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A computer-aided engineering system for feature-based design of box-type sheet metal parts
Journalof
Materials Processing Technology Journal of Materials Processing Technology 57 (1996) 241-248
ELSEVIER
A computer-aided engineering system for feature-based design of box-type sheet metal parts R. Mantripragada *, G. Kinzel, T. Altan Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Columbus, OH, USA
Received 18 August 1994
Industrial summary
This paper explores the applications of feature-based representation and design in the area of design for manufacturing to incorporate the tooling and process considerations into the early stages of design. The goal of this research is to apply the concepts of feature-based design and to develop an interactive design tool that can be used to alert the designers to potential production problems, defects and failures, and to provide them with information that can be used to explore alternative design, evaluate trade-offs, and arrive at optimal designs for the given process conditions. This paper illustrates the development of a computer-aided engineering (CAE) system that was constructed using these concepts and applied to designing box-type sheet metal parts.
1. I n t r o d u c t i o n
Sheet forming is a significant net shape manufacturing process for producing a large variety of consumer products (kitchen sinks, cans, cabinets, boxes, brackets, etc.), automotive components (body panels, fenders, etc.), and aerospace parts (body panels, wing parts, etc.). Die design in sheet forming, even after many years of practice, still remains more of an art than a science. Historically, the evolution of a sheet metal stamping from conception through part design to die design to the final die try-out has been a slow, cautious process based on the trial-and-error experience and skill of the artisan [1]. This has led several industries to invest substantial amounts of time, money, and resources to train artisans, and to pay little attention to the development of a scientific approach to cCie and part design. With the increasing complexity of parts, the introduction of new unfamiliar materials, reduced lead times and increasing emphasis on cost effectiveness, there is a growing trend towards a breakdown of experience-based design procedures. There is a need to replace the present experience based, trial-and-error techniques with cost effective, knowledge based, analytical techniques in sheet metal forming. An increase in productivity can be achieved if the part geometry, the fabrication method, the die design and the material prope, rties are specified during the design stage. * Corresponding author. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD1 0924-0136(95)02071-S
To meet these requirements, computer-aided engineering (CAE) systems are being developed and applied to sheet metal forming. This allows for a scientific approach combined with the know-how of experienced designers to develop a novel design methodology and process control strategy in sheet forming. A CAE system development for this purpose should be able to simulate the specific sheet metal forming process accurately and study the material behavior during the process. This would require the system to have a mathematical model describing the process and knowledge-based modules to incorporate the experience-based knowledge into the system. Such a system would serve as a powerful design aid tool for designers to explore alternative designs, evaluate trade-off and arrive at optimal designs at the lowest cost and shortest lead times. The objective of this paper is to describe an integrated CAE system for design evaluation, formability
Fig. 1. A typical box-type sheet metal part.
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R. Mantripragada et al. /Journal of Materials Processing Technology 57 (1996) 241-248
analysis and tooling design for the forming of box-type sheet metal parts. A typical box-type part is shown in Fig. 1. The system consists of three main modules: a mathematical-based analysis module, a design-rule based knowledge-based module and a parametric CAD package. The information flow between the various modules in the system is controlled by an integration program which forms the control unit for the system.
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2. Review of pertinent work A considerable amount of work has been done in feature-based design by several researchers using different types of techniques. Chang et al. [2] developed a feature-based expert process planning system (QTC), to develop a part-specific process planning environment. The QTC system was intended for one-of-a-kind part production and ruled out the possibility of using part family concepts of process planning, so that a featurebased design system was developed to generate new plans based on the knowledge of basic machining processes, tools and fixturing methods. Corbett et al. [3] developed a CAD-integrated knowledge-based system for the design of die cast components. The system uses knowledge-based systems, and in particular feature representation, in the area of design for manufacture die cast parts to alert the designer to any potential production problems. It uses a framebased representation to store information about features and the commercial CAD program CAEDS for display and the user interface. Similar work was also done by Yueh et al. [4], who used the CAD program Pro/Engineer and the expert system shell NEXPERT to develop a feature-based integrated design system. Significant work in the area of feature information extraction and representation was also done by Shpitalni et al. [5]. They worked on the extraction of different types of machining features. Their approach was to extract the disjointed machining regions comprising the total volume to be machined first and then to deal with each disjointed machining region independently. Another feature based design model was developed by DeVries et al. [6] for fixture design, concentrating on the selection of location elements and the identification of locating surfaces for workpiece positioning. Further literature related tothe present topic is available in Refs. [7-15].
3. Feature-based design approach Since most geometric modeling systems do not capture any deep understanding of the design, they are in general unable to support sophisticated reasoning capability. Thus, in applications such as design for manufac-
\
Slot-rectangular Bend Slot-circular Flat wall ~
--
~
Slot-irregular
Slot-triangular
Fig. 2. A typical sheet metal part constructed using features.
