How do I stop steel pulling whilst welding?

[Trevorleach]

Last week welded two lengths of 6x3 channel end to end, all nice and square, even laid the new sections on top of another length of 6x3 to make sure it had a flat surface to work on and all seemed okay, then added a gusset on the inside of the channel and it pulled the bugger out of square, had to recut the side webs and straighten before rewelding.
I should add I did the first one with the arc welder, then after seeing it had gone wrong I decided to do the next one with the mig, same results
Today welding 3x3 angle end on to some 6x6 rsj, tacked up and checked with the spirit level all nice and vertical, fully welded and again it is now out of square, can shim this with washers so no biggy, but annoying.

Got a body to build next week or so, and am now thinking holy cow batman what is it going to look like when it is done?

 

[Rrobson]

You can’t..

 

[E T]

Predicting which way and how much it's going to move an counteracting that movement by tacking the piece on out of square.
It all comes down to experience. Certain joints pull more then others, and the direction of travel also plays a part.

 

[MikeBs]

Clamping the pieces so they are rigidly held in place can minimise the deflection, but you are not going to get rid of it completely.
Sometimes I find even the tacking welds can pull them out of square.

 

[dieselsamurai]

Sometimes you need a bit of experience to judge how much it will pull so that when you weld it pulls where you want , mostly though i clamp the hell out of it and try to weld in steps if you cant alternating around the piece so that it minimises deflection.

Ive just been welding up a new tailgate for my truck and by rigidly clamping to my welding table and leaving to cool off after its come out with minimal distortion , but sometimes you try every trick in the book and it still pulls in a way you dont expect it too

 

[Erie Fred]

Sometimes I find even the tacking welds can pull them out of square.

I call them "Non Influencing tack welds" put them where they don't pull stuff out of alignment.
I sometimes design a part with a hole in it for a plug weld, as these don't usually
pull the part out of alignment.

 

[chunkolini]

my least favourite sound in the world is the crack crack of tacks loosing off on a large piece of work as I finish the welds off on the other side of it.
I was welding this up

A mate came in yelling at me to stop, he could see the end of it moving.
I managed to overweld the opposite side and pull it straight. Then worked my way round doing shorter runs
it came out straight.