Controlling Distortion

[FranV8]

Hello all,

Well, nice to be back in the garage welding again, this time with a nice X size cylinder. I'm making up a new bench for my milling machine - very similar construction to the one I made for the lathe.

This uses 30X30X3mm thick mild steel box as the frame for the top and the legs. I've been trying to make the two sides today, and had real trouble keeping the parts square after welding. Tacked one side up, then welded the sides, then bottom then top. The legs have pulled inward at the bottom.

Sorry, should have said, the joint I'm doing is the horizontal piece is welded perpendicular, end onto the side of the vertical box section.

My next attempt (the other side) - I've clamped to a sheet of 3mm steel, it's helped but when I unclamp it there is still the inexitable spring back to a not quite square condition.

Any hints/tips/secrets greatly appreciated!

 

[troppo]

You have to think about distortion when welding. Your making a bench for your milling machine, so fit a shelf or two between the legs, cut to exactly the same dimensions as your top pieces and place between the legs, this will nit prevent distortion but will stop it by being there. You need to think and plan before you weld or you can have a bitch of a time with distortion

 

[Welderpaul]

Always easier to compensate for the inevitable distortion. If you are welding (for example) two pieces together to form a 90 degree corner, set them slightly over 90 degrees, so when they are welded and pull inwards, they finish off at 90 degrees. Weld the inside last.
Same if you are welding a tee joint - prebend the long leg before welding to the side of it so when it pulls it ends up fairly straight.
30mm shs can be bend back reasonably easily if it does pull too much.
Clamping things flat is not normally enough, they will still pull when cooled and clamps removed. Always need to be bent back on themselves a bit. How much to prebend is a bit of an art and comes with experience.

 

[roofman]

i normaly tack everything 1st in a biuld that has 90 degree legs including all the bits in-between to keep it rigid..then once i check its fully square i go round all the joints welding in a X fashion if that makes any sense,left top corner then down to the bottom right corner and try not keep prolonged heat in any one corner. Never gives me proplems but these days i find my SIP 9" angle vice a true friend in getting the 90 cornes to stay square too.

 

[Drains]

Fran, ignore (almost) all the above, especially that Paul fella.

The rule is, if you're welding box section, never weld across the box, only in line with it (if you can help it). So if you're joining a rail to a leg for instance, do two welds, one each side, in the same direction as the leg, but don't do the two welds across the leg, or the leg will bend in, annoying you and looking a bit crap (if anyone else notices).
Don't let the fact that there are two unwelded sides left get to you - they were never needed in the first place as the welds you have put on will be perfect for the job.

Be aware of course that all welds shrink so what you think is square will pull out of square if you do the wrong welds first or don't use the right tacks in the right place, or leave a necessary bend allowance (like what Paul said, although it's often not required). Even tacks can pull you out of square, particularly if they're bridging gaps.

If you can avoid welding, do. Lots of weld does not necessarily make a better job.

 

[FranV8]

Thanks all - really useful. Drains - perhaps I need to know more about the order in which to put tacks. Yes, they were themselves pulling things out of square. The tacks were probably about 4mm in diameter, effectively into a vee prep.

Like the idea of tacking the lot up first - although I suspect the distortion would pull it about all over the place when full welding was done? But it would keep most of the key dimensions correct.