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Press Brake Fundamentals: The Three Types of Bending

Can you easily rattle off the difference between air bending, bottom bending and coining? If so, this article isn’t for you. If you’re a newbie to metal fabrication or are simply looking for a refresher on the essentials of bending, read on.

Air bending, bottom bending and coining are the three types of bending most often employed by precision metal fabricators. This article covers the basic definitions of each and the differences between the three.

Coining

The term “coining” comes from coin making. In order to put the Lincoln profile on a penny, machines using extremely high tonnage compress a metal disc with enough force to make the metal conform to the image inscribed on the die set.

 In the same vein, “coining” with a press brake (Fig. 1a) involves using enough tonnage to conform the sheet metal to the exact angle of the punch and die being used. In coining, the sheet metal is more than just bent, it is actually thinned by the impact of the punch and die, as it is compressed between them along the bending surfaces (Fig. 1b).

The theory behind coining 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 you want.

Fig. 1a                                                          Fig. 1b

Bottom Bending

                                                                               Fig. 2a                                                   Fig. 2b

In bottom bending or “bottoming”
(Fig. 2a) the punch and die are brought together so that the material makes contact with the punch tip and the sidewalls of the V-opening (Fig. 2b). 

It differs from coining in that the punch and die don't make full contact with the metal, and there isn’t enough tonnage used to actually imprint, or thin the metal.

Because bottom bending uses less tonnage than coining, the material doesn’t entirely conform to the bend angle of the tooling. In fact, with bottoming, the metal experiences what’s referred to as “springback,” which is what happens when it relaxes to a wider angle after being bent. So, with bottom bending, in order to get a certain angle, you need to use tooling that has a slightly more acute angle in order to account for the springback that will naturally occur once the sheet metal 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

Fig. 3a                                             Fig. 3b

With air bending (Fig. 3a), even less contact is made with the metal than with bottom bending. The tooling only touches the material at three points: the punch tip and the die shoulders (Fig. 3b). For this reason, the actual angle of the tooling is relatively unimportant. The factor that determines the bend angle is how far the punch descends into the die. The further the punch descends, the more acute the bend angle. Because the depth of stroke (and not the tooling) determines the bend angle, one can get a whole range of bend angles from one set of tooling. Your bend angle is only limited in that you can’t get 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, there will be a certain amount of springback expected in air bending, so you will likely need to bend to a slightly more acute angle in order to get the final bend you are looking for. 


And that, ladies and gentlemen, is a brief overview of the three types of bending. If you’ve never had these basics laid out for you before, we hope you’ve found this a helpful bit of foundational knowledge. Wilson Tool is here to assist you no matter how basic or complex your challenge. We were all beginners once. Give us a call or email us with your bending or punching questions.

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