How To Tac Weld Thermocouple Wire?

i need to join type-k thermocouple 24ga wire. type-k consists of one lead Nickel-Chromium (Ni-Cr) and the other lead Nickel-Aluminum (Ni-Al).
i need to tac Ni-Cr to NI-Cr, and Ni-Al to Ni-Al
can i tac using TIG and argon?
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lead
I do it in an oxy-acteylene flame - dead easy
AWEM
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really? do you get oxidation in the joint? any change to the joint composition will cause a new junction point.

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A little more explination here? Just OA torch with tiny tip? Have you done it with the #24 wire? Held in a fixture? Wires twisted?
Andrew Mawson wrote:

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you
Twist the relatively clean wires together (just to stop them moving apart) and put the tip of the twist into the reducing cone of the flame briefly until a very small ball appears, then withdraw. Depending on the wire thickness and flame size it can take from the briefest dip in/out to a second or so. I usually hold the wire by hand and dip it into the flame, but I have held the wire an inch or too from the end in a small vice and brought the torch up to it.
AWEM
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I'll try it out next week.
Andrew Mawson wrote:

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OA......, but can i use TIG to quickly fuse the wire??

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Where is your ground (return) when you TIG? You certainly can't hook the thermocouple wires up to the ground clamp unless you like a long light bulb.
Kryptoknight wrote:

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i can make a small clamp adapter, like a alligator clip. i'll set my esab 161 as low as it can go and then just pulse the current once, kinda like a spot welder, etc.

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Should be able to...this one looks to be carbon arc, though. We have a similar unit at work, much older...
http://www.omega.com/ppt/pptsc.asp?ref=TL_WELD&Nav=heas01
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That may be a capacitive discharge one. Ours forms a nice little ball at the end....
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On Sat, 17 Mar 2007 03:48:22 GMT, "Kryptoknight"

I have never done it with TIG, but capacitive discharge welding works well.
I used a home-built unit, but there's also commercial ones available. I have used the Hot Spot one and it works well but there's others. See http://www.thomasregister.com/olc/17183047/hotspot.htm
Barry
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Any details on the home brew unit?
Barry Lennox wrote:

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i'm thinking a bunch of large uF 25v caps (maybe even one of those 1farad caps the audio junkies use to filter their car amps). charge it up with 12v supply, attach alligator leads to each half of the wire close up to the join, then touch their tips together ??
i need to make straight joins for extending the cable, etc.
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I no longer have access to it, but it was pretty simple. There was a variable PSU, that could be set from about 20 volts to maybe 65 volts, this charged 1, 2 or 3 caps ( IIRC, they were about 220, 470 and 1000uF and each could be switched in or out of circuit to provide several energy levels) and a good-sized SCR to dump the cap energy on command, it used a foot switch as you often required both hands to hold everything in place.
Take a look at http://www.thomasregister.com/olc/17183047/twospec.htm to see roughly how it was used.
Elsewhere on that site you can see the electrodes that are typically used: - an earth clamp, and a modified pair of long-nose pliers.
Safety glasses are essential, if you goofed up, which was common until you got the hang of it, there was a lot of sparks and rubbish flying about!
Barry

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i just tried 2 tests.
i have stranded 24ga TC wire.
1st test was to dump (short) the charge of 12v 6600uF to the wire tips. the wire stuck, but barely.
2nd test was to try my TIG on lowest possible setting. with the fastest possible pulse i managed to only turn the wire into a mini sparkler. tig wont work.
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Maybe it could be made to work by striking an arc between the electrode and some scrap metal and them simply sticking the TC wire into the arc (or moving the arc near the wire) - but keeping the wire isolated from the welding current path?
I've also heard of a trick where a guy used only the HF from a Tig machine to weld aluminum foil (don't know if it was true or not). You would probably need a separate HF unit to be able to do that without turning on the welding current at the same time.
Another thing I might try experimenting with is adding a resistor in series with the welding current to reduce the current to something that was workable for this application. This might be pointless for a constant current inverter machine but should work with a transformer based machine.
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On Sun, 18 Mar 2007 22:38:16 GMT, "Kryptoknight"

You will need a higher voltage. Remember that energy is 1/2 x C x V (squared). So, 4 x the volts, say 48 volts, will give you 16 x the energy. I found that the lowest setting, about 20 volts, was hardly any use at all, although another user used lower voltages for very fine TC wire, maybe it was 34 or 36g.
Barry
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i just picked up a 17kuF 100vdc cap from a local eletronics store. i get my new power supply tomorrow, it can go up to 60vdc, will test welding with this.
can i used copper 1/2" dia. rod to sandwich the leads together between +v and gnd, then trigger the current with a SCR ? or do i need to use carbon rod ?
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Tip diameter needs to be a few x the wire diameter. The units I've seen seem to have 1/8" or 1/4" copper electodes.
Kryptoknight wrote:

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