STL to Rhino: turn a mesh into real NURBS surfaces
Last updated by the stl2step team
Rhino's MeshToNURB wraps each triangle in its own NURBS face. To get real surfaces instead, convert the STL to a STEP with stl2step and import it in Rhino: you get a closed polysurface whose faces are exact planes, cylinders, cones, spheres, tori and B-splines, ready to edit.
Get real NURBS surfaces from an STL in Rhino
Convert the STL to STEP
Upload the STL to stl2step and download the .step file. The first 3 conversions are free.
Import the STEP in Rhino
File, Import (or Open) and pick the .step file. Rhino brings it in as a closed polysurface whose faces are real NURBS surfaces: planes, cylinders, cones, spheres, tori and B-splines.
Edit it as NURBS
Use the solid and surface tools directly: Gumball on sub-objects, ExtractSrf, MoveFace, BooleanDifference, fillets on the recovered edges.
Rhino's own options for meshes
Rhino's MeshToNURB turns every triangle into its own planar NURBS face, joined into a polysurface: valid, but heavy and not editable as surfaces. For organic shapes, QuadRemesh followed by conversion to SubD and then NURBS gives smooth freeform surfaces, but not the exact planes, cylinders and fillets of a mechanical part.
Converting to STEP with stl2step first gives Rhino the faces the designer drew, with exact analytic surfaces where the part has them and B-splines where it is freeform.
Questions
How do I convert an STL to NURBS in Rhino?
MeshToNURB makes one NURBS face per triangle. For real surfaces, convert the STL to STEP with stl2step and import the STEP: each face becomes a proper NURBS surface.