terminating fine nichrome wire to copper?

[coppercone2]

So if you have a potentiometer thats a bastard to get, it seems that you could wind the nichrome part easily. The wiper is also precise but just a piece of metal. It can be  etched or EDM if you really annoyed.

But how do you make the bonds to the other two potentiometer parts?

Is it just crimped? Is it spot welded? Like a clarostat.

When you start dealing with broken ganged stuff, it seems appealing to know how.

It looks like its just a piece of copper wrapped around the nichrome coil. 

[David Hess]

It is crimped or welded.

[MikeK]

Crimp, yes.  Would welding even work?...The metals are too dissimilar.  Silver soldering would probably work, if that is suitable.

[Haenk]

ultrasonic welding?

[WattsThat]

Spot welded. You would be amazed at the differences in material and thicknesses that you can electrically connect that way. Not mechanically, just electrically. Mechanical reinforcement is most times an epoxy.

[coppercone2]

but the contact area is big. How do they spot weld that? It seems that it would be like unreliable.

do you just wrap nichrome around whatever, wrap copper around it, and sandwich that junction in a spot welder?

I have a spot welder but when I try to weld a copper resistor lead to a utility knife blade, I could not get them to stick, even with power levels that blow a small hole in razor blade (5 watt metal oxide resistor)

I was trying with the kweld

is my problem that I have the inverse of a good weld, which should be big copper substrate and steel lead, not copper lead and steel substrate? I guess too much energy is dumped in the copper and not enough in the steel.. when I finish my chassis I can try a steel wire to a copper sheet, rather then the opposite.. UNLESS ITs an issue with prep, I did not really prep it because I was out of alcohol and scotch brite. Don't wanna work on it more till I finish the chassis though, its kind of scary with the super caps I put on it

[enut11]

So if you have a potentiometer thats a bastard to get, it seems that you could wind the nichrome part easily. The wiper is also precise but just a piece of metal. It can be  etched or EDM if you really annoyed.

But how do you make the bonds to the other two potentiometer parts?

Is it just crimped? Is it spot welded? Like a clarostat.

When you start dealing with broken ganged stuff, it seems appealing to know how.

It looks like its just a piece of copper wrapped around the nichrome coil. 

This is a WW trimpot from an old Fluke differential voltmeter. The connection between the resistive element and the outside world seems to be just crimped.

[coppercone2]

what tool would you use to crimp that? Like how much deformation over what surface area do you need for a nichrome copper junction?

[enut11]

Tough question. Depends on the pot. Can you post some pictures?

[richnormand]

So if you have a potentiometer thats a bastard to get, it seems that you could wind the nichrome part easily. The wiper is also precise but just a piece of metal. It can be  etched or EDM if you really annoyed.

It looks like its just a piece of copper wrapped around the nichrome coil. 

This is what I use:
https://www.ebay.com/itm/354279471860?hash=item527cb3baf4:g:vagAAOSwY1tjIc9Z&amdata=enc%3AAQAHAAAAoMtaoCNPuQDf7Aqljd9FFa1rcRtnk%2Bx%2BxKs2ZXNl7hb6uNdzsa%2Brl0VXm4TExDCcU91BC54WHQ5yF%2BIUGqDbooEJ8OLN3XOoRT19RwwCUK5oXgSOj%2BtsHBrKDhkfd2cKgs3KUaKLkMt042LSS%2Ffe1UO9aWKkfo2q3DO8ns5xKYuOrVEAcWediqfV5rtPnPVyOpeq63NistpSMa6VOaD1yFk%3D%7Ctkp%3ABk9SR_qfv9GRYQ


 
Do a search on sparkle welder for jewelry.

So, in welding a large metal part to your fine nichrome wire I would put the ground wire on the large metal part and wrap a few turns of the nichrome on it or near it. Then bring the tungnsten needle near the wire but spark on the large metal part so no current flows in the wire. The proximity of the plasma discharge with tack weld the wire without destroying it.

Takes a bit of practice and adjustments. Once you get familiar with the machine and the technique it works well.

I use it to change broken filaments in ion vacuum gauges and fine stuff that would get obliterated by a standard spot weld machine.
Neat thing is you do not have to apply any pressure or touch the fine wire to be welded just have the plasma discharge near it..

Do not use near electronics. It will reset my laptop if within half a meter.

Hope that helps.

[coppercone2]

so that is like a pulsed tig type technology?

What is the electrical difference of the weld signal? That is like a very low duty cycle tig, not a capacitor load dump, if it uses tungsten? So voltage is much higher?

[richnormand]

Somewhat related to TIG as you pointed out. Mine is similar to the one in the link but they are available with a range of power and some models have a small regulator for argon, much like TIG at that point.
The tungsten needle I have is about 10 cm long and has very little wear despite a fair amount of use. You can get about one pulse per second for a while and then give it time to cool off.

The unit is definitively not for heavy industrial use but it is turning out to be quite useful under the stereo microscope.

I put a standard camera UV-block filter glass taped in front of the objectives to block some of the sputter reaching the lens surface and UV light.
In the open air and under the microscope I do wear a battery operated ND attenuator to protect my eyes from the UV and the flash (not the big stuff for MIG but small eyeglasses type that does the same).
The attenuator is sensitive enough to trigger via the scattered light with a white piece of paper on the side of the bench.

