screwthread

A screwthread is a helical ridge around a cylinder or cone that converts rotational motion into linear motion when parts are rotated relative to each other. It provides a fastener or connection by engaging with a mating thread. An external thread runs on a cylindrical surface of bolts or screws, while an internal thread runs inside nuts, bushings, or fittings. Basic thread geometry includes major (outer) diameter, minor (root) diameter, and pitch diameter, which is the effective area where mating threads engage. The lead is the distance a nut advances with one complete revolution, and a single-start thread has a lead equal to its pitch; multi-start threads have a lead greater than the pitch. Threads can be right-hand (the common convention) or left-hand.

Common thread forms and profiles include V-threads, typically with a 60-degree flank angle. Metric and various inch-based systems use V-threads in standard forms such as ISO metric (M-series) and unified thread standards like UNC and UNF. British Standard Whitworth and others historically used different angles, while power screws often employ trapezoidal (Acme) or square profiles for strength and efficiency. NPT and other pipe threads are tapered, designed to seal under compression rather than purely rely on a thread fit.

Standards and tolerances for screw threads are defined by international and national organizations (ISO, ANSI/ASME, DIN, JIS). Thread sizes, tolerance classes, and fit designations govern interchangeability and performance (for example, general-purpose fits versus precision fits). Manufacturing methods include cutting with taps and dies, forming by rolling, and milling or turning with thread-cutting tools. Screwthreads are ubiquitous in mechanical assemblies, fasteners, hydraulic and pneumatic fittings, pipe connections, and numerous precision mechanisms.