Motorcycle 'prism' tank - shrinkage problems

[Ali]

Hi Guys,

A member on a motorcycle forum I use has been building a seriously cool 'prism' type chopper tank, and is having some problems. I told him how helpful you lot are and he's asked me to call on your advice .

I won't post a link to the other forum as I'm not sure about the rules here on links.

Anyway, here's what he's building:

The sheet metal part goes to plan. He has the steel folded up by someone with a press brake, then tacks it together:

Then welds up the seams:

Then he carefully cuts out a channel in the base of the tank, and fits a tunnel so that the tank sits correctly on the backbone of the bike frame. To make the tunnel, he's built this extremely nifty tool:

Awesome.

The tunnel then gets fitted & welded in:

Here's a close up of his Tig skills - he says he's a beginner, but he knows what he's doing...

So far, despite all odds there hasn't been any major issues with the sheet metal, which I believe he has welded without filler. NOW the problems.

A filler neck needs to be fitted to the tank - got to get fuel in there somehow, right? This is a threaded tube, not sure the dimensions, but we're probably talking about a wall thickness of 3-5mm looking at the pictures. This requires a little more heat to weld in, which is where the shrinkage occurs to the sheet metal.

He admits he went for it a little on this bit, as it was going well.

Here is the shrinkage:

 

[the snooper]

that's distortion, probably as he has welded to the filler neck its expanded and when cooling has no way of moving back so it pushes the panel down

 

[Ali]

OK, I ran out of picture space.

"Below I have dotted the dip.... in the middle being the deepest... arrows showing where the dipping starts."

There were suggestions aplenty on how to fix it, he found a way but isn't completely happy with it:

So he's making another.

Here is his setup:

As I am positing this, I realise I need more information on his kit. I'll ask. I think it's an Esab DC set.

Basically, all the above taken into account, he is seeking advice for next time on how to avoid shrinkage when welding in the filler neck, and general advice on how to avoid shrinkage when welding the seams of this particular shape (even though there wasn't any apparent problem with this stage). There have been a few suggestions so far, and one he has come across involves cooling the welds with compressed air. I would think that this a Bad Idea?

The sheet metal is 1.5mm I believe.

I'd really appreciate advice to pass along to him, and I know he would be grateful.

Thanks!

 

[the snooper]

id say less heat and braze the filler neck in, heating the neck up first and only just enough heat for the brass to flow into the joint.

others will be along with more ideas I would think

 

[cumbriasteve]

It may have been an idea to put the filler tube in before the tank became an enclosed item, you would then have had access to both sides with a much better chance of correcting any distortion.

It would also have allowed the use of heat sinks.

I only did a couple when younger and always brazed them.

 

[wyn]

Maybe next time weld the filler neck in while the tank is in two halves then it can be "adjusted" back to flat from the inside with a hammer.

 

[Ali]

that's distortion not shrinkage, probably as he has welded to the filler neck its expanded and when cooling has no way of moving back so it pushes the panel down

Thanks. Sorry to be an idiot, but could you explain the difference between distortion & shrinkage?

 

[Ali]

It may have been an idea to put the filler tube in before the tank became an enclosed item, you would then have had access to both sides with a much better chance of correcting any distortion.

It would also have allowed the use of heat sinks.

I only did a couple when younger and always brazed them.

Yeah, I think that will be his plan for the next one. I suggested heat sinks - aluminium clamped to the tank panels around the filler neck? It's an awkward shape though...

Maybe next time weld the filler neck in while the tank is in two halves then it can be "adjusted" back to flat from the inside with a hammer.

But surely there is a way to do this without distortion? What about pulsing it (manually or otherwise?).

 

[ronan]

Triumph unit 650/750 frame ? He put too much heat into the sheet imho, if he welded a bit and let it cool, and had everything braced and clamped so it couldn't move, it would be better. I had the same many thing years ago with a stainless steel oil tank i made for a featherbed frame. Didn't use it anyway as it weighed a ton, looked lovely polished though.

 

[Tigman]

it looks like a lot of heat has gone into that weld !

 

[addjunkie]

Sheet almost always distorts some what, when you heat it. Thats what bodge is for......

 

[Paul.]

Good fit up, no gaps is the first thing you need, then use a 1.6 tungsten ground to a needle point, use a small filler rod no bigger than 1mm, or even a bit of 0.8 mig wire, that will melt quickly and allow you to move faster, tight arc is very important to keep the heat focused

Turn the amps up a tad, I would probably hit that at 55 to 60 amps, weld a short run then hit it with compressed air to quench it, then weld a different part of the joint (skip welding).

Try to set the job up so that the weld is running downhill a bit, (perfectly acceptable on sheet metalwork)

If the welder has pulse, turn it up to 100hz or more, this will stiffen up and focus the arc,

 

[Maxakarudy]

The low area was cause by the HAZ around the filler neck, shrinking and drawing the surrounding metal towards, creating the hollow.
As you've been advised the filler neck should been welded before the tank was sealed up, now it's a rescue job, a scim of filler won't hurt

 

[hotrodder]

Thanks. Sorry to be an idiot, but could you explain the difference between distortion & shrinkage?

