Journal of Materials Processing Technology 172 (2006) 291–298
EDM process adaptation system in toolmaking industry J. Valentinˇciˇc a , D. Brissaud b , M. Junkar a,∗ a b
University of Ljubljana, Faculty of Mechanical Engineering, Aˇskerˇceva 6, SI-1000 Ljubljana, Slovenia Institut National Polytechnique de Grenoble, Laboratoire 3S, BP 53, 38041 Grenoble Cedex 9, France Received 20 September 2002; received in revised form 21 July 2003; accepted 5 October 2005
Abstract A Design Adaptation System for machining in toolmaking (DASMT) is proposed in order to avoid the knowledge gap between product and tool designers. The focus of the presented paper on the development of the Design Adaptation System for EDM (DAS-EDM). This system helps the designer to check the viability of individual features to their machinability by EDM process. The system reveals features, which are critical for EDM machining and makes suggestions to the designer to alter the features by keeping the overall functionality of the product. © 2005 Elsevier B.V. All rights reserved. Keywords: Elecrical discharge machining; DFM; Expert systems; Toolmaking
1. Introduction From hundreds of product concepts, only about twenty will merit serious consideration [1]. This is either due to an inadequate exploration of all feasible alternative concepts or due to an ineffective integration of the product design concepts with evaluation criteria such as ease of manufacture and production cost [2]. Designing for manufacturing (DFM) philosophy is characterized by simultaneous design of a product and its manufacturing process in order to achieve the best outcome and consequently optimise the overall costs. The manufacturability concept has been developed to measure the degree of integration between the product and the process; manufacturability implies easy production and can be approximated by the capacity to achieve the desired quality and productivity while optimising costs. Basically, the design for manufacture addresses the simultaneous design of the product and best selection of the process resources and methods. In toolmaking industry, which produces dies, the design and manufacturing are specific because the dies1 are intermediate products by which the basic products or their components are produced. In general, dies consist of one or more cavities. The
∗
Corresponding author. Tel.: +386 1 4771 724; fax: +386 1 2518 567. E-mail addresses:
[email protected] (D. Brissaud),
[email protected] (M. Junkar). 1 In the text, die will be the generic word for die or mould. 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.10.019
shape of the cavity depends on the product design, and is usually extremely complex. Therefore, it is important to decompose it to individual features regarding the requested shape, tolerance and surface roughness of the cavity. A survey has been made in nine Slovenian toolmaking companies [3] to find out how well the product and die design are adapted. Results show, that product design has to be often adapted to the manufacture of the die to meet an easier and cheaper machining. In some cases, it is even necessary to change altogether the product design, in order to enable the manufacturing. Conclusion was, that industry needs an outside assistance to adapt the product design to the die manufacturing processes. DFM needs substantial expert knowledge of the manufacturing process itself. For example, knowledge on the wear process of an electrode machining of a die is proposed in Mohri et al. [4]; a process planning system for EDM operations is proposed by Lauwers and Kruth [5], and knowledge about comparison and selection of competitive technologies, namely EDM, high speed milling (HSM) or both, in dies manufacturing is the topic of Alam et al.’s contribution [6]. The second part of the latter work deals with the basic DFM techniques as rules advising the designer, guidelines assisting him/her throughout methodology, simulation software to detect problems in shape, quality or productivity and cost calculation software to control manufacturing costs. An overview is given in Boothroyd and Dewhurst [7,8]. In addition to this core knowledge, the design process itself has to be rationalised: Lee et al. [9,10] arranged concurrently the die design process, Chin and Wong [2] rationalized the design of the plastic injection moulds and Ding et al. [11] rationalised
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the EDM electrode design process. What becomes the most significant topic in DFM system is the management of the whole body of knowledge and data. Chen and Hsiao [12] proposed a collaborative data management system. Young et al. [13] proposed a system providing high-quality information on which designers can base their decisions on-line. Most authors proposed to embody information into knowledge bases and expert systems. An integrating system for the simultaneous design of the product and the adapted dies is proposed. The system would mostly be used by the product designers. Based on the main principles of DFM techniques, its originality comes from the adaptation methodology and the implementation of the professional knowledge. The Design Adaptation System for machining in toolmaking (DASMT) aims to address the embodiment and detail design phases of the product definition, as well as the conceptual design phase of the die definition at an early stage, when there is still time to make significant changes. This methodology is extremely useful, as it enables evaluation and optimisation of the design and avoids designers’ time-consuming iterations. In this paper, the principles of the proposed system are presented and it gives a complete overview of the DAS-EDM system developed for EDM technology used as a manufacturing die technique. 2. Principles of DFM system for toolmaking companies From the set of the product features fed by the designer, the DASMT should select the die manufacturing process and propose improvement and adaptation of the product according to the die manufacturing process. Engineers’ experiences, expert and scientific knowledge about the processes and a lot of case studies enabled to define a standard and accepted reasoning to evaluate die manufacturing attributes from product design attributes. Although this reasoning is not linear and cannot be inverted, a mapping from die manufacturing attributes to product design attributes could be proposed nevertheless. Finally, the system points out the critical attributes—the attributes, which may bring about additional costs and unnecessary and unfriendly machining. The general concept of DASMT is presented in Fig. 1, where the upper part of the figure presents the information flow between the product designer, the die engineer and the manufacturer, who actually makes the die. The lower part of the given figure shows the information flow between the designer and the DASMT system, as well as the information flow between the problem solvers of the systems, namely (I) system for die segmentation and determination of the machining process, (II) Design Adaptation System for EDM (DAS-EDM) and (III) Design Adaptation System for HSM (DAS-HSM). Designer designs a product, which will be produced by mass production using dies. The initial product definition is parametrically introduced to the DASMT system. Product design attributes D are introduced to the die segmentation and machining process determination problem solver, where the product design is decomposed into features. They are based on die mak-
Fig. 1. A product Design Adaptation System for the ease of die manufacturing: DASMT.
