DETECTION OF FEATURE INTERACTIONS BASED ON THE REDUCTION OF A FEATURE S REAL FACE AREA

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1 DETECTION OF FEATURE INTERACTIONS BASED ON THE REDUCTION OF A FEATURE S REAL FACE AREA Ariffin Abdul-Razak, Ph.D Department of Production & Industrial Engineering Faculty of Mechanical Engineering Universiti Teknologi Malaysia Locked Bag Johor Bahru Abstract This paper presents an object-oriented, feature-based design system which supports the integration of design and manufacture by ensuring that part descriptions fully account for any feature interactions. Manufacturing information is extracted from the feature descriptions in the form of volumes and Tool Access Directions (TADs). When features interact, both volumes and TADs are updated. This methodology has been demonstrated by developing a prototype system in which ACIS attributes are used to record feature information within the data structure of the solid model. The system is implemented in the C++ programming language and embedded in a menu-driven X- Windows user interface to the ACIS 3D Toolkit. Keywords : Computer-Aided Design (CAD), Computer-Aided Manufacturing (CAM), object-oriented programming, feature-based design, feature interactions, Tool Approach Directions (TADs) INTRODUCTION The use of features is widely recognised by researchers as a key technology in the integration of Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) [Clark,87], [Case,88], [Lawlor-Wright,89], [Abdul-Razak,91], [Latif,93], [Gindy,97]. This technology which is most commonly seen in feature-based CAD systems is thought to be an essential component in the automation of manufacture by providing a direct approach to bridging the gap between design and downstream manufacturing applications such as process planning, assembly planning and numerical control (NC) code-generation programming. One of the common problems in feature generations methods, such as feature-based design, is feature interaction [Suh,95]. The existing feature-based design systems which deal with individual features run into difficulties when features interact with each other. Features have been identified by the international research community as meaningful abstractions, which humans can use to record their understanding of the product and manufacturing process design [Brimson,86], [Dixon,87], [Cunningham,88], [Latif,91], [Gindy,91]. Typically, the designer defines features as functional primitives, which can be embedded into a product model to serve as the basis for an object representation that will improve the quality of design and link to downstream feature-

2 based applications, such as process planning. This is the motivation for using features in many of CAD/CAM applications [Luby,86], [Chung,88], [Cunningham,88], [Shah,88], [Shah,91], [Case,94a], [Gindy,95]. Feature-based technology can be divided into two areas, namely feature recognition and feature-based design. Feature recognition attempts to identify features from an internal representation (geometric model) of a part after the design of the part's shape has been completed. Some of the popular techniques used are the graph-based method [Kyprianou,80], [Joshi,88b], [Sakurai,90], [Corney,93], the syntax-based method [Choi,84], [Li,88], the rule-based method [Chamberlain,93], cell decomposition [Amstrong,82], the convex-hull algorithm [Woo,84], [Ferreira, 90], [Kim,92], geometric reasoning [Joshi,88a], [Regli,93] and volume decomposition [Shen,94], [Tseng,94]. In contrast, feature-based design deals with the geometry of the features that can be automatically constructed by defining the features parametrically without requiring the user to translate the shapes of the component into geometric primitives. Some of the reported systems using feature-based design approach are QTC [Turner,88], LUT-FBDS [Case,94b], FSMT [Chen,94a], FREDS [Wozny,94], ASU Feature Testbed [Shah,95], IFBDS [Gindy,95]. However, most of the above feature-based design systems have limitations in terms of automatically handling interactions between features. The main problem that needs to be resolved in the feature-based design process is that feature interactions must be satisfactorily modelled if true automated process planning is to be achieved [Mill,93]. Feature interactions can be defined as an intersections of one feature s boundaries with those of another feature such that either the shape or the semantics of a feature are altered from the standard or generic definition [Shah,95]. Feature interactions are important for determining the sequence of machining processes for the part as they provide a constraint on the manufacturing methods that can be employed. In other words, feature interactions can make crucial changes to the sequence of the machining processes. For example, an interaction between two features occurs when the cutting of one feature affects the subsequent machining of the second feature [Chu,96]. Several types of interaction between features have been identified and given different names by different researchers: recursion (or sits-on) and adjacency interactions [Gadh,92], volume and boundary interactions [Bidarra,93], overlapping and touching interactions [Chen,94b], intersecting and non-intersecting relationships [Lee,95], interference, adjacency and remote interactions [Regli,96], overlapping and adjacent interactions [Gindy,96a]. In other words, a feature interaction relationship may exist because of proximity, overlap, geometric tolerance or other considerations that are important to manufacturing [Mill,93] or because of volume sharing [Vandenbrande,93]. Handling feature interactions in feature-based design system is still an unsolved research issue. Research on feature interactions in the area of feature-based design approach has had only limited discussion in the literature [Su,95] but has been described widely in feature recognition research [Vandenbrande, 90], [Tseng,94], [Narang,96]. The research involves analysing the interaction relationship, decomposing the interacted features into atomic or single features and defining their relationship [Suh,95]. In the feature-based design approach, the designer adds features and monitors the interaction of the added features.

