- Email: [email protected]
Feature based integrated intelligent sequence design for cold extrusion
Journal of Materials Processing Technology 174 (2006) 74–81
Feature based integrated intelligent sequence design for cold extrusion X.Q. Zhang a,∗ , Y.H. Peng b , X.Y. Ruan b , K. Yamazaki a a
Department of Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616, USA b Shanghai Jiao Tong University, Shanghai 200030, PR China Received 20 September 2001; received in revised form 20 February 2002; accepted 12 December 2004
Abstract This paper introduces the intelligent sequence design system of multi-stage cold extrusion. It discusses the method to build the part feature modeling and to realize feasibility checking, process planning and evaluation, process parameter optimization and sequence optimal selection, mainly focuses on the feature modeling and system implementation strategy. According to the given input data, the system can generate the forgeable geometry and the basic sequence design and evaluates the sequences, depending on the initial billet size and material or the evaluation knowledge in the knowledge base. An example proves the proposed theory and method. © 2006 Published by Elsevier B.V. Keywords: Cold extrusion; CAD/CAPP/CAE; Artificial neural network; Relative process cost; Micro genetic algorithm
1. Introduction Cold extrusion is a well known manufacturing method, which is becoming increasingly popular due to recent improvement in tool and press design, and tool materials. The basic advantages of cold extrusion are good dimensional accuracy, surface quality, saving in energy and material, and improvement in mechanical properties and it also eliminates the extra post processing, such as trimming and machining. Therefore, cold extrusion is a promising area for companies exploring the potential of existing technologies to meet the new demands. In cold extrusion, forming pressure is extremely high because the metal deforms without external heating to an extremely large strain and thus, fracture of the tool, defects in the product, seizure between the billet and the tool tend to take place easily. Therefore, it is very important to design an appropriate forming sequence of multi-stage cold extrusion without causing these problems. Actual process planning usually requires very long time even for long experienced experts because there are many kinds of design rules to be applied. In recent years, the use of computers to aid design ∗
Corresponding author. Tel.: +1 530 754 7687; fax: +1 530 752 8253. E-mail address: [email protected] (X.Q. Zhang).
0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2004.12.011
and manufacturing activities in extrusion has shown that the experience based intuition and skills of designers can be considerably augmented by computerized analysis and design. To ensure the productive ability of products and processes of cold extrusion at low cost, high quality and short leading time, efficient utilization of computers and the development of expert system with the integration of CAD/CAPP/CAE techniques are considered to be the most promising way. Because the time for drafting could be shortened by CAD and the number of trial-and-error could be decreased with the aid of CAPP and computer simulation [1]. Therefore, many researchers have worked on the topics [2–9]. However, there are still many problems in these systems, such as part representation, sequence design efficiency, limited use of CAE, parameters optimisation and sequence optimal selection. Then, researches on the integration of CAD/CAPP/CAE and optimum technique and establishing efficient intelligent design system for cold extrusion by making full use of all kinds of current advanced techniques are of great significance in theory and in practice. In this paper, by gradually accumulating each individual knowledge base, an extensive feature based integrated system, including part representation and system implementation strategy, is described, and the methods for parameter optimization and sequence selection are introduced. Meanwhile, the examples of results are displayed.
X.Q. Zhang et al. / Journal of Materials Processing Technology 174 (2006) 74–81
75
ple who are experienced in cold extrusion sequence design, the foundation for the prototype system development was established.
3. Feature representation
Fig. 1. System frame of feature based intelligent sequence design for cold extrusion (ISDCE).