turing, the interaction of the human designer is necessary for data extraction and decision making. Thus to develop an intelligent CAE system for design and analysis, it is essential to devise a representation scheme for the design which allows the computer to capture and manipulate the design information, and, ultimately, to make decisions. In the sytem developed, the approach followed is to design with features. A feature is defined as a named entity with attributes of both form and function. Form attributes describe the physical geometry of the product, whilst functional attributes describe the purpose or functional aspects of the product. Typical sheet metal features include bends, flanges, holes, slots, beads, etc. This enables the designer to use a standard library of features to construct the part and incorporate useful design information during the construction stage. Every feature is treated like a record with a set of information fields that carry information about the feature and describe its relationship with other features. In addition, it may also contain some information that is specific to a particular feature, depending upon its type and functional importance. By following this kind of approach, the system has access to the complete feature information required for the analysis and evaluation of the design. Each feature can now be analyzed independently and in conjunction with other features for ease of manufacture and design criteria. Fig. 2 shows a typical sheet metal part design with features. The CAE system for the feature-based and analysis of box-type sheet metal parts has been built using the commercial CAD software platform, Pro/Engineer. Pro/Engineer was chosen for this project because of its parametric nature and its ability to enable the user to design with features. It has a standard library of sheet metal features that the designer can use to construct his/her part. Pro/Engineer associates with each feature. The following standard feature information, which enables the association of a separate identity to each feature. Another important capability of Pro/Engineer is that it allows most of this feature information to be extracted from the design using various data extraction
R. Mantripragada et al./Journal of Materials Processing Technology 57 (1996) 241-248
243
FEATURE N U M B E R : 2 INTERNAL, FEATURE ID : 80 P A R E N T S : 33 (#1) CHILDREN : 174(#3), 282(#4), 307(#5), 330(#6), 396(#7), 425(#8), 426(#9), 427(#10) 6011(#11), 648(#12), 684(#13), 770(#14), 789(#15), 850(#16) TYPE : BEND SECTION N A M E : $2D001 F E A T U R E "S D I M E N S I O N S : d5 = 90 d7 = 127 d8 = 90 d9 = 85 dlO = 1.27t'
Fig. 3. Features informationrepresentation in Pro/Engineer.
sub-routines in Pro/Develop, which is another module of Pro/Engineer. To develop an integrated CAE system for design evaluation and analysis using features, the following operations must be performed: (i) feature information representatio:a; (ii) feature information extraction; and (iii) design evaluation with features. 3.1. Feature information representation
The system developed uses Pro/Engineer sheet metal features to construct the design and every feature is associated with standard feature information that describes its type, geometry parameters, etc. that assign a specific identity to the feature. This information establishes an identity to each feature and distinguishes between similar features in the design. Fig. 3 illustrates the information representation in the CAE system for a BEND feature. 3.2. Feature information .extraction
Extraction of feature information from design using an external application program is possible using Pro/ Develop, a programmatic interface to Pro/Engineer which allows software applications to customize Pro/ Engineer and/or create new applications that can be integrated directly into t!he Pro/Engineer environment. Pro/Develop consists of a library of 'C' functions which provide a supported interface to Pro/Engineer and direct access to the Pro/Engineer database. 3.3. Design evaluation with features
Once the various sheet metal features, i.e., bends,
holes, slots, etc., present in the design are recognized, each one of them can be evaluated separately and in conjunction with other features based on mathematical models and design rules. The feature information is extracted from the design by the integration program which then calls the analysis and knowledge-based modules to perform their tasks. The integration module handles the information flow between the various modules in the system and also interacts with the user for the input and output of design information. It consists of a set of programs written in 'C' and linked to the Pro/Engineer environment.
4. System structure The system is comprised of three main modules to analyze, design and evaluate the different sheet metal features independently and in conjunction with other features, to determine the effect of one feature on the accuracy and manufacturability of the others. The modules are: (a) Analysis module: This module incorporates an accurate mathematical model and enables the designer to perform mechanics and formability-based analyses to predict stresses, strains, failures (cracks and wrinkles), defects (spring-back, dimensional tolerances, etc.) for forming operations (bending, flanging and local stretching). It also enables the user to determine the punch loads, displacements, and forces during the forming process. (b) Knowledge-based design support system module: This module contains knowledge about tooling and
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R. Mantripragada et al./ Journal of Materials Processing Technology 57 (1996) 241-248
process conditions in the early stages of design and uses the knowledge to establish a systematic methodology for the design of box-type sheet metal components. This involves starting from the final part and evaluating the various features in the part for failure, ease of manufacture and interference using both design guidelines and formability checks provided by the analysis module to determine the necessary changes required for the design. (c) CAD system: The CAE system has been developed over the commerical CAD system platform, Pro/ Engineer. This enables the user to construct his/her design using the sheet metal module of Pro/Engineer or to import it in a standard IGES format from any other CAD system and then to call the CAE system for design and analysis. Pro/Engineer was chosen as the CAD software for the system because of its parametric nature and its ability to extract feature information from the design using Pro/Develop routines and to build a customized system on top of the software (Fig. 4). (d) Integration module: This module handles the information flow between the various modules in the system and also interacts with the user for the input and output of design information. It consists of a set of programs written in 'C' and linked to the Pro/Engineer environment.
5. Design procedure Integrated with Pro/Engineer, the system is a comprehensive and powerful tool for the design of box-type
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components. The design is constructed using the various sheet metal features such as walls, bends, holes, slots, etc. in the Pro/Sheet metal module of Pro/Engineer. The evaluation procedure is illustrated by taking a few examples of sheet metal features.
