STL to CAD: turning a mesh into an editable parametric model

Why an STL is not a CAD model

STL stores a surface as a list of triangles. That is perfect for a 3D printer and useless for design work: there is no hole you can resize, because there is no hole, only a ring of triangles. The usual "convert mesh to body" command in CAD software wraps those triangles into a solid, which is heavier but no more editable. Change one dimension and you are back to sculpting.

What engineers actually want from a scan is the model someone would have drawn: a base plate, a boss, four bolt holes on a pitch circle, a fillet. That is what STL to CAD rebuilds.

How AI rebuilds a mesh

In AI Reverse Engineering, an AI agent rebuilds the part the way a careful engineer would, with measuring tools instead of guesses:

1. Survey. It checks the mesh is sound, finds its symmetry (round, mirrored or patterned) and decides what kind of part it is.
2. Measure. It cuts cross-sections through the mesh on any plane, probes distances, and fits surfaces to regions of it: planes, cylinders, cones, spheres, tori, fillet radii and freeform patches.
3. Build in steps. It writes the model as named steps ("base plate", "bolt holes", "edge fillets"), using patterns and mirrors where the part has them.
4. Check every step. After each change it compares the model with the mesh, locally and overall, and fixes what is off before moving on.
5. Report. It finishes with how closely the model matches, any walls thinner than 1 mm, and the judgement calls it made.

You can watch it work: the part builds up over a ghosted copy of your file. If a run stops, it can carry on from where it got to.

What "matches within 0.2 mm" means

After the build, the model and the mesh are placed together and the distance between their surfaces is measured. The result is stated as a tolerance: most of the surface sits within the number given. You set the tolerance you need. Regions that are not design intent, such as cast-in lettering, knurling or texture, can be left out of the match, and the report gives the match both with and without them.

This is the ground truth that vibe-CADing needs: not "looks right" but a number you can hold the model to.

Getting a good result from a scan

• Units. Make sure the mesh is in millimetres, or know what it is in. A file exported in inches reads 25.4 times too small.
• Closed meshes. A watertight scan is measured more reliably than one with holes. Most scanning software can close small gaps.
• Decimation is fine. You do not need millions of triangles. Large files work, but a sensibly decimated mesh is quicker.
• Expect texture to be set aside. Lettering, logos and knurling are not design intent; they are left out of the match and named in the report.

From the result to Fusion 360

The rebuilt part exports as STEP for any CAD package and STL for printing. For Fusion 360 there is also a script export: run it in Fusion and it rebuilds the part as sketches and features, extrudes, revolves, fillets and patterns, with the model's dimensions as Fusion user parameters where they map cleanly. Where a feature cannot be expressed in Fusion's terms, that piece comes in as an exact solid instead, so what opens in Fusion is always the part you saw.

STEP files too

A STEP file from a supplier is a solid but usually has no feature history: you can measure it, not change it. Upload it the same way and it is rebuilt as editable features.