Implementing manufacturing feature based design in CAD/CAM

Transcription

1 Implementing manufacturing feature based design in CAD/CAM T. Szecsi School of Mechanical and Manufacturing Engineering, Materials Processing Research Centre, Dublin City University, Dublin 9, Ireland Abstract The paper presents the development of a new design system that allows the composition of a design from manufacturable features. The system contains several modules: hierarchical design-for-manufacture rule system, manufacturing feature library, manufacturing feature-based design module, designer advisory module, manufacturing feature recognition module, and design analysis module. Keywords: Manufacturing Feature Based Design, Feature Extraction, Design for Manufacture 1. Introduction Conventional CAD/CAM systems normally compose designs using geometric primitives and Boolean operations performed on them. A 3D solid model, for example, can be composed using cubes, cylinders and other geometric elements, extrusions, rotational solids, etc, and operations like subtracting and intersecting volumes can be applied. More modern CAD systems implement a set of more advanced operations for creating 3D solid and surface geometry. These include surfaces passing through several generating profiles, smooth transition from one generating profile to another, volumes included between predefined surfaces, and others. However, whatever the set of such operations, the design is purely geometry-driven. The major problem using such technology is that the design does not include information for the subsequent phases of the product development cycle. Once the design is ready and is passed over to the next expert, the designer s intent is lost. Other experts can only obtain information from the design relevant to them by interpreting the geometric elements in the design and map them into features of higher order. Fig. 1. Part with pocket. The part in Figure 1, for example, contains a pocket. Although any process engineer would be able to identify the pocket in the design, the very concept of a pocket is not directly represented. It can only be obtained by mapping the combination of the existing geometric primitives to known concepts. Although this approach can be used easily with simpler designs, more complex ones make the process error prone. This is

2 especially true when the process is automated, like in a Computer Aided Process Planning (CAPP) system. With traditional design approaches, most of the information included in the design is geometry-related. However, part geometry is not the major concern of most experts in the product development cycle. From a manufacturing point of view, for example, actual part dimensions are of secondary importance. Production volume, material, accuracy and surface finish are much more important. In order to include information for other experts in the design, a technology called Feature-Based Design can be implemented [1]. Although many modern CAD/CAM systems support feature-based design, virtually all features are limited to the functionality of the design, thus they are design features [2]. Features like slots, pockets, holes, fillets, bands, steps, used in most CAD/CAM systems, are pure design features, and do not contain actual manufacturing information. Most systems do not even have the capabilities of a full-scale validation of the functionality of the design features. These systems would allow the insertion of a design feature whose functionality is not consistent with the design. Although mapping a design feature to a manufacturing feature is easier than mapping geometric primitives, the fact that manufacturability is not considered at the design phase means that later design changes due to manufacturing problems are costly. There have been numerous attempts to deal with manufacturing information in features [3, 4, 5], but the consideration of manufacturability in feature-based design is still very limited despite the fact that there is a large collection of rules and guidelines available [6]. In order to reduce manufacturing costs, a fully implemented manufacturing feature based design system is being developed at Dublin City University. 2. Manufacturing feature based design system The aim of the project is to develop a software system that enables designers to appreciate manufacturing process capabilities and limitations during the design phase. The system will be linked to a commercial CAD/CAM package Pro/Engineer that supports parametric feature-based design. The system consists of several stand-alone, but interconnected modules: 2.1. Hierarchical design-for-manufacture rule system For the last decades, a large number of design-formanufacture (DFM) rules (guidelines) have been developed [6]. In order to apply them, they have to be systematised and organised into a hierarchical rule system. Rules at the higher level of the hierarchical system are applied to more generic manufacturing features, and more specific rules are applied to more detailed features. Rules at lower levels of the hierarchy (children) inherit the characteristics of their parent rules (Fig. 2) For example, the specific feature centrehole inherits all the parameters and DFM rules from its parent, more generic features that reside at higher levels of the hierarchy. In addition to the inherited rules and parameters, a feature may have its own set. The hierarchical structure enables to minimise the number of rules used and to avoid repetition when applying the rules. It also makes the extension and modification of the rule system easier, and gives a much clearer structure. Holes Rule: Avoid non-round holes if possible Machined holes Rule: Entry surface should be perpendicular to the spindle axis Rule: Avoid non-round holes is possible Rough machined holes Centre hole Fig. 2. Hierarchical design-for-manufacture rules. The hierarchical rule system is based upon a hierarchical process classification system. All manufacturing processes are arranged in a hierarchical, tree-like structure. Although the structure of this system is different from that of the DFM rules, the two systems show similarities. The link between the rule system and the process system is established using pointers (Fig. 3). Generic and specific rules can point to both process groups and individual processes. Pointing to process groups as opposed to specific processes facilitates inheritance. Since manufacturing processes can be classified into many different systems, the one that is closest to the hierarchical structure of the DFM rules results in the