5.1. Analysis of bends This illustrates the evaluation of a sheet metal feature (bend) using the analysis module. The various bends present in the design can be evaluated separately in an interactive procedure. The term 'bending' here is restricted to mean single curvature or straight-line bending because other types of curvature bending, such as multiple-curvature bending and contour forming, lack empirical rules and process standards. Plane-strain bending or straight-line bending with a single curvature is a very common deformation mode in many box-type components. The following information is calculated and displayed to the user for every bend selected. 1. The 'bendability' assessment for a given radius, bending angle, material, and thickness involves calculating the following parameters: (i) the minimum bending ratio Rp/t or Rd/t where Rp = punch radius and Rd = die radius; (ii) The maximum tensile strain and stress. 2. Spring-back prediction for die design, shape, and dimension control. The software provides the following spring-back information: (i) Bending angle under load; (ii) Spring-back angle; (iii) Strain distribution along bending arc length; (iv) Punch displacement necessary for over bending to compensate for spring-back. 3. Estimation of bending force for the selection of press capacity, for a strength analysis, and for the design of the dies. The following are calculated: (i) The maximum punch force; (ii) Punch force versus punch stroke; (iii) Bending moment distribution along the bending arm at each punch height; (iv) Maximum forces on the die shoulders. Fig. 5 shows a sample run of the sytem for the evaluation of bends made by an air-die-bending operation. The mathematical models incorporated include press-brake bending, U-Die and wiping-die bending, rotary bending, and tractrix-die bending.
5.2. Analysis of flanges
system
Fig. 4. An integrated CAE system with deformation analysis and a knowledge-based system for optimal design of sheet metal parts.
The term 'flanging' is restricted to bending along a curved line. Every flange in the design can be evaluated separately for various defects and failure criteria to obtain the following design information. Fig. 6 illus-
R. Mantripragada et al. /Journal of Materials Processing Technology 57 (1996) 241 248
245
Fig. 5. A typical sheet metal part illustrating the evaluation of bends.
Fig. 6. A shrink-flangedpart illustrating the evaluation of flanges. trates a sample run session of the system used to evaluate shrink-flanging and provide design information to the user. 1. Formability information (i) Maximum flanging strain; (ii) Maximum flanging stress; (iii) Elastic/plastic wrinkling or fracture strain; (iv) Spring-back angle; (v) Minimum bending ratio at die shoulder ( R / T ) ; (vi) Maximum bending strain at the die shoulder; (vii) Maximum bending stress at the die shoulder. 2. Geometry information (i) Initial radius of the blank; (ii) Bending angle under load; (iii) Spring-back angle; (iv) X-Y coordinates of the deformed profile. 5.3. Feature interaction evaluation
As mentioned earlier, the analysis of bend and fange features is handled by the analysis module, as accurate mathematical models have been developed for their evaluation. All other sheet metal features are evaluated by design rules compiled from various sources because the development of mathematical models for their analysis is very complicated and not practical. The feature interaction evaluation for all sheet metal features including bends and flanges is done by this module. In
this section, the structure of the knowledge-based evaluation module and its execution is explained. This module essentially evaluates various features for ease of manufacture and the effect of the placement of one feature on the manufacturability and tolerances of the other. The set of features that are evaluated for ease of manufacture are holes, beads and slots. This involves evaluation of their dimensions and geometric shapes for safe manufacture and to prevent failures and defects. The evaluation is done in an interactive manner and the user has complete control of the process. Depending upon the class of the feature to be evaluated, the user selects one of the following three options and continues the process. Next, he/she picks the feature to be evaluated by clicking on the feature. The control is then transferred from Pro/Engineer to the integration module, which extracts the following features information from the Pro/Engineer database, as shown in Fig. 7: (1) information necessary to recognize the type and class of the feature; (2) information about the location and orientation of the feature; (3) geometric dimensions for the feature. For example, Fig. 8 shows a segment of the integration program that extracts the design information for a hole feature from the Pro-Engineer database. The program uses Pro/Develop functions to perform the information-extraction procedure.
246
R. Mantripragada et al./'Journal of Materials Processing Teehnology 57 (1996) 241 248
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Fig. 7. An example run through the system illustrating evaluation of interaction between features (hole and bend).
sel_num = pro select("surface",allow,&select, O,0); if (sel_num < = O) return(O); if (select[O].sel type = = SEL 3D SRF) { p_face = select[O].selected_ptr; id = select[O].selected_id; l i d = prodb get feat_type(p_object, id); feat_id = prodb get_surface feature(p_/ace); printf("\n ................ FEATURE P A R A M E T E R A N A L Y S I S ................... printf("\n Feature Type : HOLE '9; printf("\n The id of the feature selected is : %d ", feat_id); printf("\n The selected id of the surface is : %d ", id); feat_type = prodb_get_feat_type(p_object,feat_id); Fig. 8. A geometric feature-extraction algorithm for a hole feature.
R. Mantripragada et al./Journal of Materials Processing Technology 57 (1996) 241 24g
247
if (feat_type == FT H O L E )
{ promsg_print(MSGFIL,"\n Enter Hole type [1- Punched, 2-Threaded, 3-Extruded]"); promsg_getint( &type,range) ; switch (type)
{ case1 : min_s&e = thickness; printf("Xn The min-size is %.3f ",min_size);
size = prodb measure(p_object, option, select, sel_num, &status); if(size <= 12.7) printf("Xn Recommended tolerance on the hole = 0.0127 mm "); if ((size > 12.7) && (size <= 25.4)) printf("\n Recommended tolerance on the hole = 0.0254 mm '9; else printf("\n Recommended tolerance on the hole = 0.0381 mm "); if (size < min_size) prirltf("\n"); promsg__print(MSGFIL,"Diameter of the selected hole = %Of ", &s printf("\n The diameter of the selected hole is : %.3f ", size Frintf("\n Size of the hole is too small "); promsg_print(MSGFIL,"Size of the hole is too small, Redesign r
I else promsg_print(MSGFIL," The size of the hole is acceptable "); break; Fig. 9. A design rule for geometryevaluationof a hole feature.
