What is screw?

A screw and a bolt (see Differentiation between bolt and screw below) are similar types of fastener typically made of metal, and characterized by a helical ridge, known as a male thread (external thread). Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread (internal thread) in the matching part.

Screws are often self-threading (also known as self-tapping) where the thread cuts into the material when the screw is turned, creating an internal thread that helps pull fastened materials together and prevent pull-out. There are many screws for a variety of materials; those commonly fastened by screws include wood, sheet metal, and plastic.

A screw is a combination of simple machines—it is, in essence, an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts a wedge as it pushes into the fastened material, and the shaft and helix also form a wedge in the form of the point. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut, or object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.

A screw will usually have a head on one end that allows it to be turned with a tool. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions. Carriage bolts have a domed head that is not designed to be driven. Set screws often have a head smaller than the outer diameter of the screw. Headless set screws are also called grub screws. J-bolts have a J-shaped head that is not designed to be driven but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.The distance between each thread is called the "pitch".