Machinery's Handbook, 31st Edition
Bending Sheet Metal
1417
Bending One of the most common processes for sheet metal forming is bending, which is used to form pieces such as L, U, or V-profiles, and also to improve the stiffness of a piece by increasing its moment of inertia. Bending metal is a uniform straining process that plasti cally deforms the material and changes its shape. The material is stressed above the yield strength but below the ultimate tensile strength. The surface area of the material changes only in the bending zone. “Bending” usually refers to linear deformation about one axis. Bending may be performed by air bending, bottoming bending, or coining. Air Bending: Air bending is done with the punch touching the workpiece but not bottom ing it in the lower die. The profile of a die for air bending can have a right angle or an acute angle. The edges of the die with which the workpiece is in contact are rounded, and the radius of the punch will always be smaller than the bending radius. Bottoming Bending and Coining: Bottoming or coining bending is the process by which the punch and the workpiece bottom on the die. It is necessary to flatten the bottom bend area of the workpiece between the tip of the punch and bottom on the die in order to avoid springback. The tonnage required on this type of press is higher than in air bending. Inside Bend Radius.— Fig. 12 shows the terminology used in the bending process.
T
Bend allowance
Bend angle
Inside bend radius
Neutral axis Fig. 12. Schematic Illustration of Terminology Used in the Bending Process One of the most important factors influencing the quality of bent workpieces is the inside bend radius which must be within defined limits. Minimum Bend Radius: If the bend radius is less than R min given in Equation (30), partic ularly in harder materials, the material at the outside of the bend will tend to “orange peel.” If this orange peeling, or opening of the grain, is severe enough, the metal will fracture or crack off completely in extreme cases. The minimum bend radius, R min is given by the following formula: (30) where T = material thickness (inch or mm)and, r = percentage reduction in a tensile test for a given material (%). Maximum Bend Radius: If the bend radius is greater than R max given in Equation (31), the bend will be very hard to control and will spring back erratically. The amount of springback will worsen on thinner materials. When large radius bends are required, an allowance should always be made for this in the tolerance of the part. To achieve permanent plastic deformation in the outer fibers of the bent workpiece the maximum bend radius must be (31) where E = modulus of elasticity, lb/in 2 (N/mm 2 ); YS = yield strength, lb/in 2 (N/mm 2 ); and, T = thickness of material, inches (mm). Neutral Axis: When material is formed, the deformation in the inside fibers of the mate rial will compress during forming and the fibers of the material on the outside of the bend will expand. The material between these two regions remains neutral during forming and is referred to as the neutral axis of the material. The length of fibers along the neutral axis of the bend does not change during forming. This neutral axis is used when figuring the bend allowance for flat blank layouts. R YS TE 2 max # ^ h R T r 50 1 min = − ` j
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