- Email: [email protected]
Design reuse in a CAD environment — Four case studies
oornl:u~ I~Pial
m
engineering
Computers & Industrial Engineering 37 (1999) 105-109
PERGAMON
Design Reuse in a C A D E n v i r o n m e n t - Four Case Studies P.T.J.Andrews, T.M.M.Shahin & S.Sivaloganathan
Engineering Design Group, Department of Manufacturing & Engineering Systems, Brunel University, Uxbridge, UB8 3PH, UK e-mail: [email protected], uk
Abstract This paper presents two methods for storage and reuse of detailed engine~ing designs for efficient and adaptive reuse. The Generative Method [!], invokes techniques such as 'Interactive Feature Recognition' and a 'Parametric CSG-Tree' to produce a design model suitable for adaptive and innovative design. While the Variant Method [2], was subsequently developed to overcome the complexities of creating a generative CAD model and is better suited to the representation of families of similar designs. © 1999 Published by Elsevier Science Ltd. All rights reserved. Both methods are outlined here and demonstrated by the following industrial case studies: The Generative Method: a) Hydraulic access platform and b) an Automotive Radiator tank. The Variant Method: a) Drive-End Shield Casting and b) GMT Lathe Chuck family.
1. Introduction There are two main issues to confront when devising methods to store detailed engineering designs. Firstly, the need to store (manual-paper-based) legacy designs and secondly the ability to capture designs concurrently throughout an ongoing design process. The underlying issue here is their storage with the intention for reuse (extraction and adaptation). Research within the Engineering Design Group is based on the philosophy of creating tools that can aid the designer, incorporated with current technology, that can produce realisable results. However, this emphasis is always maintained with a view to future developments and wide applicability, hence the creation of generic methodologies. This approach has allowed such methodologies to be rigorously tested in the real (industrial) world. A key theme throughout the methods and case studies presented here is family-based product design. This is a feature that is only beginning to be addressed [3] and is of significant importance in Design Reuse.
2. Background Systems that allow the creation of evolutionary or adaptive designs, on the whole, incorporate two basic principles, Feature-based Design and Parametric-based Modelling.
2.1 Feature-based
Design
Engineering features are recognisable to all designers. They maintain a degree of standardisation in oftencomplex systems. As such they have the ability to modularise design and maintain intent throughout continuing modifications. The application of features in CAD systems is termed feature-based design. This process can be further classified into two main concepts, Form-Feature Recognition and Design by Features [4,5].
2.2 Parametric Modelling The ability to set and modify parameters in feature based models is the underlying key to their success. To the user of such systems their application is relatively simple to understand. Parameters can (usually automatically) be assigned to a feature's dimensions, or via relations. They can be applied either directly (locally) or indirectly 0360-8352/99 - see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0360-8352(99)00033-9
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Proceedings of the 24th International Conference on Computers and Industrial Engineering
(globally). Global Parameter definitions are particularly important as driving parameters, especially in assembly modelling. Generally speaking, parameter-based modelling allows the creation of User-Defined Features. Parametric Modelling has it origins in 2D graphical design, where experience in constraint satisfaction of 2D profiles has influenced the technique of transforming 2D parametric profiles into 3D features. This is the current trend for the majority of commercial parametric modelling applications.
3 Methods for Storing Detailed, Engineering Designs Design is an evolving process. Looking back through this evolution can be very beneficial when creating new, but similar designs. The study of design cases is notably useful for understanding the problems, decisions and their outcomes when creating similar designs [6]. Therefore, methods for storing the design history (or versions) are required. Work on Design Reuse within the Engineering Design Group has established two prominent methods for storing such adaptive design domains.
