When we’re making sheet metal fabrications like enclosures, cabinets and work chests, we bend a lot of metal. Bends are made mostly on a press brake by one of two techniques, bottoming or air bending. Some customers are interested in which method we use. Others trust our fabrication expertise, and that’s okay too. Bending technique does make a difference to the fabrication though. Here’s an explanation of air bending vs bottom bending.

Press Brake Principles

A press brake machine has an upper and a lower tool. The lower tool is a v-shaped channel which the upper tool fits into. To make a bend the sheet metal piece is positioned between the upper and lower tools and up against a backstop. The backstop determines the distance of the bend from the edge of the sheet.

With the sheet in place the operator brings the upper tool down. As the sheet is pushed into the v-channel it deforms, elastically at first, (meaning it would spring back,) but as the deformation progresses the bend becomes permanent.

Bottom Bending

In this method the edge of the upper tool pushes the sheet down hard into the base of the v-channel. This traps the sheet between upper and lower tools, putting both in complete contact with the sheet. The bend radius formed on the sheet is the same the radius on the edge of the upper tool, or to put it another way, the tooling sets the bend radius. This is the way bending was done for generations, until air bending emerged.

The Air Bending Technique

The big difference from bottom bending sheet metal is that the material is not pushed all the way down to the base of the v-channel. The upper tool still pushes the sheet into the channel, and the sheet still bends along the edge of the tool. Unlike in bottoming though, the bend radius isn’t created by the shape of the tools. Instead, it’s determined mostly by how far the upper tool descends into the channel. The resulting radius is larger than produced by bottoming because the sheet isn’t so tightly constrained.

Why We’d Rather Air Bend

In the fabrication shop air bending has two big advantages. First, it’s easier on the press brake. Bottoming takes a lot of force; air bending needs a lot less. That means less wear and tear on the machine and longer tool life.

Second, we don’t need to keep changing tools for each bend. Instead, we can use one set of upper and lower tools to make almost every bend we need. (There are some exceptions of course.)

Bottoming Advantages

Air bending is better for our tools and machinery, and it saves time overall, but bottoming hasn’t gone away completely. Bottoming does two things for us: it results in a tighter corner radius – even equal to the sheet material thickness – and it’s more accurate. By accurate we mean a more consistent bend angle.

In air bending the sheet takes on a natural bend radius that’s determined by thickness, hardness and other material properties. There’s always some variation throughout a batch of sheet metal though, so about the best that air bending can achieve is +/- 1°.

The Customer Impact

Given the choice, we’ll air bend your sheet metal fabrication project. It’s better for our equipment and saves time on tool changes. However, if your prints call for tight internal bend radii or the assembly needs high angular precision, we’ll go back to bottoming.

Another way to look at this is, how do you go from bottoming to air bending? Quite simply, increase the bend radius and design for more angular variation. For more specific guidance, email for advice or ask to speak with one of our specialists.