Understanding Threads Per Inch
Threads per inch are an essential measurement for determining how fasteners fit into each other, and also to determine the size of the threaded end of a screw or bolt. They are also used in measuring the lead of a threaded fastener, which is important for proper function and durability. Understanding threads per inch is a simple process, and it’s something that can be done with a ruler or a caliper. There are also several different types of threads, and each one is defined by its own standards and specifications.
The most common type of thread is the Coarse Thread (UNC) or Fine Thread (UNF). US screw sizes are commonly* classified as such by their thread pitch, and this number is typically printed on the fastener itself in the form of a two digit numerical value with a suffix that indicates the series designation and class tolerances. There are also some less common thread types such as the Sellers thread, which was developed for railroad industry use in the 1860s and ’70s, and an Aerospace inch thread that is specified for the tolerances of aircraft and space hardware.
A metric thread’s pitch is measured in millimeters rather than inches, and it’s expressed as the distance between adjacent thread crests. It can be calculated from the size of the major diameter, which can be determined by a ruler or a caliper, and the number of threads that occur in an inch. The simplest method of determining a thread’s pitch is to count the number of peaks on a screw and divide it by the length of an inch, which will give you the threads per inch value for that fastener.
In order to identify a thread’s pitch, it’s important to use either a ruler or a caliper, because the results can vary between imperial and metric measurements. While there are a few similarities between the different types of threads, it’s important that you use a gauge specifically for your desired measurement because a metric pitch gauge will not necessarily match some imperial threads.
There are several methods for identifying the thread pitch, but a caliper is often the easiest and most accurate. Use the end of the fastener that is closest to the head, and place it against a ruler or caliper so that you can clearly see and count the peaks and valleys on both sides. If you have trouble locating the threads, it’s possible to use a magnifying glass to better see and distinguish between the peaks and valleys.
Once you’ve identified the thread type, the next step is to find the size of the major diameter. This can be measured with a ruler or caliper, but it’s more accurate if you use a go/no-go gauge. This tool has multiple leaves that have a number stamped on them to indicate the pitch, and it’s important that you use the correct one for your application because a wrong gauge could damage the threads and cause misalignment or failure.