can be extracted form tolerances and design attributes as well. For example, "a thread attribute may be taken as a hole hint". This approach has been more successful in recognizing intersecting features. However, the efficiency of the approach has been argued, as there could be a huge number of traces that won't lead to valid features. Some authors have been in favor of using a hybrid of graph based and hint based FR to improve the efficiency of hint-based reasoning. In the hybrid approach, graph-based reasoning is used to find out those regions of the part that certainly lead to valid features when used by the hint based reasoner. Other existing FR approaches are volumetric decomposition, Artificial Neural Networks, and expert systems Babic et al. briefly introduces many of them. However, building feature recognition systems that function effectively on real industrial products has been elusive. A real product with hundreds of faces and end edges brings almost all the above approaches to a halt due to computational complexity. Furthermore, the features studied in these approaches are usually over simplified. The bulk of the feature recognition literature normally deals with 2.5D features (those made by sweeping a 2D profile along a linear axis). Graph representations, hint definitions or volume decompositions are much more difficult to define for 3D and free form features. The work done by Sundararajan is focused on free form surfaces, but again it is limited in application. Oversimplification is also evident even in the course of 2.5D features. For example, feature recognition algorithms usually assume sharp concave edges in the feature geometry. However, such edges are barely used in real design of mechanical components due to manufacturing constrains. Some of these issues such as the presence of filleted edges and free form surfaces in the model have been studied by Rahmani and Arezoo. Commercial feature recognition systems Few commercial feature recognition systems are also available. Though feature recognition technology can be applied for various applications, commercial software have effectively adopted feature recognition technology for recreating the feature tree from imported models so that even the imported models can be edited as if it were a native solid model. Major 3D CAD modelers have Feature Recognition to convert imported 3-D models into native feature based models. CAM software and design for manufacturing software are also built using this feature recognition technology. Few CAD/CAM software have used commercially available third-party