Guides — Die Selection

Picking a V-die for air bending

By Jon Chacon · October 22, 2026

In air bending the part never touches the bottom of the die. It rides on the two shoulders while the punch pushes it into the opening — which means the die opening is doing a lot more than holding the sheet up. It sets your inside radius, it drives your tonnage, and it decides the shortest flange you can make.

Picking the opening is the setup decision. Here’s the rule most shops start from, and when to break it.

The 8× rule

For mild steel, start with a V-opening about eight times the material thickness. Quarter-inch plate gets a 2-inch die. Sixteenth-inch sheet gets a half-inch die. It’s a starting point, not a law — but it earned its place because it balances the four things the opening controls: force, radius, flange, and how hard the bend is on the material.

What the opening controls

• Inside radius. In air bending the radius comes off the opening, not the punch — figure roughly 16% of the V in mild steel. A 2-inch die makes about a 5/16″ radius whether the punch is sharp or round, as long as the punch is smaller than that natural radius.
• Tonnage. Wider opening, less force. Narrower opening, more — a lot more, fast. The math behind that is in How press brake tonnage is calculated.
• Minimum flange. The flange has to bridge the opening and stay on both shoulders while it bends. Working rule: the shortest flange you can count on is about 70% of the opening. Shorter than that and the flange tips into the die instead of bending.
• Marking and strain. A narrow opening concentrates the strain on the outside of the bend and presses harder on the surface. A wide one is gentler on both counts.

V-die opening chart

The 8× rule worked out for the common thicknesses: the starting opening, the inside radius that opening wants to make (about 16% of the V, from the section above), and the shortest flange you can count on (about 70% of the opening). These are starting points, not laws — the next section covers when to move off them.

When to break the rule

• Go 6× for thin material, or when the print calls for a radius tighter than the 8× opening wants to make. You pay for it in tonnage and die marking, and tight bends are where material cracks — go down there knowing why.
• Go 10–12× for thick plate and high-strength material, or when you need the tonnage down — a bigger opening is the cheapest tonnage reduction there is. Same when the surface finish matters: a wide die is easy on the face of the part. You pay with a bigger radius and a longer minimum flange.

A real example

Same bend as our tonnage write-up: quarter-inch A36, 61 inches long, 90 degrees. Our #37 die has a 1.960-inch opening — right at 8× for quarter inch. It ran the bend at 84.1 tons. The #35 die, also in the rack, has a 1.260-inch opening — about 5×. Same part in that die: 130.8 tons, at 25.7 tons per foot through the tooling.

The part didn’t change. The die did. That’s why “use the die that’s already in the machine” is a decision, not a shortcut. The press brake tonnage chart has the per-foot numbers for the standard openings if you want to run the same comparison for your thickness.

Questions we get asked