Torch overheating?

[chunkolini]

A small problem but it drives me nuts.
I have a Butters 180 amp welder fitted with a Parweld 250 amp torch.
Some of my work involves doing a lot of long runs of weld back to back and things get very hot, hence the 250 amp torch, I melted the insulation on the old one.

Now and again I start to get a lot of spatter and dodgy arcs etc which makes for a stressfull workplace. If I leave it to cool down it is fine until it gets hot again. Any ideas what I can do to cure this as it drives me up the wall. I think there was a post on this matter a while back and it was linked to somehing expanding in the torch and pressuring the wire, the liner?? I dont mind taking the od brew break or finding something else to do, but I like to be in charge of my work.

Cheers all, Chunko'.

 

[malcolm]

Are you using decent contact tips and wire? I've got a batch of bad contact tips which have become mixed in with good ones. Can always tell which I've picked out as the bad contact tips seem to expand and grip the wire after a few inches of welding.

 

[Archie]

At a guess it'll be the contact tip expanding when it gets hot. I use genuine Binzel tips and have never had a problem (though I use a larger torch). Could it be that you're using cheaper tips with a hole that is slightly smaller than it should be? Or I suppose by the same logic the wire could be slightly larger than is should be! Short of neither of those two ideas being the case perhaps you could look at building yourself a water cooler?

EDIT: Too slow!

 

[rtbcomp]

Strange, I would have thought an expanding tip would increase the clearance between tip and wire, after all we heat pistions to make the gudgeon pin easier to fit.

Discuss

 

[guzziguy]

i would say expanding makes the hole bigger and shrinkage would make it smaller

 

[malcolm]

There is that. Does it depend on the thickness vs diameter? I tried to do a little calculation and I think the wall thickness would increase more than the diameter.

6mm contact tip with 1mm hole
diameter = 6mm
median diameter = 3.5mm (calculate expansion at the middle)
median circumference = 11mm
Now expand 10%
New median circumference = 12.1mm
New median diameter = 3.85mm
New median radius = 1.925mm
So radius has increased by 0.175mm

Old wall thickness = 2.5mm
Now expand 10%
New wall thickness = 2.75mm
So wall thickness increase = 0.25mm
EDIT> Wall thickness would expand both ways wouldn't it - I may be wrong with this.

Probably doesn't work anything like that in real life
Anyone have any liquid nitrogen for an experiment?

 

[Archie]

I'm going to stick my neck on the block here! If we assume the tip is a simple annulus then if it is heated uniformly it should expand in all directions i.e axially and radially. Radial expansion would take place about the neutral axis. The tip is not heated uniformly but the time taken for it to reach a point where the 'sticking' occurs is great so it would be safe to assume that the temperature throughout the tip is uniform (given the small diameter of the tip this is not unreasonable). So we would expect there to be a reduction of the inner radius since there is expansion in both directions from the neutral axis.

In the case of the piston the heating is not uniform either. However, in this case the heating takes place over a much shorter period so a large temperature difference is likely to be set up through the thickness of the annulus. In simple terms the outside of the annulus (not exactly a representative model but it demonstrates the point) where it is 'hot' will expand more than the inner radius where it is (relatively) 'cold', giving rise to thermal stresses throughout the annulus. If the expansion of the outer radius is great enough then the stresses could be large enough to 'pull' the inner radius into a larger value. The extent to which this is possible would depend on all manner of factors such as the Young's Modulus of the material and its Heat Transfer Coefficient. It would have to be done quickly as with passing time the heat gradient through the wall of the annulus would move towards equilibrium at which point the stresses reduce and the case becomes the same as the first.

Hopefully that makes some sense? I haven't really covered heat expansion except in perfect gases and rectangular beams and even then it would be assumed uniform so I'm sort of making things up as I go along! Like I said, my neck is lying on the block!

EDIT: I've been thinking about this some more and I'm not so convinced of my own reasoning. For something seemingly simple it's actually rather complex. For example something I totally ignored is that the tip has length which makes it a 3D problem which brings Poisson's ratio into the equation as well. I think it would have to be solved by dividing the thick-walled section into many thin-walled sections and integrating..... Either that or fire up an FE program!

 

[malcolm]

Had a little search on the internet (which is always right of course) and it reckons:

A hole thermally expands by the same amount that the removed cylinder of metal would have under the same temperature conditions.

Which makes sense but doesn't support the fact that wire can be gripped in the contact tip.

The contact tip does need to be replaced once it's started sticking so it must be a contact tip effect, and distortion must be permanent. I'm baffled now.

 

[malcolm]

Sorry Archie - missed that one.

The outer of the tip is always cooler as it has shielding gas passing over it. This constrains expansion and all bets are off. Therefore the tip gets smaller in the middle. QED.

 

[Archie]

Well spotted Malcolm. I didn't think about the outside being cooled! Glad we've sorted that. I can quit thinking about it and get on with what I'm supposed to be doing now!

 

[rtbcomp]

I'm happy with that, the expanding metal has to go somewhere and the hole in the tip offers less mechanical resistance than the copper round the outside.

 

[English Steve]

Glad this question came up as I have experienced similar problems with the Portamig. I had put it down to getting close to the duty cycle limit, and just gone away and had a cuppa to let things cool down. Next time it happens I'll try changing the tip and press on for a while to see if that improves things - may be a while before I get the chance to experiment though as it's all tractor work at the moment.
Thanks Chunko for asking, and all for replying

 

[chunkolini]

Sounds promsing, I will try changing the tip next time, which might be a while as I have finished the job..
Cheers all Chunko'.

 

[zardoz]

Where's TechnicAl ? im sure he would have something to say on this matter.

 

[weldequip]

It could be due to the fact that your machine is a light duty, car bodywork machine Chunko, and not really designed for wrought iron work type stuff
weldequip

 

[chunkolini]

I had wondered about that, but the thermal cutout has never kicked in while doing this type of work, I am usually not operating it at high settings just for long runs of weld, also it it seems fixed when the torch has cooled down. The job it was lately causing problems was weldng the fins up on the fish I made recently see my post ''seen on the M50''

Chunko'.

 

[Jake SS]

I had wondered about that, but the thermal cutout has never kicked in while doing this type of work, I am usually not operating it at high settings just for long runs of weld, also it it seems fixed when the torch has cooled down. The job it was lately causing problems was weldng the fins up on the fish I made recently see my post ''seen on the M50''

Chunko'.

The gun has a duty cycle as well
May be shorter than the machine
Or a problem with the internals of the gun, heating of the copper cable which may be causing a higher resistance somewhere !

 

[Benji]

The gun has a duty cycle as well
May be shorter than the machine
That is something that had never occurred to me. Makes sense though, I melted a gun without the machine even warming up. Knew I was being a bit ambitious but hadnt expected the gun to go first.