The V-die opening (also called the V-width or die width) is the distance across the top of the V-shaped groove in a press-brake bottom die. During air bending the sheet bridges this opening, supported only on the two shoulders of the V while the punch pushes down at the center. The width of that span is the single most influential variable in air bending: it sets the required tonnage, the resulting inside radius, the minimum flange that can be bent, and how much springback the operator must fight.
A wider opening lets the sheet bend over a longer span, so it bends more easily and requires far less force, but the unsupported metal forms a larger inside radius. A narrower opening does the opposite — it produces a tighter radius but demands much more tonnage and can mark the part. The relationship to force is captured in the standard air-bend tonnage estimate, F = (k × UTS × L × T²) / V, where L is bend length, T is thickness, UTS is tensile strength, k is a constant, and V is the die opening. Because V is in the denominator, doubling the opening roughly halves the required tonnage.
The die opening also fixes the natural inside radius of an air bend. As a rule of thumb the air-formed inside radius is on the order of 16% of the V-opening for mild steel (so a 12 mm V yields roughly a 2 mm inside radius). This is why you cannot freely choose radius and die independently in air bending — picking the die largely picks the radius.
The common sizing guideline is to make the V-opening about 6 to 8 times the material thickness for typical gauges, with the multiplier trending higher for thicker plate and for higher-strength materials that spring back more. For example, bending 2 mm steel, a shop would reach for roughly a 12–16 mm V-die. The opening also dictates the minimum flange length, since the sheet must reach across at least to the die shoulders — a flange shorter than about the half-width of the V cannot be formed reliably. Choosing the V-opening is therefore the first decision in setting up an air bend, balancing tonnage, radius, flange length, and springback against one another.
The die opening interacts with springback as well. A wider V produces a larger radius and a longer unsupported span, both of which leave more elastic recovery, so operators on wide dies generally overbend more to land on the target angle. The V also limits how tight a radius a given material can take without cracking: because the air-formed radius scales with the opening, achieving a very small inside radius forces a narrow V and the high tonnage that comes with it, and at some point the radius becomes too tight for the material's ductility and the outside fibers crack. Most brake tooling is supplied as multi-vee dies or as a graduated set of openings precisely so the operator can pick the V that balances all of these factors for the job at hand. In short, the V-die opening is the master variable of air bending — choose it well and tonnage, radius, minimum flange, and springback all fall into a workable range; choose it poorly and one of them inevitably goes out of bounds.