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A Progressive design and manufacturing evaluation system incorporating STEP AP224
Computers in Industry 47 (2002) 155±167
A Progressive design and manufacturing evaluation system incorporating STEP AP224 R. Sharma, J.X. Gao* School of Industrial and Manufacturing Science, Cran®eld University, Cran®eld, England, MK43 0AL, UK Received 29 March 2001; accepted 18 August 2001
Abstract Most of the important cost related design decisions are taken in the early design stages. More often than not, designers have little or no knowledge of the manufacturing dif®culties. Therefore, a design engineer typically designs components without consideration for manufacturing dif®culties. This paper describes a product data management (PDM) based system for manufacturing evaluation and analysis in the early design stages. The system is based on a process planning system and an embedded expert system to resolve the abstract data usually associated with the early design stages. The system will allow early measurement of design in terms of time, manufacturing cost and resources. Early measurement of design would be especially helpful in supporting new design techniques like simultaneous design and concurrent engineering. The system is being developed in industrial collaboration with LSC Group, UK, which is a pioneering company in the ®eld of automated process planning using STEP (standard for exchange of product data) AP224 standard. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Computer-aided process planning; Conceptual design; Product data management; Manufacturing analysis
1. Introduction In the manufacturing environment of the 21st century, there is no place for traditional `over-the-wall' approach to design and manufacturing. Time is of essence, and the success of a design project is often judged by the time taken to bring a product to market. There has been an exponential increase in the application of new software tools to remove the barriers between design and manufacturing. However, research in computer-aided design and manufacturing (CAD/ CAM) has been aimed at providing computer support for the later design stages. Automated analysis of
manufacturability and process planning during the conceptual design stage has become the focus of the research community. More often than not, the designers have little or no knowledge of the manufacturing dif®culties. The result is a design, which may meet the design speci®cations, but is costly or dif®cult to manufacture. This paper reports on the development of a product data management (PDM)integrated early design system, which also provides manufacturing evaluation and process plan generation. Some key concepts used in this project are explained below. 1.1. Progressive design
*
Corresponding author. Tel: 44-1234-750111x5417; fax: 44-1234-751-172. E-mail address: [email protected] (J.X. Gao).
Much research has been devoted to the theory of design. Extensive comparison of the various systema-
0166-3615/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 3 6 1 5 ( 0 1 ) 0 0 1 4 6 - 4
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tic modern design methods can be found in literature [1]. Product design begins with a need and ends with detailed drawings and all the accompanying information necessary for manufacturing the product. Product design is a complex and iterative process [2], and most studies have divided the design process into two, three or four stages. The classic design model described by [1] has four stages, i.e. design speci®cation, conceptual design, preliminary design (also called embodiment design or fundamental design in some studies), and detailed design. The transition of design from one stage to another is a gradual process and the design evolves from one stage to another rather than an abrupt jump from one stage to the next. Especially in the context of providing computer support for design process, it is dif®cult to separate these design stages individually. With the aim of developing a computer system to support the product design during all the stages, this research combines the ®rst three design stages together as the early design stages. A common characteristic of all the three stages of early design stage is that the design options are still being explored and compared. Design data in these stages is incomplete and abstract. The detailed design stage is more about re®ning the design selected in the early design stages. A new term has been coined to describe the evolution of design during the early design stages, i.e. progressive design. The concept of progressive design is especially bene®cial when the designer wants to use the same design tools or data across the design life cycle of a product. This will become clear and obvious in the subsequent discussions. 1.2. Computer support for early design There are various approaches for addressing the problem of product model description, representation and visualisation during early design. The representation ranges from computer-oriented methods at one end of the spectrum and human-oriented methods at the other (Fig. 1) [3]. Language based early design systems like CANDLE [4] are easy to implement but are abstract and may not be what a designer expects from a design system. Similarly, the image or interface based systems at the other end of the spectrum are computationally expensive but are the preferred mode of interaction for a designer. More about the current
Fig. 1. Computer support spectrum [3].
state of system for conceptual design of mechanical products can be found in [3]. 1.3. Manufacturability Manufacturability of a product depends upon the design domain, and a number of other related factors, like the available resources. In general terms, manufacturability can be de®ned as an indication of the effort required for manufacturing the product. The system being developed measures the manufacturability of the product in the early design stages, in terms of manufacturing time, costs and resources. Given a set of manufacturing resources and product information, the problem of manufacturing evaluation is reduced to determining whether or not the design is manufacturable. If the design is found to be manufacturable, the next step is to determine one of the evaluation metrics of manufacturability. Manufacturability metrics is usually one of the following: Boolean: this is the most basic manufacturability rating. It is a Boolean that simply reports if the part ``can'' or ``cannot'' be manufactured with the given resources. This can usually be achieved by generating a process plan. Cost: cost estimation has been an important area to study both in manufacturing and marketing communities [5]. Prompt estimation of cost with accuracy is crucial. Cost of manufacturing is a quantitative measure. Since all manufacturing operations have an associated cost, it is easy to
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arrive at a figure based on the manufacturing resources required to produce a product. The cost of raw material and its handling could also be included in the estimate. This figure may not be very accurate but is usually acceptable if it is within limits. Time: manufacturing time is also a quantitative measure of manufacturing process. Every manufacturing process has an associated time. In most cases, it is the manufacturing time that is used to calculate the manufacturing costs. The quantitative measures of time and cost, may not be directly helpful in determining if the design is good or bad or if it meets the required specifications, but it is one of the most common constraints for a designer. Qualitative measure: Ishii [6] qualified designs using adjective qualifiers: `excellent', `very good', `good', `poor', `bad' and `very bad'. These qualifiers were mapped on to a (0,1) measure for comparison. But such measures are hard to interpret and compare, especially if the rating comes from different systems or designers. Abstract: this is similar to the qualitative measures discussed above, but involves each design attribute being assigned a manufacturability index or producability index (PI) instead of a qualifying adjective like qualitative measures. As with the qualitative measures, it can become difficult to interpret or compare designs if the indexes are from different systems. 2. Previous work in manufacturability evaluation Subramanyam and Lu [7] described a framework for an AI-based computer-aided design environment to ensure manufacturability of product design for components manufactured in small and medium lot sizes. The authors emphasised on unifying and integrating the design and process planning knowledge into the design environment. The framework consisted of an explicit product model and an explicit manufacturing facility model. These models were used by an AI-based reasoning system to determine if the product design satis®ed all machining related concerns. The product and manufacturing facility model were based on a complex combination of featurebased models and geometric models.