3 simplest rule application. This means that the process classification system is developed with DFM in mind. L2 Processes Material transformation Material joining Material removal Material addition A1 D1 A2 Internal External Round Non-Round L1 Rough Drilling Boring Twist drill Centre hole Finish Entry surface should be perpendicular to tool rotation axis Maintain correspondence between tolerance and roughness Fig. 4. Parameterised feature centre-hole. Feature datum Avoid drilling deep holes Fig. 3. Pointing DFM rules to processes. The extension of the hierarchical systems is relatively simple, provided the existing classifications are not altered, but only expanded. In this case, the extension means that new rules and processes are added to the hierarchical trees without changing the remaining structures Manufacturing feature library Manufacturing features contain the following information: parameterised geometry (Fig. 4) of the feature, the description of the manufacturing process to produce the feature (including machine tool, tooling, possible fixtures, manufacturing conditions, production volume), relative cost information, design limitations, functionality rules, and links to design-for-manufacture rules. A feature also includes the description of the datum surfaces that link the feature to other, already inserted features (Fig. 5). This description also contains rules of what types of surfaces the feature can be linked to. Fig. 5. Feature data. Since a manufacturing feature contains information about cutting tools, too, limitations to feature dimensioning may apply due to the fact that the feature will have to be produced using standard tools. For example, a manufacturing feature centre-hole can only have dimensions that match one of the combinations in Table 1. Table 1. Standard centre-drill parameters [7] D1 (mm) L1 (mm) A1 (deg) A2 (deg) Features are selected from a hierarchical feature library (Fig. 6). Since inserting a manufacturing feature into the design involves manufacturing process planning, the designer is guided in his/her selection.

4 Pockets Holes Slots Forged Punched Machined Cast Design Functionality Rules DFM Rules Rough Rough+Finish Cutting Tool Cutting Conditions Machine Tool Datum Cost Fig. 6. Manufacturing Feature Library. Drill Drill+Countersink Drill+Counterbore Centre Hole 2.3. Manufacturing feature-based design module This module allows designers to compose a 3D solid model not from geometric primitives, but from a set of manufacturable features (Fig. 7), using feature libraries. Manuf. Feature Keyway Cutting tool Cutting Conditions Machine tool Fixture Datum Cost checked and possible problems are identified. Features that may cause functionality and/or manufacturing problems are rejected. This ensures that a design remains consistent with the specifications and will not cause manufacturing problems. A possible reason for functionality and/or manufacturing problems is improper feature dimensioning. Consider the length L2 of the conical surface of the centre-hole feature in Figure 9. The other dimensions of the feature were limited to a certain combination of the dimensions based on the values in Table 1. But L2 could not be assigned an exact value because its length depends on the drilled depth. However, due to functionality and manufacturing problems, L2 should not exceed the effective length of the cutting edge of the centre-drill LT. Improper dimensioning of L2 will cause functionality and manufacturing problems. If L 2 > LT, the exit surface of the centre-hole feature becomes cylindrical, thus preventing a centre from having a proper contact with the hole. This is a feature functionality problem. At the same time, attempting to drill a centre hole deeper than the cutting edge of the drill would result in the cylindrical surface of the drill (which has no cutting edge and flute) blocking the outlet of the hole during drilling, thus preventing chips to escape the hole and cutting fluid to enter the cutting zone. L2 LT Manuf. Feature Rolled Bar Manuf. Feature Centre hole Fig. 7. Designing with manufacturing features. The feature is positioned in the design by relating its datum surfaces to existing surfaces of other features in the design (Fig. 8). This link is maintained even if the design is modified and ensures its consistency. Fig. 9. Improper feature parameter. Fig. 8. Linking of feature datum. By inserting a manufacturing feature, the linked functionality and design-for-manufacture rules are Since functionality and DFM rules are not linked, it might be the case that a feature is rejected by one of them only. The centre-hole features in Figure 10 are only rejected by the functionality rules. The one on the left because the axis of the centre-hole does not coincide with the axis of the rotational body, which is the functional violation of the concept of a centre-hole.

