An interactive system for optimum and concurrent design of components for manufacture by powder metallurgy technology

An interactive system for optimum and concurrent design of components for manufacture by powder metallurgy technology

Journal of Materials Processing Technology ELSEVIER Journal of Materials Processing Technology 61 (1996) 187-192 An Interactive System for Optimum ...

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Journal of

Materials Processing Technology ELSEVIER

Journal of Materials Processing Technology 61 (1996) 187-192

An Interactive System for Optimum and Concurrent Design of Components for Manufacture by Powder Metallurgy Technology L. N. Smith and P. S. Midha Faculty of Engineering, University of the West of England, Frenchay Campus, Coldharbour Lane, Bristol BS16 1QY, UK

Abstract

This paper describes an interactive knowledge based approach for advising on the design of components to be manufactured by the powder metallurgy (PM) process. The interactive methodology employed, which involves a progressive evaluation of geometrical features, enables the system to generate design recommendations for both axisymmetric and non-axisymmetric components. The system employs a design with features approach to allow development of a solid model of the component, and rule-based inferencing is used for design interrogation. As each feature is created the user is advised with regard to its effects on the manufacturability of the component by means of PM, thereby enabling process specific knowledge to be made available at an early stage in the design. A hubbed gear is employed as an example component to illustrate the operation of the system, which includes automatic detection of unsuitable features, and a capability for automatic design modification. Systems of this type can offer potential for improved access to manufacturing knowledge; thus contributing to the concurrent design process, and resulting in reduced product development cost and improved quality. Such systems are particularly relevant to currently under-utilised production processes like powder metallurgy, which can provide significant cost and environmental benefits.

Keywords: Concurrent Design for Manufacture, Powder Metallurgy. Computer Aided Design

1. Introduction

1.1. Rationale for Concurrent Design The aim of concurrent design is to integrate the design process with all stages of the product life-cycle, thereby enabling the attainment of substantial reductions in lead-times and product development costs. Such reductions are likely to have a critical effect on the ability of manufacturing companies to compete effectively, as has been demonstrated by certain companies in the automobile sector [1]. Competitive manufacture requires minimisation of costs associated with different stages of product life-cycle, and numerous studies have concluded that the majority of these costs are determined during the design phase [2]. Therefore the application of concurrent engineering techniques such as design for manufacture (DFM), offers a powerful means of reducing overall product costs.

design on each aspect of the manufacturing system. The relevant knowledge is often vast and scattered, and is therefore difficult to apply using sequential algorithmic approach of a conventional program. However, a knowledge based system (KBS) approach is well suited to the collation and analysis of the large amounts of information required to provide solutions to DFM problems. Since the function of a KBS is to emulate the reasoning of a human expert, the knowledge is stored in a knowledge base in a high level form, as production rules. Therefore a KBS approach offers the advantage of knowledge storage in a form which can accessed and updated without a requirement for the user to have extensive computing expertise. As a consequence of these potential advantages, knowledge based systems have been the subject of extensive Artificial Intelligence research internationally, and in recent years there have been a number of successful systems developed for industrial applications [3]. DFM by powder metallurgy technology, which has a relatively well defined set of DFM rules, lends itself well to the application of KBS methods.

1.2. A Knowledge Based Approach to DFM Design for manufacture (DFM) requires consideration of process capability and other manufacturing related issues at the design stage, with a view to optimising the downstream manufacturing processes in terms of cost, quality, and productivity. The knowledge and experience about a manufacturing process can be encapsulated in a knowledge base in the form of production rules, which in turn can be used to evaluate the effect of the component 0924-0136/96/$15.00 ~ 1996 Elsevier Science S A All rights reserved PH 0924-0136(96)02485-5

1.3. The Powder Metallurgy Production Process The powder metallurgy (PM) production process offers significant cost, technological, and ecological advantages which can result in savings of up to two thirds in raw materials [4], and 50% in energy consumption [5]. Despite these advantages, manufacturing industry generally, and especially in Europe, does