2. System architecture A feature-based intelligent sequence design for cold extrusion (ISDCE), integrating CAD/CAPP/CAE and optimal techniques, is proposed after analyzing the application and tendency of computer aided process planning, and artificial intelligence technique in metal forming processes. The system architecture is shown in Fig. 1, from which we can notice that there exist two work cycles in the system. One is for the design of forging part, including part feature modeling and feasibility analysis. The other is for process planning, including intelligent sequence design module, knowledge base and database. The constructed feature modeling, knowledge base and database are necessary for the sequence design. Furthermore, both cycles are constructed in the integrated design environment. In this paper, feature-based modeling and system implementation strategy will be introduced in detail later, and the other parts of the system are described in the literature [10]. With the ISDCE system window interface, the user inputs the part geometry by using part feature modeling, and the feasibility analysis is conducted to obtain formable part for cold extrusion. Then, sequence design starts. In another window, the system shows the expansion of the search tree during the system execution. At last, the 2-D geometry data of generated sequences are finally shown in AutoCAD environment so that the user can make modification if necessary. The very important step in the system development is to study the problem domain. In the field of cold extrusion, the operation limitations and evaluation rules have been well studied and documented [11,12]. In contrast to the operation evaluation rules, the heuristics about how to choose operations and how to form a most feasible sequence are mostly unwritten. By extracting the sequence design rules from books and technical papers, by examining in-use cold extrusion sequences and by communicating with peo-
When developing an extrusion sequence design system which covers a variety of cold extruded parts, even though the parts are mostly rotational–symmetric, an appropriate geometry representation is difficult to find because the material flows and the volume is kept constant in cold forming. But the application of feature technique in the field of machining provides the chance to represent the cold extrusion part in the system. In the past 20 years, the concept of feature representation has been widely accepted and applied in process planning, feasibility analysis and mechanical design. The characteristics of the feature modeling is that the models consist of high level modeling elements that more directly correspond to shapes having clear engineering meaning. Sequence design of cold extrusion is also considered as part geometry manipulation. The geometry changes in forming are made by operations based on volume constancy and some formability limitations. In the previous research work, all the parts are cylindrical and two types of representations are adopted. The first is the boundary representation of the axial cross-section of part [3–6]. This representation is simple and convenient for volume calculation. However, it cannot effectively represent non-axisymmetric parts, and the information for operation selection is difficult to be associated with the geometry. The other representation decomposed a part into several volumetric primitives, each of which has attributes, such as height and diameter [2,7,8]. The second representation has advantages over the first since the diameter and height of primitives are key variables for choosing and evaluating operations. Furthermore, it also makes representing non-axisymmetric part possible. However, both representation methods cannot contain engineering meaning information, such as material, precision and management information. But all the information is very important for sequence design. Besides volume constancy, it has also been noticed that the geometry change of the cold extrusion distinguishes itself from machining: (1) the deformation of cold extrusion is the action of material flow, not the metal cutting. Furthermore, material flow results in the changes of material properties, which limits the deformation; (2) for most parts, especially hollow shaped parts, shape changes in the desired section cannot isolated from those in the adjacent sections; (3) the transition feature can make material flow easier. They are formed with the main feature at the same single station. No separated operation is required.
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Therefore, the feature modeling method in the field of machining cannot be directly introduced into the field of cold extrusion. In this research, the features representation is used for the part representation [13]. Based on the analysis of the peculiarity of cold extrusion parts, a feature classification system is put forward here, especially the classification of form feature and their relation are emphasized. Form feature, material feature, precision feature and management feature are four major features which characterize the metal forming process. Form feature characterizes the nominal geometry of part, in which the primary shape represents the major shape of part and the sub-feature represents alternations to the major shape. The material feature comprises the material composition, physical and mechanical properties, heat treatment and surface treatments. The material feature can be represented as forming limits (such as maximum upset strain, maximum upset ratio, maximum area reduction), friction factor and stress–strain relation. Roundness, positive/negative deviations in size and eccentric deviation in principal axis are all the factors that comprise the precision feature. Management feature contains the general information of the part. Among them, form feature is main feature and the others exist depending on it. Furthermore, the form feature often refers to the other features through pointers. Generally, form feature can be classified into main feature and subordinate feature. And they can be classified more detailed sub-features, shown in Fig. 2. A part is represented in term of main feature and subordinate feature. Meanwhile, main feature contains external and internal macro features. Each external macro feature can be composed of external primitive features, such as cylinder and cone features. Each internal macro feature can be composed of internal primitive features, such as topblind-hole and bottom-blind-hole feature. And the primitives, macros and their relation match the way in which
Fig. 2. Classification of form feature.
Fig. 3. Structure of part feature model.
the parts are perceived by engineers during designing the sequence. The features’ type and attributes are relevant to operation selection and evaluation. For example, the internal macro feature “top-blind-hole” suggests that backward extrusion could be used. The diameter and depth of the “top-blind-hole” feature are also essential in evaluating the operation. Then, a dynamic model is established to represent the part, shown in Fig. 3. The feature-based model consists of three key components: feature syntax, feature semantics and feature operations. Feature syntax defines geometric construction of part or a feature, especially the relations between form features. There are several feature relations: (1) (2) (3) (4)
Outer Increment Adjacent (OIA); Outer Reduction Adjacent (ORA); Outer Adjacent Inner (OAI); Subordinate (SUB).
The relations of features are listed in Table 1. Feature semantics describes information associated with the feature geometry, tolerance and technologies used for management and manufacturing. Only by using the feature semantics description does a feature become a meaningful geometric entity. Meanwhile, some necessary feature operations, such as “Add”, “Delete”, “Save” and “Save As” are designed and integrated with the form feature structure. With the capabilities of these three key components, the feature modeling can be used as tool for feature-based part design. To meet the requirements for feature information saving, a dynamic data structure (binary-tree structure and dynamic chain structure) for feature representation has been designed. Furthermore, an independent feature library has been established for the user to select from as well as to add, instead of feature enumeration. After the definition of the part, the feasibility analysis, which focuses on the judging of whether the part can be formed using cold extrusion from the geometry, material and mechanical properties are conducted.
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Table 1 Note of feature relations No.
Relation (argu1, argu2)
Meaning
Features
1
OIA (A, B)
Adjacent between A and B at external features and the section increases
A: cylinder, cone, . . . B: cylinder, cone, . . .
