Screws belong to a family of threaded fasteners that also includes studs and bolts. The practical applications of screws are almost endless: They are used in industries ranging from construction to electronics and can be used for almost any project that requires two or more objects to be firmly connected. The process of making these small fasteners is complex, but modern manufacturing processes have made screws cheaper and more durable and precise.
Early Screw Production
The concept of screws can be traced backed to approximately 200 B.C., but metal screws that resemble the modern style weren't developed until the Renaissance Age, between the 14th and 17th centuries A.D. These early screws were handmade, thus no two screws were ever identical. In 1586, Jacques Besson introduced the first screw-cutting machine, which would pave the way for future innovations. Job and William Wyatt filed a patent for the first automatic screw-cutting machine in 1760; it could cut roughly 10 screws per minute. In 1836, William Keane developed a thread-rolling process. Although his efforts were mostly unsuccessful, thread rolling eventually became the modern standard for screw production.
The most common material used to make screws is low to medium carbon-steel wire. Other durable and inexpensive metals that are sometimes substituted include brass, stainless steel, nickel alloy and aluminum alloy. Some screws have a finish applied to them for extra protection, which must be compatible with the raw materials of the screw. Steel can be plated in zinc, cadmium, nickel or chromium.
The mass manufacturing of screws is accomplished through cold heading and the thread rolling method. A coil of wire is fed into a machine that straightens it, then directly into another machine that cuts the wire into the desired lengths. A one- or two-punch process die cuts the head of the screw into a preset shape. This process can produce between 100 and 550 screw blanks per minute.
The screw blanks are then guided down a chute that leads to one of three different types of thread-cutting dies. If a reciprocating die is being used, the screw will be rolled between one stationary flat die and one that moves back and forth alternately to create the screw threads. A cylindrical die achieves the same basic result by rolling the screw between two or three round dies. The planetary rotary die process holds the screw blank in a stationary position while several die-cutting machines roll around the screw. All three of these methods create screws that are stronger than those created using machine-cut methods. This is because the threads are not actually cut into the screw blank but rather pressed. Thread rolling ensures that no material is lost in the body of the screw, keeping the metal strong while also creating more precisely positioned threads.
Standards for screw threads were established by the National Screw Thread Commission in 1928; the main goal was to make screws more interchangeable. A unified screw thread system was adopted in 1948 that focused on the number of threads per inch, the pitch and shape of the thread and diameter sizes. In 1966, the International Standards Organization suggested universal restrictions on metric and inch size ranges that have become the accepted global standard.