Threading

What is a thread?
A thread is a helical profile, formed as a protrusion on an external cylindrical or conical surface called a screw, which mates with a helical groove of the same profile formed on an internal cylinder or cone called a nut. Only the exact dimensions of both the profile and the helix allow for proper engagement between screw and nut.
Threads are primarily used to join two components in a fixed or adjustable but non-permanent manner.
Helix of a thread
The helix generated by the thread performs several turns or rotations around the cylindrical profile. Each turn of the helix is referred to as the pitch. Once the screw and nut are engaged, rotating either component causes the screw or nut to advance along its axis by a distance equal to the pitch for every full rotation. For this reason, thread specifications usually report, alongside the major (external) diameter and the designation of the thread, its pitch as well. Knowing the pitch of a thread is crucial not only for determining the axial displacement per rotation, but also for deriving other important information required in the manufacture of screws and nuts, whether with standard or special pitches.
Threads can be right-hand or left-hand, depending on the helix orientation. Rotating a right-hand thread clockwise results in engagement and tightening between screw and nut. Conversely, in a left-hand thread, counterclockwise rotation produces loosening and separation of the two components.
Thread profiles
Several thread profiles exist, including triangular (with different flank angles), trapezoidal, square, buttress, spherical, and others.
In the following sections, the various types of threads will be described, covering their general applications, geometry, and profile. The triangular-profile thread is the most common and widely used worldwide. Historical, geopolitical, and industrial factors contributed to a fundamental division between the metric system and the inch-based system. In Europe, with the exception of Ireland, England, and Scotland, the metric system—introduced by Napoleon Bonaparte in the 19th century—is predominantly used. In the Americas, many former Commonwealth countries, and in England, Scotland, and Ireland, the inch system is still common, where one inch corresponds to 25.4 mm.
Triangular-profile threads are further divided into different families, depending on the measurement system adopted by each country, as well as their field of application and intended use.