In the segment of a feature-extraction program illustrated below, first the feature information record for the selected surface, edge or vertex is extracted from the Pro/Engineer database. Then the id, type, and topology information for the surface selected is extracted from the record, and then the information is extracted for the feature to which the surface, edge or vertex belongs. This information is then passed on the knowledgebased module, which applies the appropriate rule, as illustrated by the segment of a program in the knowledge base shown in Fig. 9. The above procedure is applied to evaluate the various features present in the part. Design rules have been compiled from different sources and coded into the knowledge base of the system. The structure of the knowledge base is very flexible to enable additional design rules to be incorporated into the system for the existing or new sets of features.
1. Analysis module: The programs are written in FORTRAN and can be used in the UNIX and VAX environments. 2. Knowledge-based module: The design rules have been coded in 'C' and are integrated into the integration module. 3. Integration module: This module consists of a set of programs written in 'C' which extract features information from the Pro/Engineer database using Pro/Develop functions. The programs are also linked with the knowledge base. 4. CAD system: All displays and user interface work is done on Pro/Engineer which is essential to run the system. It works in a UNIX-based workstation environment. As explained above, the integrated system requires a UNIX-based workstation environment (IBM/RS 6000, Silicon Graphics Indigo, etc.,) to be used.
6. System architecture
7. Conclusions
The system developed ]~as the following structure and requires the following platforms to be used.
A feature-based approach to model sheet metal parts during the design and analysis process has been pre-
248
R. Mantripragada et al./ Journal of Materials Processing Technology 57 (1996) 241-248
sented. Features allow a structured representation of knowledge which can accommodate information about m a n y aspects of the design in a common scheme. They allow the computer to capture and manipulate the design information and, ultimately, to make decisions. The CAE system developed can be used for formability analysis and design-formanufacturing analysis of box-type parts formed from sheet metal. Its range of applicability should be sufficiently large for several manufacturing companies to be interested in using it. Also, the structure of the program is such that it will be easy to make enhancements in the future by changing well-defined modules. The knowledge-based expert system provides an environment for the design of 'hard' tooling. This environment will be used to assist the experienced designer to construct part models conveniently and to transform a new part design into a manufacturable one. The design result will be compatible and consistent with a minimum level of performance and selected manufacturing processes. This design environment will integrate the analysis modules and a knowledge-based system to check sheet metal formability. The explicit benefits that the system provides will be to ensure that the part design is compatible with selected manufacturing processes and to reduce the try-outs in process sequence determination and the set-up time for tooling. The computer-aided analysis (CAA) system will provide a scientific approach to analyze the formability of complex sheet parts formed in multiple operations (bending, flanging, stretching). These models incorporate modern concepts in process design and control, an advanced understanding of sheet material formability, the latest developments in sheet forming mechanics, modern computer technology and numerical methods. This C A A system can stand alone as a process simulator, or it can be adapted to other C A D systems for formability analysis. The basic system that has evolved would be suitable for applying to m a n y other processes. The only changes required would be the provision of appropriate process specific features in the modeling environ-
ment and the construction of domain-specific design rules defined in terms of those features.
References [1] S.P. Keeler, Sheet metal stamping technology--need for fundamental understanding, in D.P. Koistinen (ed.), Mechanics of Sheet Metal Forming, Plenum, New York, 1977, pp. 3-18. [2] T.C. Chang, D.C. Anderson and O.R. Mitchell, QTC--an integrated design/manufacturing/vision inspection system for prismatic part. Proc. ASME Computers in Engineering Conf., San Francisco, CA, USA, 1988, pp. 417-426. [3] J. Corbett and J.A.J. Woodward, A CAD integrated knowledgebased system for the design of die cast components, Ann. CIRP, 40 (1) (1991) 103-105. [4] J. Yueh, R.A. Miller and T. Altan, Die cast: design for die casting, Report No. ERC/NSM-92-65-C-D, State University, Columbus, Ohio, 1992. [5] M. Shpitalni and A. Fischer, CSG representation as a basis for extraction of machining features, Ann. CIRP, 40 (1) (1991) 157-160. [6] W.R. De Vries, X. Dong and J.M. Wozny, Feature-based reasoning in fixture design, Ann. CIRP, 40 (1) (1991) 111-114. [7] T. Altan and R.A. Miller, Design for forming and other near net shape manufacturing processes, Ann. CIRP, 39(2) (1990) 609620. [8] J.J. Cunningham and J.R. Dixon, Designing with features: The origin of features, Proc. ASME Computers in Engineering Conf., San Francisco, CA, USA, 1988, pp. 237-243. [9] G. Eshel, M. Barash and T.C. Chang, A rule based system for automatic generation of deep drawing process outline, ASME Winter Annual Meeting, Miami, Florida, 1985.
[I0] R.C. Gilman and J.M. Wozny, Feasibility and limitations of the step form feature information model as a conceptual schema for form features. Intelligent Design and Manufacturing for Prototyping, ASME, Production Engineering Division, PED v.50, ASME, New York, pp. 1-11. [11] O.D. Lascoe, Handbook of Fabrication Processes, ASM, Metal Park, Ohio, 1988. [12] PMA, Design Guidelines for Precision Metal Stamping and Fabrication, PMA Richmond Heights, Ohio, 1992. [13] C.T. Wang, G. Kinzel and T. Altan, Plane strain bending: fundamentals and applications--Part 1: Elementary bending theory, Report No. ERC/NSM-S-15-92, State University, Columbus, Ohio, 1992. [14] C.T. Wang, Mechanics of bending, flanging, and deep drawing, and a computer aided modeling system for predictions of strain, fracture, wrinkling, and springback in sheet metal forming, Ph.D. Dissertation, State University, Columbus, Ohio, 1993. [15] D.W. Wilson, J. Wang, A frame based representation for the design of sheet metal parts, Trans. NAMRI/SME, pp. 48-53.
