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.
Get an 8×T suggestion instantly
The tonnage calculator recommends a V-die width automatically based on your material thickness.
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.
Die width chart
Recommended V-die opening, approximate inside bend radius, and rough minimum flange length by thickness, for standard air bending:
| Thickness T | V-die (8×T) | Inside radius (≈V/6) | Min. flange (≈0.5V + T) |
|---|---|---|---|
| 1.0mm | 8mm | 1.3mm | 5.0mm |
| 1.5mm | 12mm | 2.0mm | 7.5mm |
| 2.0mm | 16mm | 2.7mm | 10.0mm |
| 3.0mm | 24mm | 4.0mm | 15.0mm |
| 4.0mm | 32mm | 5.3mm | 20.0mm |
| 5.0mm | 40mm | 6.7mm | 25.0mm |
| 6.0mm | 48mm | 8.0mm | 30.0mm |
| 8.0mm | 64mm | 10.7mm | 40.0mm |
| 10.0mm | 80mm | 13.3mm | 50.0mm |
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.
Check the tonnage for your die choice
Enter your V-die width directly and see the resulting force before you commit tooling.
Frequently asked questions
What is the 8×T rule?
A rule of thumb for picking a V-die opening: multiply material thickness by 8. It balances bend quality against tonnage for standard air bending and is the usual starting point before adjusting for a specific job.
What happens if I use a narrower V-die?
Tonnage rises sharply since force is inversely proportional to die width, and the tighter radius raises the risk of cracking on the outside of the bend, particularly on less ductile materials.
What happens if I use a wider V-die?
Bend accuracy and repeatability tend to suffer, the inside radius opens up more than intended, and springback becomes larger and harder to predict.
How do I work out minimum flange length?
A common estimate is 0.5 × V-die width plus material thickness. Flanges shorter than that risk tipping into the die opening rather than bending cleanly against it.