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Three terms used for forming sheet metal. AIR   BENDING :   3   point   contact   of   metal.   Punch   tip   and   die   edges.   Air bending   is   what   most   bend   allowance   formulas   are   based   on.   When forming   mild   steel   the   inside   bend   radius   is   based   upon   bottom   die   V opening.   It   is   close   to   15.6   percent   of   the   die   opening.   Therefore   a 3/8 die opening would produce an inside bend radius of .059.           .375 x .156 = .0585 (Almost a 1/16 bend radius) On   most   aluminum   alloys   the   inside   bend   radius   would   be   determined   by the   punch   tip   radius   (i.e.   if   using   a   punch   tip   with   .06   radius   on .063 5052-H32 aluminum the bend radius will be close to .06). To   stay   in   the   air   bending   range   the   inside   bend   radius   needs   to   be   no less   than   about   80   percent   of   the   material   thickness.   Anything   less would   be   considered   bottom   bending.   For   example:   Let's   take   that   .375 die   opening   which   produces   a   .0585   bend   radius   and   try   to   apply   it   to .090   cold   roll   steel.   We   take   the   .0585   and   divide   it   by   the   material thickness to see what percentage of the material the radius is. .0585 ÷ 090 = .65 The   radius   produced   is   65   percent   of   the   material   thickness,   that would   put   us   into   the   bottom   bending   range.   We   would   have   to   go   to   a larger   V   opening   for   the   die.   To   find   out   what   size   V   opening   you   need for   a   3/32   or   .094   Bend   radius   just   take   the   radius   and   divide   it   by 15.6 percent. .094 ÷  156 = .6025 .602 rounding it off to a 5/8th V die would work just fine. BOTTOM   BENDING :   This   is   where   material   is   bent   to   conform   to   a   set   of dies   that   are   doing   the   forming.   Full   surface   contact   of   metal   to   dies will   occur.   Angles   and   inside   bend   radius   of   formed   sheet   metal   are determined by the punch and die COINING: This is where actual deformation of the material occurs. The sheet metal that is compressed between the punch and die is actually thinner than it was originally before the forming operation.
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