FTM 130: Dip-Spin Coatings

Fastener Training Minute 130: Dip-Spin Coatings

This edition of the Fastener Training Minute with Carmen Vertullo was originally published July 26, 2018 as “Dip-Spin Coatings” during episode 130 of Fully Threaded Radio.

Hello everyone, this is Carmen Vertullo coming to you  from the Carver Facts Center and the Fastener Training Institute.

I’ve had several questions recently regarding dip-spin coatings on fasteners, and some of the problems that we can run into with drive systems on small fasteners and threads, and particularly thread fit. Dip-spin coatings tend to be heavier than zinc plating and other types of electroplating so we can run into some problems. When we come back, I’ll tell you what those problems are and how to effectively deal with them.

Now for the most part when we think of dip-spin coatings, we think of a couple of trade names such as Geomet or Magni and the many many flavors they come in. In older systems it was called Dacromet, or we might think of the ASTM specification versions of those coatings like ASTM F1136 or ASTM F3019. There’s another one or two out there as well. But the important thing to remember is that these dip-spin coatings are like paint, they go on either by dipping the fasteners into a solution that looks like paint and then it gets spun off or it gets sprayed on like spray paint.

It consists of some kind of a binder with a bunch of zinc in it and some aluminum in the top coat and when it goes on, it usually goes on kind of thick. Now some of the coatings can be thin enough to maybe be similar to zinc plating. When you think about plating, think of a very very thin coating. And for coating fasteners, it can be as thin as maybe two ten thousands of an inch (2/10,000), which is very thin up to four ten-thousandths (4/10,000). When we get more than about four ten thousands of an inch (4/10,000) we may have some thread interference, even on threads that have an allowance for coating.

Now, the problem is that when threads are coated, it affects the thread pitch diameter. That is the thing that we measure the primary dimension for the threads. It has an effect on the pitch diameter that is 4 times a coating thickness. So it’s important that we control the coating thickness. Now, one of the great features of some of these dip-spin of coatings is that we can get some very high salt spray tests, up in the thousand hour range. A lot of times what we’re looking for is the 1000 hours salt spray tests, highly corrosion resistant coatings. To get that though, the coatings need to be rather thick, as much as four ten thousands of an inch and over (4/10,000) and in some of the specifications coatings are as much as eight ten thousandths (8/10,000) of an inch. Sometimes they will measure them in microns versus ten thousands of an inch.

One Micron is equal to about forty millions of an inch or 1/2 of 1/10. So when we look at our thickness ratings, those are the two dimensions of the two dimensional systems that we use. So in the metric world, it’s microns and in the inch world it’s ten thousands of an inch. So all that being said we have this thick coating. When we have small fasteners we have tight tolerances on threads me of tight tolerance in the drive. And sometimes that coating is too thick. The effect on the drive. Let’s say it’s a Torx drive or 6-lobe drive, or a Phillips drive. Maybe we have too much coating inside that drive and the insert bit that we drive the fastener with won’t go in, or the part will not gauge when we try to gauge it. So we have to accommodate that. For the most part the applicators can control that but they can’t control it 100%. So we have to have an accommodation and typically for drives, that accommodation is rather than inspecting the drive with the gauge, you would normally inspect it with (if it’s a hex socket or a Torx) the bit that we’re going to use to drive the tool with or the fastener with. So we have to have those drive bits available to the applicator.

As well when it comes to thread fit, we have to accept that from time to time, maybe most of the time, the nut and the bolt will not screw together easily by hand. In other words, they won’t just spin on, it’ll take some to work to overcome that. Now we do have some ways to measure that amount of torque. It’s not very much and the end user has to accept the fact that these things are not going to spin together like zinc plated fasteners would. When we have these thick coatings, getting the end-user to understand that sometimes can be difficult. It really is no big deal, you’re going to put a wrench on that nut or bolt sooner or later. So losing the spin-on feature is really not that big of a sacrifice compared to the increase in corrosion resistance that we get from dip-spin coatings.

It’s just a matter of being able to educate the end-users on what to expect. Now with thinner coatings, if we are only looking for maybe 200 400 500 hours of salt spray, we can probably get dip-spin coatings to accommodate spin-together nuts and bolts. When we need them to spin together, and we want that thick coating, that has to be taken into account when the fastener is manufactured. We need to make those threads not necessarily out of tolerance, but on the low side of the tolerance so that we have an extra amount of allowance there to make room for the plating.

That’s about all I can say about these dip-spin coatings, and the fact that we have to have some extra accommodation when we use them. If you have any questions, you can contact me at the Carver Facts Center at Carverfact.com. Thank you for listening.

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