Press Brake V-Die Opening and Minimum Flange Calculator
Choosing the right V-die opening controls bend radius, required tonnage and the shortest flange you can form. This calculator applies the common multiple-of-thickness rule, then estimates the air bend radius and minimum flange for your part.
Your numbers
Common shop rule: 6 to 8 times thickness for thin sheet, 10 to 12 times for thicker plate.
Inside radius as a fraction of die opening. About 0.15 to 0.17 for mild steel; confirm for your material.
Minimum flange as a multiple of die opening. About 0.7 is a common rule of thumb.
Measured from the bend line to the edge of the flange.
Your results
These are shop rules of thumb. Use the die chart from your press brake and tooling supplier for tonnage limits and exact minimum flange values.
V-die opening and minimum flange vs thickness
Sweeps Material thickness from 0.5 to 10, holding your other inputs fixed.
Tooling data into your quotes
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How to use it
Enter the material thickness and select a die multiplier. For mild steel up to about 3 mm, 8 times thickness is a widely used starting point. Thinner sheet allows a narrower opening and a tighter radius, while thick plate needs a wider opening to avoid excessive tonnage and cracking. Enter your planned flange length to see the margin against the estimated minimum. A negative margin means you should redesign the flange, use a narrower die, or plan a different forming method such as a gooseneck punch or a hem die.
The rules of thumb used
In air bending the inside radius is set by the die opening, not the punch tip, and lands at roughly 15 to 17 percent of the opening for mild steel. The minimum flange is commonly taken as about 0.7 times the die opening, because the sheet must bridge the shoulders of the die. Higher strength steel, stainless and aluminium alloys differ, so adjust the two factors after test bends. Results here are estimates for planning and quoting, not a replacement for the tooling manufacturer chart.
- V = multiplier x thickness
- Inside radius is about 0.16 x V in mild steel air bending
- Minimum flange is about 0.7 x V
Reading the result and avoiding mistakes
At 3 mm thickness with an 8 x die the opening is 24 mm, the radius is about 3.8 mm and the minimum flange is about 17 mm. A 15 mm flange then shows a negative margin. Typical mistakes are designing a tight inside radius that the die cannot produce, ignoring the effect of grain direction, and forgetting that a wider die lowers tonnage but increases the minimum flange. Record the die you actually used on the job so the CNC program and the quote stay aligned.
Frequently Asked Questions
Why is 8 times thickness so common?
It balances tonnage, achievable radius and minimum flange for mild steel in the 1 to 3 mm range. Narrower dies raise tonnage and risk marking; wider dies increase radius and the minimum flange length.
What happens if the flange is shorter than the minimum?
The sheet may fall into the die opening, bend unevenly, or lose its position. Options include a narrower die, a special tool, bending before adding other features, or changing the design.
Does stainless need a different multiplier?
Stainless springs back more and is harder, so many shops use a slightly wider opening or add tonnage margin. Check the tooling chart and run a test bend.
Is the radius estimate exact?
No. It is a first estimate. Actual radius depends on material strength, thickness variation, punch nose, and bending method. Measure a sample part.
We can pull tooling and bend data into your quoting and routing process so engineering notes, die choices and setup times stay consistent across jobs.
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