A machine screw, also sometimes referred to as a machine bolt, is a screw that is typically designed to be fastened to an existing, tapped hole on a metal surface, usually in conjunction with a corresponding nut. These types of screws are not as large as the average screw, usually ranging in sizes up to three quarters of an inch (19.05 mm) but they can still be larger. Variations between machine screws mainly exist in overall size, shape of the head, slot type, length, material, and characteristics of the thread.
A major way of categorizing a machine screw is by the type of slot in the head, which will determine the kind of screwdriver that is to drive it into its hole. The two major kinds of screwdrivers used with machine screws are slotted and Phillips head screwdrivers, but there are several other minor types of screw drive holes such as Torx — six-pointed, star-shaped drive holes — that require more specialized screwdrivers. Specialized types of screw drive holes may be important when it is desired that the screws are not interfered with for safety reasons, since it is more difficult to acquire specialized types of screwdrivers. A dangerous electrical panel, for instance, is a good place for a machine screw with a specialized type of hole. Other machine screw heads can be similar to a hex or square bolt that have no screwdriver slot and must be turned with a wrench.
The shape of the head of a machine screw will determine how it lies once it is fastened to a hole. Round and pan heads will protrude from a flat surface once they are attached. Flat-headed screws are designed for holes that are bored so they lay flush with the surface when they have been screwed in. The head of a set machine screw is smaller than the diameter of the screw threads, so when it is attached it can be driven through a hole and adjusted to a desired depth.
More of the major differences between machine screws are in the characteristics of the thread. This part of any given machine screw is one of the most important, because the corresponding holes that they are fastened to will typically be tapped to fit only one specific size and type of screw thread. The system used for characterizing different threads usually follows ISO (International Organization for Standardization) standards, which have been established to keep consistent design specifications in machine parts produced throughout the world. Two of the major thread characteristics are its diameter and pitch, the latter being the distance between threads. These screw threads can also be made to be screwed clockwise (right-handed) or counter-clockwise (left-handed).
While the size of a machine screw is usually relatively small, there can still be a vast amount of variations between size, shape and length, mostly determined by their intended purpose and how strong they need to be. Custom-made screws may also be manufactured for specific machines that do not even have to follow any ISO standards. Machine screws can be made of any of a variety of metals or plastics.