3.1 The Generative Method This method is based around the Constructive Solid Geometry (CSG) representation technique [7]. CSG stores and builds a complex solid model by performing Boolean operations (such as union, subtract and intersect) on basic solid primitives. It is an efficient method of storage, although limited when it comes to modifying the complex solid. The Generative Methodology takes this process one step further by using a technique called 'Parametric CSG' [1]. The solid model is created and, using Interactive Feature Recognition [8], features are extracted from the model and stored as CSG trees. The CSG tree is then modified by the introduction of parameters. On modification of the design, the designer enters values for these parameters through a userfriendly front-end menu. These values are then substituted into the parametric CSG tree and the solid model is re-generated. Immediate benefits of such a methodology are: • The storage of only the parametric CSG tree (i.e. not the model) hence storage on computer would be minimal and hence efficient. • Process of building many models for optimisation and/or Finite Element Analysis can be fully automated by generating an ASCII-text file, with the value of the parameters of each 'run' on a separate line. A program/macro would read each subsequent line when it is time to build the next model run. • This methodology is useful for designs where many possible modifications need to be made to a product or where a part changes significantly (e.g. topological changes) for inclusion in a different product. However there are drawbacks to this methodology: • It can be time consuming to initially implement. • It is more akin to programming as modification of the CSG tree is required • It needs to re-generate the model from scratch each time a modification is required.
3.2 The Variant Method For many cases, use of the generative method may prove too time consuming or beyond the scope of the designer or software package. A Variant Methodology [2, 9] has been created to achieve the following alms: • allow for the variance of similar designs for modification and reuse, • efficiently store families (and histories) of design models in a database, • be less resource consuming than the generative method, • be comparatively simple to implement. The basics of this methodology can be outlined as follows:
1) Define Parts, Features and Parameters: As with the generative methodology, all parts (generic or otherwise), features and parameters must be defined before modelling can begin. Use of a suitable structure, e.g. Parts or Function-Means tree, can be adopted to assist in determining driving parameters for an assembly of components.
2) Create Variant CAD models: Unlike the generative method, variant CAD models [10,11] are modified (at run-time) only by modification of their parameter values and feature suppression status. No editing of data structure or scripting files is required. Which, although less flexible than the generative method, is simpler to implement. These
Proceedings o f the 24th International Conference on Computers and Industrial Engineering parametric CAD models form what is termed the Variant Master Model.
3) Create a Family Database of Designs: The master model is driven by an Intelligent Engine, i.e. the database. This can be a commercial database application or a specific user-defined program. Use of software automation techniques are used to drive the CAD application via the Intelligent Engine to evaluate modified part and assembly models.
4. Case Studies 4.1 The Niftylift Hydraulic Access Platform Niftylift are a world-wide producer of aerial access platforms. This case-study uses the Nifty!ift "Height-Rider' series of self-propelled hydraulic access platforms to produce a tool that can reduce desigin time by reusing information about existing products and components. The mainstay of Niftylift products can be categorised into various ranges of product, each having its own individual characteristics and features. A consistent design policy at Niftylift is the use of so-called 'standard-components', which can be both off-the-shelf (supplier based) or inhouse. This has the effect of reducing the number of components required in the final design. However, as is typical of many small to medium enterprises (SME's), much of their design library still exists in a primarily 2D format, making the full switch to 3D modelling (along with all of its associated benefits) more difficult.
It was decided to utilise the generative design method to create a feature-based design tool to : i) store the various ranges of access platforms, ii) allow both simple and radical changes to the designs, and iii) allow radically innovative designs of new products to be generated. MSC/Aries was used to model this design family. After creation of a nominal design, CSG tree extraction/parameterisation was undertaken for each component. The entire model was then re-packaged around an interactive, dialog-based interface, where components are generated according to values input to a series of pop-up dialog boxes. Through this mechanism a design tool to rapidly explore different boom assemblies, using existing design knowledge, was created
figure 1 - MSC/Aries CAD model of a 'HeightRider' boom assembly.
4.2 Ford Motor Company - Jaguar Radiator Tank A case-study to optimise the design of a Jaguar engine radiator tank was undertaken. Its primary objective being to investigate stress effects on the tank structure for a range of configurations and sizes, according to a designed experiment. The degree of customisation required and the need to maintain a high degree of control over the meshing and (finite-element) analysis of the model invoked the use of the generative method. Again, MSC/Aries was used to generate the model, providing CSG tree extraction and integrated macro scripting as standard. As well as using the nominal generative model to insert variable parameters, other nongeometric information, such as material properties were parameterised. Readily allowing different materials to be selected for analysis.
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Proceedings of the 24th International Conference on Computers and Industrial Engineering
..~,~
~!
Master
Figure 2 shows a sample, meshed radiator tank before undergoing finite element analysis.