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not fully exploit the potential of PM technology. The relatively low growth in PM can be attributed to a number of possible factors including historical, cultural, psychological and technical, the latter being associated mostly with the multi-disciplinary nature of the technology, the need for expensive tooling and specialised plant, and associated long lead-times. In addition, an inadequate awareness and knowledge of PM technology amongst design engineers has played some part in the relatively slow take up of this technology, and to benefit from the advantages offered by PM this obstacle must be overcome. The inherent peculiarities of PM processes include the need for filling the die with powder, followed by compaction and extraction of the part from the die, and sintering. These requirements necessitate the presence or absence of certain design features in the part. The basic design of most forms can be adequately adapted to facilitate manufacture by PM. However, the lack of widespread knowledge and basic training on "design for PM" prevents the average engineer from appraising the PM option effectively. It is therefore expected that the availability of intelligent and interactive knowledge based design tools, to detect inappropriate features and recommend new forms, would enable considerable improvements to be made in the speed and ease with which PM part designs could be developed. 1.4. Application of interactive knowledge based techniques to PM

In addition to fulfilling functional specifications, concurrent design of components also incorporates requirements of downstream elements of the production process into the design. The system described here, which is called 'Interactive Design Analyse((IDA), is aimed at providing a decision support tool when considering PM technology as a downstream production process. It employs an interactive approach for geometrical evaluation of components, by applying 'expert system' techniques to evaluate the part design, for feasibility of manufacture by uniaxial compaction PM. Each time a feature is added a geometrical analysis is performed, by means of examination of the suitability or otherwise of individual features and feature combinations present within the design. IDA is the second in a series of knowledge based systems developed by the authors to evaluate the design of components for manufacturability. The first system; 'PM Design Advisor', applied only to analysis of axisymmetric components [6]. This limitation has also been reported to apply to expert systems for PM being developed by other researchers [7]. However, the IDA system uses CAD based tools to model and analyse designs which are not limited only to components with axisymmetric geometries. The user is advised regarding the suitability of the features present for the process, and if required the system can automatically perform feature modification. The likelihood of such a modification being acceptable to the engineer will depend upon the extent to which the function of the feature concerned can be inferred from the original design. For example, in the case of the hubbed gear shown in Figure l, (which has been created with a co-axial hole to locate a shaft), IDA assumes that the function of the through hole in the hub which has its axis at 90 degrees to that of the gear, is to fasten the gear onto the shaft. Consequently the system erases the hole, and offers to replace it with a keyway which will have the same function as the hole, but will allow extraction of the gear from the die. In this way IDA simulates the processes of design analysis and modification which would be performed by a

PM manufacturer upon receipt of a concept design from an end user. By making PM process related knowledge available to the end user in this way, it is envisaged that employment of this type of system would dramatically reduce the time and costs associated with development of PM component designs.

Ho] 90 ° to axis

~r at 40 ° to axis

Fig. 1. Original gear design, not suitable for PM production. The PM 'know how' contained within the rules of the system knowledge base has been derived from part manufacturers, PM standards, and textbooks. This technique of knowledge elicitation, and subsequent representation within a knowledge base, could be employed for development of CAD based interactive design systems able to analyse a wide range of categories of potential PM part designs. Such systems would offer the following benefits:- Improved awareness amongst engineers of the ecological, materials, and cost benefits offered by powder metallurgy technology. - Identification of suitable production techniques which will allow a component with a given functionality to be manufactured at minimum cost. - Access for design engineers to process related knowledge at an early stage in design, allowing development of an optimised component design which will avoid shop floor production problems, such as tool breakage. - Reduced development lead-times for components manufactured by the PM route.

2. The Interactive Design Advisor 2.1. System architecture and function

The Interactive Design Analyser (IDA), has been implemented using the LISP interpreter built into AutoCAD ® (AutoLISP~), which enables development of application programs able to access most AutoCAD and AME solid modelling commands. LISP was chosen as a development tool both because of its capability for providing the unrestricted interaction of CAD system data required for the design process, and its suitability for development of expert system architectures. Although LISP provides a convenient means of building rule based inference procedures, TM

L,N Smith, P.S. Midha/Journal of Materials Processing Technology 61 (1996) 18~192 certain computations, such as the development of the solid model of the gear described here, require a higher speed of calculation than that provided within AutoLISP. This limitation was overcome by running C programs from within AutoLISP, via the AutoCAD Development System (ADS"). In order to develop a user friendly system, the program interacts with the user by means of dialogue boxes in addition to command line prompts. The dialogue boxes, which were created using the Dialogue Control Language (DCL) with AutoLISP functions to control their operation, were employed to ensure that the designer is aware of the features available for design, and has easy access to them. The user is able to combine the features by specifying size and position parameters, as required. Analysis of these feature combinations allows the system to advise on a range of component geometries. Because of the resulting potential for creation of complex geometries, an interactive approach is employed to analyse the geometry for formability. As each feature is added the designer is advised on the feasibility of forming the design by uni-axial compaction. Once a suitable geometry has been established, its feature parameters are automatically stored in a text file for future reference. Standard AutoCAD functions can be applied to achieve surface meshing, hidden line removal, and rendering to aid visualisation of the finished component. Also, if AutoCAD application programs for finite element analysis were available to the user, component stress analysis could be performed.