ing knowledge, such as: positioning of the dividing plane, runner system, cooling system, etc. A new set of attributes which are die design attributes D* is created and the manufacturing technology is selected for each feature of D* : either EDM or HSM. According to this selection, each die feature moves on to the appropriate technology adaptation solver: DAS-EDM or DASHSM to be examined from the manufacturing point of view. The critical attributes of the die features D∗cr are established. Since there are no critical attributes in the die design itself, the critical die design attributes are indentical to the product design attributes Dcr and are introduced to the designer. He/She then tries to adapt the design focusing on the given critical product design attributes. The system is, of course, interactive. When the product design and the die manufacturing process are tuned, the outputs of the DASMT are both the product design attributes and the die manufacturing attributes. The work of the die engineer is then quite simple since elements of the both are coherent. The die segmentation and determination of the machining process problem solver is presented in [14]. The DAS-EDM solver is described in detail in the next section. 3. The Design Adaptation System for EDM 3.1. System overview The DAS-EDM can be used as an integrated module of the DASMT system or it can operate autonomously to solve specific problems. It is a knowledge-based system that allows the designer to adapt concurrently the die definition and EDM manufacturing process. It incorporates a mapping-based environment with bi-directional communication among product knowledge, process knowledge and the designer (Fig. 2). Product knowledge is represented by design attributes to be achieved. Process knowledge is represented by several knowledge representation mechanisms: EDM machine database with their performances, performances of the EDM process and mapping functions between
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Table 1 Classification of feature shapes Tool movement 1 Direction Feature access Open Closed
Fig. 2. The DAS-EDM module.
design and manufacturing attributes. The system applies a design procedure and highlights the critical design attributes for achieving the demanded product quality and the set of manufacturing objectives simultaneously. 3.2. Die design attributes Knowledge on dies has been formalised into design attributes, which all together describe the die from the manufacturability point of view. According to the literature, industrial partner’s experience and our own experience, 10 design attributes of die definition have been selected as most significant in determination of the EDM manufacturing attributes. The design attributes are in general vectors and noted as di . Two attributes characterise the whole die: d1 represents the maximum dimensions of the die (x, y, z) and d4 is the die material. Each die feature is characterised by 7 attributes; the main one is the attribute d2 characterising the shape of the feature. Others are geometrical or technological attributes: d5 surface area, d6 depth, d7 surface roughness, d8 heat affected zone, d9 roundness of edge and d10 slope. The attribute d3 is a global attribute characterising the tolerances of the dimensions, shape and position. 3.2.1. Shape attributes of die features The shape of a feature is described by attribute d2 . In general, there are many features on a die, thus there are more than one instance of attribute d2 for each die. The feature shapes are grouped into six prototypes. According to a vast literature, a manufacturing feature can be defined as a geometrical form of the workpiece to which technological properties are associated and for which a manufacturing process is known. In the case of EDM process, a feature is a part of the die, which can be machined by an electrode (or a set of electrodes for rough and fine machining) on a given EDM machine. A clustering of six basic shapes can cover all the features machined by EDM. They are characterised by two properties: the tool movement direction
2 Combined directions
3 Combined directions
Non-basis movement
A –
B D
C E
F –
and its accessibility to the feature. In the former case, the movement of the tool could be in one linear direction, two combined directions, the combination of the three possible directions and a rotation of the tool added to the three basic x, y, z movements. In the latter case, the feature is called open when the feature can be reached from the top of the die. It is “closed”, when features cannot be reached directly from the top of the die and extra actions are needed. Table 1 summarises this classifications and Fig. 3 illustrates the prototypes as they are implemented in the software. Note that two combinations presented in Table 1 do not exist and that the ‘F’ case is very rare in toolmaking. It is performed only for modifications after errors. 3.2.2. Geometrical and technological parameters of the die features Other attributes are briefly described here. The surface area is the size of the feature surface area where machining occurs. The depth is the largest depth of the feature measured from the top (die surface) to the bottom of the feature. The surface roughness is the roughness of the highest quality surface of the feature given as roughness parameter Ra . The heat-affected zone is the smallest requested depth of the heat-affected zone among all feature surfaces. The roundness of edge consists of two data, the radius between two surfaces and the depth, where radius occurs measured from the top of the feature to the centre of the radius. The slope of the feature surfaces are described by two data: the first one is the smallest surface slope of the feature, but still bigger than 5◦ ; the other one consists of the information that the surface roughness or the heat affected zone depth is also requested for feature surfaces inclined for less 5◦ . The angle is measured from the machining direction. 3.2.3. Tolerance attributes of die features In general, a workpiece is defined by tolerances on intrinsic feature dimensions, feature shapes and feature positions. They are related either to the geometrical machine accuracy, to the machine positioning accuracy, the tool (electrode) shape accuracy or the positioning of the die onto the machine. But only the precision of the electrode positioning or orbital moving during machining has to be considered; set-up program references from workpiece location and the accuracy of the electrode shape should be mastered, and since EDM machines satisfy the ISO 11090-1 standard they have satisfactory geometrical precision. The tolerance attributes are noted d3 and have finally been splitted into five cases as shown in Fig. 4. On one hand they are
