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Join Date: Jan 2009
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# FORMULA FOR SPRINGBACK

02/14/2010 7:55 AM

Does anyone have a formula for spring back. We are pressing .25 inch thick A-36 plates into knuckle pieces for large diameter tank heads. The blank is 6' long and pie shaped with a 5' width in back and 3' width in front. The longitude's radius is 5.5' and the horizontal varies from 15' on large end to 22' on small end. We have made a die with ribs that have the compound radius. The dies are about 6 inches wider than the blank. The dies are also 1" larger than the blanks. We intend to start by over pressing by .25 inches and continue hoping to find our way the hard way. We are wondering if there is a formula to calculate the spring back.

Pathfinder Tags: head press forming tanks
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Guru

Join Date: Mar 2007
Location: City of Light
Posts: 3945
#1

### Re: FORMULA FOR SPRINGBACK

02/14/2010 1:44 PM

I would prefer to see a drawing. But any way the spring-back is possible but very difficult to compute especially in 3D. For simple flat parts I got a quite good result but not exactly due to the dispersion of material properties. For 3D it is possible to use FEA but it is not easy.

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#2

### Re: FORMULA FOR SPRINGBACK

02/14/2010 6:56 PM

A-36 is a low carbon steel, with typical behavior. 3D models are hard to define manually, so, I agree that a FEA model would be helpful if things are not going fine. There are no prompt formulas for complex geometries, just for unidimensional and beam model ones. But at the end, what most people do is to go a little further with the press to compensate the spring back effect. Remember the tension-deformation diagram of a low carbon steel? Thats what it is... you should just know that when you go beyond the Sy tension, you should be aware that the material will deform back all the way down following its elastic modulus. After all, its all about that.

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#3

### Re: FORMULA FOR SPRINGBACK

02/19/2010 12:44 AM

We pressed a few pieces and discovered the answer is 1.5 inches.

The development of the plate is all most flawless. We are getting a very small (one inch) ripple in the middle of both ends. Our erectors can correct that in the field.

We will keep working with the fixture. The problem with the ripples seems to be that the fixture has the sides pinned down and the metal is flowing inward in lieu of outward. Intend to rework fixture so the middle hits the blank first instead of last.

I appreciate everyones thoughts and ideas. Will keep posting results as we get them.

We are setting up the smaller press to do the dome portion of the heads while the larger press will do the knuckles.

Next stop will be to press 5 or 6 pieces or one quarter of the roof or bowl in one stroke of the press.

It looks like two men can do roof and bowl in one day. That's 16 hours compared to 6 men x 8 hours x 14 days when do on the old mechanical press. End result is 16 hours compared to 672 hours. Shop rate is \$35.00 per hour.

Thanks again. I will attempt to upload some photo's.