2.2. Component representation Although there has been widespread employment of CAD systems in manufacturing, these systems are often used as 2D drafting aids. In addition to component drawings produced using CAD, written details of production plans and bills of materials are often used for modelling the production process. However, the development of solid modelling design systems within modem CAD systems allows surface, materials, and production data to be directly associated with the component geometry. Therefore when geometrical analysis is to be performed, a solid model within a CAD system is a suitable method for representing the component. To progress from the solid model to generation of advice on design for manufacturability, it is necessary to identify, quantify, and analyse the features of the component, (a feature has been defined by Cunningham et. al. as 'any geometrical form or entity that is used in reasoning in one or more design or manufacturing activities'), [8]. Most of the research into extracting feature information from solid models has employed either 'feature recognition' or 'design with features'. A number of researchers have reviewed the advantages and disadvantages of these approaches, with a state of the art survey of feature based modelling provided by Allada and Anand in 1995 [9]. For the system described here, it was decided that the solid model of the component should be developed by employing a design with features approach. Design with features is a modelling technique where the solid model is constructed using a set of previously defined product features. Problems inherent with feature recognition, such as unrecognisable component geometry, or alternative design evaluations for a given component, are avoided. This approach also has the advantage that the features relate to the process concerned, and therefore information associated with them, such as production parameters or cost data is available for analysis.

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2.3. Design development and analysis IDA presents the user with a set of features for modelling the component. The primary features available are box, cylinder, and gear; while hole, circular groove, linear groove, and chamfer constitute the secondary features. For ease of operation these features are presented as a set of radio buttons within a customised dialogue box. After selecting the feature to be created the user is prompted for feature location and dimensions. This information is stored within the system as AutoLISP variables. IDA then determines if other features were already present, and if so whether any unformable geometrical characteristics result from their combination with the new feature. If the design is detected as being unformable, the system identifies the nature of the problem, erases the feature, and automatically provides the user with an alternative feature. When the design is completed, the system combines the features into one solid entity using the boolean operations of union and subtraction. Design analysis requires that the feature combinations present should be checked for manufacturing feasibility. Given the freedom with which the user can specify feature size and location, it is likely that component geometries with emergent features will be created from a combination of standard features. While it would be expected that such geometries require complex analysis algorithms, it is suggested here that application of modular interactive analysis allows significant function simplification, with a resulting improvement in software reliability. The term 'modular' refers to the modular nature of functions which check the interaction of each combination of feature pairs present. The parameters of emergent features are checked to ensure they lie within the acceptable range for PM. This is illustrated in Figure 2, where it can be seen that a 'hole' feature has been located within a 'cylinder' feature, resulting in creation of a "hollow cylinder' emergent feature. It is then necessary for the system to check the wall thickness of the hollow cylinder to ensure it is within range. Analysis of each instance of a given feature combination can be achieved using the same function, but with different parameters, as appropriate. Cylinderwall emergentfeature requiringthicknesscheck

Cylinder

Hollow cylinder

Fig. 2. Creation of hollow cylinder, requiring check of wall thickness.

2.4. Method of system operation Within the AutoLISP program, a function is devoted to the creation of each feature. To achieve this, the functions use boolean combinations of AME solid primitives, created using the

L.N. Smith, P.S. Midha/Journal of Materials Processing Technology 61 (1996) 187-192

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If it is necessary for the gear to be modified, then the process of erasing it and introducing a new gear is simplified.