Distortion is the result of shrinkage. Welds shrink and locally heating metal usually causes it to shrink too... heat a lump of metal up evenly and it'll expand in all directions and then shrink evenly as it cools with no net change in shape or dimension. Heat a spot up and it's restrained in some directions by the surrounding cooler metal. This forces it to expand more in the directions it can. As it cools it's free to shrink in all directions and so it changes shape causing distortion. Another example would be to lightly clamp a piece of bar in a vice and heat it. The vice stops it from expanding in one direction so it expands more in the other two. It shrinks evenly as it cools though and then falls out of the vice because it's now shorter in that direction. This how the 'trick' with wheel bearing races works, run a weld bead around the race, it shrinks and then (usually) falls out

As said the weld and area around it shrank as it cooled. Geometry of the part makes it fairly stiff in most areas around the filler neck and so better able to resist the stress. Result is that distortion was concentrated in the flat area that has little stiffness

As said less heat input would have helped a chunk. Another method not mentioned yet would be to tip/form a flange in the tank skin where the filler neck goes... easier to picture with a tube/filler neck into a flat sheet where dimple dies to bell the edge of the hole are commonly used. This'd stiffen the area further reducing distortion

 

[Ali]

id say less heat and braze the filler neck in, heating the neck up first and only just enough heat for the brass to flow into the joint.

others will be along with more ideas I would think

Not sure he has access to brazing gear. This crossed my mind too, only I was thinking silicone bronze with tig.

Good fit up, no gaps is the first thing you need, then use a 1.6 tungsten ground to a needle point, use a small filler rod no bigger than 1mm, or even a bit of 0.8 mig wire, that will melt quickly and allow you to move faster, tight arc is very important to keep the heat focused

Turn the amps up a tad, I would probably hit that at 55 to 60 amps, weld a short run then hit it with compressed air to quench it, then weld a different part of the joint (skip welding).

Try to set the job up so that the weld is running downhill a bit, (perfectly acceptable on sheet metalwork)

If the welder has pulse, turn it up to 100hz or more, this will stiffen up and focus the arc,

So quenching is not a bad thing in certain circumstances?

Distortion is the result of shrinkage. Welds shrink and locally heating metal usually causes it to shrink too... heat a lump of metal up evenly and it'll expand in all directions and then shrink evenly as it cools with no net change in shape or dimension. Heat a spot up and it's restrained in some directions by the surrounding cooler metal. This forces it to expand more in the directions it can. As it cools it's free to shrink in all directions and so it changes shape causing distortion. Another example would be to lightly clamp a piece of bar in a vice and heat it. The vice stops it from expanding in one direction so it expands more in the other two. It shrinks evenly as it cools though and then falls out of the vice because it's now shorter in that direction. This how the 'trick' with wheel bearing races works, run a weld bead around the race, it shrinks and then (usually) falls out

As said the weld and area around it shrank as it cooled. Geometry of the part makes it fairly stiff in most areas around the filler neck and so better able to resist the stress. Result is that distortion was concentrated in the flat area that has little stiffness

As said less heat input would have helped a chunk. Another method not mentioned yet would be to tip/form a flange in the tank skin where the filler neck goes... easier to picture with a tube/filler neck into a flat sheet where dimple dies to bell the edge of the hole are commonly used. This'd stiffen the area further reducing distortion

Thanks for the explanation. I'm liking the flange / dimple idea.

All great advice, thanks folks!

Any mileage in having an internal rib to reinforce the problem area? Or am I over-thinking this...?

 

[hotrodder]

An internal rib or rolling a stiffening bead into the flat area won't fix things by itself for example draw a line across the tank and



it highlights that the entire area around the neck has 'sunk' a bit. Too much heat input was the biggest culprit there, fix that (whether welding or TIG brazing etc) and the amount of shrinkage & resulting distortion will reduce lots

 

[Ali]

Shame it's not on the flat, you could turn a fine lip on the filler and do it from the inside.

 

[hotrodder]

Won't have anything directly to do with distortion but just had a thought which'd be worth asking him... does he know what flavour of steel the filler neck is made from and, slightly easier to answer if it was bought, what did it weld like?

I've done several motorbike tanks where the owner has supplied the cap/neck having either bought something they liked the look of or had made one. Not always but more than a few times they've been horrible to weld because whoever made the weld in fitting made it out of a free machining flavour whether it was leaded carbon steel or 303 stainless which has a load of sulphur in it. A fairly common way to deal with their nastyness it turn the juice up a bit and stuff more wire in there to combat their tendancy to uncut. If he did that...

 

[Burdekin]

Try planishing around the filler neck to stretch the metal there and bring it back flat. Will need a dolly to fit inside. Best to use a flipper to planish.

He's stretching the wrong area, it's the weld and directly around it he needs to stretch. And just go slowly so it doesn't end up all marked and dented.

This is how I think would be best to weld it. When welding it in the first most important bit is the fit is critical, no gap and just some prep to ensure the least amount of distortion. I'd then start tacking in place, each tac going round the filler and spaced at the end of each heated effected zone, wait a little for each tac to cool. If you get distortion planish at the tac when it is cool. Best would then be to weld from tac to tac and let cool, planish as you go if you get distortion, but that won't look as good as one continual weld. So if welding in one go it might be fine and just need to planish out the distortion by stretching at the weld where the stresses are from the welding.

 

[Maxakarudy]

All the advice about shrinkage/distortion is spot on, however no welding method leaves sheetmetal distorion free, you just have factor in how you are going to deal with it, it takes planning in advance and you the need to learn plannishing techniques that recover the shrinkage, but just keep it to the HAZ, everything else will fall in line if you get that right.
Fail to plan is a plan to fail