Set Equal Radius Constraints on Circles in FreeCAD

Applying an equal radius constraint to multiple circles in FreeCAD ensures that all selected circles or arcs share the same dimension without having to define individual measurements for each one. This guide explains how to use the Sketcher Workbench's equality constraint to link the radii of multiple circular entities simultaneously, allowing you to update the size of all linked circles by modifying just one dimension.

Step 1: Open Your Sketch

  1. Launch FreeCAD and open your project.
  2. In the tree view, double-click the sketch containing your circles to open it in the Sketcher Workbench.
  3. Ensure the circles or arcs you want to constrain are already drawn in the viewport.

Step 2: Select the Circles

  1. Hold down the Ctrl key (or Cmd on macOS).
  2. Click directly on the circumference edge of each circle you wish to make equal.
  3. Verify that the selected circles are highlighted (usually in green or your configured selection color). Do not select the center points; you must click the circular edges.

Step 3: Apply the Equality Constraint

  1. With the circles selected, click the Constrain equality tool in the Sketcher toolbar (represented by an equals sign =).
  2. Alternatively, press the shortcut key E on your keyboard, or navigate to Sketch → Sketcher constraints → Constrain equality from the top menu.
  3. The circles will immediately snap to the same radius, and an equality constraint marker will appear next to each entity.

Step 4: Define the Master Radius or Diameter

  1. Select one of the constrained circles.
  2. Apply a radius constraint by clicking the Constrain radius tool (shortcut: Shift + R) or a diameter constraint using the Constrain diameter tool (shortcut: Shift + D).
  3. Enter your desired measurement in the dialog box and press OK.
  4. All circles linked by the equality constraint will automatically resize to match the newly specified value.

Tips for Managing Equal Constraints

  • Arcs and Circles: The equality constraint works interchangeably between full circles and circular arcs.
  • Adding More Circles Later: To add another circle to the group, select one of the previously constrained circles along with the new circle, and press E.
  • Avoiding Redundancies: Do not apply individual radius constraints to more than one circle in an equal-constraint group, as this will trigger an over-constrained sketch error.