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Priest John and Sanchez Jose [8] developed an empirical methodology for measuring design-formanufacturing (DFM) early in the conceptual design stage and also in the later detailed design stages. Their methodology was based on identifying producability factors, which could be quanti®ed and combined by means of a simple formula. This was done using evaluation rules. Gupta ans Nau [9] described a plan-based automated manufacturing analysis system. The system was speci®cally targeted at early design of prismatic parts. The ®rst step in the analysis was to generate at least one process plan for the product. In case, at least one process plan was feasible, it was assumed that manufacturing the part was feasible and the analysis progressed to the next stage. The analysis methodology was to identify all the possible manufacturing operations required to produce the part, and using these operations, different process plans can be generated. These plans were analysed and compared to suggest improvements and to reduce the number of steps for machining. Mukherjee and Liu [10] described a conceptual design system based on sketching abstraction. A sketching abstraction is a minimal representation of a design object using functional features, quasi-links and quasi-nodes, so as to provide an intermediate representation between pure geometric and pure functional form. The system was speci®cally for conceptual design of stamped metal parts. It provided the designer with feedback on manufacturability constraints. Pham and Ji [11] described a concurrent design environment capable of providing information for manufacturability assessment including machining time. The system was based on feature recognition from Pro/Engineer1 constructive solid geometry (CSG) model. The feature information retrieved from the CSG model was then checked against the capabilities of the available resources. The detailed manufacturing information was generated only if the speci®ed requirements of each manufacturing feature could be satis®ed. The manufacturing information sequence or the process plan was generated using a manufacturing knowledge base. Other notable efforts to evaluate manufacturability have concentrated on feature accessibility, set-up costs and redesign [12,13] manufacturing process selection
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[14] and concurrent product design and process planning [15]. Based on the literature survey of existing manufacturing evaluation systems, the approaches to derive manufacturability metrics from design and resources as input can be broadly divided into two categories, i.e. 1. Direct or rule-based: in this approach, analysis is based on direct or rule-based interrogation of the design description to assess manufacturability. Rule-based systems use a set of guidelines to evaluate the feasibility of a proposed design. They may also provide recommendations to designers for violated rules. Rule-based approaches do not usually provide quantitative metrics related to manufacturability that can provide a basis for comparison. 2. Plan-based: this approach is based on generating the manufacturing process plan and then analysing this plan (or its rating) based on a pre-selected criterion. Rating-based approaches like this, typically provide a quantitative evaluation based on scales like time or costs. The major advantage is that it can compare alternative designs and optimise a design based on a selected objective. Because DFM rules are not applied, this approach, in most cases cannot provide speci®c redesign recommendations. It emerges that most manufacturing evaluation systems reviewed are feature-based, directly or indirectly. Manufacturability evaluation is either done through a priori such as DFM guidelines, or through a posteriori such as analysis tools which take the geometric or feature model as the input. The a priori approach is usually rule-based, while a posteriori analysis has often been implemented using a plan-based analysis. The analysis is usually separate from the detailed process plan generation. In most cases the analysis module and process plan generator module are completely separate. The only exception is in some planbased analysis systems. Some systems integrate design and process planning to allow simultaneous design with process planning. But even in such closely integrated systems, design and process planning are sequential activities. There are some modular systems like the OPPS±PRI system [16] which allow invoking of modules in a non-sequential manner but even here the integration is limited to sharing of a common data
model. Some systems provided redesign suggestions based on design rules. Some others provided analysis on feature accessibility and set-up costs. 3. The proposed approach One aspect of early design, which cannot be ignored while developing any computer system to support it, is the requirement of visualisation of the design. Visualisation of the product and its constituent features, is based on a complete and evaluated geometric or feature model. In a feature-based environment, the direct approach adopted by most manufacturing analysis systems, breaks down or becomes very complex to implement if the component under analysis has interacting manufacturing operations or features. This is because it is not possible for a set of ®nite rules to cater for every possible case. Also, all such approaches reported are based on feature recognition or feature mapping, which require a fully validated and completed geometric model [17]. This primary requirement of having a fully valid and complete model is precisely the restriction in early design. It is, therefore, clear from the discussion so far that both design visualisation and manufacturing evaluation require a complete and valid feature model. The proposed feature-based conceptual design system (FBCDS) is a system presently under development at the School of Industrial and Manufacturing Science at Cran®eld University, as a tool for early design using incomplete or abstract product models. The system allows the user to create a part by creating a hierarchy tree of its constituent features. The user can visualise this feature tree as a solid model and save it as an ACIS1 solid model. The system provides early feedback on various aspects of design like manufacturing costs and manufacturing effort (in units of time). This is accomplished by integrating a feature-based modeller based on the ACIS 3D kernel with a process planning system. Like most other feature-based design systems, this system is also based on feature libraries. Current computer-aided design tools do not adequately support the progressive design paradigm. The computer-aided conceptual design tools, where used, rarely use a product model, which can be directly carried over to the detailed design stages. The result is a break in the information ¯ow. Furthermore,
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transition of design from early design to detailed design involves a lot of rework as data from the early design stage has to be redeveloped using the CAD system to be used in the detailed design stage. A solution to this problem is offered by the new generation of PDM systems. PDM applications have a common purpose of providing con®guration management to engineering databases and the intent to start bridging the gaps between islands of automation. Con®guration management in this sense represents a disciplined approach to de®ne elements of product data, to control its change, and to track the changes made. For any application systems to be successfully integrated into an existing enterprise, it is mandatory that it be closely integrated with the PDM environment. Integrating a computer-aided conceptual design system with a PDM environment would provide a suitable framework for data sharing as well as implementation of work¯ow. This is the best way to automate the clerical tasks associated with progressive design and transfer of design to the next stage. This would also avoid one very important pitfallÐmanual re-entering of data in the detailed design stage. Using the PDM system, it would be possible to
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carry design from the early design stages to the detailed design stage in a seamless manner. In contrast, the detailed (conventional) CAD systems would take over where the early design system leaves. FBCDS has been very closely integrated with the PDM system. All databases and resources are held within the PDM system. This has been done with an aim to support progressive design (as de®ned in the initial discussions). The PDM system is also used to carry design over to the detail design phase. 4. The FBCDS system architecture The FBCDS system architecture is shown in Fig. 2. This system is based on the premise that the designers should be able to represent the preliminary design in terms of meaningful features which have manufacturing information associated with them. The representation of preliminary design as meaningful features allows close integration of design system with the computer-aided process planning (CAPP) system. This also eliminates any requirement of feature translation or feature recognition, allowing incomplete
Fig. 2. System architecture.
