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I have got an iron nut with a diameter of about 5 cm. I would like to solder a pin to it so I can connect a wire to it. (I want to make a capacitive sensor.)

I tried it already, but it won't attach to the nut at all.

How can I do that?

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    \$\begingroup\$ That is why I bought a 300 watt solder iron, for bus bars and such. Use 60/40 rosin core solder. Else you will have to tin plate it first. \$\endgroup\$ – Sparky256 Mar 25 '18 at 19:56
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    \$\begingroup\$ Warm the nut up on your gas/electric stove, you'll probably get it to 250 Celsius. Don't even try to add the wire until the solder starts to melt on the larger piece of metal. You'll need a jig to hold things steady until it all cools down. \$\endgroup\$ – tomnexus Mar 25 '18 at 20:20
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    \$\begingroup\$ Clarify please - are you applying any flux at all or just depending on whatever is inside your solder ? \$\endgroup\$ – Criggie Mar 26 '18 at 1:12
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    \$\begingroup\$ drill a hole in the nut and use a screw to attach a lug \$\endgroup\$ – jsotola Mar 26 '18 at 5:01
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    \$\begingroup\$ If the nut is very old or may have come from modern or old commercial or military aircraft hardware, ensure that it is not cadmium plated before attempting to heat it. \$\endgroup\$ – Spehro Pefhany Mar 26 '18 at 9:37
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You probably have two issues: getting enough heat, and surface compatibility. Iron (more likely steel, likely plated) may not solder easily, but it can be done with care to get the surfaces extremely clean and flux. Heat wise, you might use a soldering gun, heat gun, hot plate, (non-food!) oven, or even plumber's propane torch to pre-heat it. Be careful however not to get it too hot - zinc based plating is a respiratory hazard if it boils off, solder will not behave properly when a joint is far above the appropriate temperature, and excessive heat will just make the metals oxidize faster, which is your real obstacle to making a good connection..

A far better method of connection might be to cross-drill the large nut and tap it for a small machine screw which can affix your wire directly or better yet with a crimped terminal ring. Using thread cutting taps is a skill, but one worth learning. And you probably have six faces on the nut to work with if you accidentally break a tap in the first hole you try...

For anything used outdoors or in a tough environment you might also need to worry about corrosion between dissimilar metals, but this sounds a bit more like a temporary hobby or science fair project. Of course if this is a very temporary initial "does this even work" proof-of-concept, you might also just be able to wrap the wire through the nut a few times and twist tightly - but that may be unreliable, and for good reason looks a bit unprofessional.

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  • \$\begingroup\$ Do you think soldering iron tip cleaner would work well to tin the nut at normal electrical soldering temperatures, so that there is less chance of producing zinc vapour? \$\endgroup\$ – Andrew Morton Mar 26 '18 at 9:18
  • \$\begingroup\$ If you had nothing else suitable it might work. But there might be some confusion of issues - plumbing soldering isn't really done at a notably higher temperature than electronics soldering, the issue is more the residual aggressiveness of plumbing and craft fluxes. Zinc is more an issue when welding or with high temperature brazing. If I wanted a solution to last, I'd go the drill and tap route anyway, you should be able to find everything at a hardware store. \$\endgroup\$ – Chris Stratton Mar 26 '18 at 15:56
  • \$\begingroup\$ @AndrewMorton The odds of this process creating metal vapors of some kind that are at least mildly bad for you is quite high. This is true for any soldering. Protecting yourself from this is a much better strategy than hoping nothing bad is produced. Fume extraction and working in well ventilated places are common suggestions. safety.eng.cam.ac.uk/procedures/Soldering/soldering-safety \$\endgroup\$ – user52386 Mar 27 '18 at 2:12
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    \$\begingroup\$ @BaileyS ventilation is always a good idea, mostly because of flux fumes, but so is factual awareness. The boiling point of zinc is 907 C, far above soft soldering temperatures, and even above those of popular high-silver brazing (hard soldering) alloys. But base metal brazing and welding can indeed be a concern. \$\endgroup\$ – Chris Stratton Mar 27 '18 at 2:21
  • \$\begingroup\$ @ChrisStratton Lets say metal fumes. There are definitely metal oxides being freshly formed at normal soldering temperatures and possibly dispersed by flux activity. The point remains; it is really hard to know what is in there without analytical chemistry, and really dangerous to assume that the stuff solder is made of would never be in the smoke emanating from it... \$\endgroup\$ – user52386 Mar 28 '18 at 21:21
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You can try to drill a small hole such as 2.5 millimeters in it (I assume, it's big enough to do this, if it's 50mm wide) deep about 5mm and tap a M3 thread. It should be enough. Then use a crimping tool to crimp a eye-hook terminal to the wire, use teethed washers and tighten it with a short M3 screw with flat or rounded head (make sure it's not countersink head). Here's the list of pictures: enter image description here

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Here's how it should look on the end: enter image description here

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    \$\begingroup\$ I could make a funny caption for your nice photos but wont \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 26 '18 at 21:08
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    \$\begingroup\$ Feel free to do it, if you'd like :D \$\endgroup\$ – Jakey Mar 26 '18 at 21:09
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Other than brazing, arc welding or ultrasonic welding a threaded ring lug will be the easiest reliable method of attachment.