Intellig Engine ' ~ ~ ' ~ / (databaSe~ i ~
figure 2 - CAD model of a "meshed' radiator tank
4.3 L u c a s V a r i t y - Drive End Shield c a s t i n g This product is a single piece casting, used to house and mount a starter-motor to a suitable petrol/diesel engine. The casting is a compound family of fifteen different, but inherently similar designs, each containing a similarly profiled section, albeit with varying dimensions and between 2-4 lugs (or bosses) for mounting. Such similarity between designs, and the desire for simple but effective model creation is key to the variant method. Also, the lack of radical modifications further facilitates the use of this approach. Another requirement was to create a database of these designs, where a single design record can be picked, and its corresponding solid model and manufacturing drawings created, at the touch of a button. In this case it was decided to use Autodesk Mechanical Desktop
Instances
~.~, Figure 3 - Instancing Castings from the Variant Model
as the variant modelling application. This application has the disadvantage that it does not support feature suppression (at the time of undertaking). Hence the addition of suppressible features as external CAD files, which (along with the base model) make-up the master model (figure 3). These in turn, are controlled by the database software (the intelligent engine). Where features can be suppressed, resumed and arrayed (patterned) as per parameter modifications
4.4 G M T L a t h e C h u c k Manual and power operated lathe chucks are the main product of Guindy Machine Tools of Madras, India. They, as is typical of many similarly sized companies in the developing world, are in the early stages of realising and implementing the full advantages of integrated computer modelling and analysis. Again, many of their numerous past designs exist as manual drawings, which must be modelled on computer to enable efficient reuse and modification, so as to meet improving standards if GMT are to contend with their competitors. The entire chuck clan envelopes seventeen radically different types of chuck family, for a variety of applications. With each family member maintaining 3-7 family members (products). Despite a high degree of radical differences between chuck families, and even family members, the variant method, coupled with a suitable modelling package is best suited to this study, primarily due to the amount of work involved.
Proceedings o f the 24th International Conference on Computers and Industrial Engineering
As with the Lueas casting example, the aim here is to create an adaptive database of designs. Due to the large number and complexity of the components that make-up the chuck clan, Pro/ENGINEER was used as the modelling software, providing built-in part and feature suppression. Although the model can be driven entirely by the clan (assembly) models set of global parameters, individual piece parts can also be tailored. Where, unless links (relations) to the higher-level assembly parameters are broken, will still, as required, remain under their control, due to ,the hierarchical nature in which parameter relations and constraints are resolved.
Figure 4 - Instances of the Chuck Variant Model
5 Discussion & Conclusions The four case studies presented successfully demonstrate the ability of both generative and variant methods to store and modify detailed engineering designs. However, their ultimate usefulness is dependent on the designers ability to store (and thereby retrieve) these designs systematically. Ongoing research in the Engineering Design Group incorporates detailed design within the whole design process. Relating detail to more abstract forms, such as concept and function. This will enable past designs to be more meaningfully and rapidly retrieved.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Shahin T.M.M., 'Automation of Feature-Based Design and Finite Element Analysis for Optimal Design', PhD Thesis, Brunei University, UK, December 1996 Andrews P.T.J, 'Design Reuse in a CAD Environment', PhD Thesis, Brunel University, January 1999 Hoffmann C.M., 'On Semantics of Generative Geometry Representations', Advances in Design Automation - Volume 2, DE-VoI., 65-2, 1993 Shah J.J, 'Assessment of Features Technology', Computer Aided Design, Vol. 23, June 1991 Finger S. & Sailer S.A., 'Representing & Recognising Features in Mechanical Designs', Design Theory & Methodology, pp19-25, 1990 Finger S., 'Design Reuse and Design Research - Keynote Paper', Engineering Design Conference, Brunel University, UK, 1998. Requicha A.A.G., 'Solid modelling: Current Status and Research Direction', IEEE CG&A, pp 25-37, October 1983. J.J Shah, 'Assessment of features technology', Computer-Aided Design, Vol. 23 pp 331-343, June 1991 Andrews P.T.J & Sivaloganathan S., 'A Variant Model for Storing Families of Mechanical Designs', Engineering Design Conference, Brunel University, UK, 1998 Fowler J.E., 'Variant Design for Mechanical Artefacts: A State-of-the-Art Survey', Engineering with Computers, Vol. 12, 1-15, 1996 Technicom Inc , 'Parametric, Variational and Feature Based Modelling Study Report', USA, http://www.technicom.com, 1992
109
m
engineering
Computers & Industrial Engineering 37 (1999) 105-109
PERGAMON
Design Reuse in a C A D E n v i r o n m e n t - Four Case Studies P.T.J.Andrews, T.M.M.Shahin & S.Sivaloganathan
Engineering Design Group, Department of Manufacturing & Engineering Systems, Brunel University, Uxbridge, UB8 3PH, UK e-mail: [email protected], uk
Abstract This paper presents two methods for storage and reuse of detailed engine~ing designs for efficient and adaptive reuse. The Generative Method [!], invokes techniques such as 'Interactive Feature Recognition' and a 'Parametric CSG-Tree' to produce a design model suitable for adaptive and innovative design. While the Variant Method [2], was subsequently developed to overcome the complexities of creating a generative CAD model and is better suited to the representation of families of similar designs. © 1999 Published by Elsevier Science Ltd. All rights reserved. Both methods are outlined here and demonstrated by the following industrial case studies: The Generative Method: a) Hydraulic access platform and b) an Automotive Radiator tank. The Variant Method: a) Drive-End Shield Casting and b) GMT Lathe Chuck family.