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product model to be processed for manufacturing evaluation and process planning. Loop `A' in Fig. 2 shows the information ¯ow between the conceptual modeller and the process planning system. The system can be divided into four distinct segments, which are discussed below. 4.1. Feature-based conceptual design system This is basically a feature model editor (FME) [18] that provides the user facility to build up the feature hierarchy tree using a simple interface. It also includes the feature library editor and solid model viewer. A feature library based on the STEP AP224 [19] is provided as standard. The users can however create their own features and use them. Most CAD systems display the features of a component as a feature tree where the features are arranged in a de®nite hierarchy. Although, hierarchy seems to be the logical way to represent features, AP224 does not support the concept of feature hierarchy. A very simple logic has been developed to cater for hierarchy. Each feature-type is given a logical hierarchy level and a set of possible parent features. For example the base_shape feature has the hierarchy level of 0 and no possible parent features as it is supposed to be the ®rst feature in the hierarchy. It must be noted here that this methodology is in no way comprehensive. But is suf®cient for this system since the hierarchy is not important for process planning. The system offers a facility to edit the feature attributes. The validity of the feature attributes is checked against a set of design-for-manufacture rules which are developed in CLIPS. The system at present has only simple rules like checking for the shell/wall thickness. But more rules can be developed depending on requirement and application domain. This is not intended to be a full scale CAD editor. The user would need to use a conventional CAD system for detailed editing. A user con®gurable feature library has been implemented in FBCDS using the component technology. The data about the features, which includes all the attributes and parameters is entered using a graphical tool, called the feature model editor. The feature de®nition is stored in a plain ASCII ®le. Each feature in the library is identi®ed by a unique `handle' which is the feature name. Implementation of feature visua-
lisation without recompiling the complete software is done by compiling the visualisation routines of the user feature library into a component object model (COM) component. COM or COM is a standard that de®nes how to build interoperable components [20]. Components are described as reusable piece of software in binary form that can be linked with other software with relatively little effort. Using components is a much more productive way to design, build and reuse software. In order to introduce a new feature called `hole' in the library, the developer ®rst decides on the attributes of the feature, which are required to completely describe it. The user can create the feature hole in the graphical library designer by simple visual commands. This de®nition of the hole is stored in an ASCII ®le. This de®nition of the feature hole is suf®cient for the system to create feature trees and generate a process plan by parsing the feature tree. But visualising the feature requires the system to call the ACIS kernel with appropriate commands and parameters. This type of a procedure needs to be compiled into the source code of the system. Since the end user implementing and using the system would not have access to the source code, the visualisation routine of the hole is written in a separate dynamic linked library (DLL) ®le. Instead of providing the user with the source code of the entire system, he/she is provided with a template for writing a DLL for the visualisation routine. The DLL is compiled and registered in the system directory. In FBCDS system, the DLL is compiled using Visual Basic. This could however be compiled using any high level language like C/ C or FORTRAN which has the capabilities to compile DLLs. This choice of selecting the compiling too would depend on the user's capabilities and requirements. 4.2. Computer-aided process planning system The process planning system used is a specially developed version of LOCAM process planning system [21]. The system integrates with CLIPS expert system. Acronym CLIPS stands for ``C'' Language Integration Production System. It was developed in the mid 1980s in NASA's Johnson Space Centre to enable integration of state-of-the-art arti®cial intelligence and rule-based applications with existing regular applications. A new set of LOCAM enhanced
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commands have been developed to allow calling of CLIPS routines and handle the conceptual model. The process planning system uses two databasesÐthe manufacturing resource database and the logic database. CLIPS expert system rules are kept in a separate database. As discussed earlier, there are various ways to measure manufacturability. FBCDS uses the embedded process planning system to measure the manufacturability in terms of manufacturing time and costs. It also determines if the design is manufacturable with the available resources. Instead of adopting a pure rule-based or plan-based approach to analysis and plan generation, a novel hybrid approach is used. This is achieved by combining the plan-based approach and the rule-based approach. CLIPS rules facilitate evaluation of partially completed model [22]. These rules can be called from within the analysis logic of the process planning and analysis system. The output of the process planning system is also a structured database. This is novel in that it gives the user control of not only the input but also the system output. 4.3. PDM system Motiva PDM system can be thought of as the glue, which holds the entire system together. A component oriented approach has been adopted in the development of the system. The individual components of the system like the CAPP system and the design system communicate and share information via the PDM system. 4.4. Logic designer Every application is based on some form of logic. This logic is usually `built into' the system. For the purpose of CAx (where x stands for design, manufacturing, engineering, and so on) applications like CAPP and CAAP, the logic can be divided into two types: 1. Application logic: this is the logic on which the application is based. This is a broader logic and can be considered as static for the lifetime of the application.
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2. Analytical logic: this is the logic used for the engineering analysis for generating the results like the process plans or assembly plans. This logic is based on the current trends, practices, and standards. It may also be based on the application domain and the customer requirements. Although this may seem to be static at the time of development of the application, it is bound to get outdated as newer manufacturing and machining technologies emerge. For increasing the lifetime of the application as well as making it more generic, it is important to differentiate between these two types of logic. The user should be able to update analytical logic and keep it instep with the current technology. In FBCDS, the analytical logic is kept in an external database. This logic can be edited and viewed graphically as a ¯owchart using the logic designer as shown in Fig. 3. The user also has the options of having multiple logic ¯owcharts. This is extremely convenient in case the same system is to be used for different domains. For example, the user could develop the logic for process planning of prismatic parts and sheet metal parts as two separate ¯owcharts and use them as appropriate. Further ¯exibility is offered by allowing nesting of ¯owcharts. This approach to storing the logic has proved immensely successful not only in making the application ¯exible and updateable, but also helping in breaking down the domain speci®c barriers and making the application more generic. The application itself is only a shell to run the logic. Simple development of new analytical logic allows the application to be used in a completely new domain. 5. System implementation The system has been developed and implemented on Windows NT1 platform. The choice of implementation platform was based on a number of factors such as availability of development tools and ease of development and administration. The development tools used included Visual Basic and Visual C. A view of the main application is shown in Fig. 4. The system provides the user access to the PDM system, feature library, part feature tree and the solid
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Fig. 3. FBCDS logic designer.
model in four separate panes. The solid model (or its wire-frame) shown is a visualisation only and the geometry cannot be directly edited. Any changes can be made only to the feature tree of the part being designed. The changes made to the feature tree are re¯ected in the solid model. The embedded feature model editor is very userfriendly. Feature instances can be added to the part by simple drag and drop operations. The features can also be re-positioned within a part using the drag and drop operation. The application also provides facilities for cut and paste operations. Each feature in the tree has a check-box that can be used to include or exclude the feature from the analysis. This check-box allows a quick evaluation of various product con®gurations. This can be exploited by the designer to decide if the cost and time overheads of a particular feature (like a chamfer or a knurl) are justi®ed. Once a suf®cient level of information has been added to the part model, it can be evaluated or exported as a SAT ®le. SAT ®le is an industry standard ®le format that enables free
exchange of wire-frame, surface and solid geometric and topological model data between various ACISenabled applications. This ®le can later edited using a conventional detailed ACIS-enabled CAD system. The ®les and databases created are automatically checked-in the PDM system. The system can also set-up PDM product structures for standard products based on prede®ned templates. This greatly speeds up the process of starting a new design project. The user manual is available on-line. The application also has extensive on-line context sensitive help. The analysis logic is developed using a rich vocabulary of commands provided by the system. There are commands to extract data from the feature tree as well as from databases in which the data about resources and machining standards is stored. The command set is a sub-set of the commands provided by the LOCAM process planning system for automated analysis of STEP AP224 ®les. A few more commands have been added to cater for the conceptual model and calling of CLIPS rules.
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Fig. 4. FBCDS-PDM integrated early design environment.
6. Case study To understand the assistance provided by the system, it is necessary to consider the steps in building a complete model and evaluating it using the system. The example part used here is a simple stepped bolt shown in Fig. 5. The feature tree representing the stepped bolt is also shown in Fig. 5. The part is composed in terms of standard features provided by the AP224 STEP library. All these features are available in a pre-de®ned library and there is no need to de®ne any more features for this part. The part feature tree is built by selecting the feature in the library and creating an instance of it in the part. A cut and paste operation or a drag and drop operation can do this. The hierarchy of the features in the part is not very important for analysis. But if a particular
feature is excluded from the analysis by clicking on its inclusion check-box, all features, which are children of this feature, are automatically excluded. Each feature has a set of attributes. The user needs to provide suitable values for these attributes. Not all attributes need to be entered to analyse the design. Some of the attributes are derived from CLIPS rules during analysis if they have been left out. In case the user has entered a value, then this is checked against the value predicted using the rules. Evaluation of the part gives a manufacturing cost and time estimate of the design. The user can then also generate a detailed process plan if required provided suitable planning logic has been developed in the system. The planning logic is developed by associating each feature with a set machining operations required to realise the feature. The feature parameters
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Fig. 5. Feature tree and its visualisation.
or attributes are used as input for deciding the parameters machining operations like the cutting speed and feed. Fig. 6 shows the constituent features of the case study component as extracted from a AP224 ®le. This table also shows the machining operations, which are necessary to realise each feature.
