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Actual Bend Tests
This   is   the   most   accurate   way   of   determining   a   bend deduction.   For   example   bend   allowance   formulas   are   not   very accurate   if   using   them   on   thicker   materials   with   extremely small   bend   radii.   Remember   you   have   to   stay   in   the   air   bending range for these formulas. Because   bend   allowance   formulas   are   only   theoretical   they may   be   slightly   different   from   actual   results.   With   today's tolerances   slightly   may   be   too   much.   When   in   doubt   or   if tolerances   are   very   critical   (i.e.   ±.005)   it   is   best   to   pre- bend   a   piece   of   material   with   the   exact   brake   tooling   you expect   to   use   to   find   out   what   the   bend    deduction    will   be.   For Example:   Take   a   piece   of   material   that   is   exactly   2   inches wide.   Place   your   tooling   in   the   brake   and   bend   some   samples until   you   get   the   exact   angle   and   radius   you   need.   Now   bend your   2   inch   piece   and   then   measure   the   two   flanges,   outside   to outside,   and   add   them   together   then   subtract   your   2   inches. This   is   your   Bend   Deduction.   To   be   on   the   safe   side   if   bending thick   materials   with   very   small   or   very   large   radii,   exotic materials,    very    soft    (malleable    or    ductile),    very    hard (brittle)   materials   or   extremely   close   tolerances   then   you should   probably   use   the   actual   bend/test   method .   After   you have   done   this   for   a   particular   material   and   radius   keep   a   log of   these   bend   deduction   values   for   your   shop   so   you   don't   have to   go   through   this   again.   This   value   will   pretty   much   remain constant. The log can look something like this: COLD ROLL STEEL 90 Degree Bends