EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: anishkgt on June 08, 2017, 02:45:23 pm
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Hi All,
was wondering if anyone here would know which one would make a good holder for a spot welder electrode, is it copper or brass ? Would it really make a difference when using brass bar or should i go with copper bar ?
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From a resistance point of view it's no contest, copper way better then brass.
http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/ (http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/)
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Not sure what sunstone and other brands use it hold the electrodes. Was looking for something similar.
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I guess I will go with copper then. My concern is are they very soft to use screws in them like screwing a grub screw to hold the electrodes in place.
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It's worth repeating: copper alloys are far less conductive than the inexperienced expect. The relationship is far from linear -- most of the conductivity is lost with only a touch of alloying metal.
A typical brass alloy has 50-60% copper, but has far less than half the conductivity. It takes a brass alloy of 94% copper (which visually appears as regular copper, not brass) to lose only half the conductivity of pure copper.
If high conductivity is needed, it's better to plate pure copper, or laminate it rather than alloy it.
Edit: it's best to use stronger fasteners to clamp copper to copper, and just treat then as negligible contributions to conductivity.
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I have 20x20 copper bar that I plan to use as electrode holders. Which I as bought from AliExpress. I guess they are pure.
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Grub screws digging in: use full hard temper, if possible.
Also use 110 alloy or better (for above reasons).
Arrange the holder to use maximal contact between the arm and the electrode. Do not rely on the mounting screws for contact (or else you'll spot weld them instead!).
You could even arrange an all-copper clamp geometry, so that screws push against a plate which is conductive on the inside, so that all inner faces make copper-to-copper contact on the electrode. The plates can be flexible and soldered/brazed/bolted in place, or wired with heavy braid.
Also consider simply using bigger bolts. Grub screws are great for saving space and not having a head on the outside, but they're terrible for grip, for the same reason -- size!
Tim
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It's also worth noting that just specifying a "brass" bar could be a huge range of alloys, there's quite a few variants that are common in various applications.
If you're sourcing your parts for a high current device from aliexpress, be sure to do a sanity check on them before installing. I'd do a resistance measurement of the bar just to make sure it's in the right ballpark, and if you have a scale, I'd also weigh it and make sure it's on target for being pure copper. It's not hard to keep the right color and mix in other things, and copper is expensive enough that I wouldn't put it past a vendor without a good reputation. Also worth noting that if you're dealing with these fairly low resistance materials, you want to be sure you're using a good surface cleaner to make sure you're minimizing contact resistance between parts that are permanently attached.
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(https://uploads.tapatalk-cdn.com/20170608/663bfd6b96741e2a5b72bb9dc08b0863.jpg)
The cooper resistance, end to end is 0.06Ohm. I don't have a scale to weight it but the 20x20 bar I guess weighs at 300g approx.
Yes I did think of sliding the electrodes from the front left and using 6mm screw to tighten the gap between the holder holding the electrode. A 8mm steel rod goes through the hole holding the electrode head for lowering and raising it, two heads on two rods. That is is my plan.
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Unless you are really, really good at using taps, do not tap a threaded hole into pure copper. Instead, drill smooth holes and use a brass nut and machine screw to apply clamping force.
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The thread works ok but the clamping force with 3mm flat head is not enough. I will drill a smooth hole and then use a nut and bold, hopefull i should get a better clamping force that way.
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In industry, connections generally use steel nuts and bolts to hold flat, clean copper surfaces together. The holes go all the way though. Sometimes several (e.g. 4) bolts are used to spread the clamping force evenly.
You could try a Helicoil or similar. They are little stainless steel inserts to help with tapped soft metals. First you drill and tap a larger hole, then you insert the helicoil to provide a strong surface.
Copper will tarnish (depending on humidity and fingerprints), so polishing the contact surface before final assembly can be good. Make sure it stays flat. Or just keep it clean from the get go. Once assembled the joint should last OK by excluding air from the clamped area.
If you want top quality, get a local electroplating shop to nickel plate it for you.
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Anti-seize paste is a good thing, too. Even if it doesn't improve electrical conductivity, it will improve thermal conductivity!
The stuff that's made out of grease and powdered copper is ideal, here.
Tim
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What are the electrode holders of brands such as sunstone use ? they look like brass than copper and copper oxidizing so quickly i doubt copper would be be a good option for electrode holders.
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The cooper resistance, end to end is 0.06Ohm. I don't have a scale to weight it but the 20x20 bar I guess weighs at 300g approx.
If a bar is made of copper with dimensions 20x20x80 mm it would have resistance of 33,6 x10-12 Ohm end to end..
I seriously doubt it is possible to measure it's resistance ..
In this kind of equipment, contact resistance is critical... Solid copper interconnects are as good as zero ohms.. Where they connect together it is critical..
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It would probably be a low zinc brass for improved conductivity like C210 (C21000, "Gilding Brass"). You pay for it in price and machinability though.
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Pure Cu is the best electrically speaking, but you've got to think about machinability (pure copper is poor from a machinist perspective) and desired mechanical properties.
Hey - if you want the very lowest resistivity, why not just use solid silver? :)
You've got to think about the real-world tradeoffs, including price. And mechanical properties, and corrosion.
Tellurium copper is a good choice where you want high machinability with minimal change in the conductivity. Or beryllium copper where it needs to be harder than pure copper, etc.