Frame Stiffener Tool The Frame Stiffener plug-in has been developed to allow Autodesk® Inventor® users to easily place and edit stiffener plates in I-Beams and Channels generated by Frame Generator. Constrain Plane Toggle This add-in adds the option to toggle an occurrence’s work planes and or axes during the constrain command. When constraining, right click on an occurrence and press Show Planes / Show Axes.