The difference between STEP and STL

STEP (and STP) exchanges a solid: planes, cylinders, curves and the body they form. A milling toolpath is built from that definition. Volume, area and solid validity can be read from it.

STL only stores triangles. It has no unit. Shops usually assume millimetres, and that assumption is a contract, not a measurement. An inch file priced as millimetres will be wrong. Volume exists only when the mesh is watertight. Holes, flipped normals or double walls make CNC totals unavailable.

eksenix.com reads STEP with CadQuery/OCP (OpenCascade). A GLB mesh is only a preview. The original STEP stays in storage and is not replaced. STL volume uses trimesh, and only if the mesh is closed.

A multi-body STEP can become one line per solid. We do not silently treat it as a single part; a review note is attached.

Use STL for printing and STEP for milling. If tolerance matters, keep the source solid, because tessellation throws curves away.