are required.
License constraints.

Recommended for Libraries:

B-rep geometry operations are needed (NURBS processing, etc.).
Commercial product (quality assurance is critical).
Reducing development time.

Hybrid:

Build a custom Part 21 parser and use OpenCascade for geometry processing.

Q38: How do I generate STEP files programmatically?

Method 1: Generate as Text

python

with open('output.step', 'w') as f:
    f.write("ISO-10303-21;\n")
    f.write("HEADER;\n")
    # ... Write HEADER
    f.write("DATA;\n")
    f.write(f"#10 = PRODUCT('{name}','{name}','',());\n")
    # ... Write DATA
    f.write("END-ISO-10303-21;\n")

Method 2: Use a Library

OpenCascade (C++)
pythonOCC (Python)

Key Considerations:

Correctly resolve entity dependencies.
Ensure instance ID uniqueness.
Set units and precision correctly.

Q39: What are the best sample files for learning?

CAx-IF Test Cases: https://www.cax-if.org/
- Verified standard samples.
STEP modularization: https://www.stepmod.org/
- EXPRESS schemas and simple examples.
DIY:
- Create a simple cube in FreeCAD → Export as STEP.
- Inspect the contents in a text editor.
This Repository:
- Minimal STEP Analysis
- STEP File Walkthrough

Have more questions?

If your questions are still not answered:

Check terms in the Glossary.
Learn the basics in Getting Started.
Understand entity structure in the Data Model Map.
Check implementation warnings in Common Pitfalls.

📚 Next Steps

Which AP should I use? - Select the best AP for your project.

Back to README

STEP Standard FAQ (Frequently Asked Questions) ​

📑 Table of Contents ​

Basic Concepts ​

Q1: What is STEP? ​

Q2: What is the difference between STEP and STL? ​

Q3: What is the difference between STEP and IGES/OBJ? ​

Q4: What is the difference between Part 21 and Part 28? ​

Q5: What is B-rep? ​

File Operations ​

Q6: Is it okay to open it in a text editor? ​

Q7: Why are the file sizes so large? ​

Q8: Is there a binary format? ​

Q9: What is the encoding? ​

Q10: What is a typical file size? ​

AP Selection ​

Q11: Which AP should I use? ​

Q12: Is there compatibility between APs? ​

Q13: What is the difference between AP242 ed2 and ed3? ​

Persistence and Simulation (IDs/Names) ​

Q14: Can I assign persistent IDs or names to faces? ​

Q15: Does Ansys Workbench read face names? ​

Q16: Why do my face names disappear in simulation? ​

Implementation ​

Q17: What is the recommended parser library? ​

Q18: How do I traverse entities? ​

Q19: How are units handled? ​

Q20: How is precision handled? ​

Troubleshooting ​

Q21: How do I check if a file is corrupted? ​

Q22: Why are colors disappearing? ​

Q23: Why is the assembly structure breaking? ​

Q24: Why is PMI missing or unreadable? ​

Geometry and Topology ​

Q25: How do I check if a model is "water-tight" (closed)? ​

Q26: How are fillets and complex blends represented? ​

Q27: Does STEP support CSG (primitives like Blocks and Spheres)? ​

Q28: How do I handle doughnuts or "inverted" shapes? ​

Q29: What is G2 continuity and does STEP support it? ​

T-Splines ​

Q30: Does STEP support T-Splines? ​

Tools and Resources ​

Q31: Are there free STEP viewers? ​

Q32: Where can I get a validator? ​

Q33: Where is the official documentation? ​

Q34: What is CAx-IF? ​

Q35: What is LOTAR? ​

Q36: What is MBD? ​

Q37: Should I build my own parser or use a library? ​

Q38: How do I generate STEP files programmatically? ​

Q39: What are the best sample files for learning? ​

Have more questions? ​

📚 Next Steps ​

STEP Standard FAQ (Frequently Asked Questions)

📑 Table of Contents

Basic Concepts

Q1: What is STEP?

Q2: What is the difference between STEP and STL?

Q3: What is the difference between STEP and IGES/OBJ?

Q4: What is the difference between Part 21 and Part 28?

Q5: What is B-rep?

File Operations

Q6: Is it okay to open it in a text editor?

Q7: Why are the file sizes so large?

Q8: Is there a binary format?

Q9: What is the encoding?

Q10: What is a typical file size?

AP Selection

Q11: Which AP should I use?

Q12: Is there compatibility between APs?

Q13: What is the difference between AP242 ed2 and ed3?

Persistence and Simulation (IDs/Names)

Q14: Can I assign persistent IDs or names to faces?

Q15: Does Ansys Workbench read face names?

Q16: Why do my face names disappear in simulation?

Implementation

Q17: What is the recommended parser library?

Q18: How do I traverse entities?

Q19: How are units handled?

Q20: How is precision handled?

Troubleshooting

Q21: How do I check if a file is corrupted?

Q22: Why are colors disappearing?

Q23: Why is the assembly structure breaking?

Q24: Why is PMI missing or unreadable?

Geometry and Topology

Q25: How do I check if a model is "water-tight" (closed)?

Q26: How are fillets and complex blends represented?

Q27: Does STEP support CSG (primitives like Blocks and Spheres)?

Q28: How do I handle doughnuts or "inverted" shapes?

Q29: What is G2 continuity and does STEP support it?

T-Splines

Q30: Does STEP support T-Splines?

Tools and Resources

Q31: Are there free STEP viewers?

Q32: Where can I get a validator?

Q33: Where is the official documentation?

Q34: What is CAx-IF?

Q35: What is LOTAR?

Q36: What is MBD?

Q37: Should I build my own parser or use a library?

Q38: How do I generate STEP files programmatically?

Q39: What are the best sample files for learning?

Have more questions?

📚 Next Steps