pop rivets and ground washers? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 01:52:13 am
content: Can these be mixed to make a good ground on bare aluminum sheet metal jointing?

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 02:52:05 am
content: Quote from: coppercone2 on July 19, 2020, 01:52:13 amAbsolutely, clean the surface to remove any paint/oxidation/ etc. and use a good big rivet; ⁵/₃₂" or ³/₁₆", and a good PTFE di-electric grease (like this). I have done this dozens of times.Also consider the elevated dynamics of setting the rivets with a pneumatic rivet gun (like this from Arrow), it will make sure they get pulled up tighter than a pig's butt-hole...

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 03:19:16 am
content: I mean star washers. I thought that you can put washers on both sides of the rivet and pinch it together and not worry about grease dynamics so long you start clean. The idea of relying on a greased connection to join a chassis together makes me nervous.Otherwise I am inclined to use bolts. That way the grease under the washers can at least be serviced (regreased/tigthened if a low Z ohm meter shows a problem). .I guess its not going to be permanent even if you use two star washers and rivets?

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 03:39:29 am
content: Quote from: coppercone2 on July 19, 2020, 03:19:16 amDi-electric grease is an insulator, it has nothing to do with conductivity other than adding corrosion resistance. Flat washers would create larger contact surfaces as compared to the "pointy" contacts of a star washerBest would be no washer between the ring connector and the chassis--why add another metal-to-metal juncture when not needed?.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2020, 05:03:18 am
content: If possible, tin the aluminum (which can be done with ordinary solder if you abrade the surface to remove the oxide under a small pool of heat-resistant oil then solder under the oil without exposing it to the air), wipe off excess solder then drill the hole. Use a tin plated ring terminal and you've got tin to tin contact with the tin to aluminum interface hermetically sealed under the tinning and thus protected from corrosion. A little grease to exclude air and moisture wouldn't go amiss.I'd be wary of a pop-riveted tinned ring terminal to bare aluminum connection. Any mechanical stress will open up a microscopic crack at one side allowing the aluminum to oxidise. The oxide layer is thicker so will wedge the crack further open allowing the oxide to propagate further inwards towards the fastener. Your only chance of saving the day is to have initially used a conductive antioxidant grease like Noalox during assembly, which helps keep air and moisture out of the microscopic crack to start with. Don't just use any conductive grease - ones containing copper or carbon particles can accelerate the corrosion of the aluminum.Zinc is one of the better metals for electrochemical compatibility with aluminum, so if you cant use a welded in stud,its generally preferable to use a bright zinc plated nut and bolt with a star washer (+Noalox) under the nut. You can then use another nut to clamp ordinary ring terminals on top of the first nut and get a reliable connection.

author: trevatxtal
author_rank: Contributor
timestamp: July 19, 2020, 06:19:21 am
content: Star washers are poor conductors as they are steel, makes quite a difference at RF.

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 07:28:35 am
content: Quote from: trevatxtal on July 19, 2020, 06:19:21 am^This, they can be effective lock washers. However even though often used on electrical connections, it is only done by those who do not really know what they are doing...

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 08:01:37 am
content: is it stable enough for a safety ground? don't care about RF here at all

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 08:31:05 am
content: Quote from: coppercone2 on July 19, 2020, 08:01:37 amHell yeah, but (there's always a "but") what level of voltage/power is involved? What gauge wire?What is the application?

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 02:59:24 pm
content: safety ground on 10 amp chassis So like 12 AWG.The wire I can ground with a bolt and star washer, to the solid part of the assembly, from the back its a question of the chassis cover. It would be easier to put angle aluminum pieces on it with rivets, because I can't mount the chassis cover nicely without using a bad thread count (its thin). So I need something thicker, the easy choice is to cut angle irons to make a lip that I can tap screws into to hold the cover on. The cover is 2 faces of a cube. It houses a pid and switch. The part with all the components mounted to it (swtiches, indicator) is all one piece, its just a question of grounding the lid incase a wire touches it. It would only be a problem if someone moves the chassis while its plugged in but obviously it needs good treatment. It should never pass any current unless something goes horrendously wrong with the wire. For the chassis cover I plan to use chamfered screws with peened star washers underneath (like I see in old HP equipment). Or maybe if a capacitor or something blows up in the pid and electrifies the chassis cover with a conductive trace.Rivets just look nice as a joint if I added angle pieces. I can't put anything else in that location without it looking like a pressure vessel.

