Trying to fix warped wedling table

[Dmaxer]

So I picked up a used 4'x8' welding table for $50 from a pre-fabbed barn manufacturer going out of business. It is made of 1/2" plate, galvanized on the top with a frame made mostly of 2" x 3/16” sq. tube. I welded on plates for casters on the corners and installed retractable screw supports on the center two legs. Then I discovered that the center of the table top is higher than the ends. It is kind of hump backed with the center about .20" higher than the ends. I'd like to find a way to restore it to flatness.

I'm guessing that the frame assembly is somewhat responsible for the warped top. Take a look at the photos. I'm considering gouging out the welds holding the top to the frame with a plasma cutter, except for the ones in the center of the table, to see if the ends of the top spring up. There are about 36 1" to 2" welds I'd have to eliminate. Later, I could even play an O/A rosebud flame under the center of the top to see if that forces it in the desired direction. If those methods work, I would shim between the top and the frame as necessary and reweld.

I'm alternatively considering severing some of lower parts of the frame to see if weld shrinkage in that area has bowed the top. At least one of the lower connectors was welded with a gap (see photo). Or maybe there's another option I haven't considered yet. Your suggestions are appreciated.

[Sonora Iron]

I could even play an O/A rosebud flame under the center of the top to see if that forces it in the desired direction.

Don’t do that!
That is opposite of where you need to apply the heat!
Place the heat on the crown of the hump. I’d remove the gap in the frame first, then look at adding a center support in the frame.

[FusionKing]

I would seriously consider cutting the top completely loose and flipping it over.

By cutting it loose you will find out what is really going on. You are not gonna staighten it if it is welded together wrong. That doesn't happen on sheet metal, let alone 1/2" plate.
If you was to heat it to bring the top down without cutting the frame loose it will most likely bend the frame some when it comes back flat...that metal has to go somewhere. And that's providing the frame was square in the first place. I'm betting it was all the perimeter welds that BOWED it in the first place, which is a common rookie mistake. Most flat tables have very little welding done on them. You can lay one decent weld and lay a staightedge down and watch it warp as it cools almost anywhere you do it.
I would attempt to get it almost flat while loose and then let a couple of tacks to the frame do the rest for me.
If the bottom isn't galvanized (?) flipping it would be a good thing in my mind.

[Sonora Iron]

[QUOTE=FusionKing;197725]that metal has to go somewhere. QUOTE]
Contraction!

When the heated zone cools, those little molecules in the steel get closer and closer and closer! Kind of like a group hug.

[FusionKing]

If it is warped from welding the perimeter you won't bring it back by heating the middle.
It ain't gonna happen! BTDT, you will end up with a wavy POS. You need to UNDO what is wrong with it.
You may make it close enuff tho...myself I would much rather strive for perfection.
If I had wanted to put a bow in that table I would have welded it exactly like they did
One thing you can do is weld all around, at the welds only on the opposite side and then grind them off.

[Sonora Iron]

That box tube frame pretty much has the plate trapped from moving, (much).
First thing I’d do is cut the plate loose from the frame; place a straight edge on the plate to find the crown. The size of the crown would determine what method I’d attack the bow with.

[Dmaxer]

Looks like the consensus so far supports cutting the top free of the framework. I think for now I'll resist the urge to flip the top just because I want to preserve the galvanization for the future (and because it's so d**ned heavy).

FK, your idea to apply welds on the top was intriguing, but I think I'll try another approach first for the same reason.

Thanks FK and Sonora for your thoughts.

[Sonora Iron]

If you don’t want to use heat, you can always do it the tuff way. With a beam and port-a-power.

[kevin]

easy way

nobody really suggested, except sonora, its just mild steel, why make it a career, move it cold, it will go, know some one with an excavator, block 4 corners, push down the ctr, but if you use it alot for welding and tack on it alot, it will still move around

[sailor]

easy way II

For better control you could use a hydrualic jack between the plate and say the stright axle on a heavy truck or a pallet loaded with something heavy on a fork lift. Be Carefull.

[overkill 19]

Could you not just cut all sides but 1, then heat her up and let it streatch back to normal. By leaving one side tacked to the frame it won't be so likly to buckel/bow.

[Dmaxer]

Could you not just cut all sides but 1, then heat her up and let it streatch back to normal. By leaving one side tacked to the frame it won't be so likly to buckel/bow.

The frame below the tabletop is warped the same as the 1/2" plate. So, no matter whether I cut the top free from all of the welds or just a few, I'll have to shim beneath the straightened top before I reattach it to the curved frame. From that perspective, it makes more sense I think to free the plate from all of the welds. Maybe tomorrow. I'll decide what to do about the crown in the plate after I measure it free of the frame.

FK is right, there are too many welds holding the top on now. I think the excess heat from those beads and the ones holding the frame together is what bent the top in the first place. I'll not make the same mistake when reattaching the top; fewer welds applied with better timing and spacing are better.

[nocheepgas]

You might have a difficult time with 1/2", but when I made my table (3/8") I attached it to the frame by welding some nuts to the table (on the bottom of course), welded some spacers on the frame and bolted it down. This let me pull the table straight by adjusting the bolts.
Of course if you had access to a really big mill, you could just grind it flat!

[pipeline Dan]

$50

you only paid $50 for the table!, cut the warped plate off and add a new top. For me to buy all the HSS castors and plate to build that table it would cost around 2-$300, so dont waste your time with a rosebud, or cutting some of the welds and try bending it back, your wasting your time, just replace the top

[FusionKing]

you only paid $50 for the table!, cut the warped plate off and add a new top. For me to buy all the HSS castors and plate to build that table it would cost around 2-$300, so dont waste your time with a rosebud, or cutting some of the welds and try bending it back, your wasting your time, just replace the top

But dude...what would that prove??? It's the principle
Besides if he wasn't all hung up on the stinkin' galvanize, it would almost come back flat simply by flipping it and welding it back down in the same spots. And if not at least everything would stay on the table