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Mesh Union

In one sentence: Compute the Boolean union of two solid meshes, producing a single mesh that encloses the combined volume.

Overview

Workbench: Mesh
Menu: Mesh → Meshes → Boolean → Union
Command: Mesh_Union
Shortcut: none

Computes the Boolean union of two selected Mesh::Feature objects. The result is a new mesh enclosing the volume of both inputs, with the overlapping region merged into a single continuous surface.

Intuition

Mesh Union is the mesh equivalent of Part Boolean Fuse — it combines two mesh volumes into one. Unlike the Part operation (which is exact), mesh booleans are approximate — their quality depends on the triangle density at the intersection boundary.

Always run Evaluate and Repair on both inputs before attempting Mesh Union.

When to use it

  • Combining two solid mesh objects into one unified mesh for 3-D printing.
  • Creating complex shapes by uniting simpler mesh primitives.

When NOT to use it

  • Don't use it on non-solid meshes — both inputs must be watertight (closed, manifold). Run Evaluate Solid first.
  • Don't use it when exact geometry matters — use Part workbench Boolean Fuse for engineering-accurate results.

Step-by-step

  1. Select the first mesh in the model tree.
  2. Ctrl+click the second mesh.
  3. Choose Mesh → Meshes → Boolean → Union.
  4. A new mesh object appears with the union. The original meshes are retained.

Common mistakes and pitfalls

Both meshes must be solid (watertight)

If either mesh has holes, non-manifold edges, or self-intersections, the Boolean fails silently or produces a corrupt result. Run Evaluate and Repair on both meshes first.

Non-overlapping solids produce a compound mesh, not a fused solid

If the two meshes don't intersect, the union result contains both meshes as disconnected components — not a single topologically fused solid.

Python API

import Mesh
import FreeCAD as App

m1 = App.ActiveDocument.getObject("Mesh")
m2 = App.ActiveDocument.getObject("Mesh001")

result = m1.Mesh.copy()
result.unite(m2.Mesh)

feature = App.ActiveDocument.addObject("Mesh::Feature", "Union")
feature.Mesh = result
App.ActiveDocument.recompute()

See also