2
ORA (A, B)
Adjacent between A and B at external features and the section decreases
A: cylinder, cone, . . . B: cylinder, cone, . . .
3
OAI (A, B)
Adjacent between A and B at the end of the part, especially between internal and external features
A: cylinder, . . . B: top-blind-hole, bottom-blind-hole, . . .
4
SUB (A, B)
B is the subordinate feature of A
4. Sequence design strategy 4.1. Forming method In cold extrusion, the basic operations are upsetting, forward and backward extrusion according to the material flow directions. In order to handle various types of parts, a relatively detailed classification scheme should be formulated. Meanwhile, single feature is regarded as the basic unit during the sequence design in the paper. To meet the requirement, forming methods for cold extrusion are divided into single-blow process, multi-blow process, combined process and special process. Single process means that there are no any defects when one feature is formed using the selected machine. Multi-blow process is not like this. When the defects arise if the feature is formed by single-blow process, the continuous forming process is necessary to form this feature so as to obtain the preform without any defects, which is called multi-blow process. Here, single-blow and multi-blow process is divided corresponding to a single feature. However, combined process is necessary to combine the continuous process under the condition of forming limitation rules so as to reduce the forming station numbers and the production cost. Special process is used to form the subordinate feature. All the forming methods can be further divided into more detailed methods shown in Fig. 4. Furthermore, extrusion element is used to represent the forming methods, which provides more convenience for downstream geometrical sequence design [10].
A: cylinder, top-blind-hole, . . . B: screw, spur, . . .
With the strategy, the difficult problem of strategy for process planning can be divided into a number of sub-problems to be solved easily using different reasoning machines. So the structure of reasoning machine for process planning can be simplified significantly. Then, the procedure model is established, including billet selection, sequence design (geometrical and technological sequence design) and process optimization (parameter optimization and sequence optimal selection), which provides the idea for realizing the process planning in computer. Fig. 5 shows the procedure model. The functions of all the procedures are as follows: Preparation procedure is the foundation of sequence design. Its main task is to load all kinds of information necessary to the downstream procedures, which includes the information about the part, forming method, knowledge base and database. Billet selection procedure is to select the billet with reasonable shape and size in accordance with user inputting information and part feature information. The procedure is very important because it is also the base of sequence design. Sequence design procedure is key to the whole process planning strategy model. Meanwhile, the procedure is divided into geometrical sequence design and technological sequence design according to different functions.
4.2. Process planning strategy Process planning strategy is one of the most important issues in the intelligent design system. In the paper, the hierarchy-based strategy for process planning is used to select the sequence for the given part. The principle of hierarchy-based strategy is: one complicated task is divided into many sub-tasks, which can be divided into much simpler tasks until all the sub-tasks can be solved easily. After combining all sub-task solutions, the final results can be obtained.
Fig. 4. Classification of forming method for cold extrusion.
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Fig. 5. Model of process design for cold extrusion.
Geometrical sequence design selects the forming method only considering the geometry with help of the extrusion element and knowledge base. Its function is only to select the reasonable forming method for every single feature of the part and arranges their sequences in accordance with the knowledge, and any technological information is not involved. It is well known that geometrical sequence design is creative and it mainly shows the level of the expert. Technological sequence design is to evaluate and modify the sequences decided by geometrical sequence design by considering the material properties and technological formability. Each operation is responding to the part feature in geometrical sequence design. After passing the evaluation against certain standard formability constrains, such as the reduction ratio, pressure and some dimensional limits, most operations must be modified and the preforms are generated. Process optimization procedure is to obtain the best sequence and product quality. We know that pursuing the best product quality and reducing the product cost are the
most important target of current enterprises with the development of economics. Therefore, how to realize these targets is the task of current researchers. This paper puts forward a method for design optimization of process variables in multistages metal forming processes, which integrates engineering numerical analysis, artificial neural network and micro genetic algorithm. First, a series of orthogonal numerical analysis experiments are designed to investigate the effect of process variables on the damage factor, which constructs the learning samples for training the artificial neural network. Then, the interaction of revised micro genetic algorithm and artificial neural network is utilized to obtain the optimal variables corresponding to the optimal target. Significant improvements in the simulated productivity and the total calculation time saving have been observed as a result of the integrated model. Meanwhile, this method enriches the application of optimal technique in metal forming. It is well known that there are many feasible forming sequences for extruding a certain part. To select the optimal sequence, a method for
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Fig. 6. Process planning strategy based on the blackboard.
