TIG welding question - how much heat?

I've never TIG welded - only amateur MIG & stick welding......

As part of an assembly process would the following be possible:

I've got a 304 stainless steel tube with some electronics embedded in polyurethane inside it.

I want to put a 304 stainless cap on the end of the tube capable of handling say 20 kg pulling force.

When fitted to the end of the tube the gap from the cap to the polyurethane potting material will be 1/32"

How much heat would spot TIG'ing the cap onto the tube introduce to the assembly?

Cool it with a damp cloth immediately after welding?

SPOT TIG HERE | \/ ======================================|====\ | |==== \ | | | POLYURETHANE POTTING | | O | COMPOUND | | | | |==== / ======================================|====/ ^ | SPOT TIG HERE

|---Tube 1/2" OD x 1/16 wall----------|-cap-|

Should I be looking at threading the cap into the end of the tube?

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Try gluing it, or using a "security screw" or a rivet. Or laser weld it.

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I think silver brazing would cause much less heat to the polyurethane. But think it would still be too much heat.

Think about why you want the cap on the end of the tube. If it is to prevent liquid from reaching the electronics , you might find a O-ring seal would work well.


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It looks like the cap fits into the tube from your ascii art. You could drill a small conical hole through the tube in two places where you want the welds. Use a 90 degree included angle spot drill. The small diameter of the hole only needs to be about .030". Push the cap in and TIG weld at the holes. It should only take a few seconds for each weld. This would probably be safe. You could wrap a wet rag around the tube before welding but I don't think that's needed. Just make sure the person doing the welding knows what they are doing. There is another option. If the cap could be as thick as .125" then Loctite could work very well for you. Loctite 638 will bond stainless steel quite well and will fill a gap up to .010" . If you have a cap with a .125 wide contact area and .375 diameter it will take, according to Henkel, 587 lbs to pull out. I use the stuff and it certainly seems permanent to me. Eric

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1/2-20 is a readily available size for taps and dies. Here are the other standard choices:
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1/2-32 leaves ~3/4 of your wall thickness intact.

I've been collecting 32 pitch taps and dies to cut and repair electrical instrument threads. The F connector for video cables is

3/8-32, toggle switches are 15/32-32.

Not all lathes can cut 27 TPI but the others are easy. I cut threads in thin-walled tubing part way on the lathe and finish with a die, which starts straight and doesn't deform the work if it removes only a little metal to reach final size.


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Jim Wilkins

Many thanks for the suggestions guys.

It looks like TIG might be iffy.

Threading the cap and tube would be ideal but is a lot of mucking around for this job which I want to keep cheep.

I like the sound of the Loctite 638. I think I'll try it out.

There's always lots of invaluable advice here, thanks again.

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