Sheet Metal Processing - Article Example

On a stretch press, stretch forming utilizes a piece of sheet metal secured by gripping jaws along its edges. The sheet is stretched by pneumatic or hydraulic force gripping jaws that are attached to a carriage. A forming die which is a stretch form block is common tooling used and is a solid contoured piece that presses the sheet metal (Hosford, 2005). By driving the form die into the sheet, there is an increase in tensile forces that deform the sheet plastically into a new shape to plastic and elastic deformation.

By stretching a metal sheet complex shapes are produced using stretch forming equipment such as extrusion or plate on a form die. Compared to rolled or drawn parts, stretch-formed parts have better surface quality and shape control. Titanium parts for aerospace applications or aluminum parts for the automobile industry are produced using stretch-forming equipment. Moreover, sheet metal applications such as household appliances apply these stretch-formed parts.The stretch forming equipment is either longitudinal or transverse and has a die table, jaws, and hydraulic system (OCR Document, 2011).

While transverse equipment stretches along its width, the workpiece stretches by longitudinal equipment along its length. Stretch forming equipment can be interfaced to an integral front panel or console or a computer numeric control (CNC) where an operator program the radius of the jaw. Task 1c: HERF and other sheet metal forming processesHigh Energy Rate Forming (HERF) Processes are affected by the strain rates used. As strain rates increase, the flow stress also increases. Adiabatic heating increases with the temperature but makes it hard to form materials like Tungsten and Titanium alloys that require deformation under high strain rates (Hosford, 2005).

Compared to conventional processes, the energy of deformation is much higher and is applied for a very shorter time interval. In contrast with the conventional forming process, high particle velocities are produced with a corresponding large velocity of deformation. Under the extra fast application of force, metals tend to deform more readily to form large parts.

HERF Processes maintain tolerances, have high production rates and relatively lower die costs (Allwood & Shouler, 2007). However, it requires careful handling of source of energy, bigger dies to withstand high energy rates, and highly skilled personnel from design to execution. HERF processes are applied in forming of large plates (up to 25 mm thick) during ship construction and in bending thick pipes or tubes of up to 25 mm thick. Some of HERF forming processes are explosive, electro-hydraulic and electromagnetic.

Explosive forming uses gaseous mixture of explosives such as TNT, RDX, and Dynamite to replace a punch in conventional forming (Hosford, 2005). The key factors considered are cost of tooling, behavior of work material, safety considerations and the overall capital investment. Explosive forming can be the unconfined or confined type. Unconfined types have the die cavity evacuated to produce a pressure pulse of very high intensity as shown in the figure below. Figure 14: Explosive forming-Unconfined type A gas bubble expands spherically and then collapses as the metal is deformed into the die with a high velocity (120 m/s).

As the vacuum prevents adiabatic heating water acts as energy transfer medium. The process variables are affected by stand-off distance and the type and amount of explosive. The advantage of this forming process is that energy is transmitted effectively on the work while shock wave is efficiently transmitted through water. Moreover, thick and large parts are formed easily and have less chance of damage to work (Khamis, 2011). However, there is need for careful handling of explosives and to withstand shocks, dies must be larger and thicker.

These types are common in making of elliptical domes, radar dishes and ship building. In the confined system, the shock wave or pressure pulse produced indirectly contact the work piece to directly transfer energy without the need for a water medium (Allwood, 2008). Besides, the tube collapses into the die cavity and is used for flaring and bulging operations. Figure 15: Explosive forming-Confined type Confined type utilizes the entire shock wave front and is more efficient. On the contrary, is not only unsuitable for large and thick plates but also hazardous during die failure.

Electro hydraulic forming applies shock wave in the water medium produced between electrodes (Kalpakjian & Schmid, 2006). The work plate is deformed by the shock wave and collapses it into the die. Figure 16: Electro-hydraulic forming Although it is similar to explosive forming, a capacitor bank replaces the chemical explosive as a store of electrical energy. Again, it releases less energy released compared to explosive forming. This process forms thin plates, has better control of the pressure pulse and more suitable for small to medium work size (Kalpakjian & Schmid, 2006).

The process is limited by the need for vacuum and its suitability only for smaller works. It is used in thinner and small works apart from smaller cone and radar dish. Electromagnetic forming uses a capacitor bank surrounded by current carrying coils to produce opposing magnetic fields around a tubular work piece (Allwood, 2008). With the coil held firmly, the magnetic repelling force collapses the work piece into the die cavity which assumes die shape as shown in the figure below. Figure 17: Electro-magnetic forming Electro Magnetic Forming Process uses a coil that produces varying and opposing magnetic fields.

The process parameters are strength of the current, size of the capacitor bank, work material electrical conductivity and the work piece size (Merklein & Allwood, 2012). While suitable for small tubes, the process is safer, and makes it easier for operations like crimping, bending and collapsing. The process is not suitable for large work pieces and is applicable to electrically conducting materials only. This method is used for bulging thin tubes, bending of tubes into complex shapes and crimping of wires, tubes and coils.