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Metal · Sheet bending

Bend allowance and flat length calculator

Flat length = the outside flange sizes minus one bend deduction per bend, where the bend deduction BD = 2 × tan(A/2) × (R + T) − A × (R + K × T). A U channel 50 × 100 × 50 mm in 2 mm sheet, bent to 90° on a 2 mm inside radius, with K = 0.325 from DIN 6935, takes off 3.84 mm per bend: the blank is 192.33 mm, not 200.

The sheet and the bend
The angle the sheet is bent through: 90° for a right angle. A part closed to an included angle of 135° is a 45° bend.
K-factor
The part
Every bend has the same angle and radius. An angle is two flanges, a U channel three, a hat section five.
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Flat length
— mm
Bend deduction
— mm
Bend allowance
— mm
K-factor
—

Weight of the blank? The metal weight calculator gives kg for the flat length times the width, in steel, stainless or aluminium.

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How this is calculated

A bend angle, in radians BA = A × (R + K × T) bend allowance OSSB = tan(A/2) × (R + T) outside setback BD = 2 × OSSB − BA bend deduction flat = Σ outside flanges − n × BD = Σ inside flanges − n × (2 × tan(A/2) × R − BA) K (DIN 6935) = (0.65 + 0.5 × lg(R/T)) / 2 for R/T ≤ 5; 0.5 above

Check by hand: 1 mm sheet, 1 mm inside radius, 90°. DIN 6935 gives K = 0.325; BA = π/2 × (1 + 0.325) = 2.081 mm; OSSB = 2 mm; BD = 4 − 2.081 = 1.919 mm. The bend lines in the table are the centres of the bend zones, measured from the first edge of the blank.

What this page does not do

Next, usually

Metal weight calculatorThe weight of the blank, of a sheet, or of any profile per metre. Linear cutting optimizerProfiles and bars cut from stock lengths with the fewest bars. All metal toolsThe rest of the metal and profile collection.