sequence selection is proposed based on the relative process cost and internal quality factors. On one hand, the relative cost ranking of all sequences can be obtained through comparing them and many indexes have been included. Meanwhile, the existing numerical analysis software (DEFORM) is used to simulate each sequence and get the related quality factors, from which the quality-based ranking can be obtained. Therefore, this model fully integrates the advantages of these two methods, and the optimal sequence can be gained according to the user’s requirements. So the last sequence can meet the requirements of cost and quality at the same time. Here, it is not described in detail because of the page limitation. Output procedure is to output the last sequence required of some given part in accordance with some formats. In this system, the standard plates are constructed to output the last results. Then, sequences are constructed which correspond to the reverse version of actual basic operations or their combinations. Although study has been done in obtaining formability constrains for cold extrusion, there is no well documented methodology for cold extrusion sequence design. The method some designers use is to start with a simple billet and proceed to the intermediate proform. By continuing this forward chaining procedure until reaching the last part shape, a sequence is generated. In this paper, combined with the blackboard reasoning strategy, a hierarchy-and-blackboard-based model for process planning is proposed. Fig. 6 shows the system implementation strategy. In Fig. 6, environment blackboard is used to save the middle results of all sub-tasks and the last results of the whole task, and the results are described using technol-
ogy information model. Domain blackboard is used to save the basic information for process planning, such as part feature information, the forming method link, required rules and other initial information, etc. Control blackboard is to control the normal use of the whole system. Meanwhile, case based reasoning (CBR) and rule based reasoning (RBR) method are adopted at the same time. When running the system, CBR model is firstly utilized for input part. If there exists a case in the case base, the case is loaded and modified so as to obtain the last required sequence. If there is not the case, RBR model is used to obtain the new sequence for the input part. Meanwhile, the sequence is saved into the case base
Fig. 7. Bevel gear billet.
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5. Example of results According to the theoretical studies stated above, the integrated CAD/CAPP/CAE prototype system suited to sequence design and optimization for cold extrusion of ferrous material is established based on AutoCAD. To illustrate how the proposed sequence design scheme, an example is given here. Fig. 7 shows a manufactured part from metal forming handbook [14]. The original part material is 16MnCr5. Since the ISDCE currently does not possess data for this specific material, 1015 steel is selected. Related sequences are generated assuming single-stage machines are used. The part includes five form features. Three cylinders and one cone are the external feature and one top-blind-hole is the internal feature. Meanwhile, the cone is transition feature. For this part, three different billets are selected in accordance with the billet selection algorithm. Then, many different sequences are generated based on the part feature modeling, the selected billets and related knowledge and data. The screen dump of AutoCAD drawing is displayed in Fig. 8. If necessary, parameter optimization and sequence optimal selection method can be applied to obtain the best sequence the user expects.
6. Conclusions
Fig. 8. Sequence generated for the bevel gear billet.
when it produced, and the study function is accomplished. Generally, RBR is often used at the beginning of the system running, because the cases are not enough for all kinds of parts in the case base. With the running of the system, the cases can be enriched. However, this paper only gives the example produced using RBR model. There is tremendous amount of work involved in collecting sequence design knowledge and converting sequence design knowledge and converting it into computer code. Moreover, most of the sequence design related knowledge is not well documented and practices vary from company to company. In the paper, object-oriented method is used to describe the knowledge for cold extrusion. When representing knowledge, production rules are divided into different groups for different uses, including feasibility analysis, billet selection, geometrical sequence design, knowledge sequence design. Furthermore, all the knowledge is constructed into the hierarchy structure. Meanwhile, knowledge representation language is used to describe the knowledge.
The integrated CAD/CAPP/CAE prototype system of cold extrusion has been established based on part feature modeling. Here, the paper mainly discusses the part representation scheme and system implementation strategy. The system has capabilities of accomplishing the forging part design and process planning of cold extrusion on the support of feature base, knowledge base and database. Furthermore, it can also realize the optimization of process variables in multi-stages metal forming process and the optimal selection from many feasible forming sequences. All these prove that the research is successful and effective. Of course, more work is necessary to put the system into use.
References [1] X.Q. Zhang, Y.H. Peng, X.Y. Ruan, Simulation and fracture prediction for sintered materials in upsetting by FEM, J. Mater. Process. Technol. 105 (3) 253–257. [2] Q. Sun, X.Z. Zeng, X.Y. Ruan, et al., Expert system of process planning for cold extrusion, J. Shanghai Jiao Tong Univ. 23 (1989) 1–7 (in Chinese). [3] K. Sevenler, P.S. Raghupathi, T. Altan, Forming sequence design for multistage cold forging, J. Mech. Working Technol. 14 (1987) 121–135. [4] H. Kim, T. Altan, K. Sevenler, Computer-aided part and processingsequence design in cold forging, J. Mater. Process. Technol. 33 (1992) 57–74. [5] K. Lange, G.H. Du, A formal approach to designing forming sequences for cold forging, in: Proceeding of the 17th NAMRC, 1989, pp. 17–22.
