SHEET METAL AND METAL FORMING Copyright Goodheart-Willcox Co., Inc. Chapter 17 Cold Forming Metal Sheet 285 Explosive Forming Explosive forming uses a high-energy pressure pulse g from an explosive charge to form the metal. The pressure pulse can be generated by a chemical explo- sive or electrical discharge in a fluid to force the metal against the walls of a die, Figure 17-22. The liquid environment rounds off the pressure pulse generated by the detonation. This helps ensure that forces are equally distributed across the surface of the material. To prepare for explosive forming, a preform is made to provide the contours of the finished piece. The preform is placed in the die, the tank is filled with water, and an explosive charge is suspended in the water (Refer to Figure 17-22). A large ring is clamped over the outer edge of the work to ensure the seal necessary to draw a vacuum between the work and the die. The vacuum is necessary to prevent an air cushion from developing, because an air cushion would pre- vent the metal from seating in the die and assuming its proper shape. The amount of explosive used and the distance between the preform and the explosive are critical. If the explosive is too close or too strong, the pressure pulse could perforate the metal. If the explosive is too far away or too weak, the preform would not be forced against the die. One of the greatest advantages of explosive form- ing is that it requires less lead time than many other forming operations. (Lead time is the time needed to get the part into production and produce the first piece.) The longer the lead time, the longer it is before the part can be put into production. This usually leads to higher expenses. An example of explosive forming is the production of the skin for a space shuttle engine, Figure 17-23. Usually a component like this is made from several dif- ferent parts that must then be assembled. This requires more lead time and may also negatively impact the weight and quality of the component. In HERF, the metal is shaped in microseconds using pressure generated by the sudden application of large amounts of energy. HERF usually does not require expensive machinery, and there appears to be no limit on the size of sections that can be formed. Three common HERF processes are explosive forming, electrohydraulic forming, and electromagnetic forming. Goodheart-Willcox Publisher Figure 17-21. Many metals tend to spring back to near their original shape after being formed by conventional means. A—Flat sheet metal blank ready for forming. B—Metal is formed between dies. C—Metal tries to return to its original shape when the male die is removed or pressure is released. A B C When pressure is released, metal springs back Distance very critical Explosive charge Water Die Holding ring To vacuum pump Clamping bolts Metal preform Vacuum Goodheart-Willcox Publisher Figure 17-22. This diagram illustrates the principle of the explosive forming process. A small amount of low-cost explosives, in some applications, will do the work of a press that costs hundreds of thousands of dollars.