Air forming and V-die selection


May 4, 2004


Air forming, bottom bending, and coining are metal forming methods. Air forming, the most common, is a three-point operation. The actual inside radius produced is based on a percentage of the V-die width, regardless of the sharp–radius relationship.

Selecting the correct die width can be confusing because of the different methods and the different bend types—sharp, radius, and profound. All of the possible combinations for producing a bend vary the inside bend radius. The outside radius is the same no matter what method and combination you use.

V-die Selection

Historically, V-die openings were determined by operator judgment, usually a range six to 12 times the material thickness, with eight times being the optimum width. So why not use that formula now? Because those values are valid only for a one-to-one relationship between the material thickness and the bend radius. Unfortunately, many projects involve more than one-to-one relationships, such as minimum bend radius.

Using the 0.100-in.-thick material example and the standard eight times formula for die selection produces an optimum die width of 0.800 in.; most likely, the die width used would be 0.750 in. Can this die width produce a 2.000-in. radius? Probably not.

Basing die width on the part's outside radius is a better way to achieve the desired radius. The die-to-workpiece relationship remains constant, regardless of the material thickness or bend radius relationship, for example, thick material with a small radius or a large radius in thin material.

The formula for calculating a proper V-die width based on the outside radius is:

Factor (Outside radius Sine 45)
Factor is 4.00 for sharp bends in material less than 0.125 in.
Factor is 4.85 for radius bends in material less than 0.125 in.
Factor is 5.00 for sharp bends in material greater than 0.125 in.
Factor is 5.85 for radius bends in material greater than 0.125 in.

By applying this formula to the outside radius (the sum of inside radius and the material thickness), you should be able to measure and achieve the required inside radius (even if air forming). However, if you don't have the perfect V die in-house, what then? Buy more tooling? Perhaps, but not necessarily. First, try applying the 20 percent rule.

The 20 percent rule

Simply stated, the 20 percent rule is: The inside radius produced is equal to a percentage of the V- die opening factored by material type.The material factors are:

20 percent for 304 stainless
15 percent for cold-rolled steel
12 percent for 5052-H32 aluminum
12 percent for hot-rolled steel

Going back to the 0.100-in.-thick material thickness example, assume that you are producing a 0.100 inside radius at 90 degrees in cold-rolled steel.

0.100-in material thickness
0.100-in. inside radius
Optimum die width = 0.685 in.
Bend deduction = 0.172 in.

Changing that V-die width to 1.000 in. would produce the following results:

0.100-in. material thickness
0.150-in. inside radius
Selected die = 1.000 in.
Bend deduction = 0.272 in.

Changing the die width from what was seven times V-die opening (0.100 in.) to ten times V-die opening (1.000 in.) resulted in a difference of 0.100 in total bend deduction.

Will this die width produce the required minimum bend radius? Maybe, maybe not. Continuing with the same example, where the 0.100-in.-thick material turns sharp at 0.063 in. and where the required radius is a minimum bend radius, it could be perceived on the shop floor as a 1/32-in.-radius (0.032 in., 0.83 mm). Although a 1/16-in. or 1/64-in. punch tip radius would be valid, either way the inside radius (Ir) still is defined by the V-die opening. The 0.685-in. optimum V die produces, with reasonable accuracy, the minimum bend radius of 63 percent, whereas the 1.000-in.V die produces the 0.150 in. inside radius.

So what does all this mean? A real possibility exists that inaccurate parts are being produced—especially when the old style six to 12 times V-die selection process is used. The six to 12 times method allows for any die width within that range, but it will not produce a consistent inside bend radius.

Any variation of V-die width in excess of the optimum V die will produce this effect to some degree, regardless of the radius-to-material-thickness relationship. However, the sharper the punch (even in a larger V-die opening), the greater the variations in bend angle and dimension because of the ditch created at the bend line.

When designing or engineering any part headed for a press brake operation, find out what methods and operations are being employed and the tooling sizes available. If the tool radius matches the required radius, and if that radius is attainable within the methodology, the inside radius will match the bend's geometry, resulting in a perfect part every time.


Steve Benson

2952 Doaks Ferry Road N.W.
Salem, OR 97301-4468
Phone: 503-399-7514
Fax: 503-399-7514
Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC, 2952 Doaks Ferry Road NW, Salem, OR 97301, Benson also conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country. For more information, visit, or call 888-394-4362. For more information on bending, check out Benson’s new book, The Art of Press Brake: the Digital Handbook for Precision Sheet Metal Fabrication, © 2014, available at

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