The design can also be checked-in the PDM system SAT ®le format which Spatial Technology's non-proprietary geometric model ®le format. Further design activities can be done using a detailed design system based on Spatial Technology's ACIS1 3D CAD kernel.
Fig. 6. Constituent features and operation information.
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7. Further work Further developments on the system are presently ongoing. The current implementation of the editor provides for a very limited validation and model consistency veri®cation. This is not seen as a handicap at the initial stage as the model is intended for modelling only simple products. A trial copy of the system will be shipped to a few selected LOCAM customers. Feedback obtained would be critically analysed to decide on the future scope of work. The system and the analysis logic will be further developed to tackle more sophisticated machining examples. Work has also been started to make the system Web enabled. Web-enabled applications have generated a lot of interest in the academic and business world alike. The Web is a cheap and practical alternative to traditional client/server deployment. It also provides
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an immediate cross platform support on the client side. Developing a stand-alone application and then trying to enable it for the Web amounts to literally rewriting the entire application. To avoid any redevelopment of code, the FBCDS user interface has been developed as a single ActiveX1 control. Since it is possible to embed ActiveX controls in a Web page, the entire interface can be easily duplicated in a Web page. Using ActiveX1, it is possible to implement a very interactive content on the Web. Since the same ActiveX1 control is used in the main FBCDS application as well as on the Web page, the user is presented with a similar and consistent interface, for both the stand-alone application as well as the Web page, as shown in Fig. 7. Using the same ActiveX control for both, the FBCDS application and the Web page, also results in enormous saving in development time. Extending the component
Fig. 7. FBCDS stand-alone application and Web interface.
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architecture further, the functionality of FBCDS has been developed as classes. A version of these classes has been compiled as a special type of class called the Web class, which can be used in server side components used by active server pages (ASP). 8. Conclusions The computer-aided progressive design system presented here can be used in the early design stages to improve the product quality from the manufacturing point of view. The development of the system's analytic logic would also help in formalisation of company's knowledge into a knowledge base and its integration with CAPP system. It is envisaged that this system will enable the designers to evaluate their design with respect to the available manufacturing processes and resources. It gives the planner complete freedom to create his own feature library dedicated to a particular application or a part. It also provides for quick evaluation of `What±If' scenarios by relocating a feature, changing its parameters or excluding it from the analysis logic. Initial case studies reveal that the system is a highly desirable conceptual design and editing aid. It increases the ef®ciency by reducing the time taken to enter and edit the conceptual design data. Further studies will help to quantify the results. Acknowledgements The authors gratefully acknowledge the support of LSC Group, UK Ltd. during this research, especially Mr Alan Crawford and Mr Ray Muldoon, without whose support this would not have been possible. References [1] G. Phal, W. Beitz, Engineering Design: A systamtic approach, Springer, UK, 1997. [2] X.F. Zha, S.Y.E. Lim, S.C. Fok, Integrated knowledge-based approach and system for assembly planning, International Journal Computer-Integrated Manufacturing 12 (3) (1999) 211±237. [3] W. Hsu, I.M.Y. Woon, Current research in the conceptual design of mechanical products, Computer-Aided Design 30 (5) (1998) 377±389.
[4] K. Andersson, A proposal for a new product modelling language to support conceptual design, Annals of CIRP, Vol 44 (1), 1995. [5] K. Schreve, H.R. Schuster, A.H. Basson, Manufacturing cost estimation during design of fabricated parts, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 213 (1999) 731±735. [6] K. Ishii, Modelling of Concurrent Engineering Design, Concurrent Engineering: Automation, Tools and Techniques, Wiley, New York, 1993. [7] S. Subramanyam, S.C.-Y. Lu, The impact of an AI-based design environment for simultaneous engineering on process planning, International Journal of Computer-Integrated Manufacture 4 (2) (1991) 71±82. [8] W. Priest John, M. Sanchez Jose, An emperical methodology for measuring producability in early product development, International Journal of Computer-Integrated Manufacture 4 (2) (1991) 114±120. [9] S. Gupta, D.S. Nau, Systematic approach to analysing the manufacturability of machined parts, Computer-Aided Design 27 (5) (1995) 323±342. [10] A. Mukherjee, C.R. Liu, Conceptual design, manufacturability evaluation and preliminary process planning using function form relationships in stamped metal parts, Robotics Computer-Integrated Manufacture 13 (3) (1997) 253±270. [11] D.T. Pham, C. Ji, A concurrent design system for machined Parts, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 213 (1999) 841±846. [12] D. Das, S.K. Gupta, D.S. Nau, Reducing set-up cost by automated generation of redesign suggestions. ASME Computers in Engineering Conference, 1994. [13] D. Das, S.K. Gupta, D.S. Nau, Generating redesign suggestions to reduce set-up costs: a step towards automated redesign, Computer-Aided Design 28 (10) (1996) 763±782. [14] K. Ishii, S. Krizan R.A. Miller C. Lee, Hyper Q/Process: an expert system for process selection in design, in: Proccedingsof the 6th International Conference on AI. Applications in EngineeringUK, Computational Mechanics Publications, 1991. [15] D.T. Pham, S.S. Dimov, An approach to concurrent engineering, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 212 (1998) 13±27. [16] T. Dereli, I.H. Filiz, Allocating optimal positions of cutting tools on ATCs, Robotics Autonomous Systems 33 (2000) 144±167. [17] K. Case, J.X. Gao, Feature technology: an overview, International Journal of Computer-Integrated Manufacture 6 (1-2) (1993) 2±12. [18] R. Sharma J.X. Gao, A feature model editor for process planning of sheet metal products, in: Proceedings of the 15th International Conference on Computer-Aided Production Engineering UK: DMRG, 1999. [19] J. Fowler, STEP for Data management, Exchange and Sharing, Great Britain: Technology Appraisals, 1995. [20] D. Rogerson, Inside COM, Microsoft Press, USA, 1997.
R. Sharma, J.X. Gao / Computers in Industry 47 (2002) 155±167 [21] LSC. LOCAM 10.9 Reference Manual, LSC, UK, 1997. [22] W. Bowland, J.X. Gao, Embedded Knowledge-Based Functionality For Process Planning, Second International Workshop On Intelligent Manufacturing System, 1999. J.X. Gao is a lecturer at Cran®eld Univerrsity specialising in CAD/CAM/PDM. His research group attracts funding from the Engineering and Physics Research Council (EPSRC), the Department of Trade and Industry (DTI) and leading manufacturing and consultant companies in the UK. He has published 70 papers in international journals and conference
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proceedings. His current research interests are design and manufacturing knowledge management, and distributed and collaborative product development and manufacturing evaluation. R. Sharma received his MSc degree in Computer-Aided Design from Cran®eld University, UK in 1998. He is presently a PhD researcher in the same university. His research interests include integration of CAPP, PDM and MRP systems. He has been involved in development of a commercial CAPP system which can automatically generate process plans from STEP AP224 ®les.
