For nearly two millennia, threaded hardware has supported humankind’s greatest innovations. The multitude of different bolt sizes and styles might seem confusing, but it’s possible to navigate the complexities with a bit of knowledge and a little practice. The key is understanding the distinctions between imperial measurements (threads per inch) and metric ones (thread pitch).
Imperial threaded fasteners, like bolts, are typically described by their diameter, number of threads in an inch, and length. Their size is identified by a one or two digit number for smaller bolts and as a fraction for larger ones. The first number is called the gauge number, while the second is known as the threads-per-inch number or TPI.
Bolts with coarse threads are typically called UNC, while those with fine or extra-fine threads are labeled as UNF. These classifications reflect the thread’s design and application, with coarse threads more durable and suitable for lower-tolerance applications, and finer threads better suited for precision work and high tensile strength.
Depending on the desired outcome of a particular project, the proper thread can be determined by analyzing the needs of the environment or equipment where the bolt will be used. The process of generating threads is complex and can be done in many ways, including traditional subtractive methods (machining – tapping, single-point cutting, thread rolling, screw cutting, etc.); newer additive techniques (die casting, sand casting); and a combination of the above.
When choosing a bolt for an application, it’s important to know the bolt threads-per-inch and shank diameter to ensure a tight fit. A good way to determine the appropriate thread size is to use a standard thread insert tool, which is available at most home improvement and hardware stores. These devices are designed to accept all major imperial and metric hex bolts and display their threads. The display is split into sections corresponding to the various threads, so that you can easily discern which type of bolt is the correct one for your application.
Another method is to use a caliper or ruler to measure the actual threads-per-inch or the distance between thread crests on the bolt’s shank. The TPI and pitch numbers are the inverse of each other, with TPI measuring the density of the threads and pitch measuring the space between them.
A common mistake when determining bolt threads is to confuse the TPI measurement with the bolt’s length. The thread pitch, which is the distance between thread crests on a hex bolt, is the measurement that is referred to when determining the bolt length. In most cases, the thread pitch will increase as the bolt’s diameter increases. bolt threads per inch