thread

A thread is a helical groove cut or rolled into the surface of a fastener (usually a bolt or screw) or into the inside of a hole (a nut or tapped hole). The thread converts rotational motion into linear motion, or vice versa, while also creating mechanical advantage through friction and engagement. On a bolt, the male thread is the raised spiral; in a nut or hole, the female thread is the recessed spiral that receives it.

Threads come in two main forms: metric (ISO 13-1, specified in millimetres) and imperial (UNC, UNF, specified in threads per inch). A metric M10 bolt, for example, has a 10 mm nominal diameter and typically a 1.5 mm pitch (distance between adjacent crests). UNC threads are coarser; a 1/4 inch bolt is typically 20 TPI (20 threads per inch). The choice between them depends on the standard used in your industry and the equipment already in place. Aerospace and precision machinery favour metric; older industrial equipment in English-speaking regions may still use imperial.

Thread profile defines how the thread engages. Metric and UNC both use a 60-degree V-profile, but variations exist: metric has a flatter root, UNF is finer and therefore stronger in tension, Whitworth uses a rounded profile (common in older British machinery), and ACME threads are trapezoidal, used in lead screws where driving force matters more than tensile strength. Self-tapping screws have sharp or blunt points and form their own path into softer materials; machine screws require a pre-tapped hole.

Common failure modes

Stripped threads occur when the fastener is overtightened or when the material is too soft (aluminium in a plastic hole). Corrosion, especially in stainless steel bolts in brass threads or vice versa, creates galvanic couples that seize threads solid. Galling, a form of cold welding, is common with stainless fasteners; anti-seize compounds and lubricants reduce this. Loosening due to vibration can be managed with lock washers, nylon inserts (Nyloc nuts), or thread-locking compounds.

Thread pitch is critical in dynamic applications. Fine pitch threads (UNF, metric fine) offer better vibration resistance and higher clamping loads because more thread engagement is available in the same length. Coarse pitch (UNC, metric coarse) assembles faster and tolerates contamination better. In lead screws and actuators, pitch determines travel per revolution: a 2 mm pitch moves the load 2 mm per full turn of the spindle.

The term 'thread' in textiles (yarn, warp, weft) originates from the same root as the fastening thread, both describing something twisted or wound. In manufacturing, thread specifications must account for material (steel, stainless, brass), hardness, and the material being fastened. A bolt into cast iron tolerates less torque than one into steel; aluminium requires smaller fasteners or inserts to prevent thread pull-out.