Convert STEP to STL

When you need this conversion

STEP is what you send to a machine shop, and STL is what a 3D printer slicer wants. If someone has sent you a STEP file and you want to print the part, whether as a real part or as a check fixture or a mock-up, you need to convert it first. That is what this page is for.

The conversion runs on the OpenCascade geometry kernel compiled to WebAssembly, which reads the real surfaces in the STEP file and then tessellates them into triangles at the quality you choose.

Choosing the resolution

The setting is a chord tolerance, meaning the largest gap allowed between the real curved surface and the flat triangles that stand in for it. Coarse allows half a millimetre, which is quick and fine for checking size and fit. Standard allows a tenth of a millimetre and suits most printing. Fine allows two hundredths, which matters when the part has small radii or a curved surface that will actually be looked at. Flat faces come out identical at every setting, because a flat face only ever needs two triangles.

If you are printing at a normal layer height, the standard setting is usually indistinguishable from fine once the part is in your hand.

What changes in the conversion

This part is worth understanding. A STEP file knows that a hole is a cylinder of a particular diameter. An STL file only knows about triangles, so after conversion that hole is a ring of flat facets that approximates a cylinder. Nothing downstream can recover the original diameter from the STL.

That is fine for printing, because the slicer only cares about the surface. It is a problem if you were planning to machine from the STL, so keep the original STEP file. If you need to go the other way, we have an STL to STEP converter as well, and that page is honest about what a mesh can and cannot give you back.

Questions