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Bend Allowance
Bend Deduction, Setback, Bend Allowance, these terms
all across the country seem to have no consistent
meaning. They are all used to mean the same thing or
different things. I will define these terms by what is
usually accepted by most sheet metal mechanics that I
know. These terms may be used differently by your company
but for the sake of consistency in this manual here are
our definitions. By the way my favorite term so far is
fudge factor. That seems to be what we are really doing.
BEND ALLOWANCE:
Sheet metal stretches minutely as it is being formed. The
greater the inside bend radius is the less the
stretching. Also material hardness or elasticity has a
lot to do with how much it stretches. Therefore bend
allowance is that dimensional adjustment required to
allow for the forming of sheet metal. The mathematical
expression most normally used is based on the empirical
formula. Empirical means by trial and error. This is how
the formula was discovered. They kept adjusting the
numbers until they found a combination that consistently
and satisfactorily worked out. Bend allowance and
deduction formulas are used for air bending. Different
results will occur if coining or bottom bending the
material.
EMPIRICAL FORMULA:
Bend allowance = (.0078 x MT + .017453 x BR) x A°
MT = Material thickness
BR = bend radius to inside of material
A° = Angle from flat
all across the country seem to have no consistent
meaning. They are all used to mean the same thing or
different things. I will define these terms by what is
usually accepted by most sheet metal mechanics that I
know. These terms may be used differently by your company
but for the sake of consistency in this manual here are
our definitions. By the way my favorite term so far is
fudge factor. That seems to be what we are really doing.
BEND ALLOWANCE:
Sheet metal stretches minutely as it is being formed. The
greater the inside bend radius is the less the
stretching. Also material hardness or elasticity has a
lot to do with how much it stretches. Therefore bend
allowance is that dimensional adjustment required to
allow for the forming of sheet metal. The mathematical
expression most normally used is based on the empirical
formula. Empirical means by trial and error. This is how
the formula was discovered. They kept adjusting the
numbers until they found a combination that consistently
and satisfactorily worked out. Bend allowance and
deduction formulas are used for air bending. Different
results will occur if coining or bottom bending the
material.
EMPIRICAL FORMULA:
Bend allowance = (.0078 x MT + .017453 x BR) x A°
MT = Material thickness
BR = bend radius to inside of material
A° = Angle from flat