A tapped threaded screw with > 3x surface area more than a smooth surface will engage to achieve a low series resistance in the xx mOhm range.

But why use a "nut" to sense capacitance when sheet metal or aluminum foil or copper clad PCB may work better?

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other info

If the wire is long it will add inductance 6nH/mm appox for 1:10 diameter/length ratio, shielded or twisted pairs will add 100~30pF/m which then results in a resonant frequency. But for short sensor to IC, 30 mOhms contact is practical for a small screw, wide head to interface to a ring lug and crimp or soldered wire.

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  • \$\begingroup\$ Aluminium foil seems fine for debuging but seems improper for something semi-permanent. I could not find a metal sheet. \$\endgroup\$ – Svízel přítula Mar 26 '18 at 19:02
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    \$\begingroup\$ Yes of course, just for testing, Since I don't know what environment or container or what item you are sensing, then a masked copper clad PCB could be appropriate. What qty size? budget? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 26 '18 at 21:06
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May I suggest using flux, scoring the surface with fine grit sand-paper (or nail file) and using a large pistol grip style soldering gun >100watt.

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  • \$\begingroup\$ What's the purpose of scoring please? \$\endgroup\$ – Paul Uszak Mar 25 '18 at 19:35
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    \$\begingroup\$ @PaulUszak, the purpose of "scoring" with a sandpaper is to remove the deep oxide buildup from metal surface and expose bare metal. Even if the surface will be oxidized again in the process, the new thin film will be easily destroyed by active flux and/or soldering iron tip. You are welcome. \$\endgroup\$ – Ale..chenski Mar 25 '18 at 20:44
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Preheat the nut to a temperature close to or at the temperature your solder needs to melt, then solder to it. A blowtorch, a kitchen oven, a heat gun would all work for that. Avoid oxidising/tarnishing the nut more than necessary - your flux will have to work harder. Optionally, tin it (if heated above that melting point), but be careful not to get solder or flux in the threads. Tinning only the spot where you want to attach the pin is another option.

Coating the intended attachment spot with flux before preheating might be a good idea with SOME fluxes (avoid the kitchen oven then, though!). Avoid fluxes not meant for electrical connections. Make sure your nut is not galvanized, not stainless steel, and not actually made out of pot metal - the first can create fumes that make you feel funny when heated, the second will be VERY hard to solder to due to the chromium content, the third will suddenly melt. Be aware that an iron piece heated to 250°C+ will stay hot for a while, and can give you far more unpleasant burns even on short contact than a flame.

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    \$\begingroup\$ Caution is advised. A large nut like this may have special load bearing properties that may be damaged by high temperature treatments. Accidentally annealing a hardened steel nut would have rather unfortunate consequences in some situations. \$\endgroup\$ – Wossname Mar 26 '18 at 10:01
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I had a similar problem bonding a ground wire to my boat trailer. A spot of braze applied with an oxyacetylene welder provided a clean spot where good contact is achieved. Break the rules! Acid flux is ok but wash it off when the brazing is done. No welding torch? Put the nut on the kitchen stove and use a bernzomatic torch on the spot to be brazed. Solder bonds nicely to the brazed spot. If you eat organic vegan gluten-free non-GMO, you will want to wear a hazmat moon suit with a crash helmet and safety glasses when doing this. Good luck with your nuts!

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Your problem definition does not seem optimal. Do you want 1) to solder, or 2) to connect a wire, or 3) to create a capacitive sensor?

For 3) we need more info as to what you want to measure. For 2), 1) does not seem my first choice. Welding might be much simpler than soldering here. With fast welding, you do not need to heat up the entire 5 cm nut. You could, f.e., create a tiny air gap between pin and nut, and create an electric arc, then push the pin against the nut, and turn the current off. Just use a small electric welding transformer.

Even simpler, just put the wire inside the nut and screw the nut with wire on your bolt. For thread of such large size you might have to select a fairly heavy wire for a good squeeze (= contact pressure).

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