SOLBOX and SOLCYL commands. More complex features such as the gear are created by extruding a polyline to form a solid, using the SOLEXT command [10]. Once a feature is created, a list of functions are automatically called to check that the dimensions of the feature itself are within range, and then to ensure that in combination with features already present, unformable design characteristics have not been created. These functions contain sets of rules used to check feature characteristics such as height to width ratio, wall thickness, depths of blind holes, and chamfer angles. For example, a function called 'chkgear' has been developed to check the geometry of a newly created gear function. Initially the height to width ratio is checked to ensure it is not greater than a recommended minimum value e.g. 3, to prevent undesirable density variations within the compact. Then the gear modulus, which is the ratio of the pitch diameter to the number of teeth, is calculated. It is necessary to ensure that the modulus is not less than 0.5, since if it were there would be a risk of the die cavity being improperly filled. The function also calculates the root radius and outside radius for use later in the program. The program then runs a set of functions to analyse the gear in combination with other features. Here, the value of an interactive approach for simplifying component analysis becomes evident. Since the effects of the standard feature, in this case a gear, are being considered as soon as it is created, the complicating effects of secondary features on the gear are avoided. If a number of features were incorporated in the design before analysis, then removal of an unformable feature such as a gear with a low modulus, would result in the loss of any secondary features of the gear. (A chamfer of the gear profile is an example of such a secondary feature.) Re-incorporation of the secondary feature at a later stage could prove difficult. In contrast, interactive design analysis allows a sequential check to be performed firstly on the feature itself, (i.e. the gear), and then on its effects on the feature combinations already present within the design.

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2.4.1. Example component for design consultation Figure 1 shows a gear, which is an example component to be used to demonstrate operation of the system. This original design incorporates a number of geometrical characteristics which make it impractical for production using PM. The user employs the IDA feature library to create a solid model of the gear. During this process the system detects and advises on unformable features. If such a feature is detected it is automatically erased, and the system suggests an altemative feature which will provide the required functionality as well as being acceptable for PM. The resulting modified design, which is now suitable for PM production, is shown in Figure 3. Chamfer at 60 ° to axis

Fig. 3. Modified gear design, suitable for production by PM technology.

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L.N. Smith, P.S. Midha/Journal of Materials Processing Technology 61 (1996) 187-192

2.4.2 Consultation procedure The consultation procedure for IDA is outlined by the flow chart shown in Figure 4. The following procedural details relate to a consultation for the Figure 1 gear:- After running AutoCAD and setting up an acceptable zoom and viewpoint, IDA is activated by selecting 'DESIGNA' on the screen menu. The order in which the user can create features is not fixed, but it is good practice to start with creation of a primary feature. Here, a gear is created of pitch circle diameter 50, thickness 20, with 12 teeth. After the gear is created, the system applies rules to the gear parameters to ensure that the geometry is acceptable for PM. IDA states that the modulus (pcd/number of teeth) here is 4.2, which is more than 0.5 and therefore within range. Standard equations for spur gears are used to calculate the root diameter and outside diameter of the gear. The minimum acceptable wall thickness is calculated using equations incorporated into the knowledge base which relate minimum wall thickness to component height and width. These equations were derived by fitting curves to published empirical data for acceptable minimum wall thickness for parts with specified heights and widths. The next feature to be added is a cylinder used to form the hub of the gear. The cylinder is created with the same base centre point as the gear, with a height of thirty and a radius of nineteen. The system then displays the following text:'A small distance between the tooth root and central hub is undesirable because it leads to thin, and therefore fragile, tool sections. The distance between the tooth root and the central hub is 1.179 mm. This is less than 3 mm, so the gear/hub arrangement is unacceptable for PM.' This is an example of the system finding that the component violates one of the geometrical rules for PM gears. IDA erases the cylinder, and asks if automatic re-definition of the hub is required. Automatic re-definition of the hub will involve redefining the hub by reducing its radius. This will produce a component with a greater distance from the hub to the gear root, which is required to be at least 3 mm to ensure adequate tool strength. As can be seen in Figure 3, IDA re-defines the hub with a smaller radius, thereby providing a greater distance from the hub to the gear root, and asks if this is acceptable. The next feature added is a vertical hole of radius 8 through which the gear shaft will pass. An attempt is then made to form the horizontal hole shown in Figure 1, which is intended to accommodate a split pin. After the hole is created, IDA displays the following: 'The axis of the hole is not the same as the axisof the component. Modify. Forming is difficult or impossible if these axes do not coincide. The hole will be deleted.' The system does not allow creation of a hole with a horizontal axis, since such a feature would not allow extraction of the part from the die. The system asks if the function of the hole was to fasten the gear onto a shaft. If the user's response is affirmative IDA states that automatic definition of a keyway feature can be attempted, and asks the user if this is required. If the user responds with yes the system suggests a keyway width of 4 mm, and provides details relating to the way in which the keyway will be created. The component then reappears as shown in Figure 3, where it can be seen that the keyway which has been added to the design, would not prevent extraction of the part from the die. The final feature to be added is a chamfer on the top surface of the hub. After entering the parameters to create the chamfer shown in Figure 1, IDA displays the following advice:

