How to Hollow a Solid Model in FreeCAD

Hollowing out a solid model in FreeCAD is a straightforward process achieved by using the Thickness tool—commonly referred to in CAD terminology as the Shell tool. This guide provides a direct, step-by-step walkthrough for creating a hollow body with uniform wall thickness in the Part Design workbench, including how to select open faces, adjust shell parameters, and troubleshoot common geometry errors.

Step 1: Open the Part Design Workbench

  1. Open your project in FreeCAD and ensure your solid part is visible.
  2. Switch to the Part Design Workbench using the workbench selector dropdown at the top of the interface.
  3. In the Model tree, ensure the Body containing the solid model you want to hollow out is active (highlighted in bold; double-click it to activate if it is not).

Step 2: Select the Faces to Remove

The Shell (Thickness) tool requires you to define which surfaces should be open to expose the hollow interior:

  1. Hold down the Ctrl key and click on the face (or multiple faces) of your model that you want to remove.
  2. If you want a completely enclosed hollow void inside the model with no openings, do not select any faces.

Step 3: Apply the Thickness (Shell) Tool

  1. With the desired face(s) selected, click the Thickness icon in the toolbar (represented by a hollow cube) or go to the top menu and select Part Design > Apply a dress-up feature > Thickness.
  2. A parameters panel will open on the left under the Tasks tab.

Step 4: Configure the Shell Parameters

In the Tasks panel, adjust the following settings according to your design specifications:

  • Thickness: Enter the desired wall thickness value.
  • Direction (Inward vs. Outward): Check or uncheck the Inward box (or enter a negative value) depending on whether you want the walls to grow toward the inside of the model (preserving outer dimensions) or toward the outside (expanding the exterior dimensions). Inward is typically standard for hollowing models.
  • Mode: Leave the mode set to Skin for standard shells.
  • Join Type: Choose how the tool handles corners:
    • Arc: Rounds sharp interior/exterior transitions.
    • Intersection: Extends adjacent faces until they meet sharply, preserving crisp edges.

Step 5: Validate and Finish

  1. Check the 3D view to verify that the preview matches your requirements.
  2. Click OK in the Tasks panel to apply the feature.
  3. The model tree will now show a Thickness object as the latest step in your active body.

Common Troubleshooting Tips

  • Negative vs. Positive Thickness: If the feature fails or inverts your shape, toggle between positive and negative thickness values, or toggle the "Inward" checkbox.
  • Complex Fillets and Radii: Hollowing geometry with very tight fillets or complex compound curves can cause self-intersections if the wall thickness is greater than the smallest radius of a curve. If the tool generates an error, apply the shell before adding small edge fillets, or reduce your wall thickness.