This edition of the Fastener Training Minute with Carmen Vertullo was originally published October, 19th 2023 as “Everything you ever wanted to know about blind rivets” during episode 193 of Fully Threaded Radio.
Well hello everyone, and welcome to the Fastener Training Minute. This is Carmen Vertullo coming to you from Carver Labs in beautiful El Cajon California, and from the Fastener Training Institute.
Today’s lesson is one that we have probably covered before, but not as comprehensively as we’re going to cover it today. As with many of these topics, this comes from a client inquiry that ended up resulting in a very nice training session with my clients customer, because that customer did not understand the risks associated with plating socket screws. And yes part of that is hydrogen embrittlement. We’re going to talk a little bit about that, but in this case it was a different risk, and we’re going to talk about that one as well. So if you are selling, using, or plating socket screws in any way, you really want to listen to this, and when I come back I will tell you all the risks and how to control them that are associated with plating alloy steel socket head cap screws.
Well welcome back everybody this is Carmen Vertullo with the Fastener Training Minute and we’re talking about plating, or putting any kind of coatings on to socket head cap screws. Now generally when we think about the risks of plating socket head cap screws, we think hydrogen embrittlement, and that applies to both inch socket head cap screws from ASTM A574 because they’re very hard and very strong, above HRC 39: that’s our trigger point. And also in the metric world class 10.9 socket screws which are at the level of 1,200 megapascals which is also above HRC 39. So we all know there are hydrogen embrittlement risks with those, and we know how to mitigate them. We essentially have to have a good risk control strategy in play, which means first off, whose risk is it? Always remember the person who specifies the plating owns the risk, and and if that’s your customer, you need to make it clear that they own the risk. It doesn’t mean that you’re not responsible to control the risk, not only in ways that are specified, (such as maybe on the purchase order, the drawing, or the standard), and that includes things like making sure the part is in conformance in the first place. it’s not overly hard and for example making sure the part gets properly plated it gets a properly baked and it gets properly tested. So those are the ways that we can if we have a good hydrogen embrittlement risk mitigation strategy, we can absolutely prevent hydrogen little product from getting into the supply chain. Notice I did not say we can absolutely prevent hydrogen embrittlement: it can still happen. But if we do everything right, we can make sure that the part does not end up out there in the world somewhere where some damage can occur. So let’s put that aside for a minute and by the way the Fastener Training Institute and Carver Labs can assist you in all kinds of ways to help you with your hydrogen embrittlement risk mitigation strategy.
So on to the other risk that we associate with plating socket screws. And this applies primarily to inch socket head cap screws that are made to a dimensional standard ASME B18.3. Now the issue there is that those screws have a thread class or tolerance called 3A for external threats and what that means is the manufacturer has the opportunity or option to make that screw all the way up to the maximum material condition. So there is no room for plating potentially, we don’t have that issue in the metric world because all metric screws sockets and otherwise in their thread class have an allowance for plating. So we are going to put the metric aside and focus on the inch screws, because these inch screws have no allowance for plating. One of the ways that we can mitigate the chances of our plating being so thick that it busts through that maximum material condition and creates the condition where the GO thread gauge will not GO, or potentially the part will not assemble, is that we can inspect them before we send them out for plating. Now because that”2A” tolerance gives us an allowance for plating and the “3A” tolerance does not, ideally we would like these sockets screws to have a 2A tolerance, but you know what, there’s a really good chance that they do, because there’s a big overlap between 2A and 3A, and the manufacturer generally is not going to make that part right up to the edge of the requirement.
