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Connect Shapes

In one sentence: Fuse two hollow (walled) solids while opening their shared wall so their interior cavities form one continuous space.

Overview

Workbench: Part
Menu: Part → Join → Connect
Shortcut: none

Joins two walled objects (pipes, tubes, enclosures) at a shared boundary and opens the shared wall so their interiors communicate. The result is a single hollow solid with a connected interior.

Intuition

Standard Boolean Fuse treats every solid as a completely filled volume. For hollow objects — pipes, housings, thin-walled enclosures — that assumption breaks down: a plain Fuse at a pipe junction fills the interior with material, destroying the flow path.

Connect Shapes understands the hollow character of walled solids and preserves it: when two pipes are connected, the shared wall is opened so fluid (or air, or wire) can pass from one pipe into the other.

Think of it as welding a branch pipe onto a header: the walls fuse, and the opening is cut through both walls at the junction.

When to use it

  • Joining two pipes at a T-junction, elbow, or wye.
  • Connecting a branch pipe to a manifold.
  • Joining two sheet-metal enclosure bodies so their interiors are continuous.
  • Any two hollow solids where the internal cavity must remain connected.

When NOT to use it

  • Don't use it for fully solid objects — use Boolean Fuse instead. Connect Shapes adds no value when there is no interior to preserve.
  • Don't use it when the objects should not share an interior — if you want two pipes side-by-side (touching but not joined), just place them without merging.

Step-by-step

  1. Select the first walled solid in the 3D view.
  2. Ctrl+click the second walled solid.
  3. Choose Part → Join → Connect.
  4. A single solid result appears with connected interiors.

Parameters

Parameter Description
Objects List of walled solids to connect (usually two)
Refine Remove redundant edges from the result

Common mistakes and pitfalls

Objects must be walled (hollow) solids

Connect Shapes is designed for objects with an interior cavity separated from the exterior by a wall. Applied to fully solid primitives, the result is indistinguishable from a standard Boolean Fuse.

Objects must overlap through the wall

The two solids must overlap at the wall region. If they only touch at a face without the wall intersection, Connect Shapes may produce unexpected results. Ensure there is genuine wall-to-wall overlap.

Very thin walls may cause tolerance issues

OCCT Boolean operations can fail or produce degenerate faces when walls are very thin relative to the model scale. Use consistent units and keep wall thickness above the modelling tolerance.

Python API

import FreeCAD as App
import BOPTools.JoinFeatures as JF

doc = App.newDocument()

# In practice, tube1 and tube2 should be actual hollow walled solids
# (e.g., shells or objects from the Mesh workbench converted to shells).
# The JoinFeatures API is:
j = JF.makeConnect(name="TJunction")
# j.Objects = [tube1, tube2]
# j.Proxy.execute(j)
# doc.recompute()
print("Use interactively: select both walled solids, then Part → Join → Connect.")

See also