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How to use it
Enter thickness, inside bend radius, angle and the two outside flange dimensions, measured to the theoretical sharp corner as they appear on the drawing. The tool returns the flat length, i.e. the flat sheet to cut, and the bend deduction to subtract from the sum of the flanges.
The K-factor can be estimated from the r/t ratio with the DIN 6935 formula or entered manually, if you know the value calibrated on your supplier's press brake.
Formulas used
The K-factor shows where the neutral axis lies — the fibre that neither stretches nor shortens — as a fraction of the thickness measured from the inner face. With tight radii the axis moves inwards (K near 0.3); with large radii it returns towards the mid-plane (K = 0.5).
Why the K-factor must be calibrated
The real value depends on the material and its condition, the die opening, the bending method (air bending, bottoming, coining) and even the rolling direction. For series parts it's worth bending a test piece on the machine that will produce the part and deriving K from the measurement. It's one of the points we follow in the industrialisation of sheet metal parts, together with minimum radii, bending sequences and nesting.
FAQ
What K-factor should I use if I have no data?
For mild steel air-bent with an inside radius equal to the thickness you often start from K = 0.33; for stainless steel from 0.38–0.42; for aluminium from 0.40–0.45. These are indicative values to be confirmed with a test piece.
Is the angle to enter the inside angle or the bend angle?
It's the bend angle, i.e. how far the flange is rotated: 90° for a square bend, 45° for a half bend. If the drawing shows the inside angle between the flanges, the bend angle is 180° minus the inside angle.
How is the flat length calculated with several bends?
You add up all the outside dimensions of the flanges and subtract one bend deduction for each bend. If the bends have the same radius, thickness and angle, the deduction is always the same.
What's the minimum bend radius?
It depends on material, thickness and rolling direction. For mild steel you can go down to a radius equal to the thickness; hardened aluminium alloys and high-strength steels need radii of 2–4 times the thickness to avoid cracks.