http://www.advantagefabricatedmetals.com/pressbrakebendallowance.html
http://www2.thefabricator.com/Articles/Fabricating_Exclusive.cfm?ID=356
http://www2.thefabricator.com/Articles/Fabricating_Exclusive.cfm?ID=314
http://www.massey.ac.nz/~odiegel/bendworks/bending.pdf