Apply Fixed Boundary Constraints in FreeCAD FEM

Applying a fixed boundary constraint in FreeCAD’s Finite Element Method (FEM) workbench is essential for anchoring parts and preventing rigid body motion during structural simulations. This guide explains how to prepare your model, activate the FEM workbench, access the fixed boundary condition tool, and assign zero-displacement constraints to specific geometric faces or edges to ensure accurate structural analysis results.

Step 1: Open the FEM Workbench and Create an Analysis Container

  1. Open your 3D solid model in FreeCAD.
  2. Switch to the FEM workbench using the workbench dropdown selector in the top toolbar.
  3. In the FEM toolbar, click the Analysis container button (yellow sphere icon with an 'A') or navigate to Model > Analysis container to initialize the simulation environment.

Step 2: Select the Target Geometry

Before or after activating the constraint tool, identify the surface where your model should be rigidly locked:

  • For realistic physical mounting (such as bolted or welded joints), it is best practice to select faces rather than lines or vertices to avoid artificial stress concentrations.
  • Click directly on the surface in the 3D viewport to highlight it.

Step 3: Apply the Fixed Constraint

  1. Go to the top menu and select Model > Geometrical constraints > Constraint fixed, or click the Constraint fixed icon (a lock or anchor symbol) in the FEM constraints toolbar.
  2. The Task panel will open on the left side of the screen displaying the constraint properties.
  3. If you have not already selected the geometry, click the Add button in the Task panel, then click the desired face(s), edge(s), or vertice(s) in the 3D view.
  4. Verify that the correct element names appear in the list within the Task panel. If an unwanted face is added, select it and click Remove.
  5. Click OK to confirm and apply the constraint.

Step 4: Verify the Constraint in the Tree View

Once applied, a new object named FEMConstraintFixed appears inside your Analysis container in the Model Tree. Red or colored constraint markers will visually indicate the locked boundaries on the 3D model. This constraint completely immobilizes the selected region, setting all translational (and rotational, depending on the element type) degrees of freedom to zero for the solver.