X.Q. Zhang et al. / Journal of Materials Processing Technology 174 (2006) 74–81 [6] K. Nakanishi, A. Danno, O. Takata, et al., Expert system for process design of multistage cold forging, in: International Cold Group 26th ICFG Plenary Meeting, vol. 14, 1993, pp. 21.1– 21.21. [7] K. Oakada, G.B. Yang, An expert system for cold forging process based on the FEM simulation, Ann. CIRP 39 (1990) 249–252. [8] P. Bariani, W.A. Knight, An integrated CAD/CAE system for forging process design, Adv. Technol. Plast. 1 (1990) 7–12. [9] R.S. Lee, Q.C. Hsu, Development of an integrated process planningbased CAE system for cold forging, J. Eng. Manuf., Part B 206 (1992) 215–225.
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[10] X.Q. Zhang, Research on Intelligent Design System for Cold Extrustion Process and the Key Technologies. Ph.D. Dissertation Shanghai Jiao Tong University, 2000 (in Chinese). [11] C.S. Yang, Technological Practice of Cold Extrusion, National Defense Industry Publication House, Beijing, 1984 (in Chinese). [12] S.Z. Hong, Sequence Design and Die Design for Cold Extrusion, Anhui Science and Technology Press, Anhui, 1985 (in Chinese). [13] X.Q. Zhang, Y.H. Peng, X.Y. Ruan, Feature design of cold extrusion part, Die Mold Technol. 3 (2000) (in Chinese). [14] K. Lange (Ed.), In Chief, Handbook of Metal Forming, McGraw Hill, 1985.
Feature based integrated intelligent sequence design for cold extrusion X.Q. Zhang a,∗ , Y.H. Peng b , X.Y. Ruan b , K. Yamazaki a a
Department of Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616, USA b Shanghai Jiao Tong University, Shanghai 200030, PR China Received 20 September 2001; received in revised form 20 February 2002; accepted 12 December 2004
Abstract This paper introduces the intelligent sequence design system of multi-stage cold extrusion. It discusses the method to build the part feature modeling and to realize feasibility checking, process planning and evaluation, process parameter optimization and sequence optimal selection, mainly focuses on the feature modeling and system implementation strategy. According to the given input data, the system can generate the forgeable geometry and the basic sequence design and evaluates the sequences, depending on the initial billet size and material or the evaluation knowledge in the knowledge base. An example proves the proposed theory and method. © 2006 Published by Elsevier B.V. Keywords: Cold extrusion; CAD/CAPP/CAE; Artificial neural network; Relative process cost; Micro genetic algorithm
1. Introduction Cold extrusion is a well known manufacturing method, which is becoming increasingly popular due to recent improvement in tool and press design, and tool materials. The basic advantages of cold extrusion are good dimensional accuracy, surface quality, saving in energy and material, and improvement in mechanical properties and it also eliminates the extra post processing, such as trimming and machining. Therefore, cold extrusion is a promising area for companies exploring the potential of existing technologies to meet the new demands. In cold extrusion, forming pressure is extremely high because the metal deforms without external heating to an extremely large strain and thus, fracture of the tool, defects in the product, seizure between the billet and the tool tend to take place easily. Therefore, it is very important to design an appropriate forming sequence of multi-stage cold extrusion without causing these problems. Actual process planning usually requires very long time even for long experienced experts because there are many kinds of design rules to be applied. In recent years, the use of computers to aid design ∗
Corresponding author. Tel.: +1 530 754 7687; fax: +1 530 752 8253. E-mail address: [email protected] (X.Q. Zhang).
0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2004.12.011
and manufacturing activities in extrusion has shown that the experience based intuition and skills of designers can be considerably augmented by computerized analysis and design. To ensure the productive ability of products and processes of cold extrusion at low cost, high quality and short leading time, efficient utilization of computers and the development of expert system with the integration of CAD/CAPP/CAE techniques are considered to be the most promising way. Because the time for drafting could be shortened by CAD and the number of trial-and-error could be decreased with the aid of CAPP and computer simulation [1]. Therefore, many researchers have worked on the topics [2–9]. However, there are still many problems in these systems, such as part representation, sequence design efficiency, limited use of CAE, parameters optimisation and sequence optimal selection. Then, researches on the integration of CAD/CAPP/CAE and optimum technique and establishing efficient intelligent design system for cold extrusion by making full use of all kinds of current advanced techniques are of great significance in theory and in practice. In this paper, by gradually accumulating each individual knowledge base, an extensive feature based integrated system, including part representation and system implementation strategy, is described, and the methods for parameter optimization and sequence selection are introduced. Meanwhile, the examples of results are displayed.
X.Q. Zhang et al. / Journal of Materials Processing Technology 174 (2006) 74–81
75
ple who are experienced in cold extrusion sequence design, the foundation for the prototype system development was established.
3. Feature representation
Fig. 1. System frame of feature based intelligent sequence design for cold extrusion (ISDCE).
