From FreeCAD Documentation
- Now Drag&Drop with the mouse both Cylinder and Cube onto the Parts folder. They are moved into that folder.
- That is the quickest way and suitable for simple cases like this. A better way is via the use of link objects:
- Select Cube and Cylinder and then Make link either from the Context menu (-> LinkActions -> MakeLink) or the Structure panel.
- This adds two link objects. Then Drag&Drop the link objects to the Parts folder.
- Click both top surfaces of Cylinder and Cube (keep Ctrl pressed (Cmd on a Mac))
- Change to Assembly3 workbench
- Select Plane Coincidence from the Main constraints toolbar.