A Progressive design and manufacturing evaluation system incorporating STEP AP224 R. Sharma, J.X. Gao* School of Industrial and Manufacturing Science, Cran®eld University, Cran®eld, England, MK43 0AL, UK Received 29 March 2001; accepted 18 August 2001
Abstract Most of the important cost related design decisions are taken in the early design stages. More often than not, designers have little or no knowledge of the manufacturing dif®culties. Therefore, a design engineer typically designs components without consideration for manufacturing dif®culties. This paper describes a product data management (PDM) based system for manufacturing evaluation and analysis in the early design stages. The system is based on a process planning system and an embedded expert system to resolve the abstract data usually associated with the early design stages. The system will allow early measurement of design in terms of time, manufacturing cost and resources. Early measurement of design would be especially helpful in supporting new design techniques like simultaneous design and concurrent engineering. The system is being developed in industrial collaboration with LSC Group, UK, which is a pioneering company in the ®eld of automated process planning using STEP (standard for exchange of product data) AP224 standard. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Computer-aided process planning; Conceptual design; Product data management; Manufacturing analysis
1. Introduction In the manufacturing environment of the 21st century, there is no place for traditional `over-the-wall' approach to design and manufacturing. Time is of essence, and the success of a design project is often judged by the time taken to bring a product to market. There has been an exponential increase in the application of new software tools to remove the barriers between design and manufacturing. However, research in computer-aided design and manufacturing (CAD/ CAM) has been aimed at providing computer support for the later design stages. Automated analysis of
manufacturability and process planning during the conceptual design stage has become the focus of the research community. More often than not, the designers have little or no knowledge of the manufacturing dif®culties. The result is a design, which may meet the design speci®cations, but is costly or dif®cult to manufacture. This paper reports on the development of a product data management (PDM)integrated early design system, which also provides manufacturing evaluation and process plan generation. Some key concepts used in this project are explained below. 1.1. Progressive design
*
Corresponding author. Tel: 44-1234-750111x5417; fax: 44-1234-751-172. E-mail address: [email protected] (J.X. Gao).
Much research has been devoted to the theory of design. Extensive comparison of the various systema-
0166-3615/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 3 6 1 5 ( 0 1 ) 0 0 1 4 6 - 4
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tic modern design methods can be found in literature [1]. Product design begins with a need and ends with detailed drawings and all the accompanying information necessary for manufacturing the product. Product design is a complex and iterative process [2], and most studies have divided the design process into two, three or four stages. The classic design model described by [1] has four stages, i.e. design speci®cation, conceptual design, preliminary design (also called embodiment design or fundamental design in some studies), and detailed design. The transition of design from one stage to another is a gradual process and the design evolves from one stage to another rather than an abrupt jump from one stage to the next. Especially in the context of providing computer support for design process, it is dif®cult to separate these design stages individually. With the aim of developing a computer system to support the product design during all the stages, this research combines the ®rst three design stages together as the early design stages. A common characteristic of all the three stages of early design stage is that the design options are still being explored and compared. Design data in these stages is incomplete and abstract. The detailed design stage is more about re®ning the design selected in the early design stages. A new term has been coined to describe the evolution of design during the early design stages, i.e. progressive design. The concept of progressive design is especially bene®cial when the designer wants to use the same design tools or data across the design life cycle of a product. This will become clear and obvious in the subsequent discussions. 1.2. Computer support for early design There are various approaches for addressing the problem of product model description, representation and visualisation during early design. The representation ranges from computer-oriented methods at one end of the spectrum and human-oriented methods at the other (Fig. 1) [3]. Language based early design systems like CANDLE [4] are easy to implement but are abstract and may not be what a designer expects from a design system. Similarly, the image or interface based systems at the other end of the spectrum are computationally expensive but are the preferred mode of interaction for a designer. More about the current
Fig. 1. Computer support spectrum [3].
state of system for conceptual design of mechanical products can be found in [3]. 1.3. Manufacturability Manufacturability of a product depends upon the design domain, and a number of other related factors, like the available resources. In general terms, manufacturability can be de®ned as an indication of the effort required for manufacturing the product. The system being developed measures the manufacturability of the product in the early design stages, in terms of manufacturing time, costs and resources. Given a set of manufacturing resources and product information, the problem of manufacturing evaluation is reduced to determining whether or not the design is manufacturable. If the design is found to be manufacturable, the next step is to determine one of the evaluation metrics of manufacturability. Manufacturability metrics is usually one of the following: Boolean: this is the most basic manufacturability rating. It is a Boolean that simply reports if the part ``can'' or ``cannot'' be manufactured with the given resources. This can usually be achieved by generating a process plan. Cost: cost estimation has been an important area to study both in manufacturing and marketing communities [5]. Prompt estimation of cost with accuracy is crucial. Cost of manufacturing is a quantitative measure. Since all manufacturing operations have an associated cost, it is easy to
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arrive at a figure based on the manufacturing resources required to produce a product. The cost of raw material and its handling could also be included in the estimate. This figure may not be very accurate but is usually acceptable if it is within limits. Time: manufacturing time is also a quantitative measure of manufacturing process. Every manufacturing process has an associated time. In most cases, it is the manufacturing time that is used to calculate the manufacturing costs. The quantitative measures of time and cost, may not be directly helpful in determining if the design is good or bad or if it meets the required specifications, but it is one of the most common constraints for a designer. Qualitative measure: Ishii [6] qualified designs using adjective qualifiers: `excellent', `very good', `good', `poor', `bad' and `very bad'. These qualifiers were mapped on to a (0,1) measure for comparison. But such measures are hard to interpret and compare, especially if the rating comes from different systems or designers. Abstract: this is similar to the qualitative measures discussed above, but involves each design attribute being assigned a manufacturability index or producability index (PI) instead of a qualifying adjective like qualitative measures. As with the qualitative measures, it can become difficult to interpret or compare designs if the indexes are from different systems. 2. Previous work in manufacturability evaluation Subramanyam and Lu [7] described a framework for an AI-based computer-aided design environment to ensure manufacturability of product design for components manufactured in small and medium lot sizes. The authors emphasised on unifying and integrating the design and process planning knowledge into the design environment. The framework consisted of an explicit product model and an explicit manufacturing facility model. These models were used by an AI-based reasoning system to determine if the product design satis®ed all machining related concerns. The product and manufacturing facility model were based on a complex combination of featurebased models and geometric models.
