How to Compare Shapes with FreeCAD Inspection

This guide explains how to compare two different mesh or solid shapes using the Inspection Workbench in FreeCAD. You will learn how to prepare your 3D models, align them in the coordinate space, run the visual inspection tool to compute surface deviations, and interpret the resulting color-coded distance map to verify manufacturing tolerances or design changes.

1. Prepare and Align Your Models

Before comparing your shapes, ensure both geometries occupy the same global coordinate space:

  1. Open FreeCAD and import or open the two shapes you want to compare (for example, a nominal CAD model and a 3D scan, or two design iterations).
  2. Ensure both models are correctly oriented and aligned. If they are not aligned, use the Draft Workbench (Move/Rotate tools) or the A2plus / Assembly tools to superimpose them accurately.
  3. If working with pure Solid (BREP) shapes, you can compare them directly, but converting high-precision solids into fine meshes using the Mesh Design Workbench (Meshes > Create mesh from shape...) can significantly improve calculation speed and sampling density.

2. Open the Inspection Workbench

  1. Click on the Workbench drop-down menu in the top toolbar (which usually defaults to Start, Part, or Part Design).
  2. Select Inspection from the list. The workbench interface will load, providing access to visual comparison tools.

3. Run the Visual Inspection

  1. In the Model tree view on the left, select both objects simultaneously by holding down the Ctrl key (or Cmd on macOS) and clicking each shape.
  2. Select the nominal or reference shape first, followed by the shape you want to evaluate.
  3. In the top toolbar, click the Visual inspection... button (represented by an icon featuring calipers or a distance measurement over a surface).
  4. A parameter dialog will open in the Tasks tab:
    • Distance: Specify the maximum search distance threshold. Points beyond this distance will be ignored.
    • Sample density / Points: Adjust the sampling resolution if prompted, ensuring sufficient detail across complex curved surfaces.
  5. Click OK to compute the comparison.

4. Interpret the Distance Map

Once FreeCAD completes the calculation, it overlays a color spectrum onto the evaluated geometry:

  • Green: Indicates areas with zero or near-zero deviation, meaning the surfaces match within the specified tolerance.
  • Warm colors (Yellow to Red): Represent positive deviation, where the evaluated surface sits outside or above the reference geometry (excess material).
  • Cool colors (Cyan to Blue): Represent negative deviation, where the evaluated surface sits inside or below the reference geometry (missing material).
  • The legend on the screen provides the numerical scale corresponding to the colors, allowing you to quantify the exact physical discrepancy between your two shapes.