How to Apply Fillets to Edges in FreeCAD

Applying fillets to sharp edges on a FreeCAD solid body smooths transitions, reduces stress concentrations, and improves visual appeal. This process is typically handled in the Part Design workbench using the parametric Fillet dress-up feature, though it can also be accomplished in the Part workbench for imported or direct CSG solids. This guide provides a straightforward, step-by-step procedure for selecting geometry, configuring fillet parameters, and resolving common errors like edge-break failures.

The Part Design workbench is the standard environment for parametric, feature-based modeling in FreeCAD.

  1. Activate the Body: Ensure the body containing your solid is active. In the Model tree, double-click the Body (its name will appear in bold).
  2. Select the Edges: Click directly on the sharp edge or edges you want to round in the 3D view.
    • To select multiple edges, hold the Ctrl key (Windows/Linux) or Cmd key (macOS) while clicking.
    • Selecting an entire face will apply fillets to all bounding edges of that face.
  3. Open the Fillet Tool: Click the Fillet icon in the Part Design toolbar, or navigate to Part Design > Apply a dress-up feature > Fillet.
  4. Set the Radius: In the Tasks tab on the left panel, enter your desired radius in the Radius field. The 3D view will update with a live preview.
  5. Adjust Selection (Optional): Use the Add and Remove buttons in the Tasks panel if you need to modify which edges are included in the operation.
  6. Confirm: Click OK to generate the fillet feature in the model tree.

Method 2: Using the Part Workbench

For simple shapes or imported geometry (such as STEP files) that do not use a Part Design Body hierarchy:

  1. Switch to the Part workbench via the workbench dropdown.
  2. Select the solid object in the model tree, or directly select the edges in the 3D viewport.
  3. Click the Fillet tool on the Part toolbar (or select Part > Fillet).
  4. In the dialog, verify the selected edges or check the boxes corresponding to edge numbers, then specify the fillet radius.
  5. Click OK to produce a new Fillet object containing the modified geometry.

Tips for Trouble-Free Filleting

  • Apply Fillets Last: Fillets alter the topological numbering of adjacent faces and edges. Add them at the end of your modeling workflow to avoid breaking downstream sketches or features.
  • Avoid Geometry Overlaps: If a fillet fails to compute, the radius is likely too large for the surrounding geometry, causing it to consume an adjacent face or intersect another edge. Reduce the radius value to test feasibility.
  • Fillet Chains: When blending continuous edges around a corner, select the entire tangent loop to prevent complex geometry intersections where multiple fillets meet at a point.