Threads are quite amazing when you stop to think about it. Yeah I know, nobody ever stops to think about threads. But consider this: a properly made bolt subject to extreme tensile stress will break across the diameter before the threads shear. In other words, when pulled vertically, the threads are stronger than the weakest part of the bolt itself. As you tighten the threads in a joint, the metal in the body of the bolt will stretch until it eventually breaks, but the threads will not be deformed. Okay, one last thing. Consider: in the scenario above, the majority of the force is concentrated on the bottom two threads and is not spread out evenly, and the threads STILL won’t shear before the bolt breaks. Threads are cool.
External Thread Basics – Terminology
We make parts with external threads, so that’s where our focus will be. We talk a little bit about internal threads in Part 2 of our Threads Series. There’s a lot of terminology that gets thrown around when talking about threads, so I thought it might be helpful to start with a diagram of an external thread and talk about some of the most important terms.
Crest – the top of the thread.
Root – the bottom of the thread.
Flank – connects the crest and root.
Pitch – distance measured parallel to the thread axis between corresponding points on adjacent threads. So, pick a spot on the thread and measure the distance to the same spot on the next thread.
Major Diameter – diameter of the bolt as measured at the crests of the threads.
Minor Diameter – diameter of the bolt as measured at the roots of the threads.
Pitch Diameter – diameter of a theoretical cylinder passing through the threads in such a manner that the distance between the crests and roots is equal.
How to Speak Thread – Inch Version
In the interest of keeping this article reasonably short, I’m going to be talking exclusively about Unified Inch threads. (If you would like to know more about metric threads, don’t worry, we’ve got an article that covers those.) Let’s start with an example that will help illuminate how we express inch threads:
The photo is of a double-end stud with 5/8-11 UNC 3A threads. Let’s break down what this means.
The “5/8” refers to the nominal major diameter of the threads. You’ll remember from above that major diameter is the diameter of the threads as measured at the crests. We use the word “nominal” to mean “more or less”. The major diameter has a tolerance built-in and can be anywhere from 0.6129″ – 0.6250″, but we just call it “5/8” because that’s easier.
The “11” refers to the number of complete threads in one inch of threaded length or threads per inch. If you were to count the number of threads in one inch of the above bolt, sure enough, there are 11 of them.
Unified Inch threads have two major classes, UNC (coarse pitch) and UNF (fine pitch). Coarse threads have fewer threads per inch than fine threads. In the above example, “UNC” tells you that this is a coarse thread. If this were a UNF thread, it would be 5/8-18, meaning it would have a 5/8″ nominal diameter and 18 threads per inch. Every nominal inch size can have UNC and UNF versions. For example, a 3/8″ diameter can have either UNC (3/8-16) or UNF (3/8-24) threads, or a 7/8″ diameter can have UNC (7/8-9) or UNF (7/8-14) threads. Click here to see a pretty decent chart.
Finally, we need to talk about that “3A”. 3A refers to a class of thread fit for external threads. Thread fit is a measure of the looseness or tightness between mating threads when an externally threaded fastener is assembled into an internally threaded nut or tapped hole. Thread fit is a bit of a rabbit hole, and I’m not going down it here. Luckily, we have an article that tackles this topic in-depth, so click here for Part 2 of our Threads Series. Also, here’s Part 3 of our Threads Series that talks about metric thread fit.
Hopefully this article has been a helpful introduction to external threads. There’s plenty more to learn, and we’ve got plenty more to say in Parts 2 and 3.