Modern CAD systems offer many powerful functions to handle parts and assemble them. However, these functions often mask problems that only occur on the final production stage (for example, positioning difficulties for two parts before fixing). The ESKUA project aims to solve this issue by providing a tangible way to test an assembling task, as soon as possible in the design process. In this Tangible User Interface (TUI) based system, each CAD part is associated to a real world object, called interactor. Each action performed with these interactors is captured by a camera, and then visualized in the CAD software. From a usability point of view, it is very important to provide an appropriate interactor family. This paper deals with a design methodology for such a set. First, we show how an object can be characterized in the assembling context, regarding a theoretical definition of assembling task. Then, we detail how our methodology gathers together parts that share the same value for a given assembling criterion, and how it builds interactors from this analysis, as abstractions of each subset properties. Finally, we validate the proposed approach with an experimental use to find out an interactors set for mechanical parts assembling.