Welding in captivated nuts?

[knoba]

What's the best way to go about captivating m6 nuts in16swg 50mm round hollow section 304 stainless steel? The tubes are 1m long and the threads are, supposed to be, in the middle.

I'm using a MIG and stain-shield with a purge line, I've tried drilling 4mm holes either side of the main7mm bolt hole to aid penetration, but the welds look like they've only just caught the nuts...

Is there some mystery technique for doing this kinda thing?

 

[wyn]

Sounds like a very fiddly job.

The 6mm thread is real close to the 10mm (across the flats) nut. That will be a nightmare with a MIG

I'd be tempted to use some 20 or 25mm round ss bar, drill and tap the thread down the centre. drill the appropriate size hole in the tube, weld in flush at the top and grind the side down to match the tube. this should keep the welding away from the threads and give you much more to weld to. Or if you are bolting something flat to the tube don't grind it down just leave a flat surface to bolt down to.

Don't know if that would be any help.

 

[knoba]

Thanks Wyn, I hadn't thought of that as an option, but it is a very good idea. I have some OSG s/s taps and dies that are very good for that sort of thing.

So... it's another sprat to catch a mackerel sub-job. "metalwork" it's more work than metal. I still find it hard to believe how long jobs take to set-up and you're right about the MIG! It's OK but not where I want to be on stainless.

 

[malcolm]

It's common for old cars to have the thread tapped into a thick rectangular sheet which is spot welded to the panel. A variation on this would be to drill through a small rectangular section of tube, weld the nut to it while you have access, then hold it in the tube with a bolt, and plug weld through a couple of holes either side of the bolt. Variation on what wyn said I think.

 

[Welderpaul]

I've had this problem before.
I used Rivnuts/Nutserts - same idea as a pop rivet but with an integral thread in the centre of the 'rivet'.
Drill a hole in the centre of the tube (V blocks centred under a drill press) and insert the rivnut. Squeeze it up and hey presto - an M6 tapping just where you need it.
The Rivnut will leave a very narrow 'head' on the top of the hole - it depends what you are fixing onto the tube as to whether this would be a problem or not!
Every nut that is welded in would need dressing etc to remove the blueing - very fiddly and labourious!

Here Yaaa. http://www.zygology.com/products/ti...aded inserts&gclid=CJG2o-XdhZUCFQcUQgodsw-Nqw

Other way of course is just to tap it out. Even with very thin wall stainless cos it is harder than mild steel it will create a strong thread even with very little meat in the wall - depends what you are doing really!
What are you making?!?!?

 

[knoba]

malcolm:

It's common for old cars to have the thread tapped into a thick rectangular sheet which is spot welded to the panel.

I can only remember old cars exclusively having nuts and bolts (Triumphs, Type 1's and the master cylinder job on a Morris minor come to mind for some reason! - God... I'm getting old!) But, I know what you mean and thank you! it's a good idea that expands the available arrangements greatly. :thumbs:

Welderpaul: The rivnuts are a good idea too (thank you!). I'm still learning about Stainless, here, and I wanted an all stainless piece (apart from a solid oak shelf 32x630x360) to help me come to terms with working with it. It's a TV stand for a 40" LCD weighing in @ 31kg and the job is for me.

I've been recently umming and arring over a Miller Dynasty 200DX AC/DC TIG. If I acquired one of these could it / would it change the game plan?

CAD;

 

[fingers99]

I think this is how I'd do it. Get or make some stainless "penny washers". Bend them to match the i/d of the tube. Tack weld the nuts to the back of the washers (use a peice of bar, rather than thread, to hold in position). Drill the tube for the nut and make 3 additional holes for plug welds, then chean up and plug weld the washer/bolt in place.

Or, can you knock a recess in the tube to take a nut or a threaded fitting (or, better, a non threaded fitting which you can then drill and tap (to make alignement better)?

TIG (even DC) would make the job easier.

If you were doing it for someone else as a paid job I'd suggest the rivenut way -- much, much quicker.

 

[Paul.]

I agree with welderpaul on this one, nutserts are the only way to make a professional job of this, if you try to weld nuts down one side of a tube it will bow like a banana, not to mention all the cleaning required.

 

[knoba]

I've already attempted to weld all the nuts inside the tubes with little or no distortion, so maybe, for this particular job; the materials and the set-up it's a moot point. However, when I was graining some 50x3mm the heat caused the metal to warp rather like a bi-metallic spring. I've had some really bad days with this job lately...

I'm warming to the nut on washer combo sort of affair though and was quoted 3300 sobs today, all in, for the Miller including a contractors pack (pedal, torch etc.).

As far as the cleaning is concerned cant I just clean up the areas that show?

 

[fingers99]

Don't see why not. You'll get a better weld if you can apply purge (maybe a disposable cylinder and reg) inside the tube.

You could use 1200 wet or dry and autosol to clean up and polish the outsides.

A way which might be easier still (assuming you have a V block and a drill press) is to re-design and make the bolts captive.

 

[knoba]

That's thinking outside the box Fingers99. Top man! I still like the idea of the washers though. I guess I'm just trying to satisfy something in my own mind - I have a bigger project coming up and would like to use the same technique.

The Mig is set up with a T line after the flow meter so I get the same rate and the tube being welded was 'mostly sealed' with masking tape.

I'm using a Cibo Finimaster for the finishing work which does grinding, graining and polishing very quickly and nicely. I'd read somewhere that stainless needs its welds cleaning with pickling gel or electro-something-or-other to re-passivate (anti rust) the surface and prevent other "corrosive effects", but, I haven't tried either and haven't read much more on it than that.

I made a stainless steel TV aerial for my old man a little while back. He loves it but I'm just OK about it. All I had to weld was the dome caps on the ends and the stabiliser for the K bracket, but it was a real struggle (for me anyway). If I can gradually extend my; knowledge, repertoire and tooling then maybe I can be more than just OK.