Why convert STEP to STL?
STEP describes a part with exact mathematical surfaces. That is what CAD programs exchange, but 3D printing works with triangles: slicers, many online print services and most mesh tools want STL. Converting is the step that turns the exact model into the faceted one that gets printed.
How the conversion works
- The first time you open a STEP file, your browser downloads the OpenCASCADE engine (about 7 MB) — the same kernel FreeCAD uses.
- The B-rep surfaces are triangulated in a background worker with a linear deflection of 0.1 % of the bounding box and an angular deflection of 0.5 rad, in millimetres.
- The converter writes the result as binary STL (or 3MF/OBJ) and your browser downloads it.
STEP files declare their own units, so there is nothing to choose: the STL comes out in millimetres at the part's real size.
Things to know
- Assemblies become one STL. Every body is merged into a single mesh at its assembled position. If you need parts separately, export them separately from your CAD program, or use your slicer's "split to objects".
- Colours need 3MF. STL has no colour; pick 3MF to keep body colours.
- Tolerances. The mesh is fine enough for any printer, but a bore that was exactly 6.000 mm in CAD is now a polygon. Measure critical fits in CAD.
- Surfaces without volume. Some exports contain only open surfaces. They convert, but the STL will not be watertight — the summary tells you before you download.
STEP, STL or 3MF for printing?
If your slicer imports STEP, giving it the STEP file lets the slicer choose its own resolution. For everything else, STL is the universal choice and 3MF the better one when the receiver supports it: smaller, in millimetres, with colours.
On a phone
STEP conversion works in a mobile browser, but triangulating a large assembly is heavy for a phone. The Mes3D app for iPhone, iPad and Mac runs OpenCASCADE natively and can export STL, OBJ or 3MF too.