When constructing fasteners, it is important that they be made according to certain specifications. This ensures that they will fit together and be able to carry out their intended purpose. One such specification is threads per inch, or TPI. The TPI measures how many threads, or V-shaped grooves, would fit along a one-inch length of a screw or bolt. This helps engineers and technicians know what size screws will fit certain bolts, and how secure and tightly they will be able to be tightened.
Depending on the country and industry in which they operate, manufacturers may use either imperial or metric threads. Both systems are used worldwide, though metric threads are gradually displacing all other forms of screw threads. The main difference between the two is that metric threads are measured in millimeters, while the imperial threads are measured in inches. In addition to TPI, threads are characterized by their major diameter, minor diameter and pitch diameter.
The standardization of thread sizes was originally prompted by problems with interchangeability between American, British and Canadian parts during World War II. As a result, the Unified Thread Standard was adopted by the thread standards committees of Canada, Britain and America in 1949. It consists of coarse (UNC), fine (UNF) and extra fine (UNEF) threads.
In order to understand the TPI number of a particular thread, it is helpful to have a basic understanding of the geometry of a thread. The threads themselves form a series of V-shaped grooves that spiral in the same direction around the screw or bolt. The size of each thread is determined by the diameter of the major thread crest and the distance between that crest and the next. The spacing between each crest is called the pitch of the thread, and it can be measured using a standard ruler or caliper.
Knowing the TPI number of a screw is only half the battle, however. It is also necessary to be able to calculate the threads-per-inch value in order to determine how much space a screw will need in a hole or shaft. To do this, you will need to count the number of gaps between each “V” shape and then divide that by the number of spaces in a full inch. For example, if you are calculating the TPI of a 1/2″-14 NPT threaded pipe, you will need to count the number of threads in a half inch and then multiply by 2 to find how many threads there are in an entire inch.
A few other important factors to keep in mind when identifying threads include the number of starts and the class tolerance. A single start thread is referred to as a 1-start thread, while a multiple-start thread is labeled as 1+ or 2-start. A thread with a larger lead is more likely to have a larger thread pitch than a thread with a smaller lead, and vice versa. This is why a TPI calculator is so useful. threads per inch