5 types of sheet metal bending.

Sheet metal bending is a real art, based on expertise and hands-on experience. This article is going to focus on the 5 most frequent ways to perform it. The main types of bending are coining and air bending. Air bending is divided into partial air bending and bottoming. Finally, we are going to focus on hemming.

1 - Coining.

When using this technique, the punch and die angles must be equal 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 (which must have the smallest possible radius) and the die (which must have a V-width 6 times the sheet metal thickness) are required to have the same angle, in order to allow "coining" the sheet metal eliminating springback.

Pros: this technique, accurate and perfectly repeatable, is particularly suitable to form 90° angles on thin sheet metal.

Cons: 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 (usually over 1.5 mm).

2 - Air bending or 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 apply two different types of air bending: partial air bending or bottoming.

3 - Partial air bending.

It allows to achieve a certain bend angle, according to how deep the punch goes into the die. The amount of force required to bend the sheet metal depends on the width of the V-groove. With the same type and thickness of sheet metal, a wider V-groove means less resistance against the punch pressure. The width of the V-groove also determines the internal bend radius.

Pros: partial air bending requires a remarkably lower tonnage than 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.

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

4 - Bottoming.

Bottoming was the first alternative to coining, as it allowed to overcome the pressure limits of higher thicknesses. It is a special type of air bending, a kind of compromise between coining and actual air bending.

This technique, normally used to obtain 90° bends, requires a punch with a smaller angle than the die angle. 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, the punch pressing on the internal surface of the bend determines a smaller internal bend radius, thus reducing springback.

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.

Pros: more accurate bending compared to air bending.

Cons: as in coining, the amount of force required is higher than in air bending, so it is not possible to perform bottoming on all thicknesses.

5 - Hemming or flattening.

The hemming technique, which requires specific tooling, is used for different reasons, for example to create edges that are safe to handle, or to reinforce thin sheet metal.

It is usually performed in a two-step process:

• pre-bending creating an angle ranging from 35° to 26° (according to the sheet metal thickness) obtained with air bending process
• flattening process where the bend is completely or partially closed (hem).
  
  

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