1. Introduction There are two main issues to confront when devising methods to store detailed engineering designs. Firstly, the need to store (manual-paper-based) legacy designs and secondly the ability to capture designs concurrently throughout an ongoing design process. The underlying issue here is their storage with the intention for reuse (extraction and adaptation). Research within the Engineering Design Group is based on the philosophy of creating tools that can aid the designer, incorporated with current technology, that can produce realisable results. However, this emphasis is always maintained with a view to future developments and wide applicability, hence the creation of generic methodologies. This approach has allowed such methodologies to be rigorously tested in the real (industrial) world. A key theme throughout the methods and case studies presented here is family-based product design. This is a feature that is only beginning to be addressed [3] and is of significant importance in Design Reuse.
2. Background Systems that allow the creation of evolutionary or adaptive designs, on the whole, incorporate two basic principles, Feature-based Design and Parametric-based Modelling.
2.1 Feature-based
Design
Engineering features are recognisable to all designers. They maintain a degree of standardisation in oftencomplex systems. As such they have the ability to modularise design and maintain intent throughout continuing modifications. The application of features in CAD systems is termed feature-based design. This process can be further classified into two main concepts, Form-Feature Recognition and Design by Features [4,5].
2.2 Parametric Modelling The ability to set and modify parameters in feature based models is the underlying key to their success. To the user of such systems their application is relatively simple to understand. Parameters can (usually automatically) be assigned to a feature's dimensions, or via relations. They can be applied either directly (locally) or indirectly 0360-8352/99 - see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0360-8352(99)00033-9
106
Proceedings of the 24th International Conference on Computers and Industrial Engineering
(globally). Global Parameter definitions are particularly important as driving parameters, especially in assembly modelling. Generally speaking, parameter-based modelling allows the creation of User-Defined Features. Parametric Modelling has it origins in 2D graphical design, where experience in constraint satisfaction of 2D profiles has influenced the technique of transforming 2D parametric profiles into 3D features. This is the current trend for the majority of commercial parametric modelling applications.
3 Methods for Storing Detailed, Engineering Designs Design is an evolving process. Looking back through this evolution can be very beneficial when creating new, but similar designs. The study of design cases is notably useful for understanding the problems, decisions and their outcomes when creating similar designs [6]. Therefore, methods for storing the design history (or versions) are required. Work on Design Reuse within the Engineering Design Group has established two prominent methods for storing such adaptive design domains.
3.1 The Generative Method This method is based around the Constructive Solid Geometry (CSG) representation technique [7]. CSG stores and builds a complex solid model by performing Boolean operations (such as union, subtract and intersect) on basic solid primitives. It is an efficient method of storage, although limited when it comes to modifying the complex solid. The Generative Methodology takes this process one step further by using a technique called 'Parametric CSG' [1]. The solid model is created and, using Interactive Feature Recognition [8], features are extracted from the model and stored as CSG trees. The CSG tree is then modified by the introduction of parameters. On modification of the design, the designer enters values for these parameters through a userfriendly front-end menu. These values are then substituted into the parametric CSG tree and the solid model is re-generated. Immediate benefits of such a methodology are: • The storage of only the parametric CSG tree (i.e. not the model) hence storage on computer would be minimal and hence efficient. • Process of building many models for optimisation and/or Finite Element Analysis can be fully automated by generating an ASCII-text file, with the value of the parameters of each 'run' on a separate line. A program/macro would read each subsequent line when it is time to build the next model run. • This methodology is useful for designs where many possible modifications need to be made to a product or where a part changes significantly (e.g. topological changes) for inclusion in a different product. However there are drawbacks to this methodology: • It can be time consuming to initially implement. • It is more akin to programming as modification of the CSG tree is required • It needs to re-generate the model from scratch each time a modification is required.
