Bending Guide

V-Die Selection: The 8×T Rule

The fastest way to pick a V-die opening that balances tonnage, bend quality and tooling wear — plus a die-width chart and minimum flange guidance.

The V-die is the bottom tool a press brake punch forms sheet metal into. Its opening width — measured across the top of the V — has a bigger effect on bend quality and required tonnage than almost any other setup choice. Pick it too narrow and force spikes while crack risk climbs; pick it too wide and the bend loses accuracy. The 8×T rule is the standard starting point most shops use before adjusting for the specifics of the job.

What the 8×T rule says

So 3mm mild steel gets a 24mm V-die, 6mm plate gets a 48mm die, and so on. It's a starting point rather than a hard law — thinner, more ductile materials can sometimes go narrower for a tighter radius, while thick or less ductile material may need a wider opening to avoid cracking.

V-die opening rules of thumb

For air bending, use these approximate starting ratios. Thin sheet, thicker plate and heavy plate are practical descriptions, not fixed thickness boundaries; material grade and tooling guidance take priority.

These rules do not apply automatically to bottoming or coining. A wider opening reduces force but increases the formed radius and minimum supported flange.

Die width chart

Recommended V-die opening, approximate inside bend radius, and rough minimum flange length by thickness, for standard air bending:

Treat the radius and flange columns as planning estimates — always confirm against your press brake's tooling chart, since actual die geometry varies by manufacturer.

Why die width changes tonnage so much

V sits in the denominator of the bending force formula, so tonnage is inversely proportional to it: halve the V-die width and you roughly double the required force. This is why a narrower-than-recommended die for a tight radius can quietly push a job well beyond what a machine was speced for. See how to calculate press brake tonnage for the full formula.

Narrower than 8×T: sharper corners, more force, more risk

Going narrower than the 8×T recommendation gets you a tighter inside radius, which is sometimes exactly what a drawing calls for. The trade-offs:

• Tonnage requirement rises sharply — sometimes enough to need a bigger machine than the job would otherwise require.
• Crack risk increases, especially on less ductile materials or bends running across the grain direction.
• Tooling wear accelerates, since the punch tip and die shoulders see higher contact stress.

Wider than 8×T: less force, less precision

Going wider reduces tonnage and tool wear, but:

• The inside radius opens up, which may not meet a drawing's tolerance.
• Springback becomes larger and less predictable, since the material has more room to move before contacting the die.
• Repeatability across a production run tends to suffer.

Minimum flange length

If the flange being formed is too short relative to the V-die opening, the material can tip or fold into the die gap instead of bending cleanly against it. A common planning rule is:

For the earlier 3mm / 24mm example, that's roughly 15mm — anything shorter and it's worth reconsidering the die choice or the part's flange design before it goes near the brake.

Frequently asked questions