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'An angle for the chamfer of less than 60 degrees from the vertical is not recommended. This is to avoid chipping of featheredged punches, which results in manufacturing delays. The chamfer has not been created. IDA can attempt automatic redefinition of the chamfer feature. Do you wish this to occur?' A new chamfer is created, at an angle of 60 degrees to the vertical, and with a height of 2 mm. When the user has decided the design is completed, the 'Finish adding features' button is selected in the IDA dialogue box, and the system then employs the boolean operations of union and subtraction to form one component. The features present, and their parameters, are saved to a disk file for future reference, and a solid model of the finished component is available as an AutoCAD drawing. The visualization of the component can be enhanced through meshing with hidden line removal, or rendering, as shown in Figure 5. Also, analysis functions supplied with AutoCAD could be used to obtain mass properties for the component, which would be useful for cost estimation.

Fig. 5. Rendered visualisation of the PM hubbed gear, following PM design consultation.

3. Conclusion The system reported in this paper demonstrates an interactive methodology for development and evaluation of designs for component manufacture by powder metallurgy. It incorporates a knowledge based approach to provide improved access to the capabilities and benefits of the PM process, thereby minimising the need for post-design changes and downstream shop floor problems. The system incorporates feature based modelling as a suitable technique for expression of the design intent, and employs interactive geometrical analysis with, if necessary, automatic design modification, to ensure that the component geometry is suitable for production by powder metallurgy. It has been shown that this approach, in combination with a KBS, enables application at the design stage, of knowledge and expertise about the production process - powder metallurgy in the present case. This has demonstrated the value of interactive design development and rule based interrogation, as concurrent engineering design tools able to generate design recommendations for different categories of components for manufacture by processes such as powder metallurgy. The paper illustrates application of the techniques by interactively designing a gear to its original specification and modifying a number of design features to enable its production by

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means of PM technology.

[5] Neue Hiitte, volume 2, p. 41-44, (1988).

The current system allows development of a design as a solid model by freely combining up to seven features. Future work will involve expanding this feature library, and the amount of information associated with each feature, to enable development of systems for modelling components with more complex geometries. This will necessitate an increase in the range and complexity of algorithms employed for inferring the production consequences of the feature combinations. Such systems will provide an enabling tool for concurrent design of structural components, and will offer access to the economic and ecological benefits of PM technology for a range of potential applications.

[6] L. N. Smith, P. S. Midha, 'A Knowledge Based System for Design and Manufacture of Axisymmetric Components Using Powder Metallurgy Techniques', 1 l th International Conference on Computer Aided Production Engineering, IMechE HQ, London, UK, (1995).

References

[1] J. Hartley and J. Mortimer, Simultaneous Engineering - the Management Guide, 2nd edition, Industrial Newsletters Ltd, Dunstable, UK, (1995). [2] K. G. Swift, Knowledge-Based Design for Manufacture, Kogan Page Ltd, UK, (1987), ISBN 1-85091-231-9.

[7] D. C. Zenger, J. J. Bausch, D.C. Brown, R. D. Sisson, and R. Ludwig, 'Interactive Design, Intelligent Process Planning, and Automated Control for Powder Metallurgy Components', Proceedings of the 1993 Powder Metallurgy Worm Congress, Kyoto, Japan, (1993). [8] J. J. Cunningham, J. R. Dixon, 'Designing with Features: The Origin of Features.' Proceedings of the 1988 ASME International Computers in Engineering Conference and Exhibition, San Francisco, CA, USA, Vol 1, p. 237-243, (1988). [9] V. Allada, and S. Anand, 'Feature-based modelling approaches for integrated manufacturing: state-of-the art survey and future research directions', Int. J. Computer Integrated Manufacturing, vol. 8, No. 6 p. 411-440, (1995).

[3] S. Dutta, Knowledge Processing and Applied Artificial Intelligence, Butterworth-Heinemann Ltd, UK, (1993), ISBN 0-7506-1612-1.

[10] Autodesk Inc, AutoCAD Advanced Modeling Extension Release 2.1 Reference Manual, (1992), ISBN 2-88447-011-5.

[4] N. Dautzenberg, K. Linder, W. K6nig, and R. Strehl, 'Comparison of conventionally manufactured gears with PM gears manufactured by the single or double sintering process - technical and economic aspects', Technical Communication, PM '94, Paris, France, (1994).

The followingare registeredtrademarksof Autodesk,Inc; AutoCAD,Autodesk, AutoLISP.The followingare trademarksof Autodesk,Inc; AdvancedModeling Extension (AME),AutoCADDevelopmentSystem(ADS).