FTM 160: Good and bad Nuts

how to tell if a nut is a good nut or  a bad nut.

This edition of the Fastener Training Minute with Carmen Vertullo was originally published January 23, 2021 as “how to tell if a nut is a good nut or  a bad nut” during episode 160 of Fully Threaded Radio.

Well, welcome to the Fastener Training Minute brought to you by the Fastener Training Institute and the AIM Testing Laboratory here in beautiful. El Cajon, California.

It’s a stunningly beautiful day here in San Diego and it’s kind of a nutty time right now. It’s nuts. Everybody’s nuts. The world’s gone nuts, and you know what, there’s a lot of nuts in our business and our faster industry. Now you know, we just couldn’t get by without nuts in this business. And these nuts have to be good enough, though sometimes nuts aren’t that good. And sometimes we don’t quite know how to tell if a nut is a good nut or a bad nut.

When I come back I’m going to tell you a few things you need to know about how to tell if a nut is a good nut or  a bad nut.

Welcome back everybody. This is Carmen Vertullo with your Fastener Training Minute. Many of these Fastener Training Minutes come as a result of emails, and I recently received an email regarding the inspection of internal threads on hex nuts. This particular product happened to be a metric product and the end-user was confused about how to know whether or not the product was conforming when it came to inspecting the threads. Now hex nuts in the metric world come from a standard called ISO 4032. ISO 4032 leads us to ISO 3269 that tells us what we need to inspect and how to inspect it. It turns out that that is an acceptance inspection plan which basically means that we’re going to have a large sample size based on the lot size. And that plan is going to allow us to have some non-conformances, unlike what we’re used to in the inch world or the imperial world. In our ASME standards which you see equals zero inspection plans which have much smaller sample sizes and allow for zero non-conformances.

So what happened is that the client’s customer (I should say the end-user), inspected a large sample size, much larger than the sample size indicated. Interestingly enough, ISO 3269 does not specify a specific sampling plan. It gives us some examples that allows the end-user or the manufacturer to choose an appropriate one. But in this case an appropriate one might be two hundred. So that would be the number we would check with the go not go threaded plug gauge gauge and the unthreaded plain plug gauge to measure the minor diameter. That’s a fair number of fasteners to inspect.

With 200 pieces, we would accept the lot if we got four or fewer non-conformances. That means that if we found four parts that were bad in the lot, we would accept the lot.

The customer found one or two out of a thousand and they wanted to reject the lot. And it was very hard to convince them otherwise. Now as we know in the fastener world, we don’t have zero defects. It’s only available on certain features. And only if we find a way to do a kind of inspection that allows us to look at that feature 100% on every part which generally means you’re doing it with a machine or some kind of a vision system. And we really don’t need that except in cases where a robot is handling the parts such as an automotive assembly.

We don’t for example have a zero defects policy with drywall screws. A drywall screw installer picks a screw out and it’s got no drive in it. He throws on the floor and picks another one. That’s not the end of the world. But if you have a screw with no drive in it in a car assembly plant, it could shut the line down. So we have reasons for that.

Anyway back to our hex nut problem. ISO 3269 also allows for us to accept a product that is non conforming if the non-conformance does not impair the intended function or use of the product. In this case, these one or two very minor non-conformances did not affect the function of the part. And the reason we know this is because in the ISO world, when we inspect the nuts with the go and no-go thread gauge, the no-go gauge is allowed to enter two threads. In the inch world according to ASME B1.2, we are allowed to enter three threads. In the metric world, it’s two threads and that’s from ISO 1502. That’s a little bit subjective because it just says two full turns into the not go gauge with no excessive force.

Well they were getting some of the fasteners to go more than two full turns, just those very few. So we know that this is so borderline that it can’t possibly affect the intended function. So we present this to the end user. Hopefully they will realize that the problem they’re causing by not wanting to use the proper sample size or choose an acceptable sample size. And secondly, not being willing to invoke the ISO 3269 objection clause that allows them to use these non-conforming fasteners if they don’t impair the intended function, is not in their best interests. Their line is shut down there, whatever it is. The parts they are assembling are not going to get assembled.

So nuts are one of the easiest things to inspect in terms of threads. It’s the go not go gauge which in the metric world two threads are allowed for the not go gauge. The go gauge must pass all the way through. The plain plug gauge must not enter any threads. It can enter the chamfer the nut, but it must not enter any threads on they not go side. And the go plug must pass all the way through. That’s true also with the ASME B1.2 gauge for the minor diameter for nuts.

So once again, it’s a nutty world out there right now, and we’re getting a lot of nutty questions, and this was one of them that I wanted to share it with you.

I hope you enjoyed this Fastener Training Minute, I’m Carmen Vertullo, thanks for listening.

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