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A few researchers have used computer-aided systems for the bending progressive dies. Li et al. [11] and Prof. Choi et al. [12] were probably among the first to develop such systems for bending progressive dies. J H Kim et al. [13, 14] developed a fuzzy set theory method for determining the sequence of the bending operations. They modified fuzzy rules and weight factors for the rules.
These systems are mostly used as an assistant for the progressive die designer. Simultaneous bends determination is a major problem in these systems. Thus, the number of the bending stations suggested by these systems is often more than the actual industrial parts. So the die layout suggested by these systems is usually modified manually by the designer.
3 Process planning
In a CAPP system for sheet metal components, the determination of the bending sequence is one of the main problems. If N is the number of bends in a given part, then the domain of possible sequences in principle is N! [7].However, this number is usually limited due to geometrical and technical constraints. In other words, the number of possible sequences depends on the shape of the component. A flow chart of the operations which are used for computeraided bending sequence determination is shown in Fig. 1.
3.1 Mother plane
Mother plane has a very important role in bending progressive die design. The mother plane is a fixed plane which stays without any rotations throughout the bending operations. All the rotating planes are called children planes. The rules for the determination of mother planes are as follows:
– A plane surrounded by other planes
– A plane located in the center of the part –
– The largest plane in the component
Figure 2 shows the mother plane for part 1; the mother plane is colored in this figure. When the determination of a mother plane is not clear from the conditions as stated above, it is determined by the minimum number of bends between a plane and the plane in the central plane [14].
3.2 Bending classification method
The classification technique is applied to the determination of simultaneous bends which can be performed in one station. The die designers use different rules to define simultaneous bends, because several parameters affect this procedure. According to experimental studies, many factors affect the determination of the simultaneous bends. However, the following rules can be summarized [15].
Rule 1 The bends that have bend lines along one line and whose bending directions are the same (up or down bending) can be performed in one station. Thus, they are said to be in one group. In Fig. 3, according to rule 1, bends B1and B2 are in one class. But bends B3 and B4 are not in one group, since their bending directions are not similar (B3 is down and B4 is up).
Rule 2 The bends that have parallel bending lines and their bending directions are the same (up or down bending) and are placed on opposite sides of the mother plane can be performed in one station and are said to be in one group if the number of the planes between them and the mother plane are equal. In Fig. 4, the directions of bends B1 and B2 are the same and they satisfy the rest of the conditions of rule 2, hence, they are said to be in one group.
Rule 3 Related bends. In some sheet metal components, two planes can be related through geometric or dimensional tolerances. To obtain the tolerance and to comply with the positioning errors, these bends should be (or better be) performed together. For example, in the part that is presented in Fig. 5, planes A and B have parallel tolerances.Thus, they should be performed together.
In the classification procedure, first, all the bends are pided according to their bending directions (feed direction or perpendicular to it). Then in each direction, the bends are classified. In other words, the bends which are parallel and perpendicular to the feed direction cannot be formed in similar groups.