Welds pulling my steel up like a banana! How to fix?

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I thought the issue was when welding in the floor sheets, but it seems to bow up like a banana in the early stages when the frame is welded up.  I always thought it was when welding in the floor sheet.So it seems that the few little fillet welds on top of the RHS 3mm 1/8" are enough to bend it up.I am now pre cambering those bits with clamps etc.But I was wondering if something like this could be easily brought back to flatness using the oxy.  Would heating up those welds one at a time to red hot work?  Any links to tips/instructions on how to straighten stuff with an oxy?See my pics of what I mean. Attached Images
Reply:The key to prevent heat warpage in metal is to weld a small bit in one spot, then flip the part over or go to the other side, and weld a bit there. Move the heat from one side to the other, never putting too much heat in one area. If you cant move to the other side or flip the part over, you can just stop and allow the part to completely cool after short welds.Clamping the part to a table will help with heat warpage also.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:A torch & a spray bottleBuy American, or don't whine when you end up on the bread line.
Reply:I have the opposite problem, welding makes my banana bend up like steelExperience is something you get right after you need it
Reply:Best thing to do is to try to prevent/control warpage in the first place. Tack the heck out of everything to hold it in place before welding it up. Then, don't over-weld.If you need to use heat to bend things back, Google "flame bending." There's a good discussion here:http://books.google.com/books?id=UT5...ots=RRxptddVvd
Reply:Yep. Tack weld.-RuarkLincoln 3200HDHobart Stickmate LX235TWECO Fabricator 211i
Reply:If I understand your design then per cambering is probably the most cost effective solution. Even with a massive fixture you will see war page due to all the welds being on just one face of the tube section.
Reply:yeah thats about it.  pre camber clamping, works good.