There are various sheet metal forming processes which may be broadly classified as
3. Stretch forming
Shearing is the mechanical cutting of materials without formation of chips or the use of burning or melting. In shearing operation there are two blades, upper blade or punch and the lower blade or die. Shearing involves the application of shearing forces of sufficient magnitude to rupture the metal in the plane of shear. It is nothing but cutting the flat martial forms from sheet, plate or strip. Some of the shearing processes are as following.
a) Blanking: Blanking and piercing are cutting operations which involves the same basic cutting action, the primary difference being one of definition. Blanking involves cutting the material to a closed contour by subjecting it to shear stresses between punch and die. In this process, the piece being punched out becomes the workpiece and any major burrs or undesirable features are scrap.
b) Piercing: In this process, punch-out is the scrap and the remaining strip is the workpiece.
c) Lancing: Lancing is an operation in which a single line cut is made partway across the work material. No material is removed so there is no scrap.
d) Shaving: Shaving is cutting operation that involves the quality and accuracy of blanked parts by removing a thin strip of metal along the edges. Only about 100 microns (0.004 inches) of material are removed by shaving.
e) Slitting: Slitting is the shearing operation used to cut rolls of sheets into several rolls of narrower width.
f) Dinking: Dinking is a modified shearing operation that is used to blank shapes from low-strength materials, such as rubber, fiber or cloth. The die functions like a cookie cutter.
Bending is the plastic deformation of metals about a linear axis with little or no change in surface area. Metal on the outside of bend stressed in tensioned beyond the elastic limit. Metal on the inside of the bend is compressed. It is very common forming process for changing sheet and plate into channel, drums, tanks, etc.
3. Stretch forming:
Stretch forming is used in place of simple bending to produce contoured or large radius of curvature parts by stretching sheet metal sheet, strips, or profile over shaped block form. Here tensile force is applied in such a way as to stretch the material over a tool or form block. Stretch forming is used most extensively in aircraft industry to make parts of large radius of curvature, frequently with double curvature.
Drawing is primarily a cold drawing process and refers to the forming of parts where plastic flow occurs over a curved axis. Sheet metal drawing is one of the most important of all cold-working operations because a wide range of shapes, from small cups to large automobile body panels, can be readily fabricated. Drawing may be performed with or without a reduction in thickness of the metal.
Sheet metal drawing can be classified into shallow and deep drawing. When depth of the product is less than its diameter the process is considered to be shallow drawing. When the depth is greater than the diameter, it is know as deep drawing.