Convert OBJ to STEP

A faceted solid, in plain terms

Your OBJ is a surface made of flat polygons. The STEP file will be a solid made of flat faces. Where the OBJ had a smooth-looking curve, the STEP will have the flats that curve was really made of — because that is what the file contains, and the smoothness you saw on screen was shading, not geometry.

The solid is real. Engineering software will boolean it, measure it, section it and assemble it. What it will not give you is a feature tree: no sketches, no extrudes, nothing to open and edit by typing a number. That information was never in the OBJ, so what comes out is a faceted solid rather than a parametric CAD model, and nothing anywhere can turn one into the other.

Quads and n-gons, and why the face count still improves

OBJ is one of the few mesh formats that allows a face to have four corners or more, and most modelling programs write quads. Those quads are cut into triangles when the file is read, which is why the triangle count on this page is usually about double the face count your modelling program showed you.

The merge then puts them back, and more besides. A quad whose two triangles are flat becomes one face again. A whole flat panel made of two hundred quads becomes one face with one outline. So the number of faces in the STEP is frequently lower than the face count you started with in your modeller, not just lower than the triangle count.

Where the model is genuinely curved — a subdivided surface, a sculpted shape — nothing merges, because no two triangles share a plane. The merge report under the download button tells you which of the two you have.

Units: the part that catches people out

OBJ records no unit. Neither does the .mtl beside it. A model that is 2 units wide might be 2 millimetres or 2 metres, and the file has no opinion. STEP does record a unit, so the conversion has to be told one.

Modelling programs differ wildly here. Blender exports in metres by default, 3ds Max and Maya usually in centimetres, and anything made for 3D printing is normally in millimetres. Set the first unit box to what your model really used, set the second to what the CAD file should say, and read the width, depth and height under the 3D view before you convert.

A part that arrives in CAD a thousand times too large or too small is almost always this, and it is one dropdown to fix.

Materials, groups and names

An OBJ can hold several named groups, each with its own material from a .mtl file. The groups are listed above the 3D view, and you can convert one on its own or merge all of them — useful when the file is a whole scene and you only want one object out of it.

Materials do not survive. STEP can carry a colour per face, but a colour taken from a game material is not information an engineer wants, and writing it would make the file larger for no benefit. Texture coordinates have no meaning in a CAD file at all.

Models built for rendering are usually the wrong shape for CAD

This is worth knowing before you start. A model made to be looked at is frequently not a solid: surfaces are left open where the camera will never see them, walls have no thickness, and parts intersect rather than joining. None of that matters in a render and all of it matters in CAD.

The measurements under the 3D view say whether the mesh is closed. If it is not, the STEP file is written as a surface model instead of a solid — you are told, the file still opens everywhere, and most CAD programs can sew it up once the gaps are filled.