Best way to weld box section chassis joints

[harvey]

I want to weld some 25x25 box, 1.6mm, and there are a lot of joints like the one pictured. Fortunately I am able to turn the structure (a car chassis) so as to get whatever is the best angle.

Should I do the red or the blue weld first, and when I do the red weld I'm not sure exactly where I should start and stop, in other words, where the overlap should be.

Hope this makes sense

 

[rtbcomp]

I shouldn't think it makes much difference, but you might like to just tack the joints until it is all square. Tacked joints can be squared up with a hammer and they're easier to grind off if you weld something in the wrong place.

 

[fingers99]

If I read it correctly, the procedure in the Haynes Roadster book is tack everything, then move, tack what you've not tacked, then weld everything up (in as balanced a fashion as you can). I think welding one side completely will tend to pull everything out of true.

As rtbcomp says, tacks are useful because you can adjust the fit with a tap from a hammer or just grind them out completely.

 

[harvey]

I should have said I was going to tack it, really my question is about what happens next, about which order to do the main welds and about the overlapping.

 

[fingers99]

Personally, I'd approach it like this (everything tacked up first, though): inside weld on one side of car, inside weld on the other, outside weld on first side, inside weld on the other. Then do the back and front on those welds, again, side to side. The overlapping should take care of itself (maybe with a bit of assistance from a file or a die grinder).

But by the time the whole frame is tacked up, it should have a lot of triangulation in there, so you can get away with not always working to plan.

As always, go for strength and penetration, the appearance is just that, appearance.