Machinery's Handbook, 31st Edition
Bending Sheet Metal
1423
where F = bending force (lb or N)
L = length of the workpiece (inch or mm) T = material thickness (inch or mm) UTS = ultimate tensile strength of the material (lb/in 2 or MPa) W = die opening (inch or mm)
Three-Roll Bending.— Many curved sheet metal parts, such as rings, cylinders, trun- cated cones, or segments of these shapes, are impractical to produce by press forming. Such parts are best produced by a process called roll bending. In this process, the work- piece is produced from a flat blank by passing it between three staggered rolls. Depend - ing upon such variables as the composition of the work material, machine capability, or workpiece size, the shape may be formed in a single pass or a series of passes, with roll adjustments needed after each pass. Fig. 18 illustrates the basic setup for three-roll bend- ing on pyramid-type machines.
Fig. 18. Three-Roll Bending To achieve permanent deformation in the outside and inside fibers of the material, the following relationship must apply: (37) where D = outer diameter of the workpiece, inch (mm) T = material thickness, inch (mm) E = modulus of elasticity, lb/in 2 (MPa) YS = yield stress, lb/in 2 (MPa) Three-roll bending is one of the bending operations most used in industrial manu facturing. With only one set of rolls and by setting only one axis, it is possible to achieve different radii or a distribution of radii along the profile length within the bending plane. In addition to the high flexibility and thus economy of this process, it can achieve high quality standards, too, if suitable measures are used. Two-Roll Bending.— Traditionally, roll-bending machines have used three rolls to form the curve. As shown in Fig. 18, the blank passes between or is supported by a third roll, whose position can be adjusted to control the bend. The two-roll bending process shown in Fig. 19 is an alternate method that often is the best choice for high-volume production. Soft to hard materials can be used, thin-gauge material can be rolled to reasonably small diameters, and round workpieces can be made even if the blank has cutouts or holes. Two-roll bending machines have two basic frame designs: vertical and horizontal. Major parts of both designs include one urethane-covered roll, another steel roll, and an overarm that can be opened and closed to facilitate removal of a finished workpieces. On horizontal machines, the lower urethane-covered roll deforms under the pressure of the upper steel roll to impact the desired shape and size to the blank in a single pass. The diameter of the rolled workpiece is controlled by the diameter of the upper steel roll; a slip-on tube is used to change the desired diameter. The operation of vertical roll-bending machines is basically the same as that of the horizontal one, except that workpieces are loaded and unloaded vertically. T D YS E 1 1 +
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