Making the move to air bending calls for three steps. First, determine the radii you need to form in your material types and thicknesses. Second, choose the dies and punches to achieve those radii. And third, ensure your press brakes haven’t been irreversibly damaged after years of banging away and, well, giving it more cowbell.
Bottoming and coining are two separate bending methods. Bottoming requires clearance between your punch and die angle. Bottoming occurs when the punch radius is stamped into the part’s inside radius, and as the ram continues to apply pressure, the bend angle is forced to conform to the 90-degree V-die angle.
When bottoming, the punch nose radius should match the radius you ultimately achieve (though springback does still play a role; for more on this, see “The hows and whys of springback and springforward, archived on thefabricator.com), and the die angle determines the bend angle, usually 90 degrees.
Coining forces the punch nose into the material, penetrating the neutral axis. Technically, any radii may be coined, but traditionally coining has been used to establish a dead-sharp bend.
Coining uses a matched tool and die set. The punch nose radius determines your inside bend radius (which, again, is usually very small); you calculate the outside bend radius by adding the inside bend radius to the material thickness. The radius at the bottom of the die should match the outside radius of the part.
The reader from last month’s column wished to introduce air bending for good reason. Air bending is the method of choice these days. The punch descends into the die opening, and a naturally floated radius is formed as a percentage of the die width