The screwdriver is a device specifically designed to insert and tighten, or to loosen and remove, screws. The screwdriver comprises a head or tip which engages with a screw, a mechanism to apply torque by rotating the tip, and some way to position and support the screwdriver. A typical hand screwdriver comprises an approximately cylindrical handle of a size and shape to be held by a human hand, and an axial shaft fixed to the handle, the tip of which is shaped to fit a particular type of screw. The handle and shaft allow the screwdriver to be positioned and supported and, when rotated, to apply torque. Screwdrivers are made in a variety of shapes, and the tip can be rotated manually or by an electric or other motor.
A screw has a head with a contour such that an appropriate screwdriver tip can be engaged in it in such a way that the application of sufficient torque to the screwdriver will cause the screw to rotate.
Gunsmiths still refer to a screwdriver as a "turnscrew", under which name it is an important part of a set of pistols. The name was common in earlier centuries, used by cabinet makers and shipwrights and perhaps other trades.
The Cabinet-Maker's screwdriver is one of the longest-established handle forms, somewhat oval or elipsoid in cross section. This is variously attributed to improving grip or preventing the tool rolling off the bench, but there is no reason to suppose these are not rationalisations. The shape has been popular for a couple of hundred years. It is usually associated with a plain head for slotted screws, but has been used with many head forms.
Screwdrivers come in a large variety of sizes to match those of screws, from tiny jeweler's screwdrivers up.
If a screwdriver that is not the right size and type for the screw is used, it is likely that the screw will be damaged in the process of tightening it. This is less important for PoziDriv and SupaDriv, which are designed specifically to be more tolerant of size mismatch. When tightening a screw with force, it is important to press the head hard into the screw, again to avoid damaging the screw.
Some manual screwdrivers have a ratchet action whereby the screwdriver blade is locked to the handle for clockwise rotation, but uncoupled for counterclockwise rotation when set for tightening screws; and vice versa for loosening.
Many screwdriver designs have a handle with detachable head (the part of the screwdriver which engages with the screw), called bits as with drill bits, allowing a set of one handle and several heads to be used for a variety of screw sizes and types. This kind of design has allowed the development of electrically powered screwdrivers, which, as the name suggests, use an electric motor to rotate the bit. In such cases the terminology for power drills is used, e.g. "shank" or "collet". Some drills can also be fitted with screwdriver heads.
Manual screw drivers with a spiral ratchet mechanism to turn pressure (linear motion) into rotational motion also exist, and predate electric screwdrivers. The user pushes the handle toward the workpiece, causing a pawl in a spiral groove to rotate the shank and the removable bit. The ratchet can be set to rotate left or right with each push, or can be locked so that the tool can be used like a conventional screwdriver. Once very popular, these spiral ratchet drivers, using proprietary bits, have been largely discontinued by manufacturers such as Stanley, although one can still find them at vintage tool auctions. Companies such as Lara Specialty Tools now offer a modernized version that uses standard 1/4-inch hex shank power tool bits. Since a variety of drill bits are available in this format, it allows the tool to do double duty as a push drill.
Many modern electrical appliances, if they use screws at all, use screws with heads of non-standard shape in an attempt to prevent users of the device from easily disassembling them. TORX is one such pattern that has become very widespread, with suitable screwdrivers widely available in electronics stores. Other patterns of security screws are less common, though sets of security heads are, again, readily available. An example of this is the Gamebit screws used in all Nintendo consoles, but which can be readily found online.
While screwdrivers are designed for the above functions, they are commonly also used as improvised substitutes for pry bars, levers, and hole punches, as well as other tools.
There is no such thing as a "left-handed screwdriver", as the device can easily be wielded in either hand. To be sent on an errand to find a left-handed screwdriver is often a test of stupidity, or is used as a metaphor for something useless. The term "Birmingham screwdriver" is used jokingly in the UK to denote a hammer or sledgehammer.
The handle and shaft of screwdrivers have changed considerably over time. The design is influenced by both purpose and manufacturing requirements. The "Perfect Handle" screwdriver was first manufactured by HD Smith & Company that operated from 1850 to 1900. Many manufacturers adopted this handle design world wide. The "Flat Bladed" screwdriver was another design composed of drop forged steel with riveted wood handles.
Among slotted screwdrivers, there are a couple of major variations at the blade or bit end involving the profile of the blade as viewed face-on. The more common type is sometimes referred to as keystone, where the blade profile is slightly flared before tapering off at the end. To maximize access in space-restricted applications, the same edges for the cabinet variety, in contrast, are straight and parallel, meeting the end of the blade at a right angle.