MANUFACTURING FEATURES IN CUTTING SHAPES AND PUNCHING HOLES IN SHEET METAL

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1 MANUFACTURING FEATURES IN CUTTING SHAPES AND PUNCHING HOLES IN SHEET METAL M.Lohtander, J. Varis. Department of mechanical engineering, Lappeenranta University of Technology, P.O.Box , Lappeenranta, Finland Abstract By examining sheet metal parts it is rather evident that each sheet metal shape has its own manufacturing procedure. There are several manufacturing devices behind each procedure. This means that each shape can be divided into manufacturing features, which consist of shapes of sheet metal and their manufacturing methods. When creating rules and structures for manufacturing features, it is possible to prepare simple guides for engineering designers so that they can design parts which are easy to manufacture. A manufacturing guide can be a kind of database. A designing engineer may follow the guidelines of the database. It proposes different kinds of possibilities to produce sheet metal parts. This enables the design of products which are easy to manufacture. In addition, it allows the designer to diversify his or her knowledge of manufacturing methods. In this study, simple sheet metal parts and their manufacturing features are studied. For example, simple shapes such as holes can be made in several ways. Cutting a hole consists of several attributes which determine the requirements for manufacturing. This study tries to validate what kinds of factors are needed in order for the engineering designer to design parts which are simple to manufacture. Keywords: Sheet metal, sheet metal manufacturing, sheet metal features. 1 INTRODUCTION Modern design and manufacture is often distributed to several actors. For designers, this inevitably leads to a decrease in manufacturing know-how because manufacturing is situated at a distance from design. Thus the possibilities and limitations involving manufacturing cannot be taken into consideration in the design of new products because the activity is distributed. Therefore, there should be a system that allows the processing of information between design and manufacturing. Modern information technology provides ample possibilities for this. As the design process progresses, the designer can compare the effect of different shapes on manufacturing technologies and possibly also on costs. The iteration rounds are then reduced, which takes the process a step closer to the ready at once principle. The main responsibility of detailed design is increasingly being shifted from manufacturers towards businesses specialising in design services. These design companies can be located at a distance from the client and the manufacturing organisation. As the distances grow, the interaction between different operations becomes difficult. The importance of the design stage in the product life cycle cannot be over-emphasised because at that point 70 80% of the manufacturing costs are established. As open and innovative an approach as possible in the early design stage is advisable, even indispensable, in order to obtain the desired position on the market. An innovative approach may result in significant savings in material and production costs, but may also make the product very difficult to manufacture. 2 CHALLENGES IN DESIGN Computer-aided design (CAD) of sheet metal components is rather easy at present. However, the seemingly easy design process can result in problems in the manufacturing properties of sheet metal components. This is due to the fact that design software programmes have a number of automatic features that facilitate drawing a component, but do not automatically improve its manufacturability. Consequently, one should always take into account the manufacturing technologies used, and consider the consequences of selecting a certain technology. This is why designers should have sufficient experience or guides to help make the product as manufacturing friendly as possible. Manufacturing high-quality sheet metal products from the best materials, with the appropriate manufacturing methods and at a low cost is very challenging. This can, however, be seen as a possibility, because each manufacturer faces the same challenges and the one with the best response to them will end up the winner on the market. In addition to the actual designing work, designers must take into consideration the manufacturability and lead time. There are many ways to simplify the construction, decrease the number of components, cut-down on raw material use and simplify manufacture. One way is to apply manufacturing features to the design and manufacture. The feature based model is composed of a variety of features, which in the case of a sheet metal part include e.g. material, edge length and width, bending angles and tolerances. Features also provide a basis for organising manufacturing information and thus utilising it in the design, further development or quality assurance of future products [1] [2]. 3 MANUFACTURING BASED ON FEATURES Product information has traditionally been recorded in drawings, on paper, in users' manuals and in suppliers' instructions. Currently, these methods have largely been replaced by CAD models and a range of databases, which contain information on e.g. materials, production management, equipment and tools. Nevertheless, traditional drawings, CAD models and 3D models are incapable of presenting and especially transmitting information between different systems.