5 (It should be noted that if the machine tool specified is a lathe, this feature would also be rejected by the DFM rule system because it would be impossible to be machined). The feature on the right results in a too thin wall L5, thus causing functionality problems of the centre-hole.. Fig. 10. Rejected features due to functionality. The drilled-hole feature in Figure 11 is only rejected by the DFM rule system by violating the rule that Exit surfaces of drilled holes should be perpendicular to the drill axis. However, the feature does not violate any functionality rule (in fact the opening at the bottom of the hole would ensure that no air is trapped inside the hole during inserting a pin into it). L Designer advisory module This module is linked to the Manufacturing featurebased design module. Its aim is to provide assistance to designers when selecting manufacturing features. Since functionality and DFM rules are applied in real time, during the actual design process, the designer is warned if they attempt to include features that are difficult to manufacture or violate functionality rules. Alternative solutions are suggested. The module also provides process engineering background for designers Manufacturing feature recognition module Although the major concern of the system is to compose a design using validated manufacturing features, in addition to the direct feature-based design system, a feature recognition module is being developed. It will be beneficial in cases where a design is not using features. The aim of this module is to translate a 3D solid model into manufacturable features. The geometric elements of the solid model are analysed and features that are recognised as manufacturing entities are extracted and organised in a topological manner. The extracted features are mapped to existing features in the feature library Design analysis module. Fig. 11. Rejected feature due to manufacturing. Many features may be rejected by both functionality and manufacturing reasons. The centrehole feature in Figure 12 violates the functionality of the feature and causes manufacturing problems due to the entry surface not being perpendicular to the drilling axis. This module analyses a 3D solid model in terms of functionality and manufacturability. Based on manufacturing feature recognition (extraction), the functionality and manufacturability of the model is defined by applying the functionality and hierarchical design-for-manufacture rule set the same way as during the direct feature-based design process. Elements that violate the rules are highlighted and the corresponding rules are referred to. Since feature extraction lacks the benefits of exactly reflecting the designer s intent (as opposed to feature-based design), this type of analysis can not be conclusive and requires extensive manual interaction. 3. Conclusions Fig. 12. Rejected feature due to functionality and manufacturing. The manufacturing feature based design system allows the designer to compose a design using manufacturable entities. The inserted features are

6 validated from functionality and manufacturing point of view. The functionality rules ensure that the design is consistent with the functionality requirements. The manufacturing rules enable early detection of manufacturing problems during the design phase. Solutions that are likely to cause manufacturing problems are rejected, or alternative solutions are suggested. This leads to reduced manufacturing costs. Acknowledgements Dublin City University is a partner of the EUfunded FP6 Innovative production Machines and Systems (I*PROMS) Network of Excellence ( References [1] Shah JJ and Mantyla M. Parametric and Feature-Based CAD/CAM: Concepts, Techniques and Applications. John Wiley, [2] Salomons O.W. et al. Review of research in feature based design, Journal of Manufacturing Systems, 12(2), 1993, pp [3] Brunetti G and Golob B. A feature-based approach towards an integrated product model including conceptual design information. Journal of Computer- Aided Design. 32(14), 2000, pp [4] Li WD et al. Feature-based design in a distributed and collaborative environment. Journal of Computer-Aided Design. 36(9), 2003, pp [5] Basak H and Gulesim M. A feature based parametric design program and expert system for design. Journal of Mathematical and Computational Applications. 9(3), 2004, pp [6] Bralla J. Design for Manufacturability Handbook. McGraw-Hill, [7] Green RE et al. Machinery s Handbook. Industrial Press Inc., New York, 1995.