Avoiding heat distortion in tube

[breezer]

Hi all,

I'm welding some 20mm box to 22mm tube, tube horizonal top & bottom and box verical sort of like:

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. I I
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The problem is the horizontal tubes need to be as close to perfectly straight as possible. I've tried clamping some angle onto the tube with G clamps but I'm not convinced they're getting enough purchase as they are slipping on the tube. When it's cooled down & I take the clamps off it's pulled down (into the middle) at each end, although inbetween the uprights still seems perfectly flat

Thoughts so far if anyone can confirm or deny?

1 - vice grips may be batter than G clamps?
2 - move the uprights to the far edge and add a 3rd in the middle?
3 - tack a piece of box perpendicular to the 2 uprights as a brace to reduce movement?
4 - try and straighten them with the blowtorch on the back of them

Or if anyone has any other ideas I'd be grateful

Cheers

jim

 

[Welderpaul]

1. Probably
2. Yes, that would help a great deal - no unrestrained ends
3. You could do with pre-bending the horizontal tubes prior to welding. Clamp them to a piece of angle as you have been doing, but slip some flat bar in the middle before clamping it up so it bends back. Difficult to judge how much to pre bend - depends on the amount of heat going in, tube size/wall thickness etc.
4. Nah

 

[Hitch]

1... As lpng as its clamped that ought to be enough.
2....Yes, at the ends would help somewhat, you may get a very slight curve between uprights if they are a fair bit apart. Depends how big the item is.
3.... Not sure what you mean....
4.....20mm box should bend by hand enough to get distortion out. I had the same problem on the table i made last week.about 600mm of free 20mm box, easily came back straight just by plling it by hand.

 

[breezer]

Cheers lads - a few more details.

Where I'm getting the distortion is in 22mm tube (not box)

Size is 470mm tube, with 300mm box section uprights. The uprights are at 33% & 66% of 470mm

I'm thinking after tacking the verticals to the top tube if I tack a bar across them to stop them closing up it may stop the top tube pulling

Sort of making the uprights into an H section?

Cheers

Jim