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Priest John and Sanchez Jose [8] developed an empirical methodology for measuring design-formanufacturing (DFM) early in the conceptual design stage and also in the later detailed design stages. Their methodology was based on identifying producability factors, which could be quanti®ed and combined by means of a simple formula. This was done using evaluation rules. Gupta ans Nau [9] described a plan-based automated manufacturing analysis system. The system was speci®cally targeted at early design of prismatic parts. The ®rst step in the analysis was to generate at least one process plan for the product. In case, at least one process plan was feasible, it was assumed that manufacturing the part was feasible and the analysis progressed to the next stage. The analysis methodology was to identify all the possible manufacturing operations required to produce the part, and using these operations, different process plans can be generated. These plans were analysed and compared to suggest improvements and to reduce the number of steps for machining. Mukherjee and Liu [10] described a conceptual design system based on sketching abstraction. A sketching abstraction is a minimal representation of a design object using functional features, quasi-links and quasi-nodes, so as to provide an intermediate representation between pure geometric and pure functional form. The system was speci®cally for conceptual design of stamped metal parts. It provided the designer with feedback on manufacturability constraints. Pham and Ji [11] described a concurrent design environment capable of providing information for manufacturability assessment including machining time. The system was based on feature recognition from Pro/Engineer1 constructive solid geometry (CSG) model. The feature information retrieved from the CSG model was then checked against the capabilities of the available resources. The detailed manufacturing information was generated only if the speci®ed requirements of each manufacturing feature could be satis®ed. The manufacturing information sequence or the process plan was generated using a manufacturing knowledge base. Other notable efforts to evaluate manufacturability have concentrated on feature accessibility, set-up costs and redesign [12,13] manufacturing process selection
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[14] and concurrent product design and process planning [15]. Based on the literature survey of existing manufacturing evaluation systems, the approaches to derive manufacturability metrics from design and resources as input can be broadly divided into two categories, i.e. 1. Direct or rule-based: in this approach, analysis is based on direct or rule-based interrogation of the design description to assess manufacturability. Rule-based systems use a set of guidelines to evaluate the feasibility of a proposed design. They may also provide recommendations to designers for violated rules. Rule-based approaches do not usually provide quantitative metrics related to manufacturability that can provide a basis for comparison. 2. Plan-based: this approach is based on generating the manufacturing process plan and then analysing this plan (or its rating) based on a pre-selected criterion. Rating-based approaches like this, typically provide a quantitative evaluation based on scales like time or costs. The major advantage is that it can compare alternative designs and optimise a design based on a selected objective. Because DFM rules are not applied, this approach, in most cases cannot provide speci®c redesign recommendations. It emerges that most manufacturing evaluation systems reviewed are feature-based, directly or indirectly. Manufacturability evaluation is either done through a priori such as DFM guidelines, or through a posteriori such as analysis tools which take the geometric or feature model as the input. The a priori approach is usually rule-based, while a posteriori analysis has often been implemented using a plan-based analysis. The analysis is usually separate from the detailed process plan generation. In most cases the analysis module and process plan generator module are completely separate. The only exception is in some planbased analysis systems. Some systems integrate design and process planning to allow simultaneous design with process planning. But even in such closely integrated systems, design and process planning are sequential activities. There are some modular systems like the OPPS±PRI system [16] which allow invoking of modules in a non-sequential manner but even here the integration is limited to sharing of a common data
model. Some systems provided redesign suggestions based on design rules. Some others provided analysis on feature accessibility and set-up costs. 3. The proposed approach One aspect of early design, which cannot be ignored while developing any computer system to support it, is the requirement of visualisation of the design. Visualisation of the product and its constituent features, is based on a complete and evaluated geometric or feature model. In a feature-based environment, the direct approach adopted by most manufacturing analysis systems, breaks down or becomes very complex to implement if the component under analysis has interacting manufacturing operations or features. This is because it is not possible for a set of ®nite rules to cater for every possible case. Also, all such approaches reported are based on feature recognition or feature mapping, which require a fully validated and completed geometric model [17]. This primary requirement of having a fully valid and complete model is precisely the restriction in early design. It is, therefore, clear from the discussion so far that both design visualisation and manufacturing evaluation require a complete and valid feature model. The proposed feature-based conceptual design system (FBCDS) is a system presently under development at the School of Industrial and Manufacturing Science at Cran®eld University, as a tool for early design using incomplete or abstract product models. The system allows the user to create a part by creating a hierarchy tree of its constituent features. The user can visualise this feature tree as a solid model and save it as an ACIS1 solid model. The system provides early feedback on various aspects of design like manufacturing costs and manufacturing effort (in units of time). This is accomplished by integrating a feature-based modeller based on the ACIS 3D kernel with a process planning system. Like most other feature-based design systems, this system is also based on feature libraries. Current computer-aided design tools do not adequately support the progressive design paradigm. The computer-aided conceptual design tools, where used, rarely use a product model, which can be directly carried over to the detailed design stages. The result is a break in the information ¯ow. Furthermore,
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transition of design from early design to detailed design involves a lot of rework as data from the early design stage has to be redeveloped using the CAD system to be used in the detailed design stage. A solution to this problem is offered by the new generation of PDM systems. PDM applications have a common purpose of providing con®guration management to engineering databases and the intent to start bridging the gaps between islands of automation. Con®guration management in this sense represents a disciplined approach to de®ne elements of product data, to control its change, and to track the changes made. For any application systems to be successfully integrated into an existing enterprise, it is mandatory that it be closely integrated with the PDM environment. Integrating a computer-aided conceptual design system with a PDM environment would provide a suitable framework for data sharing as well as implementation of work¯ow. This is the best way to automate the clerical tasks associated with progressive design and transfer of design to the next stage. This would also avoid one very important pitfallÐmanual re-entering of data in the detailed design stage. Using the PDM system, it would be possible to
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carry design from the early design stages to the detailed design stage in a seamless manner. In contrast, the detailed (conventional) CAD systems would take over where the early design system leaves. FBCDS has been very closely integrated with the PDM system. All databases and resources are held within the PDM system. This has been done with an aim to support progressive design (as de®ned in the initial discussions). The PDM system is also used to carry design over to the detail design phase. 4. The FBCDS system architecture The FBCDS system architecture is shown in Fig. 2. This system is based on the premise that the designers should be able to represent the preliminary design in terms of meaningful features which have manufacturing information associated with them. The representation of preliminary design as meaningful features allows close integration of design system with the computer-aided process planning (CAPP) system. This also eliminates any requirement of feature translation or feature recognition, allowing incomplete
Fig. 2. System architecture.