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Fig. 3. The shape prototypes of the die features.
Fig. 4. Tolerance attributes split into five cases.
related to the fact whether it is an intrinsic tolerance of the feature or a dimension tolerance between a feature and the system of reference of the workpiece (extrinsic tolerance), and on the other hand to their position (perpendicular or parallel) to the eroding direction (Table 2).
3.3. Die manufacturing databases Two databases have been developed to embrace knowledge and data on the EDM machines, the process parameters and the performances.
Table 2 Classification of feature tolerances as given in Fig. 4
Intrinsic tolerances Extrinsic tolerances
Dimension between two surfaces perpendicular to the eroding direction
Dimension between two surfaces parallel to the eroding direction
Quality of a surface parallel to the eroding direction
a b
c d
e –
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3.3.1. The machine database The machine database is a classical database with the machine properties, useful for determination of the die manufacturing process by EDM. A machine is characterised by a name, the size of the working area m1 (in x, y, z axes), the list of the axes which can operate on the particular machine m2 (z axis: with manual positioning, z and x axes: with manual positioning, z, x, y axes, where orbital machining can operate, z, x, y, c axes: where a rotational axis c is controlled as well) and the precision of the machine m3 characterised by the positioning attributes in xy plane and z axis. 3.3.2. Database of the machining parameters and process performances The parameters which have to be set-up on an EDM machine m5 are free voltage U, electric current limit I and pulse-on time t. Each set of parameters is characterised by the smallest surface area of the feature A, which can be machined by the given values of the set-up parameters, the relative corner wear of the electrode theta, the achieved surface roughness Ra , the achieved depth of the heat affected zone HAZ and the requested machining allowance z. The database based on those parameters has been built together with the performance characteristics and their relationships. This knowledge has been extracted from best practices in EDM manufacturing. To illustrate the database, Fig. 5 shows the seven main regimes resulting from the experience of the company contributing to our investigation. 3.4. The mappings The die design attributes D from design level are mapped to die manufacturing attributes M on the manufacturing level (Fig. 6). The manufactural attributes M depend upon the design attributes D:M = F(D), where D and M are vectors of attributes D = (d1 , d2 , . . ., dp , . . ., dP ), M = (m1 , m2 , . . ., mq , . . . mQ ) and F is a vector of the mapping functions. The adaptation problem deals with the inverse problem to derive design vector D from manufacturing vector M:D = F−1 (M). For each manufacturing
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attribute mq it is necessary to define all mapping functions fr with other manufacturing attributes mq and design attributes dp :mq = fq (dp , mq ); p ∈ {1, 2, . . ., P}, q, q ∈ {1, 2, . . ., Q}, q = q , r ∈ {1, 2, . . ., R}. The knowledge of the domain has been expressed by seven functions f as shown in Fig. 6. But what is needed in a die design process are the inverse functions F−1 while f functions do not allow mathematical derivation. Functions f−1 have been achieved by implementing an EDM expert system to point out the weak points of the design from the manufacturing point of view. By implementing all the mappings into a software together with algorithms for machine selection and algorithms for process attributes selection the inverse function F−1 is obtained by the designer. He/She makes the design adaptation by reconsidering and adaptation of the critical design attributes. The knowledge used to define f functions and algorithms has been formalised based on literature, company experience, laboratory knowledge and experiments designed for that purpose. See [3] for more details about this knowledge. 3.5. The design procedure The DAS-EDM system works in two steps (Fig. 2). In the first step, the appropriate EDM machine is selected according to the given design attributes and the critical design attributes for the specific machine are established. The machine selection is important, since every EDM machine has slightly different optimal machining parameters and process performances. These data are gathered in the database of the machining parameters and process performances (Section 3.3.2). The data about each machine is assembled in the EDM machines database (Section 3.3.1). After the machine is selected the machining performances are known and optimisation of die features can be performed for an ease of manufacturing. Designer feds the rest of the design attributes to the DAS-EDM system and the system highlights the critical attributes by using knowledge from the EDM expert system.