3 One of the significant aspects that needs to be considered in order to fulfil the requirement of a feature model that can support the downstream feature-based application i.e. process planning is the capability of the system to properly maintain and update technological information in the feature descriptions, such as tool approach directions. The tool approach direction (TAD) is the direction by which any given feature is machinable. In other words, it is defined as the direction by which the given feature is accessible without interfering with the workpiece boundary [Joshi,88a]. Once features interact, the likelihood of tool approach directions changing is large. To illustrate the feature interaction problem, consider the part shown in Figure 1. In this part, Blind_Slot1 feature is destroyed if Blind_Slot2 is attached at the same axis as Blind_Slot1. As a result, a new through slot feature is created. In other words, the feature class of blind slot has been changed to through slot and the number of tool approach directions is also changed from 2 to 3. Due to the interaction between features, some of the faces that belong to a feature may be deleted, partially missing or split into several faces. The ability of a feature-based design system to deal with the issues of feature interaction and the updating of data such as tool approach directions is definitely required to support the product development process [Abdul-Razak,97]. This has become one of several research problems in feature-based modelling. Add Blind_Slot1 TAD1 Add Blind_Slot2 TAD3 TAD1 TAD2 TAD2 Figure 1 : Through slot creation after feature interactions OBJECTIVE OF RESEARCH The main objective of this research work is to establish the procedures for detecting feature interactions and updating data in the feature descriptions. To demonstrate and test the proposed procedures, tool approach directions for features are updated using both ray firing and volume sweeping techniques. The results will support the development and implementation of a feature-based design system based on objectoriented approach. In order to achieve the above objective, three major areas need to be taken into consideration. i) Conceptual design of the object-oriented feature-based system. ii) Implementation of the procedure for detecting manufacturing features which have interacted.

4 iii) Implementation of the procedure for updating the tool approach directions to determine the feasible tool approach direction. The conceptual structure of the research work is shown in the shaded part of Figure 2. Geometric Modeller (ACIS) Designer Designer Interface Feature Modeller Feature Interactions Face Area Reduction Object-Oriented Feature Classes Tool Approach Directions Ray Firing Technique Volume Sweeping Technique CAPP System Figure 2 : The conceptual structure of the research work The research is focused on the feature-based design of 2 1 / 2 -D machined parts for downstream feature-based application i.e. process planning. This limitation will help in focusing the effort on the objective of establishing the flexible feature-based design application that can be easily updated and expanded. An object-oriented methodology has been used in the construction of feature-based modelling system because the classification of features into types (i.e. slot, hole, pocket etc.) can be reflected in the object classes used for feature representation. The attributes of inheritance, encapsulation and polymorphism allow the object-oriented representation to be easily expanded. Six types of main features (which are commonly used in machining) have been identified. THE ARCHITECTURE OF THE PROTOTYPE SYSTEM The architecture of the prototype system is shown in Figure 3. There are four major modules comprising the prototype system, (i) the designer interface, (ii) the feature modeller with feature classes and solid modeller, (iii) a procedure for detection of feature interactions, and (iv) a procedure to check and update the number of feasible tool approach directions associated with each feature. The system (see Figure 3 (i)) has been developed using the ACIS 3D Toolkit, which is an ACIS package for graphical software development, based on the X Windows user interface. The designer interface allows the designer to create a feature model using the feature library. The graphic

