Engineering Strength of Material Question

Does anyone have any information/ experience with "spring back ratios"
for materials, (specifically steel).

I have a project where a slight but precise bend needs to be formed
into a piece of steel. I didn't find much help in the strength of
mateirals books and Cosmos won't deal with non-uniform material
properties, (ie. below and above the yield point) until the top most
version of Cosmos, (which we don't have).

A generic description of the problem would be: Over the last inch of a
thin strip of steel every .200 inch section needs to be progressively
formed .002" (from straight) more then the previous section.

So, the question is: Does anyone have a formula for determining how
much over bend a die should have, (at each increment) so that the
desired shape is formed?

Thanks,

Ed

 

Ed,
I looked through the Machinerys Handbook without much success
regarding your question. You may have to find the answers to your
query in an Engineering book, or maybe a Metalurgists reference
book.
I know that generally speaking, there is more information that you
would need to come up within order to obtain a decent answer. The
length of the part to be overbent, the type of alloy, the temper of
the metal, the "thin" factor wouldn't be quite good enough in this
case, you would need the exact thickness.
And after all that information is gathered, you may still find it
difficult to come up with an accurate answer. I know in the general
Tool and Die Industry, these are trade formulae, that are arrived at
through much trial and error, and these formulae are closely guarded
by proprietary junkies.
The best way I know of to accomplish what you want is to make
temporary tooling out of brass, for example, and set that tooling up
in a hand press, (clamped down), so that the pressure and motion would
be constant, and the only thing left to be arrived at is the amount of
over-bend, (X) which of course can be measured, easily enough by
fastening an indicator on the ram of the press and recording what
depth the ram is traveling down to obtain each particular bend. By
measuring the resulting bend of your part on a Comparator, and
recording the depth that you traveled, to obtain that bend, it
shouldn't be that big a deal to get the answer that you want.

I hope that this is of help to you,

G. De Angelis
Valhalla Grafix LTD

www.valhallagrafix.com

 

I have had the same issue in decades past.

The actual calibrated material properties (consistent stress/strain
curve) are absolutely required to have consistency, and it ultimately
gets down to quality control on the part of the material supplier as
to what repeatability you can get.

This means metal chemistry, built-in rolling stresses, heat treat-
stress relief, and surface finish all affect the actual yield point
and spring-back.

Bo

 

I've formed tons of spring wire 3/16 dia. requires approx. 15 Deg. overbend
if you are just wrapping it. If you are able to coin it 7-10 deg.

Ready bender allows for 20 deg. overbend when providing information
on their benders.

The tensile strength may vary greatly from different suppliers up to 15
variance so you need to allow for adjustability.

Joseph

 

You also need to keep in mind any processes that occur after forming,
such as piercing, heat treating, plating, brazing. Amen to everything
else mentioned above.

Diego

 

Ed,

New to the group, so missed previous discussion, sorry if this is not
relevant.

It just happens I was reading "Spring Design and Manufacture" by Tubal
Cain, Workshop Practice Series No. 19. Pp 35-38 has a discussion of
spring back, with some experimental results for bending round wire.
Unlikely to answer your question, but the methodology used might give
you some indications.

David

 

I think you're asking a lot from a stamping or bending process.
Expecting multiple progressions of .002" nominal bends to be done at a
speed to achieve a production-quantity from a process on a part that
has a very thin cross-section is difficult for manufacturing because
of variables from thermal changes, precision control requirements from
equipment, and even stress induced by the part's own mass over a
significant length aspect ratio. Normal production tolerances I seen
for a coined feature in thin stock were +/- 1 degree at best (figure
approx +/- .017" per inch) Your progression of .200" might hold +/-.
003 if you throw a big pile of money at toolmakers and are very
patient. Tolerance of +/-.005 is more common on a bending process..

 

I want to thank all of you for your input.

EdT

 

I want to thank all of you for your input.

EdT

 

Coined bends have somewhat better control of springback.

Also check <http://www.industrialorigami.com>