Materials Processing Technology Journal of Materials Processing Technology 57 (1996) 241-248
ELSEVIER
A computer-aided engineering system for feature-based design of box-type sheet metal parts R. Mantripragada *, G. Kinzel, T. Altan Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Columbus, OH, USA
Received 18 August 1994
Industrial summary
This paper explores the applications of feature-based representation and design in the area of design for manufacturing to incorporate the tooling and process considerations into the early stages of design. The goal of this research is to apply the concepts of feature-based design and to develop an interactive design tool that can be used to alert the designers to potential production problems, defects and failures, and to provide them with information that can be used to explore alternative design, evaluate trade-offs, and arrive at optimal designs for the given process conditions. This paper illustrates the development of a computer-aided engineering (CAE) system that was constructed using these concepts and applied to designing box-type sheet metal parts.
1. I n t r o d u c t i o n
Sheet forming is a significant net shape manufacturing process for producing a large variety of consumer products (kitchen sinks, cans, cabinets, boxes, brackets, etc.), automotive components (body panels, fenders, etc.), and aerospace parts (body panels, wing parts, etc.). Die design in sheet forming, even after many years of practice, still remains more of an art than a science. Historically, the evolution of a sheet metal stamping from conception through part design to die design to the final die try-out has been a slow, cautious process based on the trial-and-error experience and skill of the artisan [1]. This has led several industries to invest substantial amounts of time, money, and resources to train artisans, and to pay little attention to the development of a scientific approach to cCie and part design. With the increasing complexity of parts, the introduction of new unfamiliar materials, reduced lead times and increasing emphasis on cost effectiveness, there is a growing trend towards a breakdown of experience-based design procedures. There is a need to replace the present experience based, trial-and-error techniques with cost effective, knowledge based, analytical techniques in sheet metal forming. An increase in productivity can be achieved if the part geometry, the fabrication method, the die design and the material prope, rties are specified during the design stage. * Corresponding author. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD1 0924-0136(95)02071-S
To meet these requirements, computer-aided engineering (CAE) systems are being developed and applied to sheet metal forming. This allows for a scientific approach combined with the know-how of experienced designers to develop a novel design methodology and process control strategy in sheet forming. A CAE system development for this purpose should be able to simulate the specific sheet metal forming process accurately and study the material behavior during the process. This would require the system to have a mathematical model describing the process and knowledge-based modules to incorporate the experience-based knowledge into the system. Such a system would serve as a powerful design aid tool for designers to explore alternative designs, evaluate trade-off and arrive at optimal designs at the lowest cost and shortest lead times. The objective of this paper is to describe an integrated CAE system for design evaluation, formability
Fig. 1. A typical box-type sheet metal part.
242
R. Mantripragada et al. /Journal of Materials Processing Technology 57 (1996) 241-248
analysis and tooling design for the forming of box-type sheet metal parts. A typical box-type part is shown in Fig. 1. The system consists of three main modules: a mathematical-based analysis module, a design-rule based knowledge-based module and a parametric CAD package. The information flow between the various modules in the system is controlled by an integration program which forms the control unit for the system.
~
Hc~e
2. Review of pertinent work A considerable amount of work has been done in feature-based design by several researchers using different types of techniques. Chang et al. [2] developed a feature-based expert process planning system (QTC), to develop a part-specific process planning environment. The QTC system was intended for one-of-a-kind part production and ruled out the possibility of using part family concepts of process planning, so that a featurebased design system was developed to generate new plans based on the knowledge of basic machining processes, tools and fixturing methods. Corbett et al. [3] developed a CAD-integrated knowledge-based system for the design of die cast components. The system uses knowledge-based systems, and in particular feature representation, in the area of design for manufacture die cast parts to alert the designer to any potential production problems. It uses a framebased representation to store information about features and the commercial CAD program CAEDS for display and the user interface. Similar work was also done by Yueh et al. [4], who used the CAD program Pro/Engineer and the expert system shell NEXPERT to develop a feature-based integrated design system. Significant work in the area of feature information extraction and representation was also done by Shpitalni et al. [5]. They worked on the extraction of different types of machining features. Their approach was to extract the disjointed machining regions comprising the total volume to be machined first and then to deal with each disjointed machining region independently. Another feature based design model was developed by DeVries et al. [6] for fixture design, concentrating on the selection of location elements and the identification of locating surfaces for workpiece positioning. Further literature related tothe present topic is available in Refs. [7-15].
3. Feature-based design approach Since most geometric modeling systems do not capture any deep understanding of the design, they are in general unable to support sophisticated reasoning capability. Thus, in applications such as design for manufac-
\
Slot-rectangular Bend Slot-circular Flat wall ~
--
~
Slot-irregular
Slot-triangular
Fig. 2. A typical sheet metal part constructed using features.
