[Peter Dow]

Wow!

That's a lot of questions all at once!

Well thanks for attempting any answers at all. I am getting ahead of myself as usual since I haven't even chosen a welding machine to buy yet.

If you try to weld that in one go, the metal will expand ahead of you and the gap will grow. You'll want to do it in short sections, far apart from each other.

So that's what you meant by "skipping around"? Understood.

Suppose you want a 1.5mm gap.

Gap where? Between the two sheets to be welded?

If you suppose you want a gap there that means you must fill the gap with the material from a steel welding rod?

That means you are supposing it's better having a gap and filling it than to attempt to simply fuse the two sheets together using a zero gap, clamping the sheets so that they press against each other at the butt and simply applying heat via the TIG torch to fuse the two sheets together?

That's not a technique I've ever tried with my oxy-acetylene car repair since the edges to be welded were not straight edges and getting a really close fit between the body work and repair panels was out of the question.

But for this task, hopefully the sheet will come supplied cut perfectly square with nice straight edges and so maybe a close-fitting clamp and fuse weld might be possible, and simpler and quicker, with no need for a filler welding rod and a much easier clean up of the weld, doing it that way. Feasible?

Cut a piece of 1.5mm wire about 5cm. Bend it to 90*, and grind a "flat" about 1/3 through one leg. Use this to tack the material together, as a spacer. Using the broad side as your gauge, make your first tack, about a cm above it. Let it cool, turn it 90* and pull it out. Insert it again, skinny side, about 2.5-3cm below, and turn 90* to push the metal apart, and again tack above. Let it cool... repeat every 2.5-3cm, until the whole thing is tacked up.

Then weld between tacks, one near the top, one near the middle, one near the bottom, staggering where you're welding giving each time to cool.

Hmm. This seems quite time consuming. It's a 150 cm weld and tacking every 2.5-3cm means 60 - 50 tacks and your suggestion of fiddling about with a wire and letting it cool between each tack means it will take a long time. Yours doesn't seem to be a method which is suited to automating the task, allowing a welding novice, like me, to complete the task in no time, with little chance for error.

Unless the job is pretty much fool-proof on the day, I fear that I would make a mess of the weld necessitating a major re-think. I'd rather do my thinking in advance than on site.

Ideally I'd want to keep my torch in hand to tack the whole 150cm following prepared 2.5-3cm indicator marks on the sheet or clamps. Maybe complete the tacking in half an hour, which is maybe 30 seconds at most between tacks.

Then set up for the welding between the tacks and then complete that part of the weld in an hour. That sort of time scale would be acceptable.

If you can use copper (or aluminum) strips as heat sinks, this is an advantage, for all the reasons you mentioned.

I'd like to put some thought into what the best arrangement to cool the work might be. Since it is a straight vertical line it may not be too difficult to integrate a water-cooled pipe into the clamps?

Might it be possible to design a tacking strip heat sink that cools only the parts not to be tacked, allowing the tacked parts only to be easily heated to welding temperature?

I've got the time - weeks, months maybe - to think of a smart solution for this particular welding task which may involve designing and building a lot of custom tooling but which would save a lot of time on site while doing the weld.

A groove in the strip of copper or aluminum allows the root to go where it naturally would, as opposed to being "blocked" by metal in the way.

Steve S

Hmm. I'm not too keen on allowing molten metal to go where it naturally would. I really want to end up with a neat weld, not a "natural" weld with the molten metal flowing as it likes.

So grooves in the backing strip or bar where I want the tacks, yes, but perhaps fill the grooves with a high temperature insulator, to keep the molten metal in place?

On closer examination of the video, the bend / ridge in his backing sheet was more or less at right angles to the line of the weld. My guess now is that it is more likely it was an old sheet which had been bent for some other task and then later flattened for the backing sheet function.

Now back to stitch welding. Here'a nice photograph of a butt stitch weld.



from here.

So they had a gap and filled it from a welding rod.

OK, but assuming a 1mm thick steel sheet, how small is it possible to make the stitches, tacks or spot welds and how close together can they be, using a TIG (Tungsten Inert Gas) torch, with pulse?

For example can you have stitches which are 2mm long, separated with a pitch of 4mm, with a 2mm unwelded part between each stitch. Or is that not possible?

[Otto Nobedder]

I'll try to stay straight on the meat of the matter this time...

Yes, ideally with a good square cut you can simply fuse weld this bead with no filler. This is the common practice in the food and beverage industry. Do a search on "sanitary welding", and I'm sure you'll find tons of specific information, and maybe even better suggestions on your specific application.

As for the tacking, yes, laying out a series of reference marks, say 3cm apart, is the way to go, but you'll still not want to start at one end and work straight to the other. This is for efficiency as much for finish. Pick a starting point, perhaps the middle, place a tack, then tack 12cm above, then below the original tack. This way you aren't waiting for tacks to cool, and can keep going. Then, return to the starting point, and, in the same sequence, tack the mark 6cm between each of the previous tacks, and so on until it's all tacked up. Then you can start at one end, weld the space between two tacks, skip the next, and keep going until you're half done. Then go back to the start and fill in the blanks, and you're done.

As for cooling, you did say this is stainless steel? Since you'll want a back-purge anyway, at this low heat the purge gas itself can be your coolant. A pipe (thin copper or aluminum, soft) sliced in half and hammered a bit on the bench to broaden/flatten it some can be taped to the backside of the weld (the sanitary guys can suggest a high temp-low cleanup tape... I've seen it, but not used it so don't know the brand), and argon (or nitrogen, if you have access... it's cheaper, so you can flow it faster for cooling) pushed through it to both cool and backpurge the weld.

A comment on nitrogen as backpurge... 300 series stainless steels can suffer "nitrogen embrittlement", meaning for a fused weld, the inner face of the weld may be effectively "case-hardened". This only matters if the duty is pressure, or sees sudden temerature cycles.

If you're doing a tight-fit fusion weld, you wont have to worry about what the backside is doing, as long as it's shielded from oxygen.

I don't think I've asked; What is the finished product to be?

Steve S

[rake]

I don't think I've asked; What is the finished product to be?

Steve S

Scotland? It must be a distillation vessel of some sorts.

[Otto Nobedder]

I don't think I've asked; What is the finished product to be?

Steve S

Scotland? It must be a distillation vessel of some sorts.

Unlikely. In Scotland, they know to use copper for that process...