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product model to be processed for manufacturing evaluation and process planning. Loop `A' in Fig. 2 shows the information ¯ow between the conceptual modeller and the process planning system. The system can be divided into four distinct segments, which are discussed below. 4.1. Feature-based conceptual design system This is basically a feature model editor (FME) [18] that provides the user facility to build up the feature hierarchy tree using a simple interface. It also includes the feature library editor and solid model viewer. A feature library based on the STEP AP224 [19] is provided as standard. The users can however create their own features and use them. Most CAD systems display the features of a component as a feature tree where the features are arranged in a de®nite hierarchy. Although, hierarchy seems to be the logical way to represent features, AP224 does not support the concept of feature hierarchy. A very simple logic has been developed to cater for hierarchy. Each feature-type is given a logical hierarchy level and a set of possible parent features. For example the base_shape feature has the hierarchy level of 0 and no possible parent features as it is supposed to be the ®rst feature in the hierarchy. It must be noted here that this methodology is in no way comprehensive. But is suf®cient for this system since the hierarchy is not important for process planning. The system offers a facility to edit the feature attributes. The validity of the feature attributes is checked against a set of design-for-manufacture rules which are developed in CLIPS. The system at present has only simple rules like checking for the shell/wall thickness. But more rules can be developed depending on requirement and application domain. This is not intended to be a full scale CAD editor. The user would need to use a conventional CAD system for detailed editing. A user con®gurable feature library has been implemented in FBCDS using the component technology. The data about the features, which includes all the attributes and parameters is entered using a graphical tool, called the feature model editor. The feature de®nition is stored in a plain ASCII ®le. Each feature in the library is identi®ed by a unique `handle' which is the feature name. Implementation of feature visua-
lisation without recompiling the complete software is done by compiling the visualisation routines of the user feature library into a component object model (COM) component. COM or COM is a standard that de®nes how to build interoperable components [20]. Components are described as reusable piece of software in binary form that can be linked with other software with relatively little effort. Using components is a much more productive way to design, build and reuse software. In order to introduce a new feature called `hole' in the library, the developer ®rst decides on the attributes of the feature, which are required to completely describe it. The user can create the feature hole in the graphical library designer by simple visual commands. This de®nition of the hole is stored in an ASCII ®le. This de®nition of the feature hole is suf®cient for the system to create feature trees and generate a process plan by parsing the feature tree. But visualising the feature requires the system to call the ACIS kernel with appropriate commands and parameters. This type of a procedure needs to be compiled into the source code of the system. Since the end user implementing and using the system would not have access to the source code, the visualisation routine of the hole is written in a separate dynamic linked library (DLL) ®le. Instead of providing the user with the source code of the entire system, he/she is provided with a template for writing a DLL for the visualisation routine. The DLL is compiled and registered in the system directory. In FBCDS system, the DLL is compiled using Visual Basic. This could however be compiled using any high level language like C/ C or FORTRAN which has the capabilities to compile DLLs. This choice of selecting the compiling too would depend on the user's capabilities and requirements. 4.2. Computer-aided process planning system The process planning system used is a specially developed version of LOCAM process planning system [21]. The system integrates with CLIPS expert system. Acronym CLIPS stands for ``C'' Language Integration Production System. It was developed in the mid 1980s in NASA's Johnson Space Centre to enable integration of state-of-the-art arti®cial intelligence and rule-based applications with existing regular applications. A new set of LOCAM enhanced
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commands have been developed to allow calling of CLIPS routines and handle the conceptual model. The process planning system uses two databasesÐthe manufacturing resource database and the logic database. CLIPS expert system rules are kept in a separate database. As discussed earlier, there are various ways to measure manufacturability. FBCDS uses the embedded process planning system to measure the manufacturability in terms of manufacturing time and costs. It also determines if the design is manufacturable with the available resources. Instead of adopting a pure rule-based or plan-based approach to analysis and plan generation, a novel hybrid approach is used. This is achieved by combining the plan-based approach and the rule-based approach. CLIPS rules facilitate evaluation of partially completed model [22]. These rules can be called from within the analysis logic of the process planning and analysis system. The output of the process planning system is also a structured database. This is novel in that it gives the user control of not only the input but also the system output. 4.3. PDM system Motiva PDM system can be thought of as the glue, which holds the entire system together. A component oriented approach has been adopted in the development of the system. The individual components of the system like the CAPP system and the design system communicate and share information via the PDM system. 4.4. Logic designer Every application is based on some form of logic. This logic is usually `built into' the system. For the purpose of CAx (where x stands for design, manufacturing, engineering, and so on) applications like CAPP and CAAP, the logic can be divided into two types: 1. Application logic: this is the logic on which the application is based. This is a broader logic and can be considered as static for the lifetime of the application.
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2. Analytical logic: this is the logic used for the engineering analysis for generating the results like the process plans or assembly plans. This logic is based on the current trends, practices, and standards. It may also be based on the application domain and the customer requirements. Although this may seem to be static at the time of development of the application, it is bound to get outdated as newer manufacturing and machining technologies emerge. For increasing the lifetime of the application as well as making it more generic, it is important to differentiate between these two types of logic. The user should be able to update analytical logic and keep it instep with the current technology. In FBCDS, the analytical logic is kept in an external database. This logic can be edited and viewed graphically as a ¯owchart using the logic designer as shown in Fig. 3. The user also has the options of having multiple logic ¯owcharts. This is extremely convenient in case the same system is to be used for different domains. For example, the user could develop the logic for process planning of prismatic parts and sheet metal parts as two separate ¯owcharts and use them as appropriate. Further ¯exibility is offered by allowing nesting of ¯owcharts. This approach to storing the logic has proved immensely successful not only in making the application ¯exible and updateable, but also helping in breaking down the domain speci®c barriers and making the application more generic. The application itself is only a shell to run the logic. Simple development of new analytical logic allows the application to be used in a completely new domain. 5. System implementation The system has been developed and implemented on Windows NT1 platform. The choice of implementation platform was based on a number of factors such as availability of development tools and ease of development and administration. The development tools used included Visual Basic and Visual C. A view of the main application is shown in Fig. 4. The system provides the user access to the PDM system, feature library, part feature tree and the solid
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Fig. 3. FBCDS logic designer.
model in four separate panes. The solid model (or its wire-frame) shown is a visualisation only and the geometry cannot be directly edited. Any changes can be made only to the feature tree of the part being designed. The changes made to the feature tree are re¯ected in the solid model. The embedded feature model editor is very userfriendly. Feature instances can be added to the part by simple drag and drop operations. The features can also be re-positioned within a part using the drag and drop operation. The application also provides facilities for cut and paste operations. Each feature in the tree has a check-box that can be used to include or exclude the feature from the analysis. This check-box allows a quick evaluation of various product con®gurations. This can be exploited by the designer to decide if the cost and time overheads of a particular feature (like a chamfer or a knurl) are justi®ed. Once a suf®cient level of information has been added to the part model, it can be evaluated or exported as a SAT ®le. SAT ®le is an industry standard ®le format that enables free
exchange of wire-frame, surface and solid geometric and topological model data between various ACISenabled applications. This ®le can later edited using a conventional detailed ACIS-enabled CAD system. The ®les and databases created are automatically checked-in the PDM system. The system can also set-up PDM product structures for standard products based on prede®ned templates. This greatly speeds up the process of starting a new design project. The user manual is available on-line. The application also has extensive on-line context sensitive help. The analysis logic is developed using a rich vocabulary of commands provided by the system. There are commands to extract data from the feature tree as well as from databases in which the data about resources and machining standards is stored. The command set is a sub-set of the commands provided by the LOCAM process planning system for automated analysis of STEP AP224 ®les. A few more commands have been added to cater for the conceptual model and calling of CLIPS rules.
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Fig. 4. FBCDS-PDM integrated early design environment.
6. Case study To understand the assistance provided by the system, it is necessary to consider the steps in building a complete model and evaluating it using the system. The example part used here is a simple stepped bolt shown in Fig. 5. The feature tree representing the stepped bolt is also shown in Fig. 5. The part is composed in terms of standard features provided by the AP224 STEP library. All these features are available in a pre-de®ned library and there is no need to de®ne any more features for this part. The part feature tree is built by selecting the feature in the library and creating an instance of it in the part. A cut and paste operation or a drag and drop operation can do this. The hierarchy of the features in the part is not very important for analysis. But if a particular
feature is excluded from the analysis by clicking on its inclusion check-box, all features, which are children of this feature, are automatically excluded. Each feature has a set of attributes. The user needs to provide suitable values for these attributes. Not all attributes need to be entered to analyse the design. Some of the attributes are derived from CLIPS rules during analysis if they have been left out. In case the user has entered a value, then this is checked against the value predicted using the rules. Evaluation of the part gives a manufacturing cost and time estimate of the design. The user can then also generate a detailed process plan if required provided suitable planning logic has been developed in the system. The planning logic is developed by associating each feature with a set machining operations required to realise the feature. The feature parameters
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Fig. 5. Feature tree and its visualisation.
or attributes are used as input for deciding the parameters machining operations like the cutting speed and feed. Fig. 6 shows the constituent features of the case study component as extracted from a AP224 ®le. This table also shows the machining operations, which are necessary to realise each feature.
