Best way to weld this:

[MattH]

I need to weld this structure and would like advise on how to minimise distortion and the best process to use?



The important factors are that the long box sections remain as parallel and straight as possible and that the entire structure remains as flat and level as can be.
The overall size is 550mm wide and 1600mm long.
The long side boxes will have a piece of 15x40mm flat bar welded down their length but will then be machined back to true and straight before welding.

The box will be 120mmx60mmx5mm wall thickness. I have 150A MIG and a 160A DC/AC MMA available. I was thinking of tacking up with the MIG and then using the Arc to complete all the welds.

I have a 24" square cast iron surface plate that I can use to clamp to and jig up the pieces (only half the structure at a time though due to the length).

I know the MIG doesn't have the power for full penetration but that is not necessarily required, this structure will not be load bearing. It just needs to be stiff and true.

 

[J.F]

this might have some useful info.

 

[Lemo]

Stick will be much worse for distortion. Looks to be sturdy enough but If strength isn't an issue I'd be inclined to go for the mig, clean the scale off tho. Decent tacks should be plenty.The flat bar is going to cause the biggest problems. Can you get away with stitching it?

 

[Richard.]

Matt a 150 amp mig is the last thing you need because it's lack of power is going to unwantingly force lack of travel speed which in turn increases heat in put even though the penetration will be next to nothing. What you need is power. Now you say it's loading and strength is minimum requirements which is good because I would (based on your options) do it with small stitches/long tacks using the mma process. If it just needs to be strong enough to support its own self in structure this would be fine. If you weld it all around on any of those slow processes it will move to a degree. How much depends on you. Obviously clamp it down, set it square and weld it in a fashion that your moving the heat around not localising it. The desired way would be a powerfull pulsed mig, part sat in a jig and blasted up fully and quickly but not an option here by The sound of it.

 

[Kent]

You need to make the full thing flat. Can you not rig something?
Tack is all doing opposite diagonals check it. Check it again then work opposite diagonal again with the welds
Note welding closes gaps pull wise when you figure direction of travel

All flat though and plenty clamps to start off tacking whatever it takes to get that

Mig is going to pull less because it's faster will your set not do 5 mm wall?

 

[Kent]

Matt a 150 amp mig is the last thing you need because it's lack of power is going to unwantingly force lack of travel speed which in turn increases heat in put even though the penetration will be next to nothing. What you need is power. Now you say it's loading and strength is minimum requirements which is good because I would (based on your options) do it with small stitches/long tacks using the mma process. If it just needs to be strong enough to support its own self in structure this would be fine. If you weld it all around on any of those slow processes it will move to a degree. How much depends on you. Obviously clamp it down, set it square and weld it in a fashion that your moving the heat around not localising it. The desired way would be a powerfull pulsed mig, part sat in a jig and blasted up fully and quickly but not an option here by The sound of it.

It's only 5 mm that will be ok If the set is good not that I have ever worked with a 150 mig mind you
The stich weld idea is a sound one though especially were things can be chain stitched I

 

[Richard.]

I haven't used a 150amp mig much either on thickish stuff. Id of thought it would struggle without silly things like weaving. Not sure tbh. I thought sticks would be best given they can be run hot and fast.

 

[waddycall]

How about welding angle on to the ends of some of the box and bolting it together? I recently spent ages carefully making a flat bench out of 50mm box in a ladder formation. Got the first one dead flat and square. Tried building the second one on top and it moved a mile when I unclamped it. I had to start again and build the second one the same as the first - predicting how much the weld would pull and pre setting the parts out of square as necessary. It's worth bearing in mind that box isn't necessarily that straight in the first place.
Perhaps you could weld up the outer frame and bolt the inner parts in place using straight edges across the top of the two long outer sections?

 

[MattH]

Stick will be much worse for distortion. Looks to be sturdy enough but If strength isn't an issue I'd be inclined to go for the mig, clean the scale off tho. Decent tacks should be plenty.The flat bar is going to cause the biggest problems. Can you get away with stitching it?

I was planning on stitching the flat bar as there no need for seam welding it. I'd of thought that the box would distort the bar rather than the other way around due to the difference in strength. I will be machining these parts after anyway.

 

[MattH]

How about welding angle on to the ends of some of the box and bolting it together? I recently spent ages carefully making a flat bench out of 50mm box in a ladder formation. Got the first one dead flat and square. Tried building the second one on top and it moved a mile when I unclamped it. I had to start again and build the second one the same as the first - predicting how much the weld would pull and pre setting the parts out of square as necessary. It's worth bearing in mind that box isn't necessarily that straight in the first place.
Perhaps you could weld up the outer frame and bolt the inner parts in place using straight edges across the top of the two long outer sections?