turing, the interaction of the human designer is necessary for data extraction and decision making. Thus to develop an intelligent CAE system for design and analysis, it is essential to devise a representation scheme for the design which allows the computer to capture and manipulate the design information, and, ultimately, to make decisions. In the sytem developed, the approach followed is to design with features. A feature is defined as a named entity with attributes of both form and function. Form attributes describe the physical geometry of the product, whilst functional attributes describe the purpose or functional aspects of the product. Typical sheet metal features include bends, flanges, holes, slots, beads, etc. This enables the designer to use a standard library of features to construct the part and incorporate useful design information during the construction stage. Every feature is treated like a record with a set of information fields that carry information about the feature and describe its relationship with other features. In addition, it may also contain some information that is specific to a particular feature, depending upon its type and functional importance. By following this kind of approach, the system has access to the complete feature information required for the analysis and evaluation of the design. Each feature can now be analyzed independently and in conjunction with other features for ease of manufacture and design criteria. Fig. 2 shows a typical sheet metal part design with features. The CAE system for the feature-based and analysis of box-type sheet metal parts has been built using the commercial CAD software platform, Pro/Engineer. Pro/Engineer was chosen for this project because of its parametric nature and its ability to enable the user to design with features. It has a standard library of sheet metal features that the designer can use to construct his/her part. Pro/Engineer associates with each feature. The following standard feature information, which enables the association of a separate identity to each feature. Another important capability of Pro/Engineer is that it allows most of this feature information to be extracted from the design using various data extraction
R. Mantripragada et al./Journal of Materials Processing Technology 57 (1996) 241-248
243
FEATURE N U M B E R : 2 INTERNAL, FEATURE ID : 80 P A R E N T S : 33 (#1) CHILDREN : 174(#3), 282(#4), 307(#5), 330(#6), 396(#7), 425(#8), 426(#9), 427(#10) 6011(#11), 648(#12), 684(#13), 770(#14), 789(#15), 850(#16) TYPE : BEND SECTION N A M E : $2D001 F E A T U R E "S D I M E N S I O N S : d5 = 90 d7 = 127 d8 = 90 d9 = 85 dlO = 1.27t'
Fig. 3. Features informationrepresentation in Pro/Engineer.
sub-routines in Pro/Develop, which is another module of Pro/Engineer. To develop an integrated CAE system for design evaluation and analysis using features, the following operations must be performed: (i) feature information representatio:a; (ii) feature information extraction; and (iii) design evaluation with features. 3.1. Feature information representation
The system developed uses Pro/Engineer sheet metal features to construct the design and every feature is associated with standard feature information that describes its type, geometry parameters, etc. that assign a specific identity to the feature. This information establishes an identity to each feature and distinguishes between similar features in the design. Fig. 3 illustrates the information representation in the CAE system for a BEND feature. 3.2. Feature information .extraction
Extraction of feature information from design using an external application program is possible using Pro/ Develop, a programmatic interface to Pro/Engineer which allows software applications to customize Pro/ Engineer and/or create new applications that can be integrated directly into t!he Pro/Engineer environment. Pro/Develop consists of a library of 'C' functions which provide a supported interface to Pro/Engineer and direct access to the Pro/Engineer database. 3.3. Design evaluation with features
Once the various sheet metal features, i.e., bends,
holes, slots, etc., present in the design are recognized, each one of them can be evaluated separately and in conjunction with other features based on mathematical models and design rules. The feature information is extracted from the design by the integration program which then calls the analysis and knowledge-based modules to perform their tasks. The integration module handles the information flow between the various modules in the system and also interacts with the user for the input and output of design information. It consists of a set of programs written in 'C' and linked to the Pro/Engineer environment.
4. System structure The system is comprised of three main modules to analyze, design and evaluate the different sheet metal features independently and in conjunction with other features, to determine the effect of one feature on the accuracy and manufacturability of the others. The modules are: (a) Analysis module: This module incorporates an accurate mathematical model and enables the designer to perform mechanics and formability-based analyses to predict stresses, strains, failures (cracks and wrinkles), defects (spring-back, dimensional tolerances, etc.) for forming operations (bending, flanging and local stretching). It also enables the user to determine the punch loads, displacements, and forces during the forming process. (b) Knowledge-based design support system module: This module contains knowledge about tooling and
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R. Mantripragada et al./ Journal of Materials Processing Technology 57 (1996) 241-248
process conditions in the early stages of design and uses the knowledge to establish a systematic methodology for the design of box-type sheet metal components. This involves starting from the final part and evaluating the various features in the part for failure, ease of manufacture and interference using both design guidelines and formability checks provided by the analysis module to determine the necessary changes required for the design. (c) CAD system: The CAE system has been developed over the commerical CAD system platform, Pro/ Engineer. This enables the user to construct his/her design using the sheet metal module of Pro/Engineer or to import it in a standard IGES format from any other CAD system and then to call the CAE system for design and analysis. Pro/Engineer was chosen as the CAD software for the system because of its parametric nature and its ability to extract feature information from the design using Pro/Develop routines and to build a customized system on top of the software (Fig. 4). (d) Integration module: This module handles the information flow between the various modules in the system and also interacts with the user for the input and output of design information. It consists of a set of programs written in 'C' and linked to the Pro/Engineer environment.
5. Design procedure Integrated with Pro/Engineer, the system is a comprehensive and powerful tool for the design of box-type
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components. The design is constructed using the various sheet metal features such as walls, bends, holes, slots, etc. in the Pro/Sheet metal module of Pro/Engineer. The evaluation procedure is illustrated by taking a few examples of sheet metal features.
