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?
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.
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.
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.
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 m> Does anyone have any information/ experience with "spring back ratios"