So we can inspect them in a couple of different ways. One way is that we can check them with a 2A GO-ring gauge and if they GO they theoretically have room for plating. And even if the 2A ring gauge does not GO, another way that we can inspect them is with a variables gauging system, which gives us an actual number of where that thread lives in its tolerance zone, and there’s a pretty good chance, and this happens 80% of the time, that that screw has enough room for plating. So our first strategy is that we make sure that we don’t have a screw that’s very close to the edge, and where we’re going to run into this trouble in particular is very small screws, #10 and below #10-32 and 1#10-24 and fine threads in almost any size. So those are the two areas we need to be concerned about primarily. So once we know that this fastener has a room for plating, it’s safe to plate it and expect that it’s going to be OK. But then we have to know it’s going to be OK, and we do that by after it’s plated we can check it with a 3A GO-ring gauge, or we can use the variables gauging system that gives us an actual number. But we have to be careful because there are two types of checks we can make with variables gauging.
One is we can check the pitch diameter which only looks at one thread at a time and that must pass, but the requirement is that we use a a system called multi-rib rolls, which checks for the assemblability of the screw: similarly to how a threaded ring gauge would check it. So we have to use the multi-rib rolls and that will tell us for sure, it checks a series of thread pitches (about a diameter’s worth) that will tell us for sure that that part will be assemblable. Now in very small screws we have a very small allowance for plating. The screw in question was a #6-32, and it only has a 2/10,000 allowance for plating. Now as you may or may not know, whenever we apply plating to a screw thread, it increases the pitch diameter by four times the coating thickness. And in this case the customer had specified a very common coating thickness for fasteners from ASTM F1941M, and that designation was FDZN5AT.
Now “5” says we have a minimum of 2/10,000 thickness now that’s going to give us 8/10,000 on the screw, and we have a potential to bust right through that requirement for that very small screw. So we advised the customer to back off on that and use FDZN3 which is a maximum of 1/10,000 and even though there is still is some risk, it’s much less that we will end up with a situation where we’re going to have the thread not pass the ring gauge test, or the thread inspection test, or where we will have an assemblability issue. One other thing we can do is that we can send the ring gauge to the plater ,and ask the plater to check the part with our GO-ring gauge. Be very careful because sometimes the plater will end up putting your ring gauge in with the parts and then it’s ruined. So make sure you put that in a separate package. Ask me how I know about that. For the most part if they plate fasteners they are very willing to do that job and some good fastener platers actually have their own ring gauges, and they can perform that check for you. So let’s say once again we end up at a situation where we’ve done everything right and here we are oh crap, these screws won’t take the ring gauge.
One of the things that the customer needs to understand is that, because they asked you to do this, and there’s ways to present this to the customer in a very effective and training and value-added way, is that it’s their risk. So when they realize it’s their risk and we’re going to have to reprocess these screws, what are we going to do? Well one thing that you should absolutely do and the customer generally will go along with this, is OK they don’t take the ring gauge but will they screw into the hole that they’re supposed to go into? Almost 100% of the time even though they don’t take the ring gauge they will still be assemblable. That means that when the end-user made that hole or that nut that that screw is going to go into, they didn’t make it at the maximum limit either, there’s a little bit of space there.
So if the part will assemble there’s absolutely no reason not to use it, even though it may not take 3A ring gauge after plating. One other risk, and this is very rare but it can occur, especially with coatings that are dip-spin, is that sometimes we might get drive fill-in, or the drive of the screw gets too much plating in it. And then it won’t take a driver, the key or the torx bit or whatever. That doesn’t happen very often but it does happen with very small screws. The saving grace there is plating does not throw very well into internal surfaces such as the drive. But when that occurs generally it’s just a matter of being able to realize, hey, the driver may not go in easily, but it will go in. And if you’ve overdone it on the drive you’ve probably way overdone it on the thread. So that would be the least of your problems.
Well I hope that informs you about the risks associated with plating socket screws. They are very contollable and we can help you do that at Carver Labs. We can do that for you, as a matter of fact, and we do we have a nice bundle test that allows us to check your screws before they go out, and check them for you after they come back, for both dimensional issues and also for hydrogen embrittlement.
Well that is your Fastener Training Minute this has been Carmen Vertullo, thank you for listening.