2 Therefore, during the past decade research has been carried out on how a model should include not only the product geometry but also other information, such as manufacturing information. A solution to this is provided, for instance, by feature based modelling [3] [1]. It allows to take into account the limitations of a manufacturing technology and to create manufacturing information automatically [2]. Regardless of the fact that more advanced CAD systems use information on features in internal communication, features cannot, with some exceptions, be transferred outside the system. Moreover, the internal feature recognition of the software solutions will not necessarily function if the transferred product or part is not modelled with the same software version. Some attempts have been made to turn information transfer formats into industrial standards even though they are not open and, apart from a few attachments, cannot transfer anything other than geometry [4]. Feature based modelling is increasingly applied in CAD/CAM software [5], but not even these systems support open information transfer they merely save the information on features in their own, closed transfer format. Products can be classified according to geometrical, nongeometrical, assembly [6] and manufacturing features [7]. Manufacturing features are shapes generated in a particular sheet metal production method, or characteristics of a product that demand certain manufacturing measures [2]. These features are different from e.g. design features used in designing [7]. Manufacturing features are, to a certain extent, related to the physical measurements and geometry of a part, and also to the characteristics required of a sheet metal product, such as lightening, whether an angle is right and aligned, and similar features which need yet different manufacturing technologies. Sometimes the combination of certain features and shapes results in a new feature. For example, the size of the blank introduces the need to cut away the undesired material from around the part. Cutting is a manufacturing feature, which requires taking physical measures [2]. Therefore, features contain not only geometry, but also functional information for e.g. manufacture or assembly [7]. 3.1 Advantages of accurate design Manufacturing errors resulting from design are often noticed only when the first batch of the product is being manufactured. If the first batch is large, the error may produce considerable costs [2]. No changes should be made to a product without first considering their effect on manufacturability. The designer should thus stop to think about production as well as the product itself [2]. If the manufacturing volumes are great and changes to them are made gradually, the time spent on designing the product and its manufacture is rather short compared to the total manufacturing time of the products [2]. In conclusion, a great deal of time can be spent on design if the designing is efficient and reduces manufacturing costs. 3.2 Research objective The objective of this study is to research the use of feature classification and its applicability to the manufacture of a sheet metal part by cutting or punching. 4 CASE EXAMPLE Manufacturing features include the most common shapes in sheet metal parts, such as bending, marking and threads, as well as characteristics, such as size and material. Figure 1 shows the feature classification of sheet metal parts. The manufacturing feature class cutting is divided here into two sub-classes, edge and hole, opening. The edge and hole are seemingly similar features, but their manufacturing technologies may differ significantly. An edge can be cut with a guillotine shear or a corner punching machine. These processes cannot manufacture a hole or an opening in a sheet metal part. On the other hand, many processes for cutting a hole can be used to cut an edge. These methods include e.g. laser cutting and nibbling.. Figure 1: The manufacturing feature "cutting" is examined here. Cutting is divided into two sub-features, cutting an edge and cutting a hole or opening. 4.1 Case part In this study, we will use a fictitious sheet metal part as an example, Figure 2. The part has 5 edges and 3 holes, which are countersinked (a cone-like embedding is made for the screw-head). The part is made of an aluminium alloy 2 mm thick. With regard to the material, the part can be considered a regular, uncoated sheet metal product

3 In the second stage, it is established whether the openings are part of a group of whether they are individual ones. The purpose of a hole group is to ascertain whether only one punch is required with a so-called serial tool. The diameter of the openings and especially the distances between them must be defined accurately. If this is not done, it may be difficult to distinguish a set of holes close to each other from an actual group of holes. The next step is to study the geometry or other information entered into the model to establish the shape and accuracy requirements of the holes and edges. An opening in a sheet metal part may be e.g. a cutout with no specific requirements in terms of size or tolerances. Figure 2: The model part used in this study. The part has 5 edges and 3 countersinked holes. The raw material is a 2 mm uncoated aluminium alloy. The case part is spread out in Figure 3. The countersinking is performed on the same surface. The spread-out figure is composed of simple shapes, such as right, 90-degree angles and circular arches. The figure shows that the part has two lightenings, which are shaped like a rectangle. Also a tear-shaped or circular lightening could be used. Shape identification is needed for the use of standard tools. Punching tools should be used as much as possible because punching is often the least expensive way to manufacture sheet metal parts. Identifying the shape and accuracy are at the same level in feature thinking because the manufacturing technology of the shape has a definitive impact on the accuracy achieved. For instance, this is the case for the abovementioned cutouts, which the designer has dimensioned specifically as cutouts. They can be manufactured in a number of ways, but the least costly alternative may be punching. Punching can be performed with a single stroke or by nibbling, the first of which is the least expensive. A single stroke can be done with tools of different sizes. However, the size range for the cutouts must be determined so that the actual size of the opening does not deviate excessively from the design. When the part is designed, it is given a number of attributes in terms of geometry and other features. At this stage, these features are examined. They can include material, surface treatment, bending or any other aspect included in the feature classes in Figure 1. Figure 3: Case part spread out. 5 SELECTING A MANUFACTURING METHOD BASED ON MANUFACTURING FEATURES The case part can be designed with nearly any CAD system. The only requirement is a spread-out picture of the model that is saved in VRML format. This is the format needed to convert the model into X3D form. If the model is in X3D form, its features can be recognised for instance with the Pro-FMA software [8]. Features are identified with the help of two modules in the programme, which its developers [8] call the X3D edge recognize and X3D face recognize algorithms. Although the software is developed for assembly feature recognition, the above-mentioned algorithms can also be used to identify manufacturing features because the recognition processes are similar. 5.1 Stages of feature recognition Figure 4 presents the classification of manufacturing features applied in this study. Designing a part based on features starts by identifying the shape of the part and possible openings within its edges. At this point, mechanical and thermal methods are separated because certain requirements may exclude the use of any thermal methods. In such cases, thermal methods need not be included in the comparison. A similar approach is taken to examine the tolerances of the part and its conditions, which are divided into position and geometrical tolerances. Geometrical tolerances may refer to e.g. the alignment of the opening or edge. In addition, the diameter of each opening is compared to the thickness of the material. This helps to ensure, for example, that punching tools are not used to make very small holes. In the final stage, all of the features above are studied and compared to different manufacturing processes. At each stage, information is saved on aspects previously mentioned. When the process is selected, the information is compared to the process database. Based on this comparison, the system selects the processes suitable for manufacturing the part. There may be one possible manufacturing method or more. There is also the possibility that the system cannot recommend a suitable process based on the prevailing conditions. In such cases, the shape or feature rejected by the system should be redesigned. The system does not take costs into consideration at any point. Instead, it selects methods based purely on technical features and limitations. Cost comparison could be incorporated into the system, which would allow to compare the costs of each individual feature. This would enable a detailed optimization of the manufacturability of each feature with regard to costs.

