From FreeCAD Documentation
- Optionally select one or more objects.
- There are several ways to invoke the command:
- Press the Mirror button.
- Select the Modification → Mirror option from the menu.
- Use the keyboard shortcut: M then I.
- If you have not yet selected an object: select an object in the 3D view.
- The Mirror task panel opens. See Options for more information.
- Pick the first point of the mirror plane in the 3D view, or type coordinates and press the Enter point button.
- Pick the second point of the mirror plane in the 3D view, or type coordinates and press the Enter point button.
- The mirror plane is defined by the selected points and the normal of the Draft working plane.