When are Surface discontinuities not acceptable on a fastener?
This edition of the Fastener Training Minute with Carmen Vertullo was originally published July 14, 2019 as “Problems with Fastener finishes” during episode 142 of Fully Threaded Radio.
The topic came up recently, which I believe I have I spoken about before on the Fastener Training Minute and that topic has to do with ugly bolts. That means bolts that have something on them called surface discontinuities. That sounds like a big word and I guess it is a big word. When I think of surface discontinuities, I think of my own face full of wrinkles and pimple scars and bumps; things that aren’t supposed to be there, but you can’t do much about them.
When I come back. I’ll tell you all about surface discontinuities on Fasteners and how you can tell if they matter or not.
We are talking about surface discontinuities and it sounds like a complicated word, but it’s not. It’s just what it sounds like, you have a nice smooth surface and something happens to that smooth surface. It’s no longer continuous, it’s discontinuous; there’s a crack or a seam or a fold of some sort.
We call that a surface discontinuity, and fastener standards have limits for surface discontinuities in their specifications to to let us know which ones are acceptable and how much, and which ones are not. A very common type of surface discontinuity and the one that brought this subject to my attention recently, is called a burst or a shear burst. That’s when you look at the edge of a head of a screw or a bolt and it just looks like something exploded out from inside of it. It’s looks like a piece of material is missing from it, kind of like the bad edge of a chocolate chip cookie. And those are quite common, more common than you would think especially on larger bolts. Interestingly enough among the several different types of surface discontinuities, these ones have quite a large amount of allowance or tolerance where they are acceptable on a fastener. So if you happen to see the edge of a bolt or a screw and there is a surface discontinuity at the edge of it, don’t freak out, it might be acceptable.
But acceptable according to what standard?
In the inch world that standard is ASTM F788 ASTM and ASTM F788M covers metric standards, even though there’s another standard for metric surface discontinuities. We’ll deal with that one later. So the most common type of surface discontinuity is a burst or a shear burst. There are some other types of discontinuities which are less tolerable, such as cracks, especially quench cracks, none of which are allowed. Some seams are allowed and some other types of forging defects are allowed, but surface discontinuities that are classified as bursts are the most common. I have some wonderful pictures of those if you want to email me, I’ll share them with you.
So when you see a shear burst on the head of a Fastener or your customer does, we have two different scenarios. One is we’re going to look at the discontinuity and we’re going to say that’s ugly as sin. I don’t want to get rid of it. I don’t care what the standard says and if that’s the case and the customer doesn’t like the look of it, there’s probably not very much you can do about it, unless you prefer to harass that customer into keeping the product. On the other hand, the customer may need to be motivated to keep that product. They want to keep their job going or they want to keep their production line up and they need to use that particular lot of Fasteners.
So the scenario looks like this: you show them in the standard where it’s acceptable to keep that fastener in place and everyone is happy and life goes on as it did before. So keep in mind that shear bursts in terms of surface discontinuities are acceptable, and interesting lie enough, they can be quite large. the case we just looked at was on a 3/4″ tension control bolt with a big round head, and that surface discontinuity was allowed to be an eighth of an inch wide. That’s very wide and very ugly. So it’s really surprising when you learn about it how much of a surface discontinuity might be acceptable. If you need to have one of those discontinuities evaluated, we do that at AIM Test Lab, but you can actually do it yourself pretty easily. All it takes is some calipers and maybe a camera to take a photograph.
Well, that’s the least you should know about surface discontinuities. One more thing. I mentioned earlier that the metric equivalent of ASTM 788 and ASTM 788M is ISO 6157. And there are three flavors of that, 6157-1, 6157-2, or 6157-3 depending on whether you are looking at bolts or nuts or studs and screws so that’s it for surface discontinuities. This is been Carmen Vertullo with the Fastener Training Minute. Thank you for listening.