The screw (or screw bolt) is a threaded fastener with an elongated shank with a round or flat head. It is used to attach other components, such as nuts and washers, to a substrate such as wood or metal. Screws are manufactured in a wide variety of shapes, sizes and materials and have many different uses. Screws are commonly used in construction, furniture making, manufacturing and home repair to attach fasteners and parts. Screws are also used in electrical equipment, vehicles and many other applications where a secure connection is required.
Screws come in a variety of styles that include wood screws, sheet metal screws and stainless steel screws. There is also a large number of drives and types including slotted, Phillips, square and Robertson as well as hex, Torx and Pozidriv. These different types require different tools to drive or remove them.
Screw threads have been in use for about 400 years and their use expanded rapidly once the Industrial Revolution took hold. Screws have also been used in a variety of applications including clocks, firearms, armor and even food items such as pasta.
The first step in making a screw is to turn the raw material into a blank by either milling or turning on a lathe. This is followed by heat treatment to harden the material. The blank is then polished and then threaded either by rolling or cutting, or both. The finished screw is then inspected for quality and then coated with zinc, black oxide or other finishes to protect the raw surface from corrosion.
Early screws were made by hand and production was very slow. In the 1770s English instrument-maker Jesse Ramsden developed a machine that would allow for more rapid production of screw threads. This allowed for greater consistency and allowed for a much larger scale of production.
During the 1940s, the United States and Canada agreed on a standard screw thread that is currently used in most American homes and vehicles. This standard is called the Unified Thread Standard and is controlled by ASME/ANSI. The standard defines a specific thread profile and series along with allowances, tolerances and designations. The thread profile was based on the Whitworth thread form with some modifications that were intended to improve fatigue performance and the screw’s ability to pass through the substrate.
The standard also defines the screw gauge which is determined by measuring the diameter of a single thread using a tool called a screw gage. A screw’s gauge is not a direct relationship to its length, but a closer correlation is seen in terms of the thread pitch which is measured as the distance between adjacent threads. The US screw size is then calculated from the thread diameter and pitch with a multiplication value to create an imperial measurement. This system is not used in the rest of the world, which instead uses metric measurements. US screws