Report back on: "Flattening 1/4" Steel Plate?"

I just picked up a used DiAcro 12" shear. It's in pretty good

>condition, with a little surface rust. The one thing that has me >concerned (and puzzled) is that the "apron" in front of the blades is >bowed up in the middle. This is the platform that the sheet metal sits >on as you feed the stock into the cutting area. There is a ~1" wide >section before you get to the blades that's part of the casting, but >the main work area in front is a piece of plate steel (probably cold >rolled) about 18" wide by about 8" deep. It's held down on each side >by a 1/4-20 flathead screw. > >This piece bows up about 50 thou a little to the right of center, so >there is a step down to the level of the casting & fixed blade. I >haven't had time to check if it's bowed across the full depth from >front to back or just towards the front edge. There's no obvious signs >that the thing was dropped on anything, but it may be that someone >picked it up on a fork lift by the underside of the plate or something. > The forces required to bend the plate must have been substantial, and >I'm surprised the two screws weren't damaged. > >The step where the plate meets the casting will mar things when metal >is clamped by the hold down system, so I want to flatten the plate out. > My first guess is to support the side edges with wood, place a block >underneath so it doesn't deflect to far, and then start hopping up & >down on it. If I can get it to bend at all, I figure I can tune the >deflection by adjusting the thickness of the block in the middle. > >Does anyone have any idea how much force it would take to bend a plate >like this? Am I likely to be able to flatten this out as described, or >is there a better approach? I'd actually like to overshoot a bit and >leave it ever so slightly concave. That way the screws on the ends >will pull it down against the casting (there's a lip it's supposed to >sit on).

I did some calculations based on vague guesses about the steel. In theory, I'd need to bend the plate at least 1/8" in the middle to reach the yield point where it wouldn't just spring back, and it would take ~400 pounds of force to flex it that much. Certainly more than I could do by jumping up & down on it.

I talked to the head of the machine shop at work, and he volunteered to help me flatten it out using a giant DiAcro hydraulic press brake they have. The key feature is that you can control the depth of the stroke in

1 thousandths of an inch. I brought the plate in at lunch, helped him remove the dies off the brake and then set the plate up on a couple of aluminum bars supporting the ends. To push down in the middle, I had a chunk of 2x3 wood set on edge, and a piece of plywood to spread the force out over about a 3" wide stripe.

We started slowly increasing the depth of the stroke in 20 thousandths increments, watching the plate flex more and more. After each stroke, we'd check it with a straight-edge for any signs of flattening. It just acted like a giant leaf spring, well past the 1/8" depth I'd calculated. Even at about 1/2" deflection, there was no obvious sign of progress. Finally, it began to flatten out. It eventually began to develop a shallow depression in the middle where we were pushing, but by then the overall curvature was down to only 10 thousandths or so. The remaining upward curve was a bit off-center at that point, so we kept the stroke the same and moved the wood off a bit to that side. At the end, the final stroke was just about 0.800"! We were getting very concerned that the end blocks were going to kick out from under the plate because the angle was getting a bit steep. I have no idea how many pounds it took to bend the plate that far, but it was a LOT.

When finished, the overall flatness is probably down to about 5 thousandths, and it's in places where it should flatten out even further when it gets bolted back down to the press. Needless to say, I'm very pleased with the results. There is no other way I would have been able to do this without driving over it with a car, and the lack of control would have made it a very haphazard process. As it was, it took us over

3/4 of an hour carefully flexing & checking things.

Flexing a piece of 1/4" thick steel 8 tenths of an inch is an amazing sight. The only guess we have about how it got that warped in the first place is stresses that relaxed over time. There is no way that much force could have been applied to it while it was on the shear without something else suffering damage. The press is probably 40 years old or so, and there is no sign it was ever in a fire that could have warped it like that.

The only other note is that it was kind of nice using the big DiAcro press to restore my little DiAcro shear. Sort of keeps it all in the family...

Doug White

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Doug White
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I think I would completely remove the piece. Place it on concrete floor. Block up the ends with 2x4. Place a scrap 2x4 long ways at the hump. Smack lightly with a 16 pound sledge hammer. If that does nothing, hit harder. I suspect it will take quite a blow to put a long gentle dip in the plate.

Do what you must to not kink it too far.

______________________________ Keep the whole world singing . . . . DanG (remove the sevens)

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In a previous job I had to straighten long shafts on a regular basis. Some of these shafts were about 4 feet long and were actually tubes with flanges on either end. The center section was about 2 inches and the wall thickness was abouy 1/4 inch. The shafts were first gundrilled a little undersize. Then I rough turned them. The bores were honed and then I turned a little more off. They went to heat treat and when they came back I straightened them. Afterwards they were honed, then turned, then I would straighten them for the last time. They needed to be straight within .001 TIR. Some of these shafts would need to be bent about 1/4 before they would start to straighten out. It's spooky to see something bend that much. The ends really climb. And you know, from looking at the pressure gauge, how much energy will be released if something gets loose. ERS

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Eric R Snow

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