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 03:16:49 pm
content: 3/16" or 1/4" pop-rivets will work fine, just make sure the mating surfaces are bright an you use some sort corrosion preventative compound. Noalox as suggested by Ian.M is good stuff and commonly available at the big box home supply stores...

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 03:34:22 pm
content: do you think alodine + typical grease would be better then noalox? I have bothbut, what makes me worry about noalox is that its a grease. As far as I am aware thats grease with grit in it. Alodine is a surface treatment that should be more permanent?could you dip the rivets in alodine?I think I am just going to weld it for piece of mind. Might be a good location to have another try with aluminum braze lap joint too, but I think that is going to result in a situation. I just know something will go wrong if I try to braze that.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2020, 04:21:27 pm
content: If you abrade and Noalox the mating faces of the cover and angle for a diameter of the width of the angle around the rivet holes *IMMEDIATELY* before assembly you'll have the best odds of a long-lasting good electrical contact that for grounding purposes will let you treat the cover as one piece of metal. From a continuity point of view, it would be better to weld them *IF* they were both a weldable alloy or solder them with special purpose aluminum solder, but from a mechanical and appearance point of view there's too high a risk of distortion, and a certainty you'll anneal them with no way to tell how much temper they'll recover by ageing.You can substitute the Noalox with any other reputable brand of metal loaded conductive antioxidant grease rated for aluminum to aluminum electrical jointing. You *NEED* the (IIRC) zinc dust in the grease to, under clamping pressure, help break through the 'flash' oxide layer that immediately reforms on aluminum after you abrade it in an oxygen containing atmosphere.Alodine is intended to passivate aluminum and leave a permanent coating to reduce the risk of corrosion, and aid paint adhesion. Done right it is also reasonably electrically conductive, but that requires that you use the minimum possible coating thickness and assemble a gas tight electrical joint as soon as possible after Alodining, as its resistance increases with thickness and age. It would be a PITA to spot Alodine the mating faces round the rivets, but if you do so in addition to using Noalox, its likely to decrease the odds of the connection going high resistance.N.B. if either part is anodized, you'll need to chemically strip it as a good hard anodised coating is a pretty good insulator and is as hard as and resistant to many abrasives unless you use a coarse abrasive with enough pressure to disrupt the underlying metal,which would 'dish' the part around the fastening - highly undesirable when you want good clamping pressure for electrical conductivity, and don't want the panel to distort.

author: KL27x
author_rank: Super Contributor
timestamp: July 19, 2020, 06:53:47 pm
content: I have found on many occasions a screw and a star washer in a painted chassis. Is this unreliable?I assume the only electrical connection is between the bolt and the tapped hole, then through this mess to the ring terminal. The main reason for the star washer seems to be to prevent the screw from loosening.Seems like some folks think this is a fault, due to cost-cutting. And they "fix it" by grinding away some paint and make some judgment against the country of China and its citizens. But I wonder if it's not perfectly ok? What better connection than a screw tightened down in a tapped hole?For thinner sheet metal, I suppose you can't get a reliable thread, though. If you don't trust a rivet or dielectric grease, you could maybe solder it after riveting/screwing? With a solid ground wire, anyhow. Not the stranded. Doh, oops. Aluminum sheet.My vote: I'd use the star washer, because one good spot of connection will trip the breaker. That's better than a large area that might oxidize. I'd use the dielectric grease, too, to keep out the oxygen. Esp in conjunction with a rivet, the star washer could add additional spring in the connection?

