When choosing fasteners, it is important to know the bolt threads per inch (TPI) to ensure that they will fit together properly. The TPI number is typically found on the fastener’s label and indicates how many threads are in one inch. Bolts can be manufactured with either a coarse or fine thread, and the diameter of the fastener is also important to know before ordering or using it.
There are several different ways to measure the TPI of a bolt, but a Vernier caliper is the most commonly used and accurate tool. A caliper has jaws that can be adjusted to the diameter of the fastener, and a lock screw allows you to hold the caliper still while you read its measurements. First, place the caliper’s jaws on the fastener’s crest or groove, depending on if it is externally or internally threaded. Then use the lock screw to stop the caliper from moving while you count the number of threads in a half inch. Record the measurement in inches, and then multiply that number by two to get the number of threads in an inch. For example, a 1/4-20 bolt has a 14 inch diameter and 20 threads per inch.
The threads of a fastener can be either cut or rolled. Both methods have advantages and disadvantages, but rolling tends to create smoother, more uniform threads that are less likely to gall or strip, especially in hard materials. However, rolling can cause the threads to be thinner, which decreases the strength of the fastener. Both methods can be used on the same type of fastener, and the choice is often a matter of preference or the availability of equipment that is designed to produce one type of threaded fastener over another.
Bolt threads are usually either fine or coarse, and the resulting difference in their structure can have a significant impact on how quickly the fastener is installed and removed. For example, a coarse-threaded bolt can be installed faster than a fine-threaded bolt because it requires fewer rotations to reach the same location. However, a fine-threaded fastener will typically be more durable and may not need to be rotated as frequently as a coarse-threaded fastener.
Measuring the threads of a bolt is an easy task, but it must be done accurately. A caliper is the most popular measuring device for Imperial-threaded hardware, but there are also printable thread gauges available for metric bolts that can be used in a pinch. To measure the pitch of a bolt, simply place the caliper on the threaded portion and count the distance between each thread peak. The thread pitch is the number of threads in an inch, so you can also calculate it by dividing the bolt’s diameter by the number of threads. bolt threads per inch