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hauptmann (Automotive) (OP)
17 Jan 02 17:35

I am looking for a formula that will calculate the bend allowance for stainless steel, types 304 & 409, so we can accurately lay out the flat pattern for our desired bend detail.  We are using brake presses with a V-die for the bottom die and a radius punch for the top.  Typical material thickness is from 10ga to .250.  We have been successfully using the Machinery's Handbook formula for mild steel but are experiencing difficulty with stainless steel, which it does not provide any information on.  The correct formula should include material thickness, inside bend radius, and angle of bend.  Thanks in advance for any help.
BillBirch (Mechanical)
17 Jan 02 19:37
wktaylor (Aeronautics)
18 Jan 02 8:29

Sounds like Your real issue is with "allowable bend-radius" and "spring-back". The flat-pattern allowance charts are the "same" for all metals [based on thickness]... but where You start and end are the important factors due to the metals being bent!

Each alloy/temper has minimum requirements for cold bend radius... any smaller radius would cause "severe cold-working" which would lead to "immediate cracking" [at ends or outside of bend]... or in-service cracking.

Also, metals with high yield strength need to be over-bent by a certain percentage, so that when they "spring-back" the final angle will be as intended.

Since You did not specify the "hardness" [strength] of Your CRES alloys being bent, no one can help You. Most aircraft companies, and some acft repair manuals, have charts specifying values for min-bend radius and spring-back... but it is still dependent on the material. If Your alloys and tempers aren't specified, find one "close" and try those values!

Regards, Wil Taylor
haimre1 (Mechanical)
1 Feb 02 7:07
the simple formula is


so that is

d-diameter of the bend.
t- thickness of the material.
e-elongation rate.

regards, Haim Revach
Htaed (Mechanical)
15 Jul 03 17:57
I've wriiten a program for figuring BA and press brake set back. The formulas used in it are taken from over 10 years experience in the metal fabrication industry. It is used in the plant I work for and the engineers there found it very useful. If anyone is interested in it please let me know. I'd be happy to share.

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