STEP vs STL vs IGES: which file to send to whom
STEP keeps exact CAD solids, STL is a triangle mesh for 3D printing, IGES is an older surface format. Here's which one a machine shop, printer or client needs.
Send STEP to anyone who will machine, mould or edit the part: it keeps the exact CAD geometry, the assembly and real units. Send STL (or 3MF) to a 3D printer: it's a triangle mesh, which is all a printer needs. Send IGES only when someone specifically asks for it: it's an older format that often arrives as loose surfaces instead of solids. And for someone who only needs to look at the part, send a link they open in the browser rather than any file.
The short version
| STEP (.step, .stp) | STL (.stl) | IGES (.iges, .igs) | |
|---|---|---|---|
| What's inside | Exact solids and surfaces (B-rep) | Triangles approximating the surface | Mostly surfaces, sometimes solids |
| Assemblies and part names | Yes | No, one mesh | Limited |
| Units | Yes | No (most tools assume mm) | Yes |
| Colours | Yes (AP214, AP242) | No (not in the standard) | Sometimes |
| Can be edited in CAD | Yes, as solids | Hardly: it's a mesh | Often needs repair |
| Best for | Machining, moulding, CAD hand-off | 3D printing | Older systems that ask for it |
STEP: the default for engineering
STEP (ISO 10303) describes the part the way CAD does: exact curves and faces that make up solid bodies. A hole is a true cylinder, not a ring of flat triangles. That's why it's what machine shops, mould makers and other engineers want.
- AP214 is the long-standing flavour and carries colours and the assembly structure.
- AP242 is the newer one; it adds product manufacturing information (tolerances and annotations) on top. If your CAD program offers both, either works for a quote; ask the shop if they have a preference.
- A whole assembly can travel in one file, with its part names.
STL: the format for 3D printing
STL describes only the outside of the part, as a mesh of triangles. A printer slices that mesh into layers, so it needs nothing more.
- There are no units in the file. A part modelled at 40 mm can arrive as 40 m or 40 in. Most slicers assume millimetres, so export in mm.
- Curves are approximated. Export at a fine resolution, or round holes print as polygons.
- No assembly. Export each printed part as its own STL.
- Hard to change. A mesh can't easily be edited as CAD geometry, so don't send STL to someone who needs to modify the design or machine it.
3MF is the modern alternative for printing: it keeps units, colours and several parts in one file, and most current slicers read it.
IGES: only when asked
IGES dates from the early 1980s, before solid modelling was the norm. It's still requested by some older CAM and CAD systems.
- Solids often arrive as a collection of separate surfaces that the receiver has to stitch back together.
- Assembly structure and part names are limited.
- If someone asks for IGES "because that's what we always use", it's worth asking whether STEP works for them: these days it usually does.
What to send to whom
| Who's receiving it | Send |
|---|---|
| Machine shop, CNC quote | STEP (AP214 or AP242), plus a PDF drawing if tolerances matter |
| Injection moulder, tooling | STEP, usually AP242 |
| 3D printing service | STL or 3MF (many also accept STEP) |
| Another engineer editing the design | STEP, or the native file if you use the same CAD |
| A client, manager or buyer who needs to look | A review link, not a file |
| A renderer, 3D artist or website | GLB (see GLB vs FBX vs OBJ vs STEP) |
| An old system that asks for IGES | IGES |
Opening any of them without CAD
MeshMarkup's free viewer opens STEP, IGES and STL files in your browser, with nothing to install. STEP and IGES come in with part names and true size; STL opens as a single mesh.
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