shrinking 18 ga sheet?

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Hi guys I'm building a sandblasting cabinet and ran into a bit of a problem. In building the lid, I welded a rectangular frame out of 1/16" wall 1" square tubing, then plasma cut to size and welded on an 18 ga sheet. I was under the impression that multiple tacks would keep it pretty straight, but in some spots there is 5/16" or more warpage. I've been told I can straighten it by "heat shrinking" certain areas using an acetylene torch and a water quench, but the sites with advice on doing this are suggesting it for thinner metal (auto body mostly) and only doing it in dime-nickel sized spots. I have a few questions about this process that I'm hoping someone can answer for me, especially relating to use on thicker metals. I'm thinking that I want to fix the warpage before cutting the hole & welding in a frame for the glass plate, would this be a good course of action?1) would a smaller tip or rosebud be better for heating up these areas to shrink?2) how hot do I want to heat up these areas before quenching?3) do I get more movement by using a colder quench (such as ice water or ice/snow)?4) There are "lines" I want to shrink to straighten the warped sheet. Would I be better off heating a line 3/8" wide and maybe 4-5" long or doing a series of dime-nickel sized spots along a line?5) does anyone have any experience with the eastwood "twist hammer" shown here - http://www.eastwoodco.com/jump.jsp?i...emType=PRODUCT ? Does it work well with thicker materials?
Reply:Hi greenbuggy, I'm no expert on shrinking metal so take what I say with a grain of salt. I tried using a grinder with shrinking disk (from eBay) and was surprised at how it works on a Buick fender. I think the process is the same for a little thicker sheet metal. The thing to remember is to take little steps starting at the outer edges of the deformity and then work toward the center of the bend. If it is large it will take a great deal time. For heating, the old-timers used a torch and heated to a dull red glow, then quenched with a spray of water. I would not try to heat large areas and stick to small targets working around the deformity eventually toward the center. The small heating areas will eventually all pull the metal back into shape with a little on-the-job-training (practice). It isn't something easy or done in an instant. It does take time. You might even have to repeat the process more than once depending on how much shrinking is required. Here is link demonstrating the shrinking disk -- but the principle is the same as a torch.[ame]http://www.youtube.com/watch?v=X8cruauERmI[/ame]Hope that helps, SteveJunkYard Tools .com
Reply:Post us a picture with a straight edge across the bulge so we can see the extent of your problem.  You have probably shrunk the area with your welds and you may actually need to stretch the weld haz zone instead of shrinking.  The warpage will be caused by the heat from welding.  You could use hammer and dolly working the haz zones of the welds (the blue areas of the welded zones), you would hammer on dolly and ring the dolly and hammer.  As you do this you will relax the metal and stretch it back to where it was before you welded.  A little bit at a time.The Eastwood hammer is good for beating hell out of some steaks.  You can get some shrink from the hammer but you will mark up your panels with bad scars. Heat on sheet metal goes a long way.  By that I mean a little can move a lot of metal.  Go slow.  Need pictures.Last edited by 59halfstep; 03-12-2009 at 12:18 AM.Charlie