Fig. 5. Machining parameters database with the set-up parameters and the corresponding process performances (extract).
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Fig. 6. Mappings fr of the design attributes dp to the manufacturing attributes mq .
4. Software implementation The DAS-EDM has been implemented in a web environment according to the principles explained above. It was fully developed and is published as an internet applet on the site: http://www.fs.uni-lj.si/lat/dfm. The user interface of the system for adaptation of the design to EDM process has been organised around seven screens. Instructions for using the software are on the first screen. The consultation of the databases of the available EDM machines in the toolmaking company is on the second and the sixth screen. New elements can be added to the databases whenever needed, but the user needs assistance to do that properly. The design attributes (from d1 to d3 ) which are important for the machine selection are collected on the third, the fourth and the fifth screen. The screens are supported by illustrations to make the input understandable and easy to carry out. (Figs. 3 and 4 are cut outs of these screens). In the first step the designer feeds the attributes d1 , d2 , d3 relevant to all the features on the die as all of the features are usually machined on one EDM machine and the given design attributes are mapped to the manufacturing attributes, which define the machine selection. According to the given data the DAS-EDM system selects the appropriate EDM machine and colours the critical design parameters in red. It is now up to the designer to change the critical design attributes for the ease of manufacturing. After each change the system reselects the EDM machine and recalculates new critical attributes. When no fur-
ther change is possible due to design requests the second step is performed. In the second step each feature is adapted separately. On the sixth screen the machining parameters database is presented, including process performances as described in Section 3.3.2 (cut out of the sixth screen is shown in Fig. 5). The design attributes from d4 to d10 of the first feature (the features order selection is designer’s choice) are fed on the seventh screen. After feeding the data the EDM expert system selects the electrode material m4 , selects set-up parameters m5 for rough and fine machining, calculates machining depths and electrode edge wear for each set of matching parameters and calculates the required number of electrodes m5 (in one set). The results of EDM expert system are shown on the seventh screen (Fig. 7). Further explanation of the results is out of the scope of this paper. According to the results the critical design attributes in the DAS-EDM system are coloured in red. It is again designer’s choice to change critical attributes of the feature for the ease of manufacturing. After each change the EDM expert system recalculates the machining parameters and new critical attributes are established. When no further changes are possible due to design requests the adaptation of the first feature is finished and the attributes of the second feature are introduced to the seventh screen. The second step keeps repeating until the last feature on the die is adapted. Then the die design adaptation is completed and the die design can be introduced to the die engineer who can use the
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Fig. 7. The feature adaptation screen of the DAS-EDM system.
DAS-EDM data for making the die design suitable for production. 5. Conclusions In this paper, the DASMT system has been described. It belongs to the group of DFM systems, but it differs from other DFM systems as it leaves the best design solutions to the designer, who has the best knowledge about the demands for the product characteristics. Such knowledge incorporates also the knowledge about aesthetics, ergonomics, etc. In such a way, the designers’ creativity is fully supported. Instead of leading the designer through the process of design, the DASMT system only points out the weak parts of the product design from the manufacturing point of view and leaves full freedom to the designer to adapt it. The weak points of the design are revealed by expert systems. The design adaptation for the ease of manufacture is a complex task, particularly in toolmaking industry where plenty of decisions have to be coordinated. Up to now two problem solvers of DASMT system have been developed separately and each of the problem solver works autonomously. The system for die segmentation and machining process selection was presented elsewhere [14]. DAS-EDM system presented in this paper can be used only for the adaptation of the die design features which will be machined by EDM process. Already by using the DASEDM system in the design phase both, the time of coordination between the designer and tool engineer and the machining time and costs can be significantly reduced. The implementation of this approach proves that classical techniques of DFM applica-
tions such as rules, procedures and assessment can be added with dependencies analysis to achieve complex parameters network. The disadvantage of the DAS-EDM system is the connection between the product design and the die design, which is implemented in the system for die segmentation and determination of the machining process.
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