Bend Allowance:

            Sheet metal stretches minutely as it is being formed.  The greater the inside bend radius is, the less the stretching.  Material hardness and elasticity also have a lot to do with how much it stretches.  Therefore bend allowance is the dimensional adjustment required to allow for the forming of sheet metal.  Bend allowance and bend deduction formulas are used for air bending, different results will occur if coining or bottom bending the material.

 

Where  MT = Material Thickness

            BR = Inside Bend Radius

            A° = Angle from the Flat

 

Bend Allowance = (.0078 x MT + .01743 x BR) x A°

 

Using Bend Allowance:

 

To reach your developed length:  Dim 1 + BA + Dim 2 = developed length.

 

Bend Deduction:

            Bend deduction is the difference from the start of the radius to the apex of the angle times two minus the bend allowance.

 

Bend deduction = D+D-bend allowance

                              D = tan(A°/2) x (MT + BR)

 

 

The bend deduction for a 90° angle = 2(MT+BR) – Bend allowance

So, bend deduction = 2x(tan(A°/2) x (MT+BR)) – Bend allowance