نکاتی درباره استفاده از STEP برای واسط طراحی برای برنامه ریزی فرایند

This short note demonstrates the use of standard for exchange of product data (STEP) for interfacing design to process planning via a compact feature recogniser. The methodology used in development of the interface (feature recogniser) makes use of both automatic feature-recognition and feature-based design technologies in order to combine their advantages, and the STEP for the non-problematic and full information exchange. Using the abilities of the STEP, a generic configuration scheme is developed in which the features are treated as a combination of faces to which geometrical and/or technical information is glued (associated). By this way, the designer is only forced to identify the functional parts of the features when designing the part, which may simplify the component design and result in the effective memory utilisation. The feature recogniser was implemented in C on a PC and tested on a large number of examples with positive results.

Features are the fundamental elements of a product model. A feature is described as any geometric form that is used in one or more design/manufacturing activities, or it can be an information element representing a region of interest within a product [9]. Feature recognition is defined as the grouping of a set of faces on the surface of a part, such that each set corresponds to a feature. The recognition of features involves identification of higher-level features like pockets, holes, etc. from a set of lower level features such as surfaces, edges and vertices. The common methods that have been applied to the recognition of the features are syntactic pattern recognition, state transition diagrams, volume decomposition, set-theoretic or constructive solid geometry-based approach, graph-based approach, rule-based approach, neural-network-based approach, trace-based approach, etc. The feature recognition work has several drawbacks as discussed in Ref. [10]. This is probably due to difficulty of representation of a generic object on a solid modeller, and due to increased complexity of the features on the prismatic parts. One of the obstacles to widespread use of the features and the development of better feature recognition systems is the low level of support for feature data exchange via standard for exchange of product data (STEP). On one hand, the application of many systems developed for the feature recognition of 3D has not been considered thoroughly enough to verify its suitability for process planning [5]. Four approaches to using features in CAD/CAM applications have been used in previous work; human-assisted feature recognition, automatic feature recognition, design by features (DBF; sometimes called feature instancing) and feature-based design (FBD). All of the four approaches used within the feature technology have their own benefits and hindrances. Therefore, it is profitable to find or to use a mixed (hybrid) methodology that makes use of the advantages of the approaches, while eliminating the drawbacks of individual systems. In this paper, we have presented a feature recognition system, which is one of modules of an in-house process planning system. The input to the system is the STEP file created for a part which is modelled on any commercially available solid modelling environment. The part model is then translated from the STEP format into an equivalent format (structure) based on boundary representation (B-Rep) scheme that is accessible and manipulable in the application environment. Orientation of each face of the part is determined. Relationships between adjacent faces of the part are found based on ‘concavity’, and these relationships are stored in a ‘relation matrix’. By tracing the elements of this matrix, part features are extracted. Then, they are identified and recognised by using a variable coding-scheme based on the numbers of faces, edges, etc. on the part features. The methodology of the developed system is illustrated with examples throughout the paper. It has an ultimate goal of underlining the importance of combining the advantages of existing approaches for the part representation, namely automatic feature recognition and FBD systems. Employing STEP for full and non-problematic information exchange between dissimilar CAD/CAM systems is strongly recommended.

In this paper, we have presented FRS-PP, which is one module of an in-house process planning system (OPPS-PRI) running on PCs. Many components (like joshi3.stp, ss_base.stp, english_metric.stp, vand_orig.stp, etc.) (including those contained in the Design, Process Planning and Assembly Repository, available at the following URL addresses 1) have been tested with the developed system. The results are promising from computing times point of view that the system is computationally efficient. The computing time to recognise the 12 features of the example part is 29 s needed on a Pentium III-350 PC. It should be noted that as the number of vertices, edges and faces (not the number of features) increase, then the computing times necessary for the recognition of the features also increase. For instance, for a typical component with 30 features, 212 faces and 587 edges, the computing time increases up to 3 min. The tests performed on the numerous examples prove that the CAD/CAPP integration for 3D components has been successfully fulfilled by the FRS-PP system. Some of the characteristics and limitations of the system and issues to be investigated in the future research are as follows. By the use of the STEP, technological information is included in the part models. The initial blank is assumed to be a cubical billet. The work is under development for including semi-finished (irregular-shaped) parts like forged workpieces. Due to flexible and generic configuration scheme used for the identification of the features, the intersecting or combined features can be effectively recognised by the developed system, if they are made available in the feature database. The FRS-PP system combines the strength of the FBD and automatic feature recognition approach for closing the gap across CAD and CAM. In the CAD/CAPP integration, the way in which the part model is created has little importance. The system has a modular structure. By accepting the STEP files as the input, the FRS-PP do not close the front end of the CAD/CAM integration to other CAD systems (commercially available or any other in-house systems) which is one of the most important advantages over the system presented in Ref. [8]. Therefore, any CAD system capable of generating the STEP files can serve as the front end of the developed feature recognition system presented in this paper. Hence, the FRS-PP might be used as a universal feature recogniser. Future works should also be focused on the effective use of STEP, which provides a powerful platform for the global standardisation of information exchange related to automated manufacturing by allowing the exchange of both the technological and geometrical information.