5 designer interface makes the communication between system and designer easier than a system driven by text commands. (i) Designer (ii) Designer Interface ACIS 3D Toolkit Feature Classes HWU_OOFBDS Feature Modeller ACIS Solid Modeller Feature Manager (iii) Detection of Feature Interactions Procedure Determination of Tool Approach Directions (TADs) Updating TADs Procedure (iv) TADs Verification (v) Process Planning Figure 3 : The architecture of the prototype system The current feature modeller (see Figure 3 (ii)) is an interactive system that maintains an ordered list of features. The main function of the feature modeller is to build up a feature-based representation of a design. The designer may instantiate any feature at any time and is allowed to create complete feature definitions. The designer is prompted for all of the information necessary to completely instantiate the feature. The ACIS solid modeller [Spatial,95a] is embedded in the feature modeller in order to create a feature model s shape and part geometry and is chosen due to its availability and openness.

6 The implementations of feature interactions proedure (see Figure 3 (iii)) and feasible tool approach direction procedure (see Figure 3 (iv)) are discussed in the following sections. THE PROCEDURE OF DETECTION OF FEATURE INTERACTIONS This section gives an explanation of the procedure for detecting feature interactions. The interaction between features is detected by the reduction of a feature s face area. To illustrate this concept, consider a simple part shown in Figure 4. The figure shows a through hole primitive subtracted from the rectangular stock. Before subtraction, the area of each face of the rectangular stock is calculated and the faces are labelled with the real numbers 1.11, 1.12, 1.13, 1.14, 1.15 and 1.16 for front, back, top, bottom, side front and side back faces, respectively using the FaceID( ) function. As a result of the subtraction, the area of two faces, i.e. the top and the bottom faces have been reduced in area. This is determined by calculating the area of the real faces of the rectangular stock after subtraction. Top (1.13) Top (3.11) Cylindrical (3.13) Area of top face has been reduced Back (1.12) Bottom (3.12) Side back (1.16) Bottom (1.14) Front (1.11) Side front (1.15) Area of bottom face has been reduced Figure 4 : Changes of the face s area If the area of a face before subtraction is not equal to the area of a face after subtraction, it means that the face has been changed. Similarly, if the area of a face after subtraction is zero, then the face has been deleted. The difference in area of the real faces will determine which features have interacted. In other words, in the example, the reduction of top and bottom face areas means that the through hole primitive has interacted with the rectangular stock. The FeasibleTADirections( ) function is then called in order to check the tool approach directions (i.e. 2 in this example). The following is a general procedure to detect the interaction between features based on feature face area reduction. The flow chart of a procedure is shown in Figure 5 and is now summarised. i) Calculate the area of the real faces of the feature before subtraction.

7 ii) After subtraction, the CheckFaceArea( ) function is called to identify the faces of the new feature and recalculate the area of each real face. Calculate the area of real faces Subtract feature Call CheckFaceArea( ) while (feature =! NULL) call for CheckArea( ) Determine face identification Recalculate the face area for each feature Changed Deleted Unchanged Call for FeasibleTADirections( ) Figure 5 : A general procedure for detecting feature interactions iii) If the value of a face s area is reduced, then the face has interacted with another feature. iv) If the value of a face s area is zero, then the face is said to have been deleted. v) If the value of a face s area is unchanged, then the face has not interacted with other features. vi) The FeasibleTADirections( ) function is called to check a feasible tool approach directions.

