Unlike ceramics, glass and even wood, metals have the property of ductility. When a material is ductile it can be bent into an infinite variety of shapes. Take the copper flex in your computer power cord as an example: copper is a metal, so it’s solid at room temperatures, yet we can twist it around a finger without it breaking. Try doing that with a ceramic plate!
It’s this ductility that makes it possible to produce the metal products we see around us. From the body of our car to the outer housing of our refrigerator, we are surrounded by metal that has been bent into complex forms. So how is bending carried out?
The most basic form of bending is “wipe bending.” To picture wipe bending, imagine a metal sheet resting on a workbench. One side of the sheet overhangs the edge of the bench by some distance, in this example it could be 2 inches.
If the overhanging edge is pushed down with enough force, the sheet will lift up off the bench, and if viewed from the side, will probably appeared curved, but if we let it go the sheet will return to its flat shape. However, if we were to hold the sheet flat on the bench while pushing down on the overhang, the metal would bend around the edge of the bench. If we pushed down with enough force, and held the sheet in place, we would produce a 90 degree bend. Interestingly though, once we let go, the metal would spring back a little, so that the resulting bend would be less than 90 degrees.
What’s happening, at a molecular level, is that the metal on the outside of the bend, (the top surface in our simple example,) is being stretched while the metal on the inside of the bend is being compressed. It’s the ductility of the metal that allows this to happen.
Ductility depends on many factors but the most significant are the actual composition of the metal and its hardness. Copper for instance, is a soft metal and can be bent in almost limitless ways. Stainless steel however, as used in high-end refrigerators, is much harder and tends to resist bending. This resistance to bending shows up in three ways: it takes more force to bend the harder, less ductile, material, the metal tends to spring back more when the bending force is released, and there is greater chance of the metal cracking along the bend.
One of the simplest and most common metal bending machines is the press brake. A press brake stands perhaps six feet tall and has a wide opening about half way up. On the bottom side of the opening is a “die” which spans the width of the opening. Above it, mounted to a ram that moves vertically up and down, is a “punch” which also spans the width of the opening.
The punch will usually have a tapered form that will mate with a “Vee” running the length of the die. In operation a metal sheet is placed on the die and the punch is lowered down. The punch pushes the metal sheet into the Vee of the die, creating a bend in the metal.
Metal bending is a complex process with many variables and a complete treatment is beyond the scope of this introductory article. Those wishing to learn more could start with the link below:
http://www.custompartnet.com/wu/sheet-metal-forming#bending
