This is the process of shaping thin sheets of metal (usually less than 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. In all sheet-forming processes, excluding shearing, the metal is subjected to primarily tensile or compressive stresses or both. 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.
There are certain basic considerations that are common to all sheet forming. Grain size of the metal is important in that too large a grain produces a rough appearance when formed, a condition known as orange peel. For general forming, an American Society for Testing and Materials (ASTM) no. 7 grain size (average grain diameter 32 |im) is recommended. Another type of surface irregularity observed in materials such as low carbon steel is the phenomenon of yield-point elongation that results in stretcher strains or Lueder’s bands, which are elongated depressions on the surface of the sheet. This is usually avoided by cold-rolling the original sheet with a reduction of only 1 to 2% (temper rolling). Since yield-point elongation reappears after some time, because of aging, the material should be formed within this time limit. Another defect is season cracking (stress cracking, stress corrosion cracking), which occurs when the formed part is in a corrosive environment for some time. The susceptibility of metals to season cracking depends on factors such as type of metal, degree of deformation, magnitude of residual stresses in the formed part, and environment.
Anisotropy or directionality of the sheet metal is also important because the behavior of the material depends on the direction of deformation. Anisotropy is of two kinds: one in the direction of the sheet plane, and the other in the thickness direction. These aspects are important, particularly in deep drawing.
Formability of sheet metals is of great interest, even though it is difficult to define this term because of the large number of variables involved. Failure in sheet forming usually occurs by localized necking or buckling or both, such as wrinkling or folding. For a simple tension-test specimen, the true (natural) necking strain is numerically equal to the strain-hardening exponent of the material: thus, for example, commercially pure annealed aluminum or common 304 stainless steel stretches more than cold-worked steel before it begins to neck. However, because of the complex stress systems in most forming operations, the maximum strain before necking is difficult to determine, although some theoretical solutions are available for rather simple geometries.
Considerable effort has been expended to simulate sheet-forming operations by simple tests. In addition to bend or tear tests, cupping tests have also been commonly used, such as the Swift, Olsen, and Erichsen tests. Although these tests are practical to perform and give some indication of the formability of the sheet metal, they generally cannot reproduce the exact conditions to be encountered in actual forming operations.
Stretch Forming
In this process the sheet metal is clamped between jaws and stretched over a form block. The process is used in the aerospace industry to form large panels with varying curvatures. Stretch forming has the advantages of low die cost, small residual stresses, and virtual elimination of wrinkles in the formed part.
Bending
This 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). The minimum bend radius, measured to the inside surface of the bend, is important and determines the limit at which the material cracks either on the outer surface of the bend or at the edges of the part. This radius, which is usually expressed in terms of multiples of the sheet thickness, depends on the ductility of the material, width of the part, and its edge conditions.
Springback in bending and other sheet-forming operations is due to the elastic recovery of the metal after it is deformed. Determination of springback is usually done in actual tests. Compensation for springback in practice is generally accomplished by overbending the part; adjustable tools are sometimes used for this purpose.
In addition to male and female dies used in most bending operations, the female die can be replaced by a rubber pad. In this way die cost is reduced and the bottom surface of the part is protected from scratches by a metal tool. 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. The process is economical for forming long sections in large quantities.
Rubber Forming
Although many sheet-forming processes are carried out in a press with male and female dies usually made of metal, there are four basic processes that utilize rubber to replace one of the dies. Rubber is a very effective material because of its flexibility and low compressibility. In addition, it is low in cost, is easy to fabricate into desired shapes, has a generally low wear rate, and also protects the workpiece surface from damage.
The simplest of these processes is the Guerin process. Auxiliary devices are also used in forming more complicated shapes. In the Ver-son-Wheelon process hydraulic pressure is confined in a rubber bag, the pressure being about five times greater than that in the Guerin process. For deeper draws the Marform process is used. This equipment is a packaged unit that can be installed easily into a hydraulic press. In deep drawing of critical parts the Hydroform process is quite suitable, where pressure in the dome is as high as 100 MPa. A particular advantage of this process is that the formed portions of the part travel with the punch, thus lowering tensile stresses, which can eventually cause failure.
Bulging of tubular components, such as coffee pots, is also carried out with the use of a rubber pad placed inside the workpiece; the part is then expanded into a split female die for easy removal.
Deep Drawing
A great variety of parts are formed by this process, the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry. Depending on many factors, the maximum ratio of blank diameter to punch diameter ranges from about 1.6 to 2.3.
This process has been extensively studied, and the results show that two important material properties for deep drawability are the strain-hardening exponent and the strain ratio (anisot-ropy ratio) of the metal.
Spinning
This 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. Parts can be as large as 6 m in diameter. The operation may be carried out at room temperature or higher for materials with low ductility or greater thickness. Success in manual spinning depends largely on the skill of the operator. The process can be economically competitive with drawing; if a part can be made by both processes, spinning may be more economical than drawing for small quantities.
In shear spinning (hydrospinning, floturn-ing) 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. The spinnability of a metal is related to its tensile reduction of area. For metals with a reduction of area of 50% or greater, it is possible to spin a flat blank to a cone of an included angle of 3° in one operation. 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.
Miscellaneous Processes
Many parts require one or more additional processes: some of these are described briefly here. 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 is the two faces of a coin.
Coining pressures are quite high, and control of lubrication is essential to bring out all the fine detail in a design. Shearing is separation of the material by the cutting action of a pair of sharp tools, similar to a pair of scissors. The clearance in shearing is important to obtaining a clean cut. A variety of operations based on shearing are punching, blanking, perforating, slitting, notching, and trimming.
Die materials used are cast alloys, die steels, and cemented carbides for high-production work. Nonmetallic materials such as rubber, plastics, and hardwood are also used as die materials. The selection of the proper lubricant depends on many factors, such as die and work-piece materials, and severity of the operation. A great variety of lubricants are commercially available, such as drawing compounds, fatty acids, mineral oils, and soap solutions.
Pressures in sheet-metal forming generally range between 7 and 55 MPa (normal to the plane of the sheet); most parts require about 10 MPa.
