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Boolean Common (Intersection)

In one sentence: Keep only the volume that is simultaneously inside both input shapes — the geometric intersection.

Overview

Workbench: Part · Menu: Part → Boolean → Common
Toolbar: Part Boolean · Shortcut: none

Boolean Common (intersection) produces a solid containing only the material that exists in both the Base and Tool shapes. Everything not shared is discarded.

Intuition

Imagine two overlapping clay lumps. Common gives you the overlap region — the piece that would remain if you squashed the two lumps together and pulled away everything that wasn't touching.

Practical uses

  • Clipping a shape to a bounding volume — Common with a box clips any protrusions outside the box.
  • Finding the engagement region — how far does a plug enter a socket? Common shows exactly the overlapping solid.
  • Lens optics — intersecting two spheres gives a lens shape.

When to use it

  • Trimming geometry to a boundary volume.
  • Extracting the overlap between two imported parts to check fit.
  • Creating lens or intersection-based shapes.

When NOT to use it

  • Shapes that do not overlap — Common produces empty geometry (a null shape). Verify that the shapes actually intersect before using Common.

Step-by-step walkthrough

  1. Select the Base shape.
  2. Ctrl+click the Tool shape.
  3. Part → Boolean → Common.
  4. A Part::Common object appears containing only the shared volume.

Parameters

Parameter Description
Base First operand
Tool Second operand

Common mistakes and pitfalls

Empty result — shapes do not overlap

If Base and Tool have no shared volume, Common produces an empty shape. Check Placement values and ensure the shapes visually overlap.

Tangent-touching shapes may produce a degenerate result

If the shapes touch only at a single point or edge (not overlapping in volume), Common may produce a degenerate edge or vertex rather than a solid. Ensure the shapes overlap volumetrically.

Python API

import FreeCAD as App

doc = App.newDocument()

# Two overlapping cylinders — intersection is a lens shape
cyl1 = doc.addObject("Part::Cylinder", "Cyl1")
cyl1.Radius = 15; cyl1.Height = 30
cyl1.Placement = App.Placement(App.Vector(-10, 0, 0), App.Rotation())

cyl2 = doc.addObject("Part::Cylinder", "Cyl2")
cyl2.Radius = 15; cyl2.Height = 30
cyl2.Placement = App.Placement(App.Vector(10, 0, 0), App.Rotation())
doc.recompute()

common = doc.addObject("Part::Common", "Intersection")
common.Base = cyl1
common.Tool = cyl2
doc.recompute()

print(f"Intersection volume: {common.Shape.Volume:.2f} mm³")

See also