Shelling an Open-Topped Box in FreeCAD

This guide explains how to convert a solid rectangular block into a hollow, open-topped container with custom wall thickness in FreeCAD. By utilizing the Part Design workbench and the Thickness feature, you can hollow out a solid model and designate which face remains open while specifying exact dimensions for the internal or external walls.

Step 1: Create the Base Solid

  1. Open FreeCAD and switch to the Part Design workbench from the workbench dropdown menu.
  2. Click Create body, then click Create sketch.
  3. Select the XY_Plane and click OK.
  4. Use the Rectangle tool to draw a rectangle centered or constrained to your preferred dimensions.
  5. Close the sketch by clicking the Close button in the Tasks panel.
  6. In the Tasks panel or toolbar, select the Pad tool, enter your desired box height, and click OK to generate the solid box.

Step 2: Select the Open Face

  1. Rotate the 3D view to clearly see the top of your box.
  2. Click directly on the top face of the box so that it highlights (typically turning light green or yellow depending on your theme). This selected face indicates the opening of the shell.

Step 3: Apply the Thickness Tool

  1. With the top face selected, click the Thickness icon (represented by a hollowed cube) in the 3D Dress-up toolbar, or navigate to Part Design > Apply a dress-up feature > Thickness.
  2. FreeCAD will generate an initial hollowed preview with the top face removed.

Step 4: Configure Custom Thickness Settings

In the Thickness parameters panel on the left:

  • Value (Thickness): Enter your desired wall thickness (for example, 2.00 mm or 3.50 mm).
  • Mode: Choose Inward to hollow the box toward the interior without changing the exterior dimensions, or Outward to add the wall thickness to the outside of your original solid dimensions.
  • Join Type: Select Arc or Intersection based on how you want the corners to resolve. For a standard sharp-cornered box, Intersection works best.
  • Make Thickness: Click OK to apply the feature.

Troubleshooting Common Errors

  • Shape Inversion / Broken Geometry: If the feature fails, ensure your thickness value is not equal to or greater than half of the box's smallest dimension.
  • Multiple Openings: If you need more than one face removed, hold Ctrl while clicking faces before launching the Thickness tool, or use the Add face button inside the Thickness task dialog.