Screw

A screw is an externally helical threaded fastener. This means it has a spiral ridge wrapped around its body. You can tighten or release a screw by applying a twisting force, known as torque, to its head.

Screws are essential because they hold objects together with great strength. They are also classified as one of the six classical simple machines. This classification was defined by scientists during the Renaissance. Screws can be inserted into existing holes or they can create their own threads as they enter a material.

The anatomy of a screw includes several specific parts. The cylindrical portion from the underside of the head to the tip is called the shank.

The shank may be fully threaded or only partially threaded. The distance between each individual thread is called the pitch. Most screws use a right-hand thread, which means they tighten when rotated clockwise. However, some special cases require a left-hand thread. For example, the left-side pedal of a bicycle uses a left-hand thread. This prevents the counterclockwise torque from loosening the pedal while riding.

There are different types of screws based on their shape and use. A wood screw is a metal fastener used specifically for fixing wood. It features a sharp point and a tapered thread. This design allows it to cut its own thread into the wood material. Some screws have a tapered shank, while others have a non-tapered shank. Fasteners with non-tapered shanks are often called bolts. A bolt is designed to be tightened or released by torquing a nut rather than just being driven into a material. There are also set screws, also known as grub screws, which do not have a head larger than their body.

The history of the screw shows it was one of the last simple machines to be invented. It first appeared in Mesopotamia during the Neo-Assyrian period between 911 and 609 BC. Later, it appeared in Ancient Egypt and Ancient Greece. The Greek mathematician Archytas of Tarentum described the screw between 428 and 350 BC. By the 1st century BC, wooden screws were common in the Mediterranean. People used them in screw presses to extract olive oil or grape juice. For a long time, people relied on other methods like pins, nails, or leather knots to join things.

Industrial production of metal screws changed significantly in the 18th century. In 1760, brothers Job and William Wyatt patented an early screw machine in the UK. By the 1780s, their enterprise produced 16,000 screws a day with only 30 employees. This was a massive amount of industrial productivity for that time. In 1777, Jesse Ramsden invented the first satisfactory screw-cutting lathe. Later, Henry Maudslay popularized these lathes around 1797 and 1800. His machines included a leadscrew, a slide rest, and a change-gear gear train. These tools helped make machine screws a common commodity.

In the United States, screw manufacturing also saw important developments. Hardman Philips built the first American screw factory in 1821 near Philipsburg. This mill used steam and water power to manufacture blunt metal screws. While it was not a commercial success, it showed the potential for American production. Later, the development of the turret lathe in the 1840s and automatic screw machines in the 1870s reduced costs. These advancements allowed for much greater automation in machine-tool control. This made threaded fasteners much cheaper and more available for everyone.

Modern screws are defined by their various head and drive types. Many years ago, screws only used simple straight slots or square shapes. In 1908, P. L. Robertson made the internal-wrenching square socket drive practical. In 1911, the internal-wrenching hexagon drive became practical to manufacture. In the early 1930s, Henry F. Phillips popularized the Phillips-head screw.

This screw uses a cross-shaped internal drive. Today, we also use precision screws to control motion in complex machines. These precision components were central to the technical advances of the Industrial Revolution. They are still vital parts of modern tools like micrometers and lathes.

Manufacturing a screw follows a precise three-step process. First, a machine takes wire or metal rods and cuts them into pieces called blanks.

Next, the screw undergoes cold heading to create the head. This process presses the metal into shape using a die, which produces almost no waste. If the screw is a slotted type, a slotting machine cuts the slot afterward.

The third step is threading, which is often done via thread rolling. Finally, a coating like zinc is applied through a process called galvanizing. This coating helps to prevent corrosion and rust over time.