If you’re a newbie to metal fabrication, here’s a refresher on the essentials of bending. Air bending, bottom bending and coining are the three main types employed by precision metal fabricators.

Coining

The term ‘coining’ comes from coin-making. To put Lincoln’s profile on a penny, machines using extremely high tonnage compressed a metal disc with enough force to make it conform to the image inscribed on the die set. Similarly, coining with a press brake involves using enough tonnage to conform the sheet metal to the exact angle of the punch and die.

In coining, the metal is more than just bent, it is actually thinned by the impact of the punch and die as it is compressed between them. The theory is that with enough tonnage, your sheet metal will bend to the precise angle of your tooling, so your tooling should be an equal match to the angle required.

Bottom bending

In bottom bending or ‘bottoming’, the punch and die are brought together so the material makes contact with the punch tip and the V-opening sidewalls. It differs from coining in that the punch and die don’t fully contact the metal, and tonnage isn’t enough used to actually imprint or thin the metal.

Because bottom bending uses less tonnage than coining, the material doesn’t entirely conform to the tooling’s bend angle. In fact, the metal experiences “springback”, naturally relaxing to a wider angle after bending. With bottom bending, to get a certain angle, you need tooling that has a slightly more acute angle to account for springback once the sheet is released. For example, you may need your punch and die to be at 88° to achieve a 90° finished form. Different materials and thicknesses result in different amounts of springback.

Air bending

With air bending, even less contact is made with than with bottom bending. The tooling only touches the material at the punch tip and die shoulders, so the actual tooling angle is relatively unimportant. The bend angle is determined by how far the punch descends into the die. The further the punch descends, the more acute the bend angle. Because the depth of stroke determines the bend angle, one set of tooling can deliver a range of bend angles. However, the bend angle cannot be equal to, or smaller than, the angle of your punch and die.

Since tonnage doesn’t produce the bend in air bending, you don’t need as much as with coining. And as with bottom bending, air bending will entail some degree of springback, so you may need to bend to a slightly more acute angle to get the desired bend.

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