4 Figure 4: Main stages of the process selection based on feature classes and features in cutting and punching sheet metal parts. After the process selection, a cost comparison can be carried out between the processes.

5 5.2 Processes for edges and holes and their parameters Cutting shapes and punching holes in a sheet metal product seems a very simple task. From a technical point of view this is true in many cases, but decisions involving the manufacturing technology and method applied can be difficult to make. For example, the size of the hole in proportion to the material thickness has a significant impact on the choice of technology. Likewise, tolerance requirements have an effect on the technology with which the desired accuracy can be achieved. Requirements concerning the quality of the shear surface also affect the choice of technology. If the hole cannot be cone-shaped, punching will most likely have to be excluded. If the part has no requirements apart from a certain shape and non-toleranced shapes, the simple choice between laser cutting and nibbling may be very difficult. No universal model for selection exists the choice is usually casedependent. In manufacturing holes, just as in cutting edges, the effect of tolerances and surface treatment must be taken into consideration. With thermal methods, the coating may be damaged. The coating may then need to be repaired in the damaged areas. Cutting and punching holes mechanically reduces the risk of damaging the surface. Due to the shape of the part, for instance a corner punching machine could be an alternative to laser or nibbling. However, in such cases curves must be cut with a laser or a tool with exactly the right rotation diameter. In theory, it is also possible to use a straight tool, but the result is not necessarily very good. Another problem may be deciding when to change the tool manually. This question is often thought to be related to batch size, but it can also be posed for individually manufacture parts with certain quality requirements. This is related to questions on the operating time of the machine, i.e. how long the process takes overall and how much individual process stages cost. In other words, is it less costly to use a slower process or stage in order to achieve the most economical solution overall? 6 CONCLUSIONS From the simple observation that behind each shape of a sheet metal product lies a restricted group of machines, we can deduce that sheet metal shapes can be classified into manufacturing features. Thus, each shape or feature corresponds to a feature class or feature. By preparing rules and restrictions for the feature classes and features, the design of sheet metal products can be made more manufacturing friendly. This study focuses on feature classification in cutting and punching holes in a sheet metal part. The classification of cutting and punching is a seven-stage process. Each stage involves making a number of choices regarding the product. These choices should be made based on shapes and features created in the design stage. It is thus a step-by-step process towards the selection of a manufacturing method. All conditions involved are finally compared to the possibilities of the processes. Based on the comparison, certain manufacturing processes are proposed to the user for manufacturing the part in question. At this point, the choice is based solely on technological aspects. Cost factors, which play an important role in the process selection, can be involved in the comparison after that. The grounds for rejection and approval should be carefully examined, because the system is a good servant when it functions well but a bad one when it does not. It may lead to choosing the wrong solutions merely because the system recommends them. On the other hand, the system may be used very little or not at all if users consider its proposals irrational. There are many manufacturing methods for edges and holes in sheet metal parts. Each process has a number of variables and parameters that impact manufacturability. These variables, in turn, are affected by several features of the part, such as material or the location of bends and holes. Due to the above, the operation of the system and its functions and logic should be carefully prepared and their validity ensured from as many viewpoints as possible. 7 REFERENCES [1] Mäntylä, M., Nau, D., Shah, J., Challenges in Feature-Based Manufacturing Research. Communications of the ACM, volume 39, number 2, pp ISSN [2] van der Net. A.J Designing and Manufacturing Assemblies. Eindhoven University of Technology. Doctoral dissertation. ISBN [3] Boothroyd, G., Dewhurst, P., Winston Knight, W., Product Design for Manufacture and Assembly. Second Edition. New York. Marcel Dekker pp ISBN X. [4] Haftl, L Need to Share Files? DWF IT. File format solves CAD exchange problems. American Machinist. January. pp [5] McCollough, T Toward Automating CAM. Machine Design. 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