8 vii) The same procedure is applied to each feature in the linked list to ensure all feature interactions are detected. In order to enable the checking of feature s face area every time the defined feature is called, the FeatureManager class has been developed. This class maintains a linked list of feature objects. IMPLEMENTATION OF FEATURE INTERACTION PROCEDURE This section uses an example to illustrate how the procedure to detect the interaction of features has been implemented in the prototype system. Due to feature interactions, some of the faces that belong to a feature may be changed in area, be deleted or split into several faces. To illustrate the implementation of feature interactions procedure, consider the part shown in Figure 6. This part consists of two through slot and through hole features. Top (1.13) ThruSlot_1 Side front (2.15) Back (1.12) Side back (1.16) Side back (2.16) Side front (1.15) Front (1.11) Bottom (1.14) ThruSlot_2 Bottom (2.14) ThruHole Figure 6 : Example of reduction of the face area Firstly, the rectangular stock is created. Then, the ThruSlot_1 feature is subtracted from the rectangular stock. ThruSlot_1 interacts with the rectangular stock by reducing the area of top, front and back faces of the rectangular stock. The top face has been split into two different faces, each of which has the same label. Next, the ThruSlot_2 is subtracted from the rectangular stock. The areas of the split top and side front faces of the rectangular stock have been reduced and also the area of the side front face of ThruSlot_1 is changed. In other words, ThruSlot_2 has interacted with the rectangular stock and ThruSlot_1. Finally, the ThruHole is subtracted. But this time, the area of the bottom face of the rectangular stock and the area of bottom

9 face of ThruSlot_1 have been changed. This means that ThruHole has interacted with the rectangular stock and ThruSlot_1. The steps for detecting feature interactions for this part are described as follow and illustrated in the flow chart shown in Figure 7. After feature subtraction Call for CheckFaceArea( ) Check the face area Determine faces that have been changed/deleted Call for Feature Update Procedures (i.e. FeasibleTADirections( ) or SweepVolume( )) Determine and update feasible TADs Yes More depression features? No Stop Figure 7 : The steps for detecting feature interactions and updating TADs i) Calculate the area of each faces of the rectangular stock with six faces in total, i.e front, back, top, bottom, side front and side back faces. ii) Subtract ThruSlot_1 from the rectangular stock. This will cause the system to recalculate the area of each face of the rectangular stock. Similarly, the areas of each face of ThruSlot_1 are also calculated. From the recalculation, determine which faces have been reduced in area. iii) Three faces are determined to have been reduced in area, i.e. front, back and top faces. In other words, ThruSlot_1 has interacted with the rectangular stock.

10 iv) Subtract the ThruSlot_2 feature. The same procedure is applied to recalculate the area of the rectangular stock faces and ThruSlot_1 s real faces. Again, the area of ThruSlot_2 s real faces are also calculated at this stage. The results show that the areas of the side front face and the top face of the rectangular stock, and the side front face of ThruSlot_1 have been reduced. This means that the ThruSlot_2 feature has interacted with the rectangular stock and ThruSlot_1. v) Finally, ThruHole is subtracted from the rectangular stock. The same procedure is applied to recalculate the area of the rectangular stock s faces, ThruSlot_1 s faces and ThruSlot_2 s faces. From the recalculation, the bottom face of the rectangular stock and the bottom face of ThruSlot_1 are shown to have been reduced. This means that the ThruHole feature has interacted with the rectangular stock and ThruSlot_1. After the procedure to determine the face area for each feature is finished, the system will call the FeasibleTADirections( ) function to check and update the feasible tool approach direction for the newly created or interacted features. This is described in the following section. IMPLEMENTATION OF THE FEASIBLE TOOL APPROACH DIRECTIONS PROCEDURE This section gives the result of updated tool approach directions based on the above example in order to show how the prototype system is able to update a number of TADs after feature interactions. Figure 8 shows how the TADs of the features can be updated. ThruSlot_1 TAD_1 TAD_3 TAD_2 ThruSlot_2 TAD_1 TAD_1 TAD_3 ThruHole TAD_1 TAD_2 TAD_2 TAD_2 Figure 8 : Updating a number of TADs of the example

