How to Blend Surfaces in FreeCAD Curves Workbench

Blending surfaces smoothly in FreeCAD is essential for achieving professional, organic CAD models with high-grade visual and aerodynamic quality. This guide demonstrates how to install the Curves workbench, prepare your source geometry, and utilize specialized tools such as the Blend Surface and Gordon Surface tools to create G1 (tangential) and G2 (curvature) continuous transitions between separate geometric faces.

1. Install and Activate the Curves Workbench

The Curves workbench is an external add-on that provides advanced NURBS modeling capabilities.

  1. Open FreeCAD and navigate to Tools > Addon manager.
  2. Locate Curves in the list of workbenches.
  3. Click Install, then restart FreeCAD when prompted.
  4. Select Curves from the workbench dropdown menu.

2. Prepare the Adjacent Edges

Smooth surface blending requires clean boundary curves. Before generating the blend:

  1. Identify the two existing surfaces or solids you wish to bridge.
  2. Ensure the edges bounding the gap are clean and uninterrupted.
  3. If your edges belong to complex solids, extract them as sub-shapes or profile curves using the Curves > Sub-shape Binder or the Extract Subshape tool to isolate the boundary paths.

3. Generate the Blend Surface

The dedicated Blend Surface tool creates a transition patch between two surface boundaries while matching boundary continuity.

  1. Hold Ctrl and select the two opposing boundary edges where the blend will connect.
  2. Click the Blend Surface icon in the Curves toolbar.
  3. FreeCAD will generate an initial NURBS patch between the selections.
  4. Select the newly created BlendSurface object in the tree view and inspect its Data tab in the Property View:
    • Continuity: Set the continuity level for each boundary (Edge 1 and Edge 2). Choose G1 (Tangent) to eliminate sharp creases, or G2 (Curvature) for aesthetic surfaces where the rate of curvature change must remain continuous.
    • Scale/Weight: Adjust the tangent scale factors (often labeled as Scale 1 and Scale 2 or Tangent Length) to increase or decrease the influence that adjacent surfaces exert on the blend patch.

4. Alternate Approach: Gordon Surface for Complex Boundaries

When a simple two-edge blend lacks shape control, use a Gordon Surface to blend along a network of guide curves.

  1. Create or extract two sets of curves: Ribs (profile curves running across the gap) and Rails (boundary curves running along the edges of the surfaces to be joined).
  2. Ensure the ribs physically intersect the rails at all junction points.
  3. Select all intersecting curves in the tree view.
  4. Click the Gordon Surface tool.
  5. In the properties panel, link the adjoining surface faces under the boundary continuity options to enforce G1 or G2 curvature alignment across the shared boundaries.

5. Inspect Surface Smoothness

To confirm the blend is visually smooth and free from undesirable wrinkles:

  1. Switch to the Surface or Draft workbench.
  2. Enable the Zebra Tool to project parallel stripes across your model.
  3. Inspect the stripe transitions across the blend seams:
    • Discontinuous lines indicate G0 continuity (a sharp edge).
    • Lines that meet at an angle indicate G1 continuity (smooth angle, but sudden curvature shift).
    • Lines that flow seamlessly without sharp bends indicate G2 continuity (ideal curvature blend).