INTERCHANGE
From FORM to Rhino: what a 3DM file keeps.
For legacy FORM shapes, export 3DM files that Rhino can open. Analytic and imported STEP bodies do not support this route. NURBS curves and surfaces keep native definitions; boxes, spheres, and cylinders can export as BRep solids. Other current shapes, including boolean results, export as meshes. Save FORM JSON too: FORM does not currently import Rhino edits back from 3DM.
How do I export a FORM model to Rhino?
- Inspect the model in FORM and give the parts and layers meaningful names.
- Ask your assistant to save an editable FORM source copy before the handoff.
- Ask your assistant to export 3DM and report the representation of each object. This route supports legacy shapes; analytic and imported STEP bodies cannot export 3DM.
- Open the resulting file in Rhino and verify the millimeter scale against a dimension you know.
- Check object types, open edges, and surface or solid status before doing more modeling.
FORM runs its 3DM writer locally in the browser and loads it when needed for export. Creating the file does not require Rhino to be installed. Continuing work inside Rhino requires your own Rhino installation.
Which objects remain editable CAD geometry?
| FORM object | 3DM representation | What to expect |
|---|---|---|
| NURBS curve | NURBS curve | Control points, weights, degree, and knots define the curve. |
| NURBS patch or loft | NURBS surface | Editable surface geometry; no added thickness. |
| Box, sphere, cylinder | BRep solid | Native supported primitive geometry. |
| Other shapes and boolean results | Mesh | Faceted geometry, not reconstructed smooth CAD surfaces. |
Names, layers, colors, and millimeter units are retained. FORM source information can be embedded as reference metadata, but that is not Rhino construction history. Changes made in Rhino do not update the embedded FORM recipe.
Retaining a NURBS definition means retaining its mathematical representation. It does not mean every calculation is exact arithmetic, every surface is closed, or every receiving workflow preserves a procedural model history.
Why can two similar-looking objects export differently?
A viewport displays sampled geometry. A smooth-looking surface may come from a retained NURBS definition, while a boolean result may be stored as a faceted solid. The picture alone does not reveal the underlying representation.
A BRep also needs topological relationships between faces, edges, and boundaries. McNeel’s BRep structure guide explains why surface equations alone do not describe a complete trimmed solid.
Choose your construction with the next tool in mind. If native surface editing matters, inspect a small test export early. Increasing viewport resolution does not convert a mesh fallback into NURBS.
Can I use this route to get a STEP file?
FORM supports direct STEP export for analytic solids and supported imported bodies. This is separate from the legacy 3DM workflow described here. Rhino also supports STEP interchange, so supported native surfaces and BReps handed to Rhino can be considered for a subsequent STEP export there.
Do not assume mesh fallbacks become smooth solids along that path. Inspect object types in Rhino and verify the STEP file in the receiving application. A single open surface still lacks thickness, even when the file extension changes.
What if I want to return to FORM later?
Keep the original FORM JSON and the exported 3DM as separate deliverables. FORM can reopen its JSON source; it cannot currently read an edited 3DM back into the workspace. Continue later Rhino edits from your Rhino file.
If the assistant needs to make changes directly to a Rhino document, consider a Rhino MCP integration. Exporting a file and remotely controlling Rhino are different workflows.
Written by the FORM team. Product details can change; the linked references and current workspace describe the available workflow. Tell us if something needs correcting.