2. System architecture A feature-based intelligent sequence design for cold extrusion (ISDCE), integrating CAD/CAPP/CAE and optimal techniques, is proposed after analyzing the application and tendency of computer aided process planning, and artificial intelligence technique in metal forming processes. The system architecture is shown in Fig. 1, from which we can notice that there exist two work cycles in the system. One is for the design of forging part, including part feature modeling and feasibility analysis. The other is for process planning, including intelligent sequence design module, knowledge base and database. The constructed feature modeling, knowledge base and database are necessary for the sequence design. Furthermore, both cycles are constructed in the integrated design environment. In this paper, feature-based modeling and system implementation strategy will be introduced in detail later, and the other parts of the system are described in the literature [10]. With the ISDCE system window interface, the user inputs the part geometry by using part feature modeling, and the feasibility analysis is conducted to obtain formable part for cold extrusion. Then, sequence design starts. In another window, the system shows the expansion of the search tree during the system execution. At last, the 2-D geometry data of generated sequences are finally shown in AutoCAD environment so that the user can make modification if necessary. The very important step in the system development is to study the problem domain. In the field of cold extrusion, the operation limitations and evaluation rules have been well studied and documented [11,12]. In contrast to the operation evaluation rules, the heuristics about how to choose operations and how to form a most feasible sequence are mostly unwritten. By extracting the sequence design rules from books and technical papers, by examining in-use cold extrusion sequences and by communicating with peo-
When developing an extrusion sequence design system which covers a variety of cold extruded parts, even though the parts are mostly rotational–symmetric, an appropriate geometry representation is difficult to find because the material flows and the volume is kept constant in cold forming. But the application of feature technique in the field of machining provides the chance to represent the cold extrusion part in the system. In the past 20 years, the concept of feature representation has been widely accepted and applied in process planning, feasibility analysis and mechanical design. The characteristics of the feature modeling is that the models consist of high level modeling elements that more directly correspond to shapes having clear engineering meaning. Sequence design of cold extrusion is also considered as part geometry manipulation. The geometry changes in forming are made by operations based on volume constancy and some formability limitations. In the previous research work, all the parts are cylindrical and two types of representations are adopted. The first is the boundary representation of the axial cross-section of part [3–6]. This representation is simple and convenient for volume calculation. However, it cannot effectively represent non-axisymmetric parts, and the information for operation selection is difficult to be associated with the geometry. The other representation decomposed a part into several volumetric primitives, each of which has attributes, such as height and diameter [2,7,8]. The second representation has advantages over the first since the diameter and height of primitives are key variables for choosing and evaluating operations. Furthermore, it also makes representing non-axisymmetric part possible. However, both representation methods cannot contain engineering meaning information, such as material, precision and management information. But all the information is very important for sequence design. Besides volume constancy, it has also been noticed that the geometry change of the cold extrusion distinguishes itself from machining: (1) the deformation of cold extrusion is the action of material flow, not the metal cutting. Furthermore, material flow results in the changes of material properties, which limits the deformation; (2) for most parts, especially hollow shaped parts, shape changes in the desired section cannot isolated from those in the adjacent sections; (3) the transition feature can make material flow easier. They are formed with the main feature at the same single station. No separated operation is required.
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X.Q. Zhang et al. / Journal of Materials Processing Technology 174 (2006) 74–81
Therefore, the feature modeling method in the field of machining cannot be directly introduced into the field of cold extrusion. In this research, the features representation is used for the part representation [13]. Based on the analysis of the peculiarity of cold extrusion parts, a feature classification system is put forward here, especially the classification of form feature and their relation are emphasized. Form feature, material feature, precision feature and management feature are four major features which characterize the metal forming process. Form feature characterizes the nominal geometry of part, in which the primary shape represents the major shape of part and the sub-feature represents alternations to the major shape. The material feature comprises the material composition, physical and mechanical properties, heat treatment and surface treatments. The material feature can be represented as forming limits (such as maximum upset strain, maximum upset ratio, maximum area reduction), friction factor and stress–strain relation. Roundness, positive/negative deviations in size and eccentric deviation in principal axis are all the factors that comprise the precision feature. Management feature contains the general information of the part. Among them, form feature is main feature and the others exist depending on it. Furthermore, the form feature often refers to the other features through pointers. Generally, form feature can be classified into main feature and subordinate feature. And they can be classified more detailed sub-features, shown in Fig. 2. A part is represented in term of main feature and subordinate feature. Meanwhile, main feature contains external and internal macro features. Each external macro feature can be composed of external primitive features, such as cylinder and cone features. Each internal macro feature can be composed of internal primitive features, such as topblind-hole and bottom-blind-hole feature. And the primitives, macros and their relation match the way in which
Fig. 2. Classification of form feature.
Fig. 3. Structure of part feature model.
the parts are perceived by engineers during designing the sequence. The features’ type and attributes are relevant to operation selection and evaluation. For example, the internal macro feature “top-blind-hole” suggests that backward extrusion could be used. The diameter and depth of the “top-blind-hole” feature are also essential in evaluating the operation. Then, a dynamic model is established to represent the part, shown in Fig. 3. The feature-based model consists of three key components: feature syntax, feature semantics and feature operations. Feature syntax defines geometric construction of part or a feature, especially the relations between form features. There are several feature relations: (1) (2) (3) (4)
Outer Increment Adjacent (OIA); Outer Reduction Adjacent (ORA); Outer Adjacent Inner (OAI); Subordinate (SUB).
