Where OBJ files come from, and why that matters here
Wavefront Technologies developed OBJ in the 1980s for its Advanced Visualizer animation software. The file is plain text: v lines for vertices, vt for texture coordinates, vn for normals and f for faces, which may have more than three corners. Materials live in a separate .mtl file.
Today OBJ is a common export of rendering, sculpting and 3D scanning tools. Those programs think in scenes and pictures, so an OBJ often has no real-world size, may lie on its back, and can carry millions of triangles. Each of those habits needs a fix before the mesh becomes a useful CAD body.
Size and orientation: OBJ stores neither
The viewer shows OBJ numbers as millimetres. A model built in metres then reads far too small, and one built in inches reads 25.4 times too small. Measure a known edge with Measure, then set the real size with Edit → Size or use Inches → mm (×25.4). The scale STL page shows each option.
Many rendering tools export OBJ with Y up, while CAD and slicers usually build along Z, so the part arrives lying on its side. Edit → Rotate about X by 90 degrees stands it upright. The STEP keeps whatever orientation you see when you press Download.
Triangle count and file weight
Each triangle costs about 1 KB in the faceted STEP. A sculpted or scanned OBJ with half a million triangles is far too heavy for most CAD work. Edit → Reduce triangles brings it down to a percentage you set; our sphere test at 30 % kept the volume within 2 %. The faceted body describes shape; keep the OBJ with its MTL and images for rendering.
Close the holes in scanned OBJ files
Scans and game assets often have open edges: a missing bottom, gaps around the eyes of a figure, a seam that never closed. A shell with holes cannot become a closed solid. Info lists holes, open edges, non-manifold edges and flipped triangles, and Edit → Repair handles the simple cases. In our test an open box came out watertight at 8000 mm³, and the message after Repair says what it fixed and what is still open.
OBJ to STEP: the order of operations
- Drop the .obj; you can drop its .mtl together with it.
- Fix size and rotation in Edit.
- Reduce triangles if Info shows a very large count.
- Run Repair when the mesh check is not watertight, so the shell closes into a solid.
- Press Download STEP and check it in the STEP viewer.
The OBJ stays on your computer for all five steps; JavaScript in this tab does the work.
Quads, loose objects and other OBJ surprises
The STEP is built from triangles, so a four-sided OBJ face reaches CAD as two triangular faces. A model of 10,000 quads therefore counts as about 20,000 triangles in Info, and the STEP grows to match.
An OBJ can also hold several objects: a character and its base, or a part and a stand. They all go into the same STEP. If you need only one, Cut with a plane can split it off; Save part 1 writes that piece as STL, and STL to STEP turns that piece into a STEP body. Grouped objects that overlap should be joined with Union all parts first, which needs a watertight mesh.
Other targets for an OBJ
For a slicer, OBJ to STL skips the CAD step entirely. For web and AR viewers, OBJ to GLB packs the mesh into one binary file. The OBJ viewer opens the file without converting it, and the OBJ file guide covers how the format stores vertices and faces.
Questions and answers
Can an OBJ become a real solid in CAD?
It becomes a closed faceted solid, which CAD can import as a body. It will not have true curved surfaces or editable features.
How do I check that the STEP is correct?
Open it in the STEP viewer here and compare the size and triangle count with the OBJ. Then import it in your CAD program; that last step we have not tested by hand.
Can I convert OBJ to STP instead of STEP?
STP and STEP are the same format with two extensions. Rename the downloaded file to .stp if your workflow expects that ending.
Why does my OBJ look tiny after conversion?
OBJ has no unit, and the tool reads numbers as millimetres. A model made in metres therefore shows far too small; scale it in Edit before you convert.
Why does my STEP have twice as many faces as the OBJ?
The OBJ was made of quads. Each four-sided face is split into two triangles, and every triangle becomes one STEP face.
Are textures kept in the STEP?
Plan on shape only. The faceted STEP is built from the triangles, so keep the original OBJ with its MTL and images for rendering.