11 After the ThruSlot_1 feature is subtracted from the rectangular stock, the procedure for updating tool approach directions is called. The prototype system determines that the number of TADs for ThruSlot_1 is 3. However, when ThruSlot_2 is subtracted from the rectangular stock, the prototype system updates a number of TADs for ThruSlot_1 from 3 to 4 and for ThruSlot_2 from 3 to 2. The number of feasible TADs for ThruHole remains 2. The feasible tool approach direction is checked by calling the FeasibleTADirections( ) function after each feature is subtracted from the rectangular stock. CONCLUSIONS The prototype system is able to detect the feature interactions by identifying changes in the area of a feature s faces. The reduction of a face area suggests that it might be possible to machine the feature from that direction (i.e. move the cutter into the feature through the material removed from the face). However, this does not mean that it is always possible. Figure 9 illustrates the situations where the reduction of face area create an unfeasible tool approach direction. TAD_1 TAD_1 3rd. through slot Ray_1 Unfeasible TAD_3 Ray_1 Feasible TAD_3 Ray_2 Ray_3 Ray_2 Ray_3 TAD_2 1st. through slot TAD_2 2nd. through slot (i) Unfeasible TADs even the area of a face is reduced (ii) Feasible TADs because of non-physical interaction Figure 9 : Feature interactions situations It also shows that non-physical interaction might also have an effect on the number of feasible tool approach directions. In Figure 9 (i), even when the second through slot feature is created, the number of feasible tool approach directions associated with it is determined to be 2. This is because Ray_3 determines that TAD_3 is unfeasible. However, when the third through slot feature is subtracted (see Figure 9 (ii)), the number of feasible tool approach directions for the second through slot feature is changed to 3. It is important to note that even though the second and third through slots do not physically interact, the addition of second through slot causes the number of tool approach directions associated with it to be updated. This is because the updating procedure is applied to each feature in the feature list when a feature interaction is detected.

12 The importance of feature interactions is that they provide constraints on the manufacturing methods that can be applied. The feature interactions can be considered as providing geometric or tolerance constraints on the manufacturing methods that can be applied in producing a component. The results of feature interactions are related to the planning methods to be used in machining a component. Despite the importance of feature interactions many problems remain unsolved. Even commercial systems, such as Pro/ENGINEER and Solid Edge, which use the concept of a feature-based design approach, are incapable of detecting feature interactions and so cannot take intelligent action when feature interaction occurs. This research work has developed the procedures for detection of feature interactions and updating the number of TADs automatically, to overcome the problems of feature interactions. This research work has focused on detecting the interaction between features in feature-based model and the updating of the feasible tool approach directions whenever interactions occur. Feature interactions are detected based on the reduction of a feature s real face area, where both ray firing and volume sweeping techniques are used to demonstrate how feature data can be updated. The concepts of object-oriented programming have been used to allow the feature classification to directly reflect a hierarchy of feature types. The output information obtained from the system can be used by a process planning system to assist in developing machining sequences for a part. REFERENCES [Abdul-Razak,91] Abdul-Razak,A., "Design by Features for Rotational Machined Components", M.Phil. Thesis, Loughborough University of Technology, [Abdul-Razak,97] Abdul-Razak,A., "Detection of Feature Interactions in an Object-Oriented Feature-Based Design System", Ph.D Thesis, Heriot-Watt University, [Armstrong,82] Armstrong,G.T., A Study of Automatic Generation of Non-Invasive NC Machine Parts from Geometric Models, Ph.D Thesis, Department of Mechanical Engineering, University of Leeds, [Bidarra,93] Bidarra.R.E. and Teixeira, J.C., Intelligent Form Feature Interaction Management in a Cellular Modelling Scheme, Proceedings of the 2nd. Symposium on Solid Modelling and Applications., pp: , May 19-21, [Brimson,86] Brimson,J.A. and Downey,P.J., Feature Technology : A Key to Manufacturing Integration, CIM Review, pp:21-27, Spring 1986.

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