Project an Edge into a FreeCAD Sketch

Referencing existing geometry is a fundamental technique in parametric modeling, allowing new sketch elements to align precisely with previously generated shapes. In FreeCAD, you project an edge from an existing 3D body into an active sketch using the External Geometry tool found in the Sketcher workbench. This operation creates an associative, non-rendering construction line directly linked to the 3D model, allowing you to attach standard geometric constraints such as coincident, distance, or tangency between your new sketch profile and the existing 3D feature.

Step-by-Step Guide to Projecting an Edge

  1. Activate the Sketch: Open the sketch you wish to edit by double-clicking it in the Model tree. Ensure you are inside the Sketcher workbench edit mode.

  2. Select the External Geometry Tool: In the Sketcher toolbar, locate the External Geometry icon (represented by a 3D box with a highlighted red edge). Alternatively, you can press the keyboard shortcut X.

  3. Pick the 3D Edge: Hover your cursor over the 3D edge on the model that you want to reference. The edge will highlight. Click the left mouse button to select it.

  4. Identify the Projected Element: The selected edge is projected onto your sketch plane and will appear as a magenta (purple) line or arc. It also creates selectable endpoints (vertices) at each end of the projected segment.

  5. Constrain New Geometry to the Edge: Use the projected magenta line or its vertices just like any other native sketch geometry. You can apply constraints like coincident, parallelism, perpendicularity, or dimensions directly to it. These lines serve strictly as reference geometry and will not create solid material or interfere with operations like Pad or Pocket.

  6. Exit the Sketch: Once your new geometry is fully constrained against the projected reference, press the Close button in the Tasks panel or hit Esc to exit the sketch.

Best Practices and Considerations

  • Same Body Requirement: In the PartDesign workbench, the External Geometry tool can directly reference features belonging to the same active Body. To reference an edge from a different Body, first create a SubShapeBinder or ShapeBinder to bring that geometry into the active Body context before opening the sketch.
  • Topological Naming: Be aware that direct references to generated 3D edges depend on FreeCAD's underlying geometry index. Modifying upstream features that significantly alter the face or edge numbering may detach or invalidate the projected external geometry.