The relations of features are listed in Table 1. Feature semantics describes information associated with the feature geometry, tolerance and technologies used for management and manufacturing. Only by using the feature semantics description does a feature become a meaningful geometric entity. Meanwhile, some necessary feature operations, such as “Add”, “Delete”, “Save” and “Save As” are designed and integrated with the form feature structure. With the capabilities of these three key components, the feature modeling can be used as tool for feature-based part design. To meet the requirements for feature information saving, a dynamic data structure (binary-tree structure and dynamic chain structure) for feature representation has been designed. Furthermore, an independent feature library has been established for the user to select from as well as to add, instead of feature enumeration. After the definition of the part, the feasibility analysis, which focuses on the judging of whether the part can be formed using cold extrusion from the geometry, material and mechanical properties are conducted.
X.Q. Zhang et al. / Journal of Materials Processing Technology 174 (2006) 74–81
77
Table 1 Note of feature relations No.
Relation (argu1, argu2)
Meaning
Features
1
OIA (A, B)
Adjacent between A and B at external features and the section increases
A: cylinder, cone, . . . B: cylinder, cone, . . .
2
ORA (A, B)
Adjacent between A and B at external features and the section decreases
A: cylinder, cone, . . . B: cylinder, cone, . . .
3
OAI (A, B)
Adjacent between A and B at the end of the part, especially between internal and external features
A: cylinder, . . . B: top-blind-hole, bottom-blind-hole, . . .
4
SUB (A, B)
B is the subordinate feature of A
4. Sequence design strategy 4.1. Forming method In cold extrusion, the basic operations are upsetting, forward and backward extrusion according to the material flow directions. In order to handle various types of parts, a relatively detailed classification scheme should be formulated. Meanwhile, single feature is regarded as the basic unit during the sequence design in the paper. To meet the requirement, forming methods for cold extrusion are divided into single-blow process, multi-blow process, combined process and special process. Single process means that there are no any defects when one feature is formed using the selected machine. Multi-blow process is not like this. When the defects arise if the feature is formed by single-blow process, the continuous forming process is necessary to form this feature so as to obtain the preform without any defects, which is called multi-blow process. Here, single-blow and multi-blow process is divided corresponding to a single feature. However, combined process is necessary to combine the continuous process under the condition of forming limitation rules so as to reduce the forming station numbers and the production cost. Special process is used to form the subordinate feature. All the forming methods can be further divided into more detailed methods shown in Fig. 4. Furthermore, extrusion element is used to represent the forming methods, which provides more convenience for downstream geometrical sequence design [10].
A: cylinder, top-blind-hole, . . . B: screw, spur, . . .
With the strategy, the difficult problem of strategy for process planning can be divided into a number of sub-problems to be solved easily using different reasoning machines. So the structure of reasoning machine for process planning can be simplified significantly. Then, the procedure model is established, including billet selection, sequence design (geometrical and technological sequence design) and process optimization (parameter optimization and sequence optimal selection), which provides the idea for realizing the process planning in computer. Fig. 5 shows the procedure model. The functions of all the procedures are as follows: Preparation procedure is the foundation of sequence design. Its main task is to load all kinds of information necessary to the downstream procedures, which includes the information about the part, forming method, knowledge base and database. Billet selection procedure is to select the billet with reasonable shape and size in accordance with user inputting information and part feature information. The procedure is very important because it is also the base of sequence design. Sequence design procedure is key to the whole process planning strategy model. Meanwhile, the procedure is divided into geometrical sequence design and technological sequence design according to different functions.
4.2. Process planning strategy Process planning strategy is one of the most important issues in the intelligent design system. In the paper, the hierarchy-based strategy for process planning is used to select the sequence for the given part. The principle of hierarchy-based strategy is: one complicated task is divided into many sub-tasks, which can be divided into much simpler tasks until all the sub-tasks can be solved easily. After combining all sub-task solutions, the final results can be obtained.
Fig. 4. Classification of forming method for cold extrusion.
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Fig. 5. Model of process design for cold extrusion.
