Sheet metal forming operations are usually associated with conventional press-working which include cutting the blanks from sheet metal and then bending or forming the blanks or drawing them into desired shapes. Sheet metal forming processes are generally used on workpieces having high ratio of surface area to thickness. Unlike the processes such as forging and extrusion or rolling, the thickness of the workpiece metal in sheet metal forming operations is generally prevented from being reduced to avoid necking or tearing of the metal. When the thickness of the rolled metal is less than 6 mm, it is called sheet metal and above that, plate. For contour roll forming for making channels, panels, door and picture frames, pipes and tubings, sheet metal coils are available in thickness varying from 0.125 mm (40 SWG) to 6 mm (3 SWG). Steel almirah and cabinets are made from sheet metal having thickness 0.559 mm (24 SWG) to 1.219 mm (18 SWG). Sheets are available for presswork either in the form of flat pieces or as strip in coils.
Sheet metal forming operations include shearing, blanking, punching, bending, stamping, drawing or deep drawing, embossing, spinning, roll forming, etc. These are described in the following. Examples of sheet metal formed products include file cabinets, almirahs, car bodies, beverage cans, kitchen utensils, hopper, cannisters, pipes, etc. Metals for sheet metal working should have sufficient ductility to avoid breaking or cracking during operation. For operations such as deep drawing carried out in several steps (or draws), intermediate annealing in between the draws may be necessary to eliminate the effects of strain hardening and to ensure enough ductility of the metal for further cold-working. To avoid rupture, the sheet metal is usually bent across grains between 45 and 90° .
The mutually perpendicular bends in sheet metal products should be made 45° to the grain orientation (as at (a)] to avoid cracking of the metal when bend is made parallel to the grain alignment [as at (b)].
When necessary to bend with the grain, a larger radius is required for a bend. Blanks with rough edges (as sheared on machines) may crack at the edge during forming and hence edges of the blanks should be ground smooth. Work hardening steels, such as stainless steels, limit the amount of drawing and bending that may be performed in one operation on the conventional presses. High energy forming and high speed presses may be of help in this regard for effective forming of work hardening metals over a larger range of sizes.
For large-volume production, automatic presses with coil stock and roll feeders are required. These are capable of handling metals up to 1 mm thickness on routine basis. Even thicker coil stock can be handled with the use of roll straightners.
Metals Used for Sheet Metal Forming (Press-working)
- Low carbon steel is the most commonly used metal because of its good formability, economy and weldability. Steels with carbon contents 0.05 to 0.08. and manganese 0.25 to 0.50 are suitable for deep drawing.
- Aluminium (commercially pure and its alloys, aluminium bronze)
- Copper and copper-nickel alloy (constantan or monel type)
- Monel metal
- Stainless steel (low carbon for drawing purposes).
For more information on metal sheets.
Lubrication in Press-working
Lubrication in press-working operations reduces friction between the workpiece and the die, reduces press tonnage required, increases die life and gives a more uniform pressed product free from abrasions and scratches. Lubricants may be in the form of oil or water soluble. Deep drawing or drawing of heavy gages of steel sheet needs viscous lubricant such as made from white lead and mineral oil. Shallow draws may be lubricated with light grease or mineral oils. Copper and its alloys are lubricated with water soluble mixtures such as soap chips and hot water.
See More: Tube Drawing
See More: Wire Drawing
See More: Roll Design