The design can also be checked-in the PDM system SAT ®le format which Spatial Technology's non-proprietary geometric model ®le format. Further design activities can be done using a detailed design system based on Spatial Technology's ACIS1 3D CAD kernel.
Fig. 6. Constituent features and operation information.
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7. Further work Further developments on the system are presently ongoing. The current implementation of the editor provides for a very limited validation and model consistency veri®cation. This is not seen as a handicap at the initial stage as the model is intended for modelling only simple products. A trial copy of the system will be shipped to a few selected LOCAM customers. Feedback obtained would be critically analysed to decide on the future scope of work. The system and the analysis logic will be further developed to tackle more sophisticated machining examples. Work has also been started to make the system Web enabled. Web-enabled applications have generated a lot of interest in the academic and business world alike. The Web is a cheap and practical alternative to traditional client/server deployment. It also provides
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an immediate cross platform support on the client side. Developing a stand-alone application and then trying to enable it for the Web amounts to literally rewriting the entire application. To avoid any redevelopment of code, the FBCDS user interface has been developed as a single ActiveX1 control. Since it is possible to embed ActiveX controls in a Web page, the entire interface can be easily duplicated in a Web page. Using ActiveX1, it is possible to implement a very interactive content on the Web. Since the same ActiveX1 control is used in the main FBCDS application as well as on the Web page, the user is presented with a similar and consistent interface, for both the stand-alone application as well as the Web page, as shown in Fig. 7. Using the same ActiveX control for both, the FBCDS application and the Web page, also results in enormous saving in development time. Extending the component
Fig. 7. FBCDS stand-alone application and Web interface.
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architecture further, the functionality of FBCDS has been developed as classes. A version of these classes has been compiled as a special type of class called the Web class, which can be used in server side components used by active server pages (ASP). 8. Conclusions The computer-aided progressive design system presented here can be used in the early design stages to improve the product quality from the manufacturing point of view. The development of the system's analytic logic would also help in formalisation of company's knowledge into a knowledge base and its integration with CAPP system. It is envisaged that this system will enable the designers to evaluate their design with respect to the available manufacturing processes and resources. It gives the planner complete freedom to create his own feature library dedicated to a particular application or a part. It also provides for quick evaluation of `What±If' scenarios by relocating a feature, changing its parameters or excluding it from the analysis logic. Initial case studies reveal that the system is a highly desirable conceptual design and editing aid. It increases the ef®ciency by reducing the time taken to enter and edit the conceptual design data. Further studies will help to quantify the results. Acknowledgements The authors gratefully acknowledge the support of LSC Group, UK Ltd. during this research, especially Mr Alan Crawford and Mr Ray Muldoon, without whose support this would not have been possible. References [1] G. Phal, W. Beitz, Engineering Design: A systamtic approach, Springer, UK, 1997. [2] X.F. Zha, S.Y.E. Lim, S.C. Fok, Integrated knowledge-based approach and system for assembly planning, International Journal Computer-Integrated Manufacturing 12 (3) (1999) 211±237. [3] W. Hsu, I.M.Y. Woon, Current research in the conceptual design of mechanical products, Computer-Aided Design 30 (5) (1998) 377±389.
[4] K. Andersson, A proposal for a new product modelling language to support conceptual design, Annals of CIRP, Vol 44 (1), 1995. [5] K. Schreve, H.R. Schuster, A.H. Basson, Manufacturing cost estimation during design of fabricated parts, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 213 (1999) 731±735. [6] K. Ishii, Modelling of Concurrent Engineering Design, Concurrent Engineering: Automation, Tools and Techniques, Wiley, New York, 1993. [7] S. Subramanyam, S.C.-Y. Lu, The impact of an AI-based design environment for simultaneous engineering on process planning, International Journal of Computer-Integrated Manufacture 4 (2) (1991) 71±82. [8] W. Priest John, M. Sanchez Jose, An emperical methodology for measuring producability in early product development, International Journal of Computer-Integrated Manufacture 4 (2) (1991) 114±120. [9] S. Gupta, D.S. Nau, Systematic approach to analysing the manufacturability of machined parts, Computer-Aided Design 27 (5) (1995) 323±342. [10] A. Mukherjee, C.R. Liu, Conceptual design, manufacturability evaluation and preliminary process planning using function form relationships in stamped metal parts, Robotics Computer-Integrated Manufacture 13 (3) (1997) 253±270. [11] D.T. Pham, C. Ji, A concurrent design system for machined Parts, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 213 (1999) 841±846. [12] D. Das, S.K. Gupta, D.S. Nau, Reducing set-up cost by automated generation of redesign suggestions. ASME Computers in Engineering Conference, 1994. [13] D. Das, S.K. Gupta, D.S. Nau, Generating redesign suggestions to reduce set-up costs: a step towards automated redesign, Computer-Aided Design 28 (10) (1996) 763±782. [14] K. Ishii, S. Krizan R.A. Miller C. Lee, Hyper Q/Process: an expert system for process selection in design, in: Proccedingsof the 6th International Conference on AI. Applications in EngineeringUK, Computational Mechanics Publications, 1991. [15] D.T. Pham, S.S. Dimov, An approach to concurrent engineering, in: Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture (Part B) 212 (1998) 13±27. [16] T. Dereli, I.H. Filiz, Allocating optimal positions of cutting tools on ATCs, Robotics Autonomous Systems 33 (2000) 144±167. [17] K. Case, J.X. Gao, Feature technology: an overview, International Journal of Computer-Integrated Manufacture 6 (1-2) (1993) 2±12. [18] R. Sharma J.X. Gao, A feature model editor for process planning of sheet metal products, in: Proceedings of the 15th International Conference on Computer-Aided Production Engineering UK: DMRG, 1999. [19] J. Fowler, STEP for Data management, Exchange and Sharing, Great Britain: Technology Appraisals, 1995. [20] D. Rogerson, Inside COM, Microsoft Press, USA, 1997.
R. Sharma, J.X. Gao / Computers in Industry 47 (2002) 155±167 [21] LSC. LOCAM 10.9 Reference Manual, LSC, UK, 1997. [22] W. Bowland, J.X. Gao, Embedded Knowledge-Based Functionality For Process Planning, Second International Workshop On Intelligent Manufacturing System, 1999. J.X. Gao is a lecturer at Cran®eld Univerrsity specialising in CAD/CAM/PDM. His research group attracts funding from the Engineering and Physics Research Council (EPSRC), the Department of Trade and Industry (DTI) and leading manufacturing and consultant companies in the UK. He has published 70 papers in international journals and conference
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proceedings. His current research interests are design and manufacturing knowledge management, and distributed and collaborative product development and manufacturing evaluation. R. Sharma received his MSc degree in Computer-Aided Design from Cran®eld University, UK in 1998. He is presently a PhD researcher in the same university. His research interests include integration of CAPP, PDM and MRP systems. He has been involved in development of a commercial CAPP system which can automatically generate process plans from STEP AP224 ®les.