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Sheet-metal forming
McGraw-Hill Science & Technology Encyclopedia: Sheet-metal forming
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The shaping of thin sheets of metal (usually less than ¼ in. or 6 mm) by applying pressure through male or female dies or both. Parts formed of sheet metal have such diverse geometries that it is difficult to classify them. Sheet forming is accomplished basically by processes such as stretching, bending, deep drawing,
embossing, bulging,
flanging, roll forming, and spinning. In most of these operations there are no intentional major changes in the thickness of the sheet metal.
See also Metal forming.
Stretch forming is a process in which the sheet metal is clamped between jaws and stretched over a form block. The process is used primarily in the aerospace industry to form large panels with varying curvatures.
Bending is one of the most common processes in sheet forming. The part may be bent not only along a straight line, but also along a curved path (stretching, flanging). In addition to male and female dies used in most bending operations, the female die can be replaced by a rubber pad (Fig. 1). The roll-forming process replaces the vertical motion of the dies by the rotary motion of rolls with various profiles. Each successive roll bends the strip a little further than the preceding roll.
Bending process with a rubber pad. (a) Before forming. (b) After forming.
While many sheet-forming processes are carried out in a press with male and female dies usually made of metal, there are some processes which utilize rubber to replace one of the dies. The simplest of these processes is the Guerin process (Fig. 2).
Guerin process, the simplest rubber forming process. (a) Before forming. (b) After forming.
A great variety of parts are formed by the deep-drawing process (Fig. 3), the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry.
Many parts require one or more additional processes. Embossing consists of forming a pattern on the sheet by shallow drawing. Coining consists of putting impressions on the surface by a process that is essentially forging, the best example being the two faces of a coin. Shearing is separation of the material by the cutting action of a pair of sharp tools, similar to a pair of
scissors.
See also Coining.
The spinning process forms parts with rotational symmetry over a mandrel with the use of a tool or roller. There are two basic types of spinning: conventional or manual spinning, and
shear spinning. The conventional spinning process forms the material over a rotating mandrel with little or no change in the thickness of the original blank. In shear spinning (hydrospin-ning, floturning) the
deformation is carried out with a roller in such a manner that the diameter of the original blank does not change but the thickness of the part decreases by an amount dependent on the mandrel angle. Shear spinning produces parts with various shapes (
conical, curvilinear, and also
tubular by tube spinning on a cylindrical mandrel) with good surface finish, close tolerances, and improved mechanical properties.