3.2 The Variant Method For many cases, use of the generative method may prove too time consuming or beyond the scope of the designer or software package. A Variant Methodology [2, 9] has been created to achieve the following alms: • allow for the variance of similar designs for modification and reuse, • efficiently store families (and histories) of design models in a database, • be less resource consuming than the generative method, • be comparatively simple to implement. The basics of this methodology can be outlined as follows:
1) Define Parts, Features and Parameters: As with the generative methodology, all parts (generic or otherwise), features and parameters must be defined before modelling can begin. Use of a suitable structure, e.g. Parts or Function-Means tree, can be adopted to assist in determining driving parameters for an assembly of components.
2) Create Variant CAD models: Unlike the generative method, variant CAD models [10,11] are modified (at run-time) only by modification of their parameter values and feature suppression status. No editing of data structure or scripting files is required. Which, although less flexible than the generative method, is simpler to implement. These
Proceedings o f the 24th International Conference on Computers and Industrial Engineering parametric CAD models form what is termed the Variant Master Model.
3) Create a Family Database of Designs: The master model is driven by an Intelligent Engine, i.e. the database. This can be a commercial database application or a specific user-defined program. Use of software automation techniques are used to drive the CAD application via the Intelligent Engine to evaluate modified part and assembly models.
4. Case Studies 4.1 The Niftylift Hydraulic Access Platform Niftylift are a world-wide producer of aerial access platforms. This case-study uses the Nifty!ift "Height-Rider' series of self-propelled hydraulic access platforms to produce a tool that can reduce desigin time by reusing information about existing products and components. The mainstay of Niftylift products can be categorised into various ranges of product, each having its own individual characteristics and features. A consistent design policy at Niftylift is the use of so-called 'standard-components', which can be both off-the-shelf (supplier based) or inhouse. This has the effect of reducing the number of components required in the final design. However, as is typical of many small to medium enterprises (SME's), much of their design library still exists in a primarily 2D format, making the full switch to 3D modelling (along with all of its associated benefits) more difficult.
It was decided to utilise the generative design method to create a feature-based design tool to : i) store the various ranges of access platforms, ii) allow both simple and radical changes to the designs, and iii) allow radically innovative designs of new products to be generated. MSC/Aries was used to model this design family. After creation of a nominal design, CSG tree extraction/parameterisation was undertaken for each component. The entire model was then re-packaged around an interactive, dialog-based interface, where components are generated according to values input to a series of pop-up dialog boxes. Through this mechanism a design tool to rapidly explore different boom assemblies, using existing design knowledge, was created
figure 1 - MSC/Aries CAD model of a 'HeightRider' boom assembly.
4.2 Ford Motor Company - Jaguar Radiator Tank A case-study to optimise the design of a Jaguar engine radiator tank was undertaken. Its primary objective being to investigate stress effects on the tank structure for a range of configurations and sizes, according to a designed experiment. The degree of customisation required and the need to maintain a high degree of control over the meshing and (finite-element) analysis of the model invoked the use of the generative method. Again, MSC/Aries was used to generate the model, providing CSG tree extraction and integrated macro scripting as standard. As well as using the nominal generative model to insert variable parameters, other nongeometric information, such as material properties were parameterised. Readily allowing different materials to be selected for analysis.
107
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Proceedings of the 24th International Conference on Computers and Industrial Engineering
..~,~
~!
Master
Figure 2 shows a sample, meshed radiator tank before undergoing finite element analysis.
