STEP & IGES: The Complete Guide to 3D CAD Exchange Formats
When mechanical engineers from different companies need to share a 3D model, they face a fundamental problem: CAD software is proprietary. A part designed in CATIA will not open natively in SolidWorks; a CREO assembly will not import cleanly into Inventor. STEP and IGES were both created to solve exactly this problem — serving as neutral interchange formats that let 3D geometry travel between incompatible CAD systems.
What Is IGES?
IGES (Initial Graphics Exchange Specification) is one of the oldest CAD interchange standards in existence, developed by the US National Bureau of Standards (now NIST) and standardized as ANSI Y14.26M in 1980. Born from the defense and aerospace industries' need to share technical drawings across contractors and suppliers, IGES became the default interchange format for decades.
An IGES file is ASCII text with a rigid columnar structure: 80 characters per line, divided into five sections (Start, Global, Directory Entry, Parameter Data, Terminate). It stores:
- Geometric entities: curves, surfaces, B-spline bodies, trimmed surfaces, solid BREP (Boundary Representation)
- Annotation entities: dimensions, text, notes, drawing views
- Structure entities: groups, transformations, coordinate systems
IGES supports both wireframe geometry (curves and edges) and surface geometry (NURBS surfaces), but its implementation of solid bodies is inconsistent across vendors. An IGES file that imports perfectly in one CAD system may come in as disconnected surfaces rather than a solid in another — the perennial frustration of IGES exchange.
IGES uses the .igs or .iges extension. File sizes tend to be large because of the verbose ASCII format.
What Is STEP?
STEP (Standard for the Exchange of Product model data), formally known as ISO 10303, is the modern successor to IGES. Development began in the 1980s and the core standard was published in 1994. Unlike IGES, STEP is not a single format but a family of Application Protocols (APs) that define exactly what data is exchanged:
- AP 203 (Configuration Controlled 3D Design): the most common for mechanical parts and assemblies, focusing on solid geometry and product structure
- AP 214 (Automotive): adds color, layers, and reference data; common in automotive supply chains
- AP 242 (Managed Model Based 3D Engineering): the current generation, combining AP 203 and AP 214 plus PMI (Product Manufacturing Information) — GD&T annotations embedded in 3D
STEP files use the .stp or .step extension. The file format (called STEP Physical File / SPF) is a structured ASCII format using EXPRESS schema language, still text-based but far more compact and semantically rich than IGES.
A STEP part file looks like:
ISO-10303-21;
HEADER;
FILE_DESCRIPTION(('Open CASCADE STEP translator 6.7'), '2;1');
FILE_NAME('part.stp','2025-04-25T14:00:00',(''),(''),'',' ','');
FILE_SCHEMA(('AUTOMOTIVE_DESIGN'));
ENDSEC;
DATA;
#1=PRODUCT('Bracket','Bracket','',#2);
#2=PRODUCT_CONTEXT('',#3,'mechanical');
...
ENDSEC;
END-ISO-10303-21;
STEP vs IGES: Which Should You Use?
| Feature | IGES | STEP AP 203/214 |
|---|---|---|
| Year introduced | 1980 | 1994 |
| Standard | ANSI/US | ISO (international) |
| Solid bodies | Unreliable | Reliable BREP |
| Assemblies | Limited | Full assembly hierarchy |
| Colors/layers | No | Yes (AP 214) |
| PMI/GD&T | No | Yes (AP 242) |
| File size | Large (verbose ASCII) | Smaller (compact ASCII) |
| Modern software support | Declining | Universal |
| File extension | .igs, .iges | .stp, .step |
Use STEP for all modern exchanges. IGES is a legacy format — still used when older software cannot export STEP, but STEP is more reliable for solid geometry, assembly structure, and metadata. Most new projects specify STEP AP 203 as the default exchange format.
