STEP, IGES or STL: Choose the Data a Parts Supplier Needs
The right geometry format depends on what the supplier has to do with the data, and on what the receiving software can actually read. STEP can carry parts, assemblies and product and manufacturing information (PMI). IGES exchanges geometry between CAD and CAM systems. STL turns a shape into flat facets that approximate it. A file extension proves neither that a file is authoritative nor that it translated correctly, so agree the format with the supplier, state the purpose of each file, and check the import before relying on it.
What each format carries
NIST describes the STEP format (ISO 10303, also called Part 21, with the extensions .stp, .step or .p21) as used to represent parts, assemblies and PMI for exchange between CAD, manufacturing, analysis and inspection software. PMI consists of annotations and attributes that define product geometry and specifications, including geometric dimensioning and tolerancing (GD&T). NIST lists several application protocols, among them AP242 for managed model-based 3D engineering and AP203 for configuration-controlled 3D design of mechanical parts and assemblies.
NIST also separates two kinds of PMI. Semantic PMI represents dimensional tolerances, geometric tolerances and datum features in a computer-interpretable way, so downstream manufacturing and inspection software can use it. Graphic PMI is the exact visual appearance of the annotations. It is not intended to be computer-interpretable and carries no representation information. A file that shows tolerances on screen may therefore still hold none that software can read.
Autodesk's export help describes IGES as an ANSI standard format for digital exchange of information between CAD and CAM systems, and STEP as an international 3D exchange format developed to overcome limitations of earlier conversion standards. It describes STL as a format that reduces a model into multiple flat facets that approximate its shape. A mesh of facets is an approximation, and converting a mesh does not restore the original design intent.
Autodesk adds that an exported design does not maintain associativity with the original design. The recipient receives a copy that no longer updates when the source changes. These are examples from one vendor's software and do not describe every translator.
Match the format to the task
A format-purpose map helps decide what to send. It is a recommended planning aid, not a standard.
| Format | What it can carry | Where it fits | What it does not establish |
|---|---|---|---|
| STEP | Solid geometry, assemblies, and PMI where the sender's software writes it | A precision quotation where the supplier needs exact geometry, and possibly machine-readable tolerances | That tolerances were written as semantic PMI, or that the receiving software read them |
| IGES | Geometry exchanged between CAD and CAM systems | A recipient whose software accepts it and that does not need PMI | PMI or product structure beyond what the translation preserves |
| STL | A faceted approximation of the shape | Visualization, or processes that work from a mesh | Exact analytic surfaces, features or tolerances |
| Native CAD file | The original model and its history | Only where the owner chooses to share it | A requirement. Native files are not mandatory for a quotation |
Choose by the data scope you are authorized to share and by what the receiving workflow can use. A requirement to hand over native files would be a decision about intellectual property and not a technical necessity. Which of two conflicting files governs is a separate question, covered in when the 3D model and 2D drawing disagree on a part.
Build an exchange manifest
Send a short manifest with the files so both sides know what each one is for. It is a recommended aid, not a form required by any standard.
| Field | What to state |
|---|---|
| Intended use | What the supplier should do with each file, such as quote, review geometry or read tolerances |
| File type | Native, neutral (STEP or IGES) or mesh (STL) |
| Protocol and version | For STEP, the application protocol and version, for example AP242 or AP203 |
| Source and revision | The model's source, revision and date |
| Units and configuration | Units, the configuration exported and the coordinate origin |
| PMI type | Semantic, graphic or none |
| Import checks | What the receiver verified after importing |
| Exceptions | Anything repaired, simplified or lost in translation |
Verify what survived the exchange
A file that opens has not been validated. Ask the receiver to confirm units, overall dimensions and the critical features on the imported geometry, and to say whether tolerances arrived as semantic PMI, as graphic PMI or not at all.
NIST publishes a free STEP file analyzer and viewer for this kind of check. It reports semantic PMI, graphic PMI and validation properties, and checks for basic STEP format errors. Validation properties are quantities computed by the native CAD software to validate the import into another system. NIST's note on the tool is that STEP file errors should be fixed so the file conforms to the schema and can work with other software. Use such a tool, or an equivalent the supplier trusts, as one check among several.
Send the request
State what each file is for, name the protocol and version, list the units and configuration, and record every import exception. If the supplier asks for more than you can share, agree a narrower format instead of sending the native file by default.
For a part from KTSU, provide the exchange manifest and the drawing package when asking what documentation is available. The public excavator undercarriage parts collection identifies the product family. It does not establish which formats any supplier accepts or that a particular file will translate.
The package is ready when each file has a stated purpose, the protocol and version are named, the import has been checked, and the exceptions are written down.