The threads of screws, bolts, and fasteners are helical protrusions that extend the length of the part. The way in which the helix is cut, and how closely the threads are spaced together determines how well the screw or fastener will mate with another part. These factors are regulated to a set standard to achieve predictable, reliable performance. Threads are also produced to a specific diameter, allowing them to be cut using standard tools. The number of thread crests that occur in 1″ of thread is measured to determine the threads per inch, or TPI.
TPI is used with American threads, while metric fasteners use a different measurement called Thread Pitch (see below). TPI can be converted to a similar value for metric fasteners using conversion tables, calculators, or a simple mathematical formula.
Generally speaking, the higher the TPI, the finer the thread, and the greater the strength of the fastener. However, there are other factors to consider when selecting a fastener, such as the material in which the screw or bolt is made, the size of the head of the screw or bolt, and how the screw or fastener will be used.
Measuring the TPI of a screw or bolt is relatively straightforward, and can be done with a pitch gauge, a caliper, or even by hand. To measure the TPI of a screw, lay it flat on a surface so that the helix is parallel with the edge of the surface. Using a caliper, place it next to the first thread and read the number of thread crests in one inch of the screw or bolt. Repeat this step for each of the threads, and calculate the number of threads per inch from the results.
In addition to TPI, the major/pitch diameter of a screw is a critical factor to consider when selecting the right fastener for the job. The major/pitch diameter is the maximum diameter of the threads as viewed from the side. The pitch is the distance between adjacent thread crests, and can be found by counting the number of threads from the bottom of the head or measuring the distance between thread crests with a caliper.
In the United States, the screw thread standards are regulated by the ASME B1.1-1989 specification. The specifications define the shape of an external and internal thread, and specify the number of starts, class tolerances, and the major/pitch diameter. While the USA has not yet moved to metric sizes, other nations have. In fact, the unified thread standard was developed in response to problems with interchanging hardware between America, Canada, and the UK during World War II. The UTS, or Unified Thread Standards, were endorsed by the Screw Thread Standardization Committees of Canada, the UK, and the US and published on November 18, 1949. The UTS consists of Coarse Threads (UNC), Fine Threads (UNF), and Special Threads. The majority of screws manufactured today are made to the UTS standard, and are categorized as either UNC or UNF based on their TPI, as shown in our table below. threads per inch