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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