What Is the Thickness Tool in FreeCAD?

The Thickness tool in the FreeCAD Part Design workbench is a feature used to transform a solid body into a hollow shell with defined wall boundaries. By removing selected faces and offsetting the remaining geometry inwards or outwards, it creates thin-walled structures essential for industrial design and 3D printing. This article covers the primary purpose of the Thickness tool, its core settings, common practical use cases, and key modeling practices to avoid operation failures.

Primary Purpose of the Thickness Tool

In 3D CAD modeling, solid shapes are fully dense by default. The Thickness tool (often called a "Shell" tool in other CAD software) automates the process of hollowing out these solids. Instead of manually sketching and pocketing an interior void to create uniform walls, the Thickness tool computes an offset of the part's existing geometry, leaving walls of a precise measurement.

How the Tool Operates

When applied to a solid body, the Thickness tool operates based on specific user inputs:

  • Removed Faces: Selecting one or more faces designates them as open surfaces. The tool removes these faces and hollows out the rest of the body, creating an open vessel or container. If no faces are selected, the tool creates a completely enclosed hollow cavity within the solid.
  • Thickness Value: This defines the distance between the original surface and the newly generated inner or outer surface.
  • Offset Direction (Mode):
    • Inward: Keeps the original exterior dimensions intact and hollows the model into its interior. This is the most common setting for maintaining external design constraints.
    • Outward: Expands the outer surfaces by the thickness amount while keeping the original solid geometry as the internal void.
  • Join Type: Defines how adjacent offset faces meet at edges. Intersection creates sharp corners, while Arc rounds the transitions based on the offset path.

Common Use Cases

The Thickness tool is essential for several standard design workflows:

  • Enclosures and Casings: Designing housings for electronics, consumer products, and appliance shells that require consistent wall thicknesses for structural integrity.
  • Containers and Vessels: Quickly generating cups, vases, bottles, and storage bins from simple solid shapes.
  • Injection Molding Design: Ensuring uniform wall thickness across a plastic component to avoid warping, sink marks, and uneven cooling.
  • Tubes and Manifolds: Creating hollow pathways and ducting along complex swept shapes.

Best Practices and Avoiding Errors

The underlying geometry kernel (OpenCASCADE) can fail if the requested geometry becomes mathematically invalid. To prevent failures:

  • Apply Before Fillets: Complex fillets and chamfers often cause the thickness calculation to self-intersect. It is best to apply the Thickness tool to simple, sharp-cornered geometry and add fillets afterward.
  • Mind the Minimum Radius: The thickness value cannot be larger than the smallest internal radius or curve of the object, as this causes the offset surfaces to self-intersect.
  • Work Within an Active Body: The Thickness tool is part of the Part Design workflow and requires an active, single contiguous Body to function properly.