How do I choose a V die opening?
Choose the die opening from the sheet thickness. For mild steel the usual starting point is V = 8t. A smaller opening such as V = 6t produces a tighter inner radius but needs more tonnage; a larger opening such as V = 12t reduces tonnage but increases the radius and the minimum flange length.
The table below applies the V = 8t rule. Tonnage values are for air bending mild steel of about 450 MPa tensile strength over a one metre bend length, and include no safety factor.
| Thickness t (mm) | Die opening V (mm) | Inner radius ≈0.16V (mm) | Min flange ≈V/2+t (mm) | Tonnage (t per m) |
|---|---|---|---|---|
| 0.5 | 4 | 0.6 | 2.5 | 4.1 |
| 1 | 8 | 1.3 | 5.0 | 8.1 |
| 1.5 | 12 | 1.9 | 7.5 | 12.2 |
| 2 | 16 | 2.6 | 10.0 | 16.3 |
| 3 | 24 | 3.8 | 15.0 | 24.4 |
| 4 | 32 | 5.1 | 20.0 | 32.6 |
| 5 | 40 | 6.4 | 25.0 | 40.7 |
| 6 | 48 | 7.7 | 30.0 | 48.9 |
| 8 | 64 | 10.2 | 40.0 | 65.2 |
| 10 | 80 | 12.8 | 50.0 | 81.4 |
| 12 | 96 | 15.4 | 60.0 | 97.7 |
| 16 | 128 | 20.5 | 80.0 | 130.3 |
| 20 | 160 | 25.6 | 100.0 | 162.9 |
How does material strength change the tonnage?
Bending force is proportional to tensile strength, so multiply the mild steel figure by the ratio of the material tensile strength to 450 MPa. Stainless steel 304 needs roughly 1.44 times the mild steel tonnage; aluminium 6061-T6 needs roughly 0.69 times.
| Material | Tensile strength (MPa) | Multiplier |
|---|---|---|
| Mild steel (S235 / 1018) | 450 | 1.00× |
| High-strength steel (S355) | 510 | 1.13× |
| Stainless steel 304 | 650 | 1.44× |
| Aluminium 6061-T6 | 310 | 0.69× |
What is the minimum flange length?
The minimum flange is about half the die opening plus the material thickness, because the flange has to rest on the die shoulders. A shorter flange can be produced with a narrower punch tip or a die with reduced shoulder width, but the die opening still sets the inner radius.
This is a rule of thumb, not a tolerance. Confirm the exact value against the die drawing and the punch tip width you intend to use, because the punch tip must also clear the upstand.
When should the die opening be larger than 8t?
Use a larger opening when the required tonnage exceeds the capacity of the machine or the tooling, when the material is high-strength or thick, or when a larger inner radius is acceptable. Increasing the opening is the most direct way to reduce bending force.
Use a smaller opening when the drawing requires a tight inner radius or a short flange, accepting the higher tonnage. Whichever direction you move, re-check the tonnage against the rated capacity of both the press and the tooling before running the job.
Frequently asked questions
These are the questions buyers ask most often about V die opening selection and bending tonnage.
- What is the V = 8t rule?
- It is a starting rule for selecting the die opening: the V opening should be about eight times the sheet thickness for mild steel. It gives a reasonable balance between inner radius and required tonnage for general work.
- How do I calculate press brake tonnage?
- Use F = 1.42 x tensile strength x bend length x thickness squared, divided by 1000 times the die opening, in kilonewtons. Divide by 9.81 to convert to tonnes. The constant assumes air bending, not bottoming.
- How much more tonnage does stainless steel need than mild steel?
- Stainless steel 304 at about 650 MPa needs roughly 1.44 times the tonnage of mild steel at 450 MPa, because bending force is proportional to tensile strength. Use the material multiplier table on this page.
- What is the smallest flange I can bend?
- As a rule of thumb about half the die opening plus the material thickness, because the flange must rest on the die shoulders. A narrower punch tip or a reduced shoulder width lets you form shorter flanges.
Not sure which die opening your job needs?
Send the drawing and the material specification. We will confirm the die opening, the radius and the tooling profile.
Email roger@sharktoolings.com