[gnabgib]

Ok I made a rectangular frame with mitred corners from 35x35x(1.37in)1.6mm(0.06in) thickness steel. Overall size is 1800x800(5.9ftx2.6ft) and it has been sitting around waiting for me to finish up. Yesterday I put 5 pieces of 20x20x1.6mm across the short span and these were a jammed in fit but the long edges of the frame still distorted eg. pulled in after welding(55amps) and I did step weld all these pieces. I understand all about expansion and contraction with welding but don't get how this happens, when the corners have been completed. Does this mean the cross pieces have permanently contracted in length and do I next time weld up these first before finishing the perimeter frame. My welding table isn't finished yet so can't resort to that.

[Oscar]

It doesn't mean the cross pieces have contracted in length, it just means that the welds have pulled on the adjacent steel. The long spans having more "pulling give" than the cross-member has in stretch, eventually gave in. It could have been as simple as not giving them enough tacks, you taking too long to run each stitch weld, long-arc'ing it and dumping in extra heat, or anything in between.

[LtBadd]

It doesn't mean the cross pieces have contracted in length, it just means that the welds have pulled on the adjacent steel. The long spans having more "pulling give" than the cross-member has in stretch, eventually gave in. It could have been as simple as not giving them enough tacks, you taking too long to run each stitch weld, long-arc'ing it and dumping in extra heat, or anything in between.

I would add that steel doesn't shrink very much, more then likely you did a good job cutting the cross pieces so that there wouldn't be any gap, so even the slightest shrinkage caused them to fit tight

[gnabgib]

It doesn't mean the cross pieces have contracted in length, it just means that the welds have pulled on the adjacent steel. It could have been as simple as not giving them enough tacks, you taking too long to run each stitch weld, long-arc'ing it and dumping in extra heat, or anything in between.

I was expecting to avoid the longer perimeter pieces pulling in by fitting the cross pieces in tight. So presumably nowhere for them to go.
I used non-sequential tacks on the cross pieces and then left it all for at least 30mins, then welded one side non-sequentially, allowed it to cool and turned it over to do the other side the same way.

[gnabgib]

I would add that steel doesn't shrink very much, more then likely you did a good job cutting the cross pieces so that there wouldn't be any gap, so even the slightest shrinkage caused them to fit tight[/quote]

Yes, but cutting in tight was to avoid the 'pull-in'. I wanted them tight. The cross-pieces were welded in as I was forming a complete panel.
By the way, this was all 1.6thick(0.6in)square tubing

[Oscar]

I would have done the same thing, cut them and make them fit very snug to try and avoid any pulling. What size are the fillets? Pics? If the fillet is larger than the base metal by much, it will usually account for pulling.

[Rick_H]

When I make something similar I tack both sides, clamp to a table if possible, then weld some nice small beads, staggered flipping back and forth from top to bottom.

Did the side rails have a round edge? You'll be surprised how much they move when you weld that joint.

All metal moves no matter how tight the joint is...clamps and careful planning of where to tack, for instance I tack at the corners, adjacent then put small 1" beads on the interior, the outside flat surfaces are nearly last....

[gnabgib]

I would have done the same thing, cut them and make them fit very snug to try and avoid any pulling. What size are the fillets? Pics? If the fillet is larger than the base metal by much, it will usually account for pulling.

I was undecided about size of wire to use, in trying 1.6 I got some undercutting so decided on 2.4mm. The fillet ended up at 3-4mm so yes probably should have tried harder at getting the 1.6 to work.
This job is for myself and in a position where the 'defect' would be unsightly or a problem. Now just trying to understand what I did wrong to cause this end result.

[gnabgib]

When I make something similar I tack both sides, clamp to a table if possible, then weld some nice small beads, staggered flipping back and forth from top to bottom.

Did the side rails have a round edge? You'll be surprised how much they move when you weld that joint.

All metal moves no matter how tight the joint is...clamps and careful planning of where to tack, for instance I tack at the corners, adjacent then put small 1" beads on the interior, the outside flat surfaces are nearly last....

The 35x35(1.37in) has maybe a 5mm(3/16in) radius and 20x20mm was welded flush with one edge of the 35x35, so there was a gap from zero to 3-4mm at that face.
For future reference, when doing something similar, would it make sense to fix in any internal pieces first before finishing the 2 end parts of a frame? I realize that these things are best clamped up and done on a properly set up welding table.