Intellig Engine ' ~ ~ ' ~ / (databaSe~ i ~
figure 2 - CAD model of a "meshed' radiator tank
4.3 L u c a s V a r i t y - Drive End Shield c a s t i n g This product is a single piece casting, used to house and mount a starter-motor to a suitable petrol/diesel engine. The casting is a compound family of fifteen different, but inherently similar designs, each containing a similarly profiled section, albeit with varying dimensions and between 2-4 lugs (or bosses) for mounting. Such similarity between designs, and the desire for simple but effective model creation is key to the variant method. Also, the lack of radical modifications further facilitates the use of this approach. Another requirement was to create a database of these designs, where a single design record can be picked, and its corresponding solid model and manufacturing drawings created, at the touch of a button. In this case it was decided to use Autodesk Mechanical Desktop
Instances
~.~, Figure 3 - Instancing Castings from the Variant Model
as the variant modelling application. This application has the disadvantage that it does not support feature suppression (at the time of undertaking). Hence the addition of suppressible features as external CAD files, which (along with the base model) make-up the master model (figure 3). These in turn, are controlled by the database software (the intelligent engine). Where features can be suppressed, resumed and arrayed (patterned) as per parameter modifications
4.4 G M T L a t h e C h u c k Manual and power operated lathe chucks are the main product of Guindy Machine Tools of Madras, India. They, as is typical of many similarly sized companies in the developing world, are in the early stages of realising and implementing the full advantages of integrated computer modelling and analysis. Again, many of their numerous past designs exist as manual drawings, which must be modelled on computer to enable efficient reuse and modification, so as to meet improving standards if GMT are to contend with their competitors. The entire chuck clan envelopes seventeen radically different types of chuck family, for a variety of applications. With each family member maintaining 3-7 family members (products). Despite a high degree of radical differences between chuck families, and even family members, the variant method, coupled with a suitable modelling package is best suited to this study, primarily due to the amount of work involved.
Proceedings o f the 24th International Conference on Computers and Industrial Engineering
As with the Lueas casting example, the aim here is to create an adaptive database of designs. Due to the large number and complexity of the components that make-up the chuck clan, Pro/ENGINEER was used as the modelling software, providing built-in part and feature suppression. Although the model can be driven entirely by the clan (assembly) models set of global parameters, individual piece parts can also be tailored. Where, unless links (relations) to the higher-level assembly parameters are broken, will still, as required, remain under their control, due to ,the hierarchical nature in which parameter relations and constraints are resolved.
Figure 4 - Instances of the Chuck Variant Model
5 Discussion & Conclusions The four case studies presented successfully demonstrate the ability of both generative and variant methods to store and modify detailed engineering designs. However, their ultimate usefulness is dependent on the designers ability to store (and thereby retrieve) these designs systematically. Ongoing research in the Engineering Design Group incorporates detailed design within the whole design process. Relating detail to more abstract forms, such as concept and function. This will enable past designs to be more meaningfully and rapidly retrieved.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Shahin T.M.M., 'Automation of Feature-Based Design and Finite Element Analysis for Optimal Design', PhD Thesis, Brunei University, UK, December 1996 Andrews P.T.J, 'Design Reuse in a CAD Environment', PhD Thesis, Brunel University, January 1999 Hoffmann C.M., 'On Semantics of Generative Geometry Representations', Advances in Design Automation - Volume 2, DE-VoI., 65-2, 1993 Shah J.J, 'Assessment of Features Technology', Computer Aided Design, Vol. 23, June 1991 Finger S. & Sailer S.A., 'Representing & Recognising Features in Mechanical Designs', Design Theory & Methodology, pp19-25, 1990 Finger S., 'Design Reuse and Design Research - Keynote Paper', Engineering Design Conference, Brunel University, UK, 1998. Requicha A.A.G., 'Solid modelling: Current Status and Research Direction', IEEE CG&A, pp 25-37, October 1983. J.J Shah, 'Assessment of features technology', Computer-Aided Design, Vol. 23 pp 331-343, June 1991 Andrews P.T.J & Sivaloganathan S., 'A Variant Model for Storing Families of Mechanical Designs', Engineering Design Conference, Brunel University, UK, 1998 Fowler J.E., 'Variant Design for Mechanical Artefacts: A State-of-the-Art Survey', Engineering with Computers, Vol. 12, 1-15, 1996 Technicom Inc , 'Parametric, Variational and Feature Based Modelling Study Report', USA, http://www.technicom.com, 1992
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