Machinery's Handbook, 31st Edition
Stretch Forming
1431
Stretch Forming Stretch forming is a metal forming process in which a blank of sheet metal is formed by the simultaneous application of tensile loads to the material over a die in order to form large contoured parts in the required shape. During stretch forming, the sheet blank is subjected to both elastic and plastic deforma tion . The most appropriate measure of formability for stretch forming is the strain- hardening exponent, or n value: (49) where k = a constant. c k n ε = A high value of n is desired if the sheet is to show good stretch formability. In assessing the formability of sheet metals while forming a workpiece, a technique called circle grade analysis (CGA) is used to construct a forming limit diagram of the sheet metal to be used. Two methods are used in stretch forming: simple stretch forming, also called the block method , and stretch-wrap forming, also called tangential stretch forming . Simple Stretch Forming.— In the simple stretch forming process, the sheet blank to be formed is clamped between two gripping jaws located on opposite ends, and the tool moves into the clamped sheet blank as shown in Fig. 22.
Form block
Fixed adjustable grippers
Blank Formed workpiece
F,v
a) Starting Position
b) Ending Position
Fig. 22. Simple Stretch Forming. The most common stretch presses are oriented vertically; the form die rests on a press table that can be raised to the sheet by a hydraulic ram. As the form die is driven into the sheet, the tensile forces increase and the sheet plastically deforms into a new shape. At the beginning of the process, the sheet blank first drapes itself around the form block, following its contours. Due to the large contact area between form block and blank, the frictional forces prevent a deformation of the sheet in this region. Stretch-Wrap Forming.— In this method also, a sheet blank is gripped from two opposite ends and stretched into the plastic region before being wrapped over a punch, so that the whole cross section of the material undergoes a uniform plastic deformation; then the die is brought down to complete the operation. The main difference from the simple stretch forming process is that both the form block and the gripping jaws are movable (Fig. 23). Stretch formed parts are typically large and possess large radial bends. The shapes that can be produced range from a simple curved surface to complex non-uniform cross sec tions. Stretch forming is capable of shaping parts with very high accuracy and smooth sur faces. Ductile materials are preferable, the most commonly used being aluminum, steel, and titanium. Typical stretch formed parts are large curved panels such as door panels in cars or wing panels on aircraft.
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