I could cap the ends of the box and then use dowels and bolts to hold it all together but I'd rather weld it if feasible.

 

[MattH]

I haven't used a 150amp mig much either on thickish stuff. Id of thought it would struggle without silly things like weaving. Not sure tbh. I thought sticks would be best given they can be run hot and fast.

I think with a bevel on the pieces that butt to the side rails my MIG would cope but it would be right on the edge of it's performance.
What sort of Amps would you consider for this job?

As Kent said I could tack the entire thing together with opposing welds before commencing with the heavier welds.

 

[MattH]

I haven't mentioned Tig as I've not really practised with it much but I do have a Transformer TIG that will go to about 140A, I hadn't considered it as I presume this is to low.

Although I don't need much of an excuse to buy a decent DC inverter TIG!

 

[chevy2]

The flat welds across the top will only cause a little distortion so long as its well clamped down,
its the fillet welds that will quickly pull it out of square so do all the flats first to gain some strength to the frame before doing any fillets,
dont do all left fillets for example as it'll pull that way,
do a left side then right side fillet on the opposite side of the frame and so on.

 

[outofthefire]

I was planning on stitching the flat bar as there no need for seam welding it. I'd of thought that the box would distort the bar rather than the other way around due to the difference in strength. I will be machining these parts after anyway.

Putting heat in one side of the box will make it distort, especially welds going across the width of the box. With that in mind I personally would only weld the upper and lower joints if you want to make the rest of the joints neat fill them with sealer. 120x60x5 usually has a large radius on the edges so you'll have quite a gap to fill, I'd personally mig it maybe using two or three passes

 

[MattH]

You need to make the full thing flat. Can you not rig something?
Tack is all doing opposite diagonals check it. Check it again then work opposite diagonal again with the welds
Note welding closes gaps pull wise when you figure direction of travel

All flat though and plenty clamps to start off tacking whatever it takes to get that

Mig is going to pull less because it's faster will your set not do 5 mm wall?

The plan was to clamp to the surface plate as much as possible.

 

[Hitch]

One of the most important things with something with lots of weld, is to make sure fit-up is good... if you cut each piece 1mm small to make it easy to put together, it will shrink back badly Leaving the sides bowed inwards.

I'd tack it together flat and weld it up without much of a worry to be honest.... The only worry would be putting the flat on the outside....if thats not on the centre line it may well buckle up to the closer side...

 

[Richard.]

If the idea is to reduce distortion then you need to pick a process that allows you to travel quickest. Time yourself on all 3 on that thickness. Id be putting my money on mma been the best option given your mig is below spec really. The added bonus of mma is you will only need to put down a few stitches and it will be stronger than welds produced from an undergunned mig.

 

[MattH]

One of the most important things with something with lots of weld, is to make sure fit-up is good... if you cut each piece 1mm small to make it easy to put together, it will shrink back badly Leaving the sides bowed inwards.

I'd tack it together flat and weld it up without much of a worry to be honest.... The only worry would be putting the flat on the outside....if thats not on the centre line it may well buckle up to the closer side...

It's not going to be centred, but if it does buckle or twist then that will be taken out when it's machined.
The fit up with be very tight. all the cross members will be milled square and straight on the mill and will all be exactly the same length.

 

[MattH]

If the idea is to reduce distortion then you need to pick a process that allows you to travel quickest. Time yourself on all 3 on that thickness. Id be putting my money on mma been the best option given your mig is below spec really. The added bonus of mma is you will only need to put down a few stitches and it will be stronger than welds produced from an undergunned mig.

I actually prefer using MMA and seem to have more of a feel for it than MIG. I will be trying various methods on offcuts before a single tack goes into the frame.

 

[Kent]

I think with a bevel on the pieces that butt to the side rails my MIG would cope but it would be right on the edge of it's performance.
What sort of Amps would you consider for this job?

As Kent said I could tack the entire thing together with opposing welds before commencing with the heavier welds.

I dint say or mean that last thing you quoted me on
What I said was opposing tacks then when you weld it follow the same opposing principle with consideration of travel direction welds pull in line with travel direction
The more you weld on it they're it pulls. The bigger the gap you fill
the slower you get it done.
The less you clamp it