5.1. Analysis of bends This illustrates the evaluation of a sheet metal feature (bend) using the analysis module. The various bends present in the design can be evaluated separately in an interactive procedure. The term 'bending' here is restricted to mean single curvature or straight-line bending because other types of curvature bending, such as multiple-curvature bending and contour forming, lack empirical rules and process standards. Plane-strain bending or straight-line bending with a single curvature is a very common deformation mode in many box-type components. The following information is calculated and displayed to the user for every bend selected. 1. The 'bendability' assessment for a given radius, bending angle, material, and thickness involves calculating the following parameters: (i) the minimum bending ratio Rp/t or Rd/t where Rp = punch radius and Rd = die radius; (ii) The maximum tensile strain and stress. 2. Spring-back prediction for die design, shape, and dimension control. The software provides the following spring-back information: (i) Bending angle under load; (ii) Spring-back angle; (iii) Strain distribution along bending arc length; (iv) Punch displacement necessary for over bending to compensate for spring-back. 3. Estimation of bending force for the selection of press capacity, for a strength analysis, and for the design of the dies. The following are calculated: (i) The maximum punch force; (ii) Punch force versus punch stroke; (iii) Bending moment distribution along the bending arm at each punch height; (iv) Maximum forces on the die shoulders. Fig. 5 shows a sample run of the sytem for the evaluation of bends made by an air-die-bending operation. The mathematical models incorporated include press-brake bending, U-Die and wiping-die bending, rotary bending, and tractrix-die bending.
5.2. Analysis of flanges
system
Fig. 4. An integrated CAE system with deformation analysis and a knowledge-based system for optimal design of sheet metal parts.
The term 'flanging' is restricted to bending along a curved line. Every flange in the design can be evaluated separately for various defects and failure criteria to obtain the following design information. Fig. 6 illus-
R. Mantripragada et al. /Journal of Materials Processing Technology 57 (1996) 241 248
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Fig. 5. A typical sheet metal part illustrating the evaluation of bends.
Fig. 6. A shrink-flangedpart illustrating the evaluation of flanges. trates a sample run session of the system used to evaluate shrink-flanging and provide design information to the user. 1. Formability information (i) Maximum flanging strain; (ii) Maximum flanging stress; (iii) Elastic/plastic wrinkling or fracture strain; (iv) Spring-back angle; (v) Minimum bending ratio at die shoulder ( R / T ) ; (vi) Maximum bending strain at the die shoulder; (vii) Maximum bending stress at the die shoulder. 2. Geometry information (i) Initial radius of the blank; (ii) Bending angle under load; (iii) Spring-back angle; (iv) X-Y coordinates of the deformed profile. 5.3. Feature interaction evaluation
As mentioned earlier, the analysis of bend and fange features is handled by the analysis module, as accurate mathematical models have been developed for their evaluation. All other sheet metal features are evaluated by design rules compiled from various sources because the development of mathematical models for their analysis is very complicated and not practical. The feature interaction evaluation for all sheet metal features including bends and flanges is done by this module. In
this section, the structure of the knowledge-based evaluation module and its execution is explained. This module essentially evaluates various features for ease of manufacture and the effect of the placement of one feature on the manufacturability and tolerances of the other. The set of features that are evaluated for ease of manufacture are holes, beads and slots. This involves evaluation of their dimensions and geometric shapes for safe manufacture and to prevent failures and defects. The evaluation is done in an interactive manner and the user has complete control of the process. Depending upon the class of the feature to be evaluated, the user selects one of the following three options and continues the process. Next, he/she picks the feature to be evaluated by clicking on the feature. The control is then transferred from Pro/Engineer to the integration module, which extracts the following features information from the Pro/Engineer database, as shown in Fig. 7: (1) information necessary to recognize the type and class of the feature; (2) information about the location and orientation of the feature; (3) geometric dimensions for the feature. For example, Fig. 8 shows a segment of the integration program that extracts the design information for a hole feature from the Pro-Engineer database. The program uses Pro/Develop functions to perform the information-extraction procedure.
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sel_num = pro select("surface",allow,&select, O,0); if (sel_num < = O) return(O); if (select[O].sel type = = SEL 3D SRF) { p_face = select[O].selected_ptr; id = select[O].selected_id; l i d = prodb get feat_type(p_object, id); feat_id = prodb get_surface feature(p_/ace); printf("\n ................ FEATURE P A R A M E T E R A N A L Y S I S ................... printf("\n Feature Type : HOLE '9; printf("\n The id of the feature selected is : %d ", feat_id); printf("\n The selected id of the surface is : %d ", id); feat_type = prodb_get_feat_type(p_object,feat_id); Fig. 8. A geometric feature-extraction algorithm for a hole feature.
R. Mantripragada et al./Journal of Materials Processing Technology 57 (1996) 241 24g
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if (feat_type == FT H O L E )
{ promsg_print(MSGFIL,"\n Enter Hole type [1- Punched, 2-Threaded, 3-Extruded]"); promsg_getint( &type,range) ; switch (type)
{ case1 : min_s&e = thickness; printf("Xn The min-size is %.3f ",min_size);
size = prodb measure(p_object, option, select, sel_num, &status); if(size <= 12.7) printf("Xn Recommended tolerance on the hole = 0.0127 mm "); if ((size > 12.7) && (size <= 25.4)) printf("\n Recommended tolerance on the hole = 0.0254 mm '9; else printf("\n Recommended tolerance on the hole = 0.0381 mm "); if (size < min_size) prirltf("\n"); promsg__print(MSGFIL,"Diameter of the selected hole = %Of ", &s printf("\n The diameter of the selected hole is : %.3f ", size Frintf("\n Size of the hole is too small "); promsg_print(MSGFIL,"Size of the hole is too small, Redesign r
I else promsg_print(MSGFIL," The size of the hole is acceptable "); break; Fig. 9. A design rule for geometryevaluationof a hole feature.