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 19, 2020, 07:52:17 pm
content: Quote from: KL27x on July 19, 2020, 06:53:47 pmYou are overthinking this entire issue, just scrub up the mating surfaces, then use some sort of corrosion preventative and some sort of robust fastener to snug it all up securely. It really doesn't matter as indicated by your finding "...on many occasions a screw and a star washer in a painted chassis." (which though not completely "unreliable" is not the way I would do it).BTW in that scenario the star washer has much more to do with locking the fastener than electrical conductivity.The fact is that over a long term--say 5 to 20 years depending on climate and use¹-- connections such as this aremaintenance itemsthough in the real world no one actually "maintains" them until they fail...------------------------------------------¹ - 5 years or even less here in Florida, by the sea; 20 years or more in the Arizona desert.

author: bdunham7
author_rank: Super Contributor
timestamp: July 19, 2020, 07:58:23 pm
content: Is this connection or device subject to any significant mechanical strain, movement or vibration?

author: coppercone2
author_rank: Super Contributor
timestamp: July 22, 2020, 04:28:37 am
content: damn its quite frankly easier to leave it on a bench strewn out lol

author: coppercone2
author_rank: Super Contributor
timestamp: July 22, 2020, 04:30:14 am
content: Quote from: KL27x on July 19, 2020, 06:53:47 pmI thought the star washer pierces the oxide and digs into it to form a indent thats kinda gas tighter. Well I guess there should not be oxide in the first place, but maybe the residual grease + the crater formed by the crown washer edge ends up filled with a grease film and maintains the electrical connection better then a big flat that does not really crater but rests on peaks? Maybe the irregular penetrations the star washer makes have a probability of being gas tight (significant)? But I have seen them rusted to shit, and replaced them, especially on chemistry lab equipment, but I never inspected it under a microscope to see if there was any non oxidized metal there.I see what the equipment makers do on things like grates/weird structures is put a dedicated labeled grounding screw and other structural screws. So we don't like it from a EMI prospective but its safe, so long you pay attention to that screw. Its annoying if you like making your own boxes because its really nice to be able to rivet a corner piece on to make a flare, without bending metal, so you can get by only doing cutting operations.. and buying angle metal... but its not safe. I dislike bending complex shapes/large bits. Sheet metal bender takes up alot of room and its a extremely ugly machine. Welding them is a bitch too, unless you have something more advanced then gas. Its hard because the bracket takes way more thermal mass then the panel and it melts.Maybe I will give ARC-aluminum a try. I don't want to invest in a spool gun for mig aluminum or tig. The nasty result from my gas weld attempts is making a literal sea of metal shavings from burrs. And its crazy im practically strapping fire bricks to things with bungee cords at this point. My skill at making a flat joint between two pieces of Al is borderline acceptable on gilligans island, but right angles are bad and dissimilar metal thickness welds are preposterous. I had to run some of these corner joints on a power saw before I got the die grinder to them! Maybe aluminum arc welding will be good at dissimilar metal thickness welding on the flange brackets..I can live with shady panel corner joints because I tested them before and they are very durable after cleaning (it can take more abuse them I can give it) , so I know the process is good. But adding the flanges is getting to be too much. Of course if I spent some money on getting the right material thickness it would work fine, but thats not acceptable.A floating metal flange is not acceptable andparticularly dangerousbecause its a good place to run wires under to prevent the lid from smashing them... and its also a great place to rest your hands on if you happen to peer into equipment and naturally you might want to put a tool there if you determine its a good stable flat surface on a busy work bench. Perfect combo to get wasted if you put one hand on a flange and the other hand on the chassis. I welded them on all sides and did metal removal work so the next flange piece can fit in and to make sure a crack has a difficult time getting through all of them. Granted its tertiary because you should not work on plugged in open equipment and you should only use one hand to probe if its turned on (clipped ground while off) but still.. its worse then a floating live heat sink (hate that shit). So, I might say its stupid NOT to bend it. Maybe I should get a bender machine. Not to mention a heavy flange is great heat sinking point for a transistor.. ughAnd unlike gas, the arc process might result in brittle aluminum (I think), so maybe its better to just remove metal all day... safer maybe