STEP File
A STEP file is a neutral CAD file, defined by the international standard ISO 10303, that stores a part's exact solid geometry so it can move between different CAD and CAM programs. It is the preferred format for CNC machining because it keeps true curves, flat faces, and units instead of approximating them with triangles.
Updated 28 Sep 2026 · 4 min read · By Wyntek
What a STEP file stores
STEP stands for Standard for the Exchange of Product model data. It is a family of ISO standards numbered ISO 10303, maintained by the ISO committee TC 184/SC 4. The file most people call a STEP file is defined in Part 21: a plain-text exchange structure that opens with the line ISO-10303-21; and holds a header section followed by numbered data records.
For a mechanical part, those records usually describe a boundary representation (B-rep) solid: faces, edges, and vertices, with each face lying on an exact surface such as a plane, a cylinder, or a freeform spline surface. A 10 mm hole is stored as a true cylinder with a 10 mm diameter, not as a polygon. A STEP file can also carry multi-part products, part names, colours, and the length unit.
What it does not usually carry is the design history. Most exports write the finished shape without the sketches and features that built it, so the receiving program gets a solid it can measure and machine but not edit feature by feature.
AP203, AP214, and AP242
STEP defines application protocols (APs): data models for particular industries and uses. Three are common for mechanical parts, and the main CAD programs offer at least AP203 and AP214 as export options.
| Protocol | Full name | What it carries | Background |
|---|---|---|---|
| AP203 | Configuration controlled 3D design | Solid geometry and product structure, but no colour | Used mainly in aerospace and defence |
| AP214 | Core data for automotive mechanical design processes | Adds colours and layers | Used mainly in the automotive industry |
| AP242 | Managed model-based 3D engineering | Combines AP203 and AP214 and adds product manufacturing information (PMI), such as tolerances, and tessellated geometry | First published in 2014; edition 4 in 2025 |
For a part going to a machine shop, AP214 and AP242 both carry the solid geometry the shop needs. Choose AP242 if you want tolerances and other PMI to travel with the model, and still state critical dimensions separately unless you know the receiving software reads PMI.
Units in STEP files
Unlike STL and OBJ, a STEP file records its length unit, so a part designed in inches opens at the correct size in a metric program. SOLIDWORKS, for example, writes the document's unit of length into AP203 and AP214 exports. It still pays to check one known dimension after import, because a model built at the wrong scale in CAD exports at the wrong scale too.
When to use STEP
- CNC machining. CAM software plans toolpaths on exact surfaces, and inspection measures against true dimensions. A mesh gives both only an approximation.
- Sending a model to someone who uses different CAD software. STEP is neutral, so neither side needs the other's native format.
- Archiving designs for the long term, a use AP242 explicitly covers.
- 3D printing, when the printing software accepts it. Slicers work from meshes and convert STEP to triangles on import, so STL or 3MF works just as well for printing.
Common export mistakes
- Exporting surfaces instead of a closed solid, which leaves gaps the receiving program cannot fill.
- Including hidden bodies, construction geometry, or reference parts that are not meant to be made.
- Exporting a whole multi-part model when only one part is wanted, or a single part when the others matter.
- Relying on the model alone for threads. A plain solid export does not say which holes are threaded, so state thread sizes separately.
How to export a clean STEP file
- Check that each part is a single closed solid.
- Select only the bodies you want made.
- Choose AP214 or AP242 as the format version.
- Confirm the export unit.
- Open the exported file in a viewer and check it before sending.
At Wyntek
STEP is the recommended format for CNC machining at Wyntek, which is available on Pro plans and up in aluminium and steel. Upload the STEP file, choose a tolerance class (±0.2 mm or ±0.008 in standard, ±0.1 mm or ±0.004 in tight, ±0.05 mm or ±0.002 in precision for suitable features), and you see the price in US dollars before you submit.
Send a STEP file for machining