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How to flatten sheet metal?

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  • #16
    Bump, because now I have to deal with 1"x13" strips of mild steel that were cut with that big shear thing where you step on a pedal and a big blade comes down. The pieces have a twist in them.

    Will heating the piece up with a propane torch while holding it against something flat work? I recall it worked on a piece of 1/8" aluminum.


    • #17
      Originally posted by beanbag
      Bump, because now I have to deal with 1"x13" strips of mild steel that were cut with that big shear thing where you step on a pedal and a big blade comes down. The pieces have a twist in them.

      Will heating the piece up with a propane torch while holding it against something flat work? I recall it worked on a piece of 1/8" aluminum.
      No, as stated previously, you have to over bend/twist.

      When you do get it flat you'll find it will have developed a sight curve , from stress introduced by shearing.

      If you want straight & flat sheet metal it needs to be slit (balanced stresses) not sheared

      I used to be indecisive. Now I'm not so sure , but I'm not a complete idiot - some bits are still missing


      • #18
        Your getting into autobody kind of work here. Like mentioned you need to go past flat to get something back strait so putting the piece on something flat and doing anything to it won't make it flat.

        Your gonna have to find high and low spots and work them out either by hand if the material is thin enough or with pliers, hammer, vise, etc. If the material is twisted evenly from one end to the other you may be able to grab one end with the vise and use pliers on the other end to twist it back. Nice and easy, do a little, check, do a little, check.


        • #19
          Almost ANY type of stress will be OK to relieve the stress causing the bend... if there is enough.

          Maybe you can just bend it back, going slightly over bend to get it to spring back to flat. You have to exceed the yield point to get it to "take a set" the way you want.

          Stretching is particularly good, but work-hardens the material. Grab both ends and pull.

          pressing in a planishing press (the bumps noted in my other post) stresses teh surface and can be used, but only if you have the press.

          Just flattening against something is not workable, since it can only get within the "springback" amount, not all the way flat.

          if you heat it enough, it will stress relieve and may lay down flat.

          if you have a jeweler's roll set, rolling slightly and pulling as it comes off the rolls might work well.

          To hammer it flat needs skill, you have to stretch the "short" areas to match the others, which will flatten it. Think the old-time skilled saw-straighteners, sight the blade, tap it in a few places, and voila, it is straight.

          it is a common problem.

          BTW, that sandblasting actually made the blasted surface alightly longer than the untouched surface, hence the bend..... perhaps blasting in certain places might bring the twist back flat!
          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
          Not responsible for clerical errors. Or those made by lay people either.
          Number formats and units may be chosen at random depending on what day it is.
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          • #20
            Originally posted by Black_Moons
            Iv read about methods of using heat to remove expanded sections (outward dents basicly), by heating with a torch and then rapidly cooling with a damp rag.
            3M also puts out a type of disk designed for this.. I forget the name however.
            Yeah, i was looking at the you tube vids on that the other day ..fascinating

            They call it "oil canning" when the metal has stretched and keeps returning to the same shape

            couple of vids there somewhere

            One with a nine inch grinder with a stainless disk on abrasive ..spins and heats up metal ..that must be the 3M disc

            The other, with guys heating up tiny spots with an oxyacetylene totch ..whilst putting water on it immediately afterwards .

            there's also a guy using a pin welding gun that can do the heat up without the pins

            Another one shows blocks of dry ice used to shrink a panel dent

            There is also a new thing, were the guys heat up the dented panel with a heat gun ..then spray computer cleaning co2 rattle can into the centre of the heated area freeze it...looks like it works a treat ..

            The last one would have cured VPT's girlfriends car issues..Doesn't involve damaging the paint.

            all the best./markj


            • #21
              I know I recently had a larger piece of 316L stainless that had an expanded area in the center of it. I hammered around the edges of it for what seemed like hours, and it did little to flatten the piece. Finally, I heated the thing in the center, with a MAPP gas torch and slapped a wet shop towel on the plate.
              When I removed the rag, the piece was almost completely flat. I then clamped one end down to the bench, and using a 120grit "flap wheel" on my angle grinder, polished the discoloration out of the metal. Apparently the heat from using the flap wheel was enough to stress relieve the plate because, when I got the whole piece polished, it was nearly "dead flat". Great, now all I had left was the other six plates, and I'm done....
              No good deed goes unpunished.


              • #22
                Flat Metal

                Saltmine hit the nail on the head with his "Heat Shrink" approach. Used this many times through the years on various automotive projects. Now a days folks use stainless steel disks in a hand grinder to produce the heat rather than a torch. I have also used the disk method with good results, information can be found on the web about their use and techniques.
                Having spent most of my adult life in the metals process industry in a whole range of areas, I can tell you that the reason shape problems whic have been described exist is because the metal is longer in the area of concern. Center buckle, oil can, edge wave etc. are all length issues. The most common industrial way to remove these shape problems is by using a leveler. A leveler is a machine with a bank of nested rollers, top and bottom. The OD size of the rools depends of the gauge of material being worked. A give roll diameter has a range in which it works the best. By pulling the material through the rolls and adjust the pressure applied to various sections of the strip, the short portions of the strip are stretched to the same length as the defect. The industry term is to bend and stretch the material beyond it's elastic modulus.
                For the average guy the key is to understand why the shape defect exists, and not to induce more problems by hammering in the wrong area.