SHEET METAL AND METAL FORMING Copyright Goodheart-Willcox Co., Inc. Chapter 17 Cold Forming Metal Sheet 281 17.2.5 Bulging Bulging, g g Figure 17-13, is sometimes referred to as entrapped rubber forming. It is used in the manufac- ture of many products. In bulging, rubber is used to transmit the pressure needed to expand the metal blank or tube against the die. Fluid pressure may be used to form the bulge when making more compli- cated pieces. 17.2.6 Stretch Forming Stretch forming is a technique in which a metal blank g is gripped on opposite edges with clamps and then lightly pulled. This forces the metal to wrap around a form block of the desired shape. See Figure 17-14. The piece is trimmed to final shape after the forming operation is completed. When stretch forming was first devised, the opera- tor had to continually adjust the machine and check the fit of the part to the die. Computer-controlled machines now do the work of numerous older machines, and all parts produced are uniform in size, shape, and thickness. 17.2.7 Bending In the bending operation, the surface area of the work g is not appreciably changed. The metal is merely bent into the desired shape. Types of conventional bending machines include the press brake and tube benders. Press brakes range in size from entry-level machines that are manually or CNC controlled, Figure 17-15, to large tandem press brakes, Figure 17-16. The large tandem press brakes are used in the aerospace, construction, lighting column, windmill, and defense industries. Start Finishshni Fi Form block Form block A B B B-B A-A Grip Work A B C Allegheny Ludlum Steel Corp. Figure 17-14. The stretch-forming process. A—Relative position of the work. Form block and clamps at the start and finish of a typical stretch-forming operation produce a raised rib. B—Typical shape produced by stretch forming over a form block. C—A stretch-formed shape is similar to the bottom of a canoe. The section will be riveted or welded to other sections to make the canoe. Goodheart-Willcox Publisher Figure 17-13. The bulging or entrapped rubber forming process. A rubber bladder is hydraulically expanded within a die. Hold down Knockout rod Punch Work being bulged Two-piece die (hinged) Base Rubber Die insert