A thread is the ridge found along a part, such as a screw, that is helical in structure and helps to convert rotational motion into linear motion. Threads can be on the internal or the external portion of a cylindrical part, and may also be sloping. Basically, when you screw a screw into a nut or bolt, the thread rotates with the screwing motion, thus pushing the screw further into the nut or bolt and securing it in place. Not only does the thread on the screw play a role in this motion, but so does the thread on the nut or bolt into which the screw is moving. But we will get into that more, shortly. Threads come in different types, and knowing which type you are looking for is very important for equipment to function properly.
One of the first distinctions to be made when assessing thread type is whether it is a male or a female thread. To determine the difference, you will look at the actual location of the threads themselves, those helix ridges on the part. If the ridges that circle the part are on the outside, then it is a male thread, if they are on the inside, then it is a female thread. In other words, external thread ridges are male and internal thread ridges are female. Most commonly, a male thread connects with a female thread. In this way the ridges line up and the threads are rotated into each other. This mechanism is what makes the simple screw work, as described above.
A second distinction to make concerning the type of thread you have or need is whether it is a tapered thread or a parallel thread. The main difference between the two can be found across the length of the thread. When a thread is tapered, it narrows in diameter over the length of the part. When a thread is parallel, the diameter of the thread remains the same across the length of the part. Usually, the distinction between tapered and parallel threads can be made just with the naked eye. However, for finer differences, a caliper can be used to measure the diameter at different points along the part length. Again, like male and female threads, knowing whether the thread you are working with is tapered or parallel will help you ensure that the part into which the ridge fits is appropriately matched.
Beyond both the male vs. female and tapered vs. parallel distinctions, there are a couple of features of threads that help identify different types. These features include pitch size and diameter. The pitch size of a thread is defined as either the number of threads per inch on a part or the distance between each individual thread on a metric thread. A pitch gage is the ideal tool for measuring the pitch size of a thread. Thread diameter is defined as the diameter across the thread. On a male thread, diameter refers to the outside diameter and on a female thread, diameter refers to the inside diameter. As mentioned above, the thread diameter is used to determine whether it is tapered or parallel.
Finally, the last way in which to distinguish between different types of threads is by the thread type standard. The thread type standard, sometimes called the unified thread standard (UTS), defines the form and series of a grouping of threads. The thread type standard accounts for the allowances, tolerances, and designations among different types of threads. Some examples of different thread type standards include NPT, PT, and G.
Now you know a little bit more about the world of threads. This important helical feature comes in particular locations, sizes, and shapes making up a number of different classifications and types. Make sure that you know what type of thread you are working with by using these different definitions.