FTM 150: Thread Crests

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.

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