Software Support
Every major CAD platform supports STEP import and export:
| Software | STEP Import | STEP Export | IGES Import | IGES Export |
|---|---|---|---|---|
| SolidWorks | ✅ | ✅ | ✅ | ✅ |
| CATIA V5/V6 | ✅ | ✅ | ✅ | ✅ |
| Autodesk Inventor | ✅ | ✅ | ✅ | ✅ |
| PTC CREO | ✅ | ✅ | ✅ | ✅ |
| Siemens NX | ✅ | ✅ | ✅ | ✅ |
| Fusion 360 | ✅ | ✅ | ✅ (import) | ❌ |
| FreeCAD | ✅ | ✅ | ✅ | ✅ |
| Blender | Via plugin | Via plugin | ❌ | ❌ |
| Onshape | ✅ | ✅ | ✅ | ✅ |
Free and open-source tools:
- FreeCAD: native STEP and IGES import/export via OpenCASCADE geometry kernel
- Open CASCADE Technology (OCCT): the underlying geometry library used by FreeCAD, BRL-CAD, and many others — handles STEP/IGES directly
- CAD Exchanger (free community tier): specialized STEP/IGES viewer and converter
- STP Viewer (free): lightweight Windows viewer for STEP files
Common Issues When Exchanging STEP/IGES Files
Surface vs Solid Bodies
The most frequent problem: a part exported as a STEP solid arrives as disconnected surfaces. This usually means:
- The sending software's STEP exporter did not close all surface loops into a watertight solid
- The receiving software's STEP importer is more strict
Fix: In the sending software, run a "Heal Geometry" or "Check Geometry" function before exporting. In SolidWorks: Tools → Evaluate → Check. In FreeCAD: Part → Check Geometry.
Missing Features
STEP stores BREP geometry (the final shape), not parametric history. When you import a STEP file into SolidWorks, you get a "dumb solid" — you cannot edit the parametric sketch that created it. This is a fundamental limitation of neutral exchange formats. Tools like SpaceClaim or Ansys Discovery are specifically designed for editing imported STEP solids without a parametric history.
Unit Scale
STEP files store units in the header (millimeters vs inches is the most common mismatch). If a part imports at 25.4× the expected size, it was exported in inches but your software assumed millimeters. Most CAD software prompts for unit handling on import — choose carefully.
Assembly Structure
STEP AP 203 supports assemblies (PRODUCT_DEFINITION_FORMATION references), but not all exporters implement this correctly. If an assembly STEP file imports as a single merged solid instead of individual parts, the exporting software did not write assembly structure data.
Converting STEP/IGES to Other Formats
STEP → OBJ/STL (for 3D printing or rendering):
- FreeCAD: import STEP, then export as STL or OBJ
- Blender (via STEPper plugin): imports STEP files
- Online converters: CAD Exchanger, AnyConv handle STEP → OBJ/STL
STEP → GLTF (for web 3D):
- Blender with STEP plugin: import STEP → export glTF
- Open CASCADE + online pipeline
STEP → PDF (for sharing technical documentation):
- Most CAD software: File → Save As → 3D PDF
- Tetra4D Enrich (Adobe Acrobat plugin) creates interactive 3D PDFs from STEP
STEP in Manufacturing and Supply Chains
STEP AP 242 with PMI (Product Manufacturing Information) is driving the "Model Based Definition" (MBD) initiative — the idea that the 3D STEP model itself carries all manufacturing instructions (tolerances, surface finish, material specs) embedded as annotations, replacing 2D drawings as the primary deliverable.
Major manufacturers (Boeing, Airbus, Lockheed, automotive OEMs) now specify STEP AP 242 as the mandatory format for supplier submissions, marking a decisive shift away from IGES in high-value industrial supply chains.
Understanding STEP and IGES — when to use each, what problems to expect, and how to resolve them — is essential for anyone working in mechanical engineering, product design, or manufacturing who needs to share 3D data across organizational boundaries.
Related conversions
Frequent conversions across the catalogue: