TPT March 2014

Article

Tracto-Technik GmbH & Co KG

The geometry of the bending components is critical for the choice of a bending process. The dimensions of the part have to be adaptable for the bending machine size. The geometry of the components can be described by the criteria bending angle, bending factor, intercept factor and torsion factor. They are important for the choice of the best process. The first criterion is the bending angle of the bending arc, which has three possible categories. Category 1 is from 0° to 180°. These angles can be reached by most processes and machines. Work pieces with an angle between 180° and 360° are defined as category 2. Greater bending angles are classified as category 3. In order to produce bending parts with bending angles of the categories 2 and 3, additional tools or machine measurements are necessary. Furthermore, the kinematics of the machine, the clearance and the free space should be taken into account in order to prevent a machine crash. The maximum strain the material experiences through the bending process is considered by the bending factor. The bending factor B is defined as the quotient of the average bending radius R and the outer diameter H (B = R / H) [7] . The technical limit of the bending factor is determined by the profile material and the profile cross-section. A minimum bending factor of B = 0.5 can be achieved geometrically. The bending factor is subdivided in three categories. Category 1 includes the production of bending radii, which are five times greater than the profile outside diameter. At the outer radius, there is a maximum elongation of 10 per cent. Therefore, almost all deformable materials can be used for category 1. This usually requires no separate assessment of the bending method to prevent wrinkles on the inner arc. Category 2 describes radii between two and five times the tube outside diameter. To obtain the cross-section, materials with sufficiently high uniform elongation must be used and measures are taken to support the cross-section. Therefore, for closed cross section geometries mandrels and inner fluid pressures are common methods for that. Bending factors below the two times outer diameter of the tube (category 3) call for special precautions to avoid geometrical differences in the inner arc. The inner contour of the tube should be supported more effectively. Furthermore, the choice of the material is limited strongly by the formability of the material. The newly defined intercept factor (figure 4) describes the complexity of the bending geometry. Rays start from the ends of a component. These rays surround the limits of 2D bending parts in one plane of the Cartesian coordinate system. Bending parts over multiple bending planes should have a pair of rays in each plane of the Cartesian coordinate system. With the exception of the angles which are upright to one another in an

Figure 1: Matrix for the choice of a bending process [5]

product, bending geometry and bending process is necessary. During the project the existing standards has been collected and evaluated in order to categorise semi-finished products by their bending-specific parameters. Next was the evaluation of typical bending geometries, which were likewise categorised in different quality categories. In the following, the definition of special criteria for the quality of tube and profile bending parts are described. Criteria for bending quality An important sub-ordinate target on the way to reach the new standard is the creation of criteria for different categories of bending work pieces regarding their quality. This quality standard was defined independently from the production process. The quality of the bending work pieces was first defined by a validation of the reached quality of the work pieces with the quality of the semi-finished part before the bending process. For this validation, knowledge from the universities and different bending companies was used . A bending component is defined by nine different criteria (see figure 2). The first four criteria describe the geometry of the bending part. With the help of the wall factor, the cross section and the wall thickness deviation, the semi-finished product – tube or profile – is defined. The criteria shape and positional tolerances and surface factor define the complexity of the work piece. Bending components can be sorted in four categories with a rising complexity in manufacturing progress.

Table 1: Criteria for bending parts [6] *1: Deformation by torsion the bending part about the axis of gravity

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March 2014