ImportExport 3DM

FreeCAD Import/Export module for Rhino .3dm files. Geometry is read and written as true NURBS — control points, weights, degree, and knot vectors are preserved exactly, with no tessellation.

FreeCAD 1.1+ and rhino3dm ≥ 8.0.0 required.

Supported geometry

Import

rhino3dm type	FreeCAD result
Brep (multi-face solid)	Part::Feature via OCCT
NurbsSurface	Part::Feature with BSplineSurface shell
NurbsCurve	Part::Feature with BSplineCurve wire
SubD	Diagnostic only — NURBS not preserved by originating app
Mesh	Diagnostic only — NURBS not preserved by originating app

Export

FreeCAD surface type	3DM output
BSplineSurface	NURBS surface — control points, rational weights, knots copied exactly
Plane	NURBS surface via OCC toNurbs()
Cylinder	Rational NURBS surface (exact); native r3.Cylinder optional (see preferences)
Cone	Rational NURBS surface (exact); native r3.Cone optional
Sphere	Rational NURBS surface (exact); native r3.Sphere optional
Torus	Rational NURBS surface (exact); native r3.Torus optional
Other analytic surfaces	NURBS via OCC toNurbs() / toBSpline()

Whole objects are traversed correctly: selecting an App::Part, a PartDesign Body, or a group exports all the geometry nested inside it. The final solid of a PartDesign Body is exported once.

NURBS fidelity. Rational geometry (cylinders, cones, spheres, tori, and any circular edges) is written with its true control-point weights, so circles stay circular on re-import. Periodic (closed) surfaces and curves are converted to clamped form first, and circular/arc edges are written as exact degree-2 rational arcs rather than polyline approximations.

The native-primitives behaviour is controlled by a Preferences option (see below; it is off by default because the bounded-NURBS path round-trips trim boundaries more reliably).

Preferences

Open Edit → Preferences → Import-Export → ImportExport 3DM:

Option	Default	Effect
Export Cylinder / Cone / Sphere / Torus as native primitives	Off	When on, writes analytic surfaces as exact rhino3dm primitives (smaller files). When off (default), they are written as exact bounded rational NURBS, which reconstructs trimmed faces more reliably on re-import.
Import: create groups	On	Mirrors each named .3dm group as an App::DocumentObjectGroup so an object's faces and boundary curves stay together.
Import: try to make shell/solid	Off	After collecting a group's surfaces, attempt Part.makeShell() (and makeSolid() if closed); falls back to individual surfaces.

Controlling the imported tree structure — Import: create groups

This is the option to reach for if you don't want the grouped tree layout.

• On (default): every named group in the .3dm becomes an App::DocumentObjectGroup. Useful when each exported object carries its own group, but it can produce a group that wraps a single same-named object.
• Off: no container objects are created. Surfaces are placed directly under the top-level Part and keep their own names; boundary curves are labelled {groupName}_{curveName} so the association stays visible without nesting.

Set it from the preferences page, or from the Python console:

FreeCAD.ParamGet(
    "User parameter:BaseApp/Preferences/Mod/ImportExport_3DM"
).SetBool("ImportCreateGroups", False)

Requirements

rhino3dm ≥ 8.0.0 must be installed into FreeCAD's own Python interpreter.

FreeCAD 1.1 on macOS

/Applications/FreeCAD_1.1.app/Contents/Resources/bin/python \
  -m pip install "rhino3dm>=8.0.0"

Other platforms

Find FreeCAD's Python interpreter via the FreeCAD Python console:

import sys; print(sys.executable)

Then install:

/path/to/freecad/python -m pip install "rhino3dm>=8.0.0"

Installation

Manual install (macOS / Linux)

cd ~/Library/Application\ Support/FreeCAD/Mod   # macOS
# or
cd ~/.local/share/FreeCAD/Mod                    # Linux

git clone https://github.com/KeithSloan/ImportExport_3DM.git

Restart FreeCAD. The importers and exporter appear automatically in File → Open / File → Import / File → Export.

From the FreeCAD Addon Manager

Not yet listed — install manually for now.

Usage

• Import: File → Open or File → Import — select a .3dm file and choose 3DM from the format dropdown.
• Export: File → Export — choose 3DM.

Report View diagnostics

Version and progress information is printed to the FreeCAD Report View during import and export. The module version and rhino3dm version are printed at module load time.

Blender NURBS pipeline

A companion Blender extension exports NURBS geometry directly to .3dm:

Blender_Export_3DM

This enables a lossless Blender → 3DM → FreeCAD NURBS pipeline:

Blender NURBS surface / Surface Psycho patch
    ↓  Blender_Export_3DM
.3dm file
    ↓  ImportExport_3DM (File → Open)
FreeCAD Part::Feature (exact BSplineSurface)
    ↓  KS_CurvesWB Import commands (optional)
Editable NurbsSurfaceFP / NurbsCurveFP objects

Sample Rhino files

Test .3dm files are in testCases/. Additional Rhino sample files: https://www.rhino3d.com/download/opennurbs/6/opennurbs6samples

Rhino API reference: https://developer.rhino3d.com/api/rhinocommon/

Changes

0.3.0

• Export now traverses containers. Selecting an App::Part, a PartDesign Body, or a group exports all nested geometry (previously such selections could produce a nearly empty file). The whole selection list is exported, not just the first object.
• Rational weights preserved. NURBS surfaces and curves are written with their true control-point weights and created as rational where needed, so cylinders, cones, spheres, tori and circular profiles no longer come back with straight/flattened edges.
• Exact rational arcs. Circular and arc edges are exported as exact degree-2 rational arcs instead of high-degree polynomial approximations.
• Periodic geometry handled. Closed surfaces/curves are converted to clamped form before writing.
• Import: degenerate trimmed faces fall back to the untrimmed surface. When trim reconstruction produces an invalid, zero-area face, the importer repairs it or falls back to the (valid, correctly curved) untrimmed surface rather than leaving a collapsed face. (import3DM 0.1.13)

Acknowledgements

• Test cases kindly supplied by Jonne Neva (cheezebreeze), EdWilliams, Sven

Developers

• Keith Sloan
• Chris Grellier

License

GNU Lesser General Public License v2.1 — see LICENSE.