A quick look at the insides shows a bank of big caps, an inverter supply after the mains and a few large scr or MOSFETs. Voltage must be fairly high to have a reliable spark over a mm with a definite crack sound and plasma ball of the same size.
Construction quality is so-so as you would expect at this price point but.... it does work.

[langwadt]

tried with an aggressive flux like for plumbing? with that stainless solders with regular tin-lead

[coppercone2]

I have not tried it at all but it does not look like they soldered it in the factory

[David Hess]

tried with an aggressive flux like for plumbing? with that stainless solders with regular tin-lead

I have done it with silver brazing, but the high temperature tends to melt the thin nichrome wire.

A better way I have seen is to electroplate a thick layer of copper onto the nichrome wire and then use low temperature solder.

[coppercone2]

tig

DO you think a tig welder can do this if its set to the correct parameters? I have a broke tig welder I have been avoiding because acetylene is working fine, but if it can do fine spot welds I would be interested in trying to fix it again. Or a mod to add to a tig welder.

I can take another shot at a board or replace a entire PCB because the cost of the extra china spot welding machine would balance out the board replacement.

[SeanB]

Copper will not weld to steel, it is used as a shield when you do not want to have the weld stick to a surface when filling just for that. Typical methods of terminating those nichrome wires are to laser weld, or simply have the nickel wire used to make the connection, with the nichrome wound around a few times, then a small nickel sleeve crimped on, or spot welded to the lot after it all has been done. The nickel wire is solderable, and accepts regular solder with regular rosin flux, and your solder joint is not close to the fragile nichrome.

Most common method is to take the nickel wire, flatten to a more or less wide oval, and wind the nichrome around the bottom half, and fold the top over, pressing firmly, with the lead of the nichrome in the middle. Then a drop of epoxy or paint to protect the junction, and away you go. Used in the genuine Bourns multiturn pots for the high values, where the fine insulated nichrome wire is wound around a length of enamelled copper wire, and then cut to length, with the ends unwound to attach the lead, then the lead and the copper core are dipped with a tiny amount of epoxy to hold the fragile wire, before being assembled. Comes in the cost, those hand made things are expensive to make in volume. Must be a whole floor of machines winding them, then another floor of cubicles where they get assembled.

[LaserSteve]

For example, folded Ni foil or Ni slip fit tube and spot weld the Ni over a tightly wrapped coil of the Nichrome to the work.

Our fine wire thermocouple welder at work uses a 30,000 uf
Low ESR Electrolytic and a 0.5 to 24 VDC charging supply.
A pulse transformer drives an SCR's gate via a small series resistor. A burst of gate pulses is used to keep the SCR triggered during a weld.

One electrode is usually one side of the work. The other side is some sort of carbon and copper filled sintered block, possibly tungsten that just won't weld under normal energies.

Feed wire to the load is a heavy, very flexible arc welding cable.

We weld a TC alloy very similar to Nichrome
To copper all the time. Not a pretty weld, but tough and conductive. For equal diameter wires, usually a nichrome bead forms around the copper. I know I have a good weld if I
apply a second, smaller pulse, and nothing happens but possibly the leads flopping around with no bang or flash.


If I had to do it all day I'd buy a Sunstone microtig with the retracting electrode and argon gas flood.

There are Sunstone clones on the Bay and Zon, but a true Sunstone has a double pulse. First pulse cleans out any oxide,  second pulse is the weld. Self retracting tips work well with fine wire, as stated above, melt the local material just under or beside the fine wire.

Retracting tip clones are Often at  few hundred Dollars, vs say 3K for a low end Sunstone. The Sunstone is worth every penny.

https://sunstonewelders.com/products/

We have one at work in a sister department. However due to a persistent rumor that argon is optional, and frying the optional  resistance welding  tweezers with max power on both pulses is fine, I usually have to order handpiece parts before I can use it. The graduate students are in a hurry and they forget that less is often more.

Jewelers use pulsed Nd:Yag lasers for the fine stuff.
Lots of local jewelers have them for repairs.  Not that expensive for them, but worth its weight in silver in less then a year of use.

I never do any of these procedures with out at
Least OD4 or OD5 dark eye protection.
Steve

[Berni]

You can use wire ferules to make a crimp connection to a strange wire material.

The ferules squeeze down from all sides when crimped so even if you put a thick and thin wire in there it will make a solid connection. You can always test how good the connection is by trying to forcefully pull the wire out of the crimp. If you want to really make sure it wont come out you could even wrap the thin wire around the thick wire before applying the crimp.

Best of all is that the tools for doing it are inexpensive and found in just regular hardware stores. It is also a nice tool to have around since ferules make working with stranded cables so much nicer. (They are even required by electrical code when terminating stranded wire into screw terminals as doing so without can be a fire hazard)

[coppercone2]

I guess you can use a ferrule if you crimp it without the substrate inside the end of the crimp and glue it on, but in the case of these potentiometers it looks like the material has the plastic core inside of it, which made me think weld