Understanding Threads Per Inch
The threading on fasteners like bolts and nuts is designed to match up and engage with each other smoothly, creating a strong and secure connection. When this doesn’t happen, the bolt and nut may seize or strip the threading, rendering them unusable. To ensure this doesn’t happen, it’s important to know how to read the threading on the fasteners and understand the meaning of the terms used to describe them. One of the most important terms in this regard is threads per inch, which describes how many threads are present in a given length of fastener.
While metric threads are defined in terms of their pitch, which is the distance between two consecutive thread crests, inch-based standards usually use the reverse logic: how many threads occur per a given distance. This measurement is called threads per inch, or TPI, and it is often affixed to the diameter of the thread in order to form a common imperial screw size. For example, a 14-20 screw has 20 TPI.
TPI is also often used as a measure of the relative roughness of an imperial thread, with higher TPI numbers denoting coarser threads and lower TPI numbers denoting finer ones. However, these designations are not universal and many hardware companies choose to label their products with different classifications, such as the less common UNC and UNF, instead of relying solely on TPI measurements.
To determine TPI, one simply needs to identify a section of the screw that is an inch long and count how many thread peaks are within this length. This can be done by eye, if the screw is easy to see, or by using a ruler or caliper with a built-in thread gauge. This process is typically much easier for metric threads, as their TPI values are standardized to a particular value and will appear in a consistent manner on all gauges.
Once TPI is determined, it’s necessary to find the thread’s major diameter and pitch, which can be found on a thread chart. This can be done by identifying the number of peaks that are an inch wide, or measuring the distance between the top and bottom of the thread crests using a high-precision caliper. The resulting number will then indicate the major diameter and TPI, which can be combined with the screw number to create a complete callout.
Despite the fact that most of the world has switched to the metric system and ISO metric threads, some countries have resisted the change and continue to cling to their old inch-based standards, such as the United States. However, with the globalization of the manufacturing industry, there is increasing pressure to switch to the metric system. As a result, the US is slowly phasing out its UTS-based screw sizes in favor of the more universal ISO metric sizes. To help speed this transition, manufacturers are introducing the ability to produce parts that can be used in both UTS and ISO-based systems, making them interchangeable.threads per inch