Use Rhino’s native 3DM format for the entire design and editing phase, and only export to STL once the piece is genuinely finished and ready to send to a printer or casting bureau. The two formats aren’t interchangeable options, they serve different stages of the same project, and converting too early is the mistake that actually costs students time.
Why 3DM stays the working file for as long as possible
A 3DM file stores your model as NURBS, exact mathematical surfaces, along with your full construction history: layers, named views, construction curves, sub-objects you might reference later, and (depending on how you’ve built) the parametric relationships between parts. Nothing about it is fixed. You can go back into a shank’s profile curve and adjust it, resize a bezel, or nudge a claw’s angle, and the surfaces around it can still respond cleanly, because the underlying geometry is still exact.
None of that survives a trip to STL and back. Once a design decision might still change, however small, it should still be living in 3DM.
What STL actually is, and why it’s a one-way trip
STL describes a shape purely as a mesh of flat triangles, with no history, no layers, no construction curves, no exact surfaces underneath. It’s a snapshot of the outer shape at the tolerance you exported it at, nothing more.
That makes it exactly what a printer or casting bureau needs, most printing hardware and slicing software reads mesh formats natively, and it makes it a poor working file, because there’s no clean way back. You can technically re-import an STL and try to model against it, but you’re now working against a faceted approximation instead of an exact surface, and any edit is a rebuild, not an adjustment. Treat an STL you’ve already exported as a finished output, not a file you continue working in.
The mistake this actually prevents
The failure mode isn’t confusing the two formats in the abstract, it’s exporting to STL earlier than the design actually is finished, because a bureau asked for “the file” and STL felt like the safe universal answer. Once that happens, the next round of client feedback, a slightly wider band, a stone swapped for a different size, means rebuilding from the last 3DM you kept rather than editing the file you’ve been sending around. Keep 3DM as the only file you actually edit, and treat every STL export as disposable: regenerate it fresh from the current 3DM each time the bureau needs an updated file, rather than trying to patch an old export.
Once you are ready to export for real, the tolerance you set at that point (and the wall-thickness checks that should happen before it) are their own decision: see how to 3D print jewellery from a Rhino file for that step.
If you’d like this worked through on a real design rather than in the abstract, one-to-one tuition covers exactly this: Jewellery CAD, taught one-to-one.