Machinery's Handbook, 31st Edition
1424
Bending Sheet Metal
Fig. 19. Two-Roll Bending When sheet metal is inserted between the two rolls, the urethane roll wraps the blank tightly against the steel roll. The urethane transmits applied force in all directions, as does hydraulic fluid in a hydroforming presses, exerting high pressure; since the urethane roll is driven, true curvature of the sheet metal is obtained. This radius remains uniform, whether the shape being formed is a complete circle or only a segment. Variations in diam- eter of the final workpiece are due to variations in and natural spring-back of the material. Since the workpiece is completely controlled, flatting and kinking are eliminated. In most applications, there is no need for pre-forming or cutting off blank ends; flat spots on leading and trailing edges are minimal: one to four material thicknesses, depending on the material. Drawing The drawing of metal, or deep drawing, is the process by which a punch is used to force sheet metal to flow between the surfaces of a punch and a die. Many products made from sheet metals are given the required shape by using a drawing operation. A blank is first cut from flat stock, and then a shell of cylindrical, conical or special shape is produced from this flat blank by means of one or more drawing dies. Most drawn parts are of cylindrical shape, but rectangular, square, and specialized shapes are sometimes produced. With this process, it is possible to get a final part, using minimal operations and generating minimal scrap, that can be assembled without further operations. Mechanics of Deep Drawing.— As the material is drawn into the die by the punch, it flows into a three-dimensional shape. The blank is held in place with a blank holder using a fixed force. High compressive stresses act upon the metal, which without the offsetting effect of a blank holder, would result in a severely wrinkled workpiece. Wrinkling is one of the major defects in deep drawing; it can damage the dies and adversely affect part assembly and function. The prediction and prevention of wrinkling is very important. There are a number of different analytical and experimental methods that can help to predict and prevent flange wrinkling, including finite element modeling (FEM). There are many important variables in the deep drawing process, but they can be classi fied as either material and friction factors, or tooling and equipment factors. Important material properties such as the strain-hardening coefficient ( n ) and normal anisotropy ( R ) affect deep-drawing operations. Friction and lubrication at the punch, die, and workpiece interfaces are very important in a successful deep drawing process. Unlike bending operations, in which metal is plastically deformed in a relatively small area, drawing operations impose plastic deformation over large areas, and stress states are different in different regions of the part. As a starting point, consider what appear to be three zones undergoing types of deformation: 1) The flat portion of the blank that has not yet entered the die cavity (the flange) 2) The portion of the blank that is in the die cavity (the wall) 3) The zone of contact between the punch and the blank (bottom) The radial tensile stress is due to the blank being pulled into the female die, and the com pressive stress, normal in the blank sheet, is due to the blank holder pressure. The punch transmits force F to the bottom of the cup, so the part of the blank that is formed into the
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