Unit Two Die Design,模具与数控专业英语,2.5 Bending Die Design,模具与数控专业英语,Bending is of the most common forming operations We merely have to look at the components in an automobile or an applianceor at a paper clip or a file cabinetto appreciate how many parts are shaped by bending Bending is used not only to form flanges，seams，and corrugations but also to impart stiffness to the part（by increasing its moment of inertia）,模具与数控专业英语,The terminology used in bending is shown in Figure 2-12 Note that，In bending，the outer fibers of the material are in tension，while the inner fibers are in compression Because of the Poissons ratio，the width of the part（bend length，L）in the outer region is smaller, and in the inner region it is larger than the original width This phenomenon may easily be observed by bending a rectangular rubber eraser.,模具与数控专业英语,Figure 2-12 Bending terminology,模具与数控专业英语,Minimum bend radii-vary for different metals, generally, different annealed metals can be bent to a radius equal to the thickness of the metal without cracking or weakening. As R/T decreases (the ratio of the bend radius to the thickness becomes smaller), the tensile strain at the outer fiber increases, and the material eventually cracks (Figure 2-13).,模具与数控专业英语,Figure 2-13 Poisson effect,模具与数控专业英语,The minimum bend radius for various materials is given in Table 2-2 and it is usually expressed in term of the thickness, such as 2T, 3T, 4T and so on.,模具与数控专业英语,Table 2-2 Minimum Bend Radius for Various Materials at Room Temperature,Tthickness of material,模具与数控专业英语,Bend allowance as shown in Figure 2-12 is the length of the neutral axis in the bend and is used to determine the blank length for a bent part However the position of the neutral axis depends on the radius and angle of bend（as described in texts on mechanics of materials） An approximate formula for the bend allowance，Lb is given by Lb = (R + kT ) (2-1),模具与数控专业英语,Where Lb bend allowance, in (mm); the bend angle ( in radians)，deg; T the sheet thickness，in (mm); R the inside radius of bend，in (mm); k0.33 when R is less than 2T and is 0.50 when R is more than 2T.,模具与数控专业英语,Bend methods are commonly made use of in press tool. Metal sheet or strip, supported by a V block (Figure 2-14a), is forced by a wedge-shaped punch into the block. This method, termed V bending, produces a bend having an included angle which may be acute, obtuse, or of 90. Friction between a spring-loaded knurled pin in the vee of a die and the part will prevent or reduce side creep of the part during its bending3.,模具与数控专业英语,(a) V die (b) Wiping dieFigure 2-14 Bending methods,模具与数控专业英语,Edge bending (Figure 2-14b) is cantilever loading of a beam. The bending punch forces the metal against the supporting die. The bend axis is parallel to the edge of the die. The workpiece is clamped to the die block by a spring-loaded pad before the punch contacts the workpiece to prevent its movement during downward travel of the punch.,模具与数控专业英语,Bending Force can be estimated by assuming that the process is one of simple bending of a rectangular beam The bending force in that case is a function of the strength of the material. The calculation of bending force is as follows:,模具与数控专业英语,P = (2-2)Where Pbending force, tons(for metric usage, multiply number of tons by 8.896 to obtain kilonewtons); Kdie opening factor: 1.20 for a die opening of 16 times metal thickness, 1.33 for an opening of eight times metal thickness; Llength of part, in; Sultimate tensile strength, tons per square in; Wwidth of V or U die, in; Tmetal thickness, in.,模具与数控专业英语,For U bending (channel bending) pressures will be approximately twice those required for V bending，edge bending requires about one-half those needed for V bending. Springback in that all materials have a finite modulus of elasticity, plastic deformation is followed, when bending pressure on metal is removed, by some elastic recovery (see Figure 2-15) .,模具与数控专业英语,In bending, this recovery is called springback. Generally speaking, such a springback varies in sheet from 0.5to 5, depending upon finite modulus of elasticity, modes of bending, clearance of die and so on, phosphor bronze may spring back from 10to 15.,模具与数控专业英语,Methods of reducing or eliminating springback in bending operations can be made according to the following operations, shown in Figure 2-16, and parts produced in bending die are also overbent through an angle equal to the springback angle with an undercut or relieved punch.,