How to Select the Right V-Die Opening: The 8×T Rule Explained

Choosing the correct bottom V-die is one of the most important decisions in sheet-metal bending. Get it wrong and you fight springback, cracking and inconsistent angles all day.

The Rule of Thumb

For air bending, size the V-opening at roughly 6× the material thickness for thin sheet (up to ~3 mm) and up to 12× for thick plate (over ~10 mm). A widely used middle-ground default is 8× thickness. So for 2 mm sheet, a 12–16 mm V is a sound starting point.

Why It Matters

• Inside radius: A wider V produces a larger inside radius. As a guide, the natural air-formed radius is roughly 16% of the V-opening.
• Bend force: A narrower V dramatically increases required tonnage; doubling the V roughly halves the force.
• Springback: Wider openings tend to increase springback, which you compensate by over-bending.

Practical Tips

Always confirm your die can handle the calculated tonnage, keep the shoulder radii in good condition to avoid marking, and when in doubt, start wider and step down. A 4-way die block or an adjustable roller die lets you dial in the ideal opening without a rack full of tools.