## 2 Materials and Methods
### CAD Feature Recognition Method  
Feature recognition is the procedure used to extract features from a geometric model. As illustrated in Figure 2, through this step, it is possible to retrieve information from a 3D CAD model and to connect product features with the DFW rules. A feature recognition procedure begins by defining the types of features to be identified. Currently, a shared methodology for feature classification is not yet available because it depends on the application scenario [35]. In the past, researchers have defined multiple categories of aspects that were used in this paper to realize a feature recognition framework. Many studies were focused on the feature extraction for machining operations [36; 37; 38]. However, feature recognition for welded products has had its share of interest within the academic world [39; 40; 41]. Figure 4 illustrates the types of features used in this research and their relationships. Figure 3: Example of W025 DFW guideline syntax and picture. Before proceeding with the description of the method, it is worth noting that the application of the feature recognition system is dependent on the way in which the 3D CAD model is developed. Indeed, two scenarios are possible when a welded product is modeled using a 3D CAD system. The first scenario, which is the one considered in this study, refers to a 3D model where the weld bead (or the weld point for resistance welding) and the manufacturing information are included (see Figure 5). Indeed, commercial CAD packages include functionalities to input welds and represent them through solid volumes or annotations in 3D representations of products. The user usually selects a pair of faces, and the system proposes alternative weld bead shapes to join the faces. The beads are created as associative features for drawings and are taken into account in computations of mass properties of products. Figure 4: Feature recognition framework (class diagram). Figure 5: Computer-aided design (CAD) modeling of welded products with weld beads and product manufacturing information. The second scenario refers to a 3D model where the weld bead is not modeled and all the necessary information for the manufacturing phase is included only in the 2D drawing issued for manufacturing purposes. This second scenario is still a standard practice in many design departments, even though the newly developed computer-aided packages provide a dedicated environment for the modeling of the welded product and the possibility to include welding PMI within the 3D model. Moreover, the fact that complex welded products require a collaborative design approach with the involvement of several operators and often the file exchange through a neutral file from and to different CAD tools makes this practice more widespread. Indeed, in most cases, neutral files (i.e., stp, jigs, or stl) do not allow storage of PMI, and all the welding data reported in the 3D model is lost when converting. However, this second scenario is not considered in this research due to the limits in the analysis of the welding features and parameters. It is worth noting that some researchers have tried to rebuild welding features from a neutral file (i.e., STP) even if some important information needs to be manually inputted by the user [41]. Based on this limitation, only new product development processes or design reviews that start from native files with the PMI related to the welding process are considered. The first type of feature to recognize belongs to the cluster \"component feature\" and refers to each item (part) of the assembly. A component feature is a physical attribute used to represent the component [42]. It describes the most relevant characteristics of a component, such as its material (i.e., material feature), mass, volume, and area. Each part of the welded assembly has only one component feature (i.e., only one mass, only one volume, only one material).