Handling customer complaints about the crests of the threads on fasteners
This edition of the Fastener Training Minute with Carmen Vertullo was originally published March 19, 2020 as “Thread Crests” during episode 150 of Fully Threaded Radio.
What do I do if my customer complains about the crests of the threads on my fasteners?
I got a recent email where where the question was: My customer is complaining because the crest on the threads of these screws or nuts did not look smooth and uniform, they looked more rounded and flat.
So if you stop to think about how a thread is made, in most cases external threads are rolled and internal threads are usually cut but sometimes rolled. If we were to make that thread to the theoretical maximum diameter that it could be on the bolt or the theoretical smallest minor diameter it could be on the nut, that crest would be sharp. We don’t really want a sharp crest because that makes it vulnerable to thread nicks, and that’s actually kind of dangerous and possibly injurious to the person handling the screw.
So the crest is usually going to have some kind of a rounded off or flat shape, because when we roll threads it’s kind of hard to get that perfect. In some cases it must look nice and be smooth and have a form, but in the case of commercial fasteners, the threads are made to ASME b18.1. 1. The only requirement is that it must be dimensionally in conformance to either the major diameter or the minor diameter on nuts. The thread form at the crest is not specified: It can be flat, It can be rounded, it can actually have a double crest, or it can be sort of not very uniform looking at all.
The reason is because for commercial Fasteners, it doesn’t matter to the function of the Fastener that the crest be perfect. Now, obviously we have some requirements depending on the fastener standard, that we don’t have nasty laps, at least laps that go down below the pitch line of the thread, or close to the pitch line. There are some other requirements like that, but it is not cause for rejection because it doesn’t affect the function of the fastener.
Well next time you have a customer that might complain that they don’t like the way the crests of these threads look, you can tell them that they don’t have to look that great. Maybe they will accept that and maybe they won’t, but at least there won’t be a cause f or rejection that they can stand on.
Well I hope you know a few things about thread crests that you didn’t know before.
This has been Carmen Vertullo with the faster training minute. Thanks for listening.