How to Check Interference in FreeCAD Assemblies

Checking for mechanical interference between components is a crucial step in mechanical design to ensure that parts fit together without colliding in the physical world. This guide outlines the most reliable methods to detect and analyze collisions between parts in FreeCAD, covering the native Boolean Intersection method in the Part Workbench as well as the automated interference detection tools found in popular assembly environments like A2plus and the official FreeCAD Assembly Workbench.

Method 1: The Universal Boolean Common Method

The most dependable way to detect interference without relying on third-party workbenches is using a Boolean Intersection in the Part Workbench. This approach works in any version of FreeCAD.

  1. Switch to the Part Workbench using the workbench dropdown selector.
  2. Hold Ctrl (or Cmd on macOS) and select the two parts in the Tree View that you want to test for interference.
  3. Click the Intersection (Common) tool in the toolbar (or select Part > Boolean > Common).
  4. Inspect the 3D view and Tree View:
    • No Interference: The operation produces an empty shape with a volume of 0 mm³, or FreeCAD displays an empty result.
    • Interference Detected: A new Common solid is created at the exact intersection. This solid represents the overlapping material.
  5. Select the resulting Common object and check its Shape > Volume property under the Data tab to see the exact volume of the collision.
  6. Press Ctrl + Z to undo the operation once verified, or delete the Common feature to revert your assembly to its original state.

Method 2: Using the A2plus Assembly Workbench

If you use the A2plus add-on workbench to assemble your models, you can run automated interference checks across two parts or the entire assembly.

  1. Open your assembly in the A2plus Workbench.
  2. Select the two parts you want to test, or deselect all objects to run a global check across the entire model.
  3. Click the Check all parts for interference button on the A2plus toolbar (or choose it from the A2plus menu).
  4. FreeCAD will calculate the intersections:
    • If collisions are found, A2plus highlights the overlapping volumes directly in the 3D viewport and lists the conflicting part pairs in a dialog box or in the Report View.
    • If no interference is found, a prompt will confirm that no overlapping solids exist.

Method 3: Using the Integrated Assembly Workbench (FreeCAD 1.0+)

In FreeCAD version 1.0 and later, the native Assembly Workbench includes integrated clash and clearance inspection.

  1. Switch to the Assembly Workbench.
  2. In the Tree View, select the components you want to evaluate.
  3. Locate and click the Check Interference or Collision Detection icon on the toolbar.
  4. Review the collision report generated in the Task panel, which displays pairs of intersecting objects along with the severity and location of the overlaps.

Tips for Accurate Collision Detection

  • Ignore Fastener Threads: Modeled cosmetic threads often trigger false interferences. Suppress complex thread geometry or use nominal cylinder diameters when testing for collisions.
  • Refine Mesh/Tolerance Settings: If FreeCAD produces errors during Boolean operations on complex curved surfaces, navigate to Edit > Preferences > Part Design > Shape view and adjust the deflection settings to generate a more precise shape representation.