How to Use External Geometry in FreeCAD Sketcher

The External Geometry tool in FreeCAD allows you to reference edges, vertices, and curves from an existing 3D body directly within an active sketch. This guide provides a direct, step-by-step walkthrough explaining how to locate the tool, project outside features into your sketch as construction geometry, and use those references to create accurate parametric constraints.

What the External Geometry Tool Does

When modeling in the PartDesign or Sketcher workbenches, sketches often need to align with features already created on a 3D model. The External Geometry tool creates a linked copy of an existing 3D edge or vertex inside your active sketch. These linked elements appear in magenta and function as reference geometry. They do not form closed profiles for operations like pads or pockets, but they can be constrained to just like standard sketch elements.

Step-by-Step Guide to Using the Tool

  1. Open or Create a Sketch: Select a planar face on an existing solid or choose a datum plane, then create a new sketch (or double-click an existing sketch in the tree view to open it).

  2. Activate the External Geometry Tool: In the Sketcher toolbar, locate and click the Create an edge linked to external geometry icon (represented by an icon showing a cube edge being projected to a sketch line). Alternatively, press the default keyboard shortcut X.

  3. Select Geometry to Project: Hover your mouse cursor over the 3D model outside your sketch. Valid edges or vertices will highlight as you move over them. Left-click the specific edge, arc, or vertex you want to reference.

  4. Verify the Linked Element: The selected feature will appear immediately inside your sketch as a magenta (purple) line or point. You can repeat the clicking process to add multiple external references while the tool remains active.

  5. Exit the Tool: Press the Escape key or right-click anywhere in the 3D view to deactivate the External Geometry tool.

  6. Apply Constraints: You can now apply standard geometric and dimensional constraints (such as Coincident, Distance, or Tangent) between your new sketch elements and the magenta reference geometry.

Best Practices

  • Topological Naming: Whenever possible, reference stable features such as datum planes, datum lines, or base coordinate axes rather than complex generated fillets or chamfers to prevent broken references if earlier model parameters change.
  • Deletion: If an external reference is no longer needed, simply click on the magenta line or point within the sketch and press Delete.