What an STL holds
An STL is a flat list of triangles, each with three corners and a direction. No unit, no colour, no history, and no faces: a cylinder exported to STL is a ring of narrow flat strips.
The strips are not random, though. Every corner of every strip lies on the cylinder they were cut from. That is what makes the surface recoverable: fit a cylinder to those corners, and if every one of them lies on it, that is the surface.
What the conversion produces
Neighbouring triangles that bend only slightly are grouped into regions, and each region is fitted with the simplest surface that holds it: a plane, cylinder, cone, sphere or torus. Whatever none of those holds — a sculpted grip, a scanned shape, a blend — becomes a smooth B-spline patch. The faces are sewn into a closed, watertight solid.
| You get | You do not get |
|---|---|
| A closed, watertight solid body | A feature tree or sketches |
| Planes, cylinders, cones, spheres and tori as true surfaces | Fillets and chamfers as editable features |
| Smooth B-spline patches on organic regions | Dimensions more exact than the mesh was |
| Holes you can measure, mate to and select as one face | The designer's intent — nominal sizes, tolerances |
Nothing is accepted on trust. A surface is kept only if every vertex of its region lies within one ten-thousandth of the part's size from it — 6 µm on a 60 mm part — and the finished solid's volume is measured against the mesh. In the rare case a region cannot be built even as a freeform patch, its triangles stay as flat faces and the result says how many.
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What a mesh cannot give back
- The feature tree
- A hole comes back as a cylindrical face, not as a hole feature. Changing it from 8 mm to 10 mm is a direct edit on the face (most CAD systems can do that) or a remodel — not a number in a dialog.
- Nominal sizes
- The radius is the one the mesh records. A bore modelled at 8.00 mm and exported coarsely may measure 7.98. The surface is exact to the mesh, and the mesh is only as exact as its export.
- Variable fillets and blends as fillets
- A constant-radius round comes back as a cylinder or torus. A variable-radius blend comes back as a freeform patch: correct in shape, not editable as a fillet.
If you need to change the design rather than use the shape, the original CAD is still the fastest route when it exists. When it does not, the converted solid is a much better starting point than a mesh: its holes and faces can be selected and measured directly while you remodel.
Getting the best result
- Export the mesh as finely as you can. Surfaces are fitted to the vertices, so a finer export gives radii closer to the modelled ones.
- Check the mesh is closed. An open or self-intersecting mesh may not sew into a solid, and the failure is confusing when it happens at the far end.
- Keep units in mind. STL records none; the result is written in millimetres.
- If the other side only needs to see the shape, send the mesh. A supplier who reads STL does not need a STEP.