FreeCAD: Mirror and Boolean Operations (freecadhub)

Continuing the @freecadhub course. This exercise introduced two operations that change how you think about building complex parts — instead of modelling everything in one continuous feature tree, you can model parts separately and combine or mirror them.

New Tools

Mirrored — reflects features or a body across a plane or axis. You define one half of a symmetric part, then mirror it. The mirror is live — edit the original half and the mirrored side updates. Far cleaner than sketching both sides and trying to keep them equal with constraints.

Boolean — combines two separate FreeCAD Bodies into one solid using set operations:

• Union (Fuse) — merges two bodies into a single solid; overlapping volume is resolved
• Cut (Subtract) — removes one body’s volume from another
• Intersection (Common) — keeps only the volume where both bodies overlap

In Part Design, Boolean specifically refers to merging Bodies. The two bodies must overlap or touch — Boolean on non-touching solids produces nothing useful.

The Exercise — Symmetric Link Part

The part is a symmetric link: two rounded lobes with through-holes connected by a rectangular spine.

Two bodies are built separately, each capturing a different aspect of the final shape:

• Body — the stepped cross-section: a spine with flanges at top and bottom, Padded along its full length and Mirrored to produce the symmetric form. This defines the profile in the front view — 8mm base, 12mm raised spine, 16mm total height.
• Body001 — the stadium/pill outline with two ⌀17mm circles sketched inside it, Padded as a solid block. This defines the top-view envelope — 60mm × 60mm overall.

Boolean Common keeps only the volume where both bodies intersect. The pill shape of Body001 clips the ends of Body to produce the rounded lobes. The two circles in Body001 punch through as the through-holes. Neither body alone describes the part — the final geometry only exists at their intersection.

What Clicked

Multi-body modelling is the right approach for complex parts. Trying to build everything in one feature tree gets fragile — features deep in the tree affect everything above them. Breaking a part into logical bodies and combining with Boolean keeps each piece manageable and editable independently.

Mirror before Boolean. The workflow is: sketch half → Pad → Pocket → Mirror → then Boolean with the other body. Doing it in this order keeps the feature tree readable.

Boolean is in the Part Design menu, not the Sketcher. It operates on Bodies, not on sketch geometry. This distinction matters — mirroring sketch elements and mirroring bodies are different operations at different levels of the tree.