FTM 106: How do I measure the external thread minor diameter

Fastener Training Minute 106: How do I measure external thread minor diameter

This edition of the Fastener Training Minute with Carmen Vertullo was originally published June 20, 2016 as “How do I measure the external thread minor diameter?” during episode 106 of Fully Threaded Radio.

Well, hi everybody, it’s Carmen Vertullo with your Fastener Training Minute coming to you from the Carver FACT Center here in San Diego and the Fastener Training Institute.

I had an interesting inquiry today regarding external thread minor diameter measurement.There are a few things that you need to know about this and when we return I’ll tell you what they are.

Today we’re discussing something called external thread minor diameter. Now for most people who inspect threads, it’s one of those things that you look at and you say it must be important, but I don’t measure it. Why is that? Well, when we talk about the minor diameter of an external thread were talking about the root, that is the smallest part of the thread and you can imagine what it would take to measure that. How do you get in there? What kind of tool can touch that minor diameter on both sides in such a way to measure it?

There really isn’t an adequate tool, and often times we see, especially on larger threads, inspectors trying to measure that with a set of calipers. You simply can’t measure the minor diameter with a set of calipers. An Optical Comparator would be the proper tool to use to measure thread minor diameter, especially if we want to look at one of the very important features of thread minor diameter, which is the root radius.

These external thread roots are not supposed to be sharp, in most cases they are required to have a root radius based on whether it is either a UNR thread or a UNJ thread. Details of that are a different topic and a different Fastener Training Minute.

In any case I was asked by someone recently in an email for a specific number on an external thread minor diameter because it wasn’t in the IFI Fastener Handbook , and the IFI version of ASME B18.1.1 where these dimensions come from leave that dimension out of the table. This is because we normally don’t inspect to it, it is a reference dimension, not something that you can accept or reject a fastener based on.

Minor diameter is important but we don’t measure it. It’s got a root radius which we do measure and you can’t get it out of the recent IFI Fastener Handbook. If you have an older version of the IFI Fastener Handbook such as the 7th or 6th edition, you will find this tables. The actual original as published by ASME B18.1.1 will have it.

So now, you know pretty much what you need to know about thread minor diameter.

This is Carmen Vertullo with your Fastener Training Minute.

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