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Old 03-11-2009, 02:42 AM
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Default shrinking 18 ga sheet?

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?
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Old 03-11-2009, 08:34 AM
fatfrank fatfrank is offline
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If I am reading this correctly, the sheet metal is warped and you want to try and flatten it out using a torch?

On thin sheet like that, I think you would make more of a mess with it using a torch. The warpage developed from shrinking of the metal due to welding. You would be better off using a hammer and dolly to try and get it back to flat. Use a lot of light taps to knock down the high spots with the dolly behind it.

The heat/quench only works for me when trying to straighten out tube or pipe, not sheet metal.
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Old 03-11-2009, 09:53 AM
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I saw this video a couple of moths ago. Is this the kind of thing you are thinking about? it's 18 gauge
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Old 03-11-2009, 10:56 AM
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Greenbuggy, I've not had reason to try it on sheet metal so fatfrank might be right but I did rescue my gate frame after discovering it was trying to imitate a banana. The frame is 18 feet long & I had no place to build it except on the driveway, and the drive is sloped and has a break in the angle of drop at about the halfway point. So not only did the welding heat warp it but I was fighting a the un-level surface too with no way to clamp it flat.

Since this is a cantilever gate it had to roll in a relatively straight line - didn't have to be perfect but it did need to aim in the general direction of the latch post.

1. here's the original bend towards the street.

2. got it redirected back to the latch post

3. by heating several spots on both rails & hitting them with the rag

4. Here is the latch side gap before heat. This picture is kinda misleading because that post that's showing is not the latch post; it came later.

5. After heat. It really doesn't look too sanitary when you sight down it but it works like it's supposed to & looks good once it was skinned.

Point is, the heat to cherry red & then cooling it quickly method does work. I think you could correct your door but it might take some practice.
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Old 03-11-2009, 11:27 AM
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Could you run a couple of stiffners, side to side, when you add the window frame? Then tack and hammer the highspots as you go. Start on the highs or lows and work to the outside. You may have to zipcut some of the orignal outside tacks. I have used heat to straighten out thicker steel but on 18g it could be very fustrating. Clamping everthing down with a strongback may help, and stagger the tacks as you go. Are you going to seal weld all the joints later for dust control? If so the additional heat will have the same effect
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Old 03-11-2009, 01:08 PM
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You might concider zip cutting the tacks where the warpage is the worst flatning out the tin and re welding it.
Shrinking may not be the way to go if the problem is bad. After you have tried shrinking other options get harder.

When tacking start in the center of the long edge. Check for distortion then tack !/2 way to the corner check and correct as needed . Once you have all sides lightly tacked start dividing the spaces . Check for distortion after each tack and correct as soon as you see it.
After you do this a few times you will get the feel for it.
You can reverse bow the frame and the welds will pull it straight.
You do need experience before that works well as it is the distortion you introduce that you want to cancel and that depends on your welding tequnique.
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Old 03-11-2009, 02:16 PM
fatfrank fatfrank is offline
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Cutter, I find the flame straightning works fine for tube, just not so good on sheet metal.
Here is a 2X4 14g tube that I heated to draw back to straight.
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Old 03-11-2009, 02:18 PM
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This one shows where I heated it.
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Old 03-11-2009, 06:33 PM
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So what'd you do? Throw a bucket of water at it?
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Old 03-11-2009, 06:56 PM
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Quote:
Originally Posted by fatfrank View Post
If I am reading this correctly, the sheet metal is warped and you want to try and flatten it out using a torch?

On thin sheet like that, I think you would make more of a mess with it using a torch. The warpage developed from shrinking of the metal due to welding. You would be better off using a hammer and dolly to try and get it back to flat. Use a lot of light taps to knock down the high spots with the dolly behind it.
My problem with this technique, is that instead of small, pinched high spots I have 2 distinct, large "waves" in the metal. I don't have a small or a line of high spot, I have two 4-5"x20" sections that are at least 3/16" or more higher than it should be. because these "waves" have some springiness to them I don't really think that a hammer & dolley is going to do much, which is why I was curious to try shrinking the panel and see if the wave evens out some, I'm not looking for perfection but I don't want to get seasick when I look at the sandblaster lid either.

Quote:
Originally Posted by terry lingle View Post
You might concider zip cutting the tacks where the warpage is the worst flatning out the tin and re welding it.
Shrinking may not be the way to go if the problem is bad. After you have tried shrinking other options get harder.
This is a hard proposition given that the panel is already seam welded onto the frame, as well I suspect that if I went thru the trouble of cutting the seam weld the sheet would return to straight or at least pretty close to straight not having the strain of the welds/frame holding it in place



Quote:
When tacking start in the center of the long edge. Check for distortion then tack !/2 way to the corner check and correct as needed . Once you have all sides lightly tacked start dividing the spaces . Check for distortion after each tack and correct as soon as you see it.
After you do this a few times you will get the feel for it.
You can reverse bow the frame and the welds will pull it straight.
You do need experience before that works well as it is the distortion you introduce that you want to cancel and that depends on your welding tequnique.
Good info. I planned my tacks & later welds around what I thought was right but was evidently wrong, tacking every 6" all the way around then tacks in half (3") third that and then short 1" welds once it was completely tacked in place

Unfortunately I realized I'm out of acetylene so this project has been put on hold for a couple days
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