In the segment of a feature-extraction program illustrated below, first the feature information record for the selected surface, edge or vertex is extracted from the Pro/Engineer database. Then the id, type, and topology information for the surface selected is extracted from the record, and then the information is extracted for the feature to which the surface, edge or vertex belongs. This information is then passed on the knowledgebased module, which applies the appropriate rule, as illustrated by the segment of a program in the knowledge base shown in Fig. 9. The above procedure is applied to evaluate the various features present in the part. Design rules have been compiled from different sources and coded into the knowledge base of the system. The structure of the knowledge base is very flexible to enable additional design rules to be incorporated into the system for the existing or new sets of features.
1. Analysis module: The programs are written in FORTRAN and can be used in the UNIX and VAX environments. 2. Knowledge-based module: The design rules have been coded in 'C' and are integrated into the integration module. 3. Integration module: This module consists of a set of programs written in 'C' which extract features information from the Pro/Engineer database using Pro/Develop functions. The programs are also linked with the knowledge base. 4. CAD system: All displays and user interface work is done on Pro/Engineer which is essential to run the system. It works in a UNIX-based workstation environment. As explained above, the integrated system requires a UNIX-based workstation environment (IBM/RS 6000, Silicon Graphics Indigo, etc.,) to be used.
6. System architecture
7. Conclusions
The system developed ]~as the following structure and requires the following platforms to be used.
A feature-based approach to model sheet metal parts during the design and analysis process has been pre-
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R. Mantripragada et al./ Journal of Materials Processing Technology 57 (1996) 241-248
sented. Features allow a structured representation of knowledge which can accommodate information about m a n y aspects of the design in a common scheme. They allow the computer to capture and manipulate the design information and, ultimately, to make decisions. The CAE system developed can be used for formability analysis and design-formanufacturing analysis of box-type parts formed from sheet metal. Its range of applicability should be sufficiently large for several manufacturing companies to be interested in using it. Also, the structure of the program is such that it will be easy to make enhancements in the future by changing well-defined modules. The knowledge-based expert system provides an environment for the design of 'hard' tooling. This environment will be used to assist the experienced designer to construct part models conveniently and to transform a new part design into a manufacturable one. The design result will be compatible and consistent with a minimum level of performance and selected manufacturing processes. This design environment will integrate the analysis modules and a knowledge-based system to check sheet metal formability. The explicit benefits that the system provides will be to ensure that the part design is compatible with selected manufacturing processes and to reduce the try-outs in process sequence determination and the set-up time for tooling. The computer-aided analysis (CAA) system will provide a scientific approach to analyze the formability of complex sheet parts formed in multiple operations (bending, flanging, stretching). These models incorporate modern concepts in process design and control, an advanced understanding of sheet material formability, the latest developments in sheet forming mechanics, modern computer technology and numerical methods. This C A A system can stand alone as a process simulator, or it can be adapted to other C A D systems for formability analysis. The basic system that has evolved would be suitable for applying to m a n y other processes. The only changes required would be the provision of appropriate process specific features in the modeling environ-
ment and the construction of domain-specific design rules defined in terms of those features.
References [1] S.P. Keeler, Sheet metal stamping technology--need for fundamental understanding, in D.P. Koistinen (ed.), Mechanics of Sheet Metal Forming, Plenum, New York, 1977, pp. 3-18. [2] T.C. Chang, D.C. Anderson and O.R. Mitchell, QTC--an integrated design/manufacturing/vision inspection system for prismatic part. Proc. ASME Computers in Engineering Conf., San Francisco, CA, USA, 1988, pp. 417-426. [3] J. Corbett and J.A.J. Woodward, A CAD integrated knowledgebased system for the design of die cast components, Ann. CIRP, 40 (1) (1991) 103-105. [4] J. Yueh, R.A. Miller and T. Altan, Die cast: design for die casting, Report No. ERC/NSM-92-65-C-D, State University, Columbus, Ohio, 1992. [5] M. Shpitalni and A. Fischer, CSG representation as a basis for extraction of machining features, Ann. CIRP, 40 (1) (1991) 157-160. [6] W.R. De Vries, X. Dong and J.M. Wozny, Feature-based reasoning in fixture design, Ann. CIRP, 40 (1) (1991) 111-114. [7] T. Altan and R.A. Miller, Design for forming and other near net shape manufacturing processes, Ann. CIRP, 39(2) (1990) 609620. [8] J.J. Cunningham and J.R. Dixon, Designing with features: The origin of features, Proc. ASME Computers in Engineering Conf., San Francisco, CA, USA, 1988, pp. 237-243. [9] G. Eshel, M. Barash and T.C. Chang, A rule based system for automatic generation of deep drawing process outline, ASME Winter Annual Meeting, Miami, Florida, 1985.
[I0] R.C. Gilman and J.M. Wozny, Feasibility and limitations of the step form feature information model as a conceptual schema for form features. Intelligent Design and Manufacturing for Prototyping, ASME, Production Engineering Division, PED v.50, ASME, New York, pp. 1-11. [11] O.D. Lascoe, Handbook of Fabrication Processes, ASM, Metal Park, Ohio, 1988. [12] PMA, Design Guidelines for Precision Metal Stamping and Fabrication, PMA Richmond Heights, Ohio, 1992. [13] C.T. Wang, G. Kinzel and T. Altan, Plane strain bending: fundamentals and applications--Part 1: Elementary bending theory, Report No. ERC/NSM-S-15-92, State University, Columbus, Ohio, 1992. [14] C.T. Wang, Mechanics of bending, flanging, and deep drawing, and a computer aided modeling system for predictions of strain, fracture, wrinkling, and springback in sheet metal forming, Ph.D. Dissertation, State University, Columbus, Ohio, 1993. [15] D.W. Wilson, J. Wang, A frame based representation for the design of sheet metal parts, Trans. NAMRI/SME, pp. 48-53.