A screw thread is defined as a uniform pattern of continuous elevation and depression either on external or internal part over a desired length of the an cylindrical object or component. Internal threads are refer to those formed on nuts or tapped machine holes and external threads are those on bolts, screws or studs.
Screw threads are primarily formed by two key methods: Cut Thread and Rolled Thread. Each of these are explained in brief.
Cut Thread : The threads are machined/cut from the surface of the bar and subsequently the material is removed from the surface with a decrease in mass from the original bar stock. The diameter of the bar stock required is same as that of the end diameter.
Roll Thread : Bar stock of a slight lesser diameter as that of the final product is taken and pressed between two rolling dies with pre-engraved opposite threads. In this process, material is not removed perhaps it is merely displaced by a rolling action of the threaded dies.
Depending upon the use, screw threads types are ISO Metric Threads, American Nation Unified Thread Series, V Threads, 60 Degree Stub Threads, Square Threads, Acme Threads, Stub Acme Threads, Buttress Threads, Knuckle Threads, & Whitworth Threads. Among these ISO metric threads and American National Unified Threads are two most commonly used screw thread profiles.
Distance as measured between one successful elevation (a.k.a crest) and subsequent depression (a.k.a root) is one screw thread. Screws threads are measured by pitch thread distance in metric system or by number of threads in imperial or US system.
In metric system, the threads are defined for a component by pitch; distance between two threads i.e between two consequent crest or two consequent roots. For example, M20-2.5P is a measurement of a metric sized bolt with thread pitch as 2.5mm as defined by ISO 724 standard. Whereas in imperial or US System, number of threads are accounted per inch of the length of the bolts. For example, Bolt, 1/2"-13UNC refers to 13 Unified National Coarse threads per inch of the bolt's length and 1/2" is the size of the bolt diameter.