Geometrical sequence design selects the forming method only considering the geometry with help of the extrusion element and knowledge base. Its function is only to select the reasonable forming method for every single feature of the part and arranges their sequences in accordance with the knowledge, and any technological information is not involved. It is well known that geometrical sequence design is creative and it mainly shows the level of the expert. Technological sequence design is to evaluate and modify the sequences decided by geometrical sequence design by considering the material properties and technological formability. Each operation is responding to the part feature in geometrical sequence design. After passing the evaluation against certain standard formability constrains, such as the reduction ratio, pressure and some dimensional limits, most operations must be modified and the preforms are generated. Process optimization procedure is to obtain the best sequence and product quality. We know that pursuing the best product quality and reducing the product cost are the
most important target of current enterprises with the development of economics. Therefore, how to realize these targets is the task of current researchers. This paper puts forward a method for design optimization of process variables in multistages metal forming processes, which integrates engineering numerical analysis, artificial neural network and micro genetic algorithm. First, a series of orthogonal numerical analysis experiments are designed to investigate the effect of process variables on the damage factor, which constructs the learning samples for training the artificial neural network. Then, the interaction of revised micro genetic algorithm and artificial neural network is utilized to obtain the optimal variables corresponding to the optimal target. Significant improvements in the simulated productivity and the total calculation time saving have been observed as a result of the integrated model. Meanwhile, this method enriches the application of optimal technique in metal forming. It is well known that there are many feasible forming sequences for extruding a certain part. To select the optimal sequence, a method for
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Fig. 6. Process planning strategy based on the blackboard.
sequence selection is proposed based on the relative process cost and internal quality factors. On one hand, the relative cost ranking of all sequences can be obtained through comparing them and many indexes have been included. Meanwhile, the existing numerical analysis software (DEFORM) is used to simulate each sequence and get the related quality factors, from which the quality-based ranking can be obtained. Therefore, this model fully integrates the advantages of these two methods, and the optimal sequence can be gained according to the user’s requirements. So the last sequence can meet the requirements of cost and quality at the same time. Here, it is not described in detail because of the page limitation. Output procedure is to output the last sequence required of some given part in accordance with some formats. In this system, the standard plates are constructed to output the last results. Then, sequences are constructed which correspond to the reverse version of actual basic operations or their combinations. Although study has been done in obtaining formability constrains for cold extrusion, there is no well documented methodology for cold extrusion sequence design. The method some designers use is to start with a simple billet and proceed to the intermediate proform. By continuing this forward chaining procedure until reaching the last part shape, a sequence is generated. In this paper, combined with the blackboard reasoning strategy, a hierarchy-and-blackboard-based model for process planning is proposed. Fig. 6 shows the system implementation strategy. In Fig. 6, environment blackboard is used to save the middle results of all sub-tasks and the last results of the whole task, and the results are described using technol-
ogy information model. Domain blackboard is used to save the basic information for process planning, such as part feature information, the forming method link, required rules and other initial information, etc. Control blackboard is to control the normal use of the whole system. Meanwhile, case based reasoning (CBR) and rule based reasoning (RBR) method are adopted at the same time. When running the system, CBR model is firstly utilized for input part. If there exists a case in the case base, the case is loaded and modified so as to obtain the last required sequence. If there is not the case, RBR model is used to obtain the new sequence for the input part. Meanwhile, the sequence is saved into the case base
Fig. 7. Bevel gear billet.
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5. Example of results According to the theoretical studies stated above, the integrated CAD/CAPP/CAE prototype system suited to sequence design and optimization for cold extrusion of ferrous material is established based on AutoCAD. To illustrate how the proposed sequence design scheme, an example is given here. Fig. 7 shows a manufactured part from metal forming handbook [14]. The original part material is 16MnCr5. Since the ISDCE currently does not possess data for this specific material, 1015 steel is selected. Related sequences are generated assuming single-stage machines are used. The part includes five form features. Three cylinders and one cone are the external feature and one top-blind-hole is the internal feature. Meanwhile, the cone is transition feature. For this part, three different billets are selected in accordance with the billet selection algorithm. Then, many different sequences are generated based on the part feature modeling, the selected billets and related knowledge and data. The screen dump of AutoCAD drawing is displayed in Fig. 8. If necessary, parameter optimization and sequence optimal selection method can be applied to obtain the best sequence the user expects.
6. Conclusions
Fig. 8. Sequence generated for the bevel gear billet.
when it produced, and the study function is accomplished. Generally, RBR is often used at the beginning of the system running, because the cases are not enough for all kinds of parts in the case base. With the running of the system, the cases can be enriched. However, this paper only gives the example produced using RBR model. There is tremendous amount of work involved in collecting sequence design knowledge and converting sequence design knowledge and converting it into computer code. Moreover, most of the sequence design related knowledge is not well documented and practices vary from company to company. In the paper, object-oriented method is used to describe the knowledge for cold extrusion. When representing knowledge, production rules are divided into different groups for different uses, including feasibility analysis, billet selection, geometrical sequence design, knowledge sequence design. Furthermore, all the knowledge is constructed into the hierarchy structure. Meanwhile, knowledge representation language is used to describe the knowledge.
The integrated CAD/CAPP/CAE prototype system of cold extrusion has been established based on part feature modeling. Here, the paper mainly discusses the part representation scheme and system implementation strategy. The system has capabilities of accomplishing the forging part design and process planning of cold extrusion on the support of feature base, knowledge base and database. Furthermore, it can also realize the optimization of process variables in multi-stages metal forming process and the optimal selection from many feasible forming sequences. All these prove that the research is successful and effective. Of course, more work is necessary to put the system into use.
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