The art of sheet metal bending.

Bending techniques and procedures play a key role in obtaining a perfect permanent sheet metal deformation by flexing it.

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Sheet metal bending: what is it?

Bending is a type of metal fabrication which, through mechanical actions, deforms sheet metal by applying a force that is higher than the metal's strength. This force bends the sheet metal beyond the yield point, deforming it plastically.

Bending sheet metal, especially when done by means of a press brake, is one the fastest, cheapest and most flexible ways to obtain prototypes and even small product lots: this is why it is frequently used both in light construction - from crankcases, metal furniture, household devices, indoor and outdoor furniture, to any type of machines, even in the aviation and aerospace sector - and in heavy construction, for structural elements for civil and industrial engineering, posts for the energy and telecommunication industries.

Press brakes work on a vertical axis with upper and lower tools, designed to bend sheet metal using one or both of the two main types of bending:

• coining
• air bending (3-point bending).

Air bending is divided into:

• partial air bending
• bottoming.

There is also an additional type of bend, which is called flattening or hemming.

Let's focus on them.

Coining.

When using this technique, the punch and die must be an equal match to the angle you want to achieve (usually 90°). By applying a high force, the punch is forced to "penetrate" into the sheet metal, pressing it against the internal surface of the die. In coining, the punch and the die are required to have the same angle, in order to allow "coining" the sheet metal eliminating springback.

This technique, accurate and perfectly repeatable, is particularly suitable to form 90° angles on thin sheet metal. One of the downsides to coining is that it is necessary to use different tools for each desired angle and a very high tonnage is required, so it cannot be performed on certain thicknesses.

Air bending (3-point bending).

In this type of bending, the material only touches the tool at three points:

• the punch tip
• the two die shoulders.

Depending on how stroke is managed, we can have two different types of air bending:

Partial air bending.

It allows to achieve a certain bend angle, according to how deep the punch goes into the die. The width of the V-groove determines both the amount of force needed to bend the sheet metal (with the same type and thickness of sheet metal, a wider V-groove means less resistance against the punch pressure) and the dimension of the internal bend radius.

Partial air bending requires a remarkably lower tonnage compared to other types of bending, allowing to also bend sheet metal with great thickness. It also allows to obtain different bend angles using the same tools.

However, in air bending it is not possible to accurately find the right punch penetration point to compensate springback, so, in order to ensure a high quality standard, a high-tech press brake is required.

Bottoming.

It is a special type of air bending, a sort of compromise between coining and proper air bending. When using this technique, normally only to obtain right-angle bends, the punch tip angle must be smaller than the die angle and the punch must penetrate into the die until the punch tip presses the sheet metal against the bottom of the die, causing it to flex, and forcing it to bend at an angle smaller than the die angle.

Even in bottoming, the width of the V-groove affects the amount of force needed to deform the sheet metal, but, as the material needs to be pressed against the bottom of the die, a higher amount of force is required in bottoming than in partial air bending.

In addition, because of the punch pressing on the internal surface of the bend, the internal bend radius is determined by the punch tip radius rather than the width of the v-groove.

In time, bottoming was the first alternative to coining, as it allowed to overcome the pressure limits of higher thicknesses. The desired angle and springback compensation are obtained "playing" on sheet metal flexing (negative springback) because the punch tip presses against the center of the angle obtained at the end of the stroke. Since it does not require a high degree of stroke control, this type of bending is still really frequent..

Other type: hemming of flattening.

The hemming technique is usually performed in a two-step process:

1. pre-bending at 26°-35° (obtained through air bending)
2. flattening process where the bend is completely or partially closed.

Hemming or flattening – performed with specific press brakes – allows to reinforce sheet metal.