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Screwdrivers are tools used to press screws into wood, metal, or other materials by turning them at the driver-screw interface. Two general types of screwdrivers exist: power drivers and manual drivers. Manual drivers are turned by handPower drivers feature motors, usually electric motors, to turn the driver head. The type of head on the driver will also vary significantly according to what type of screw is being used. The two most common heads are Phillips heads and flat heads, though many other types exist.
Flathead screwdrivers feature a flat, narrow surface that is placed into a straight slot on the screw head. The width of the screwdriver's blade will dictate which screws it can be used with; a driver with a head that is too narrow will not be able to apply enough torque to the screw, and one that is too wide may obstruct the screw from turning as it makes contact with other materials. Larger driver heads may also not fit into narrower screw heads. The length of the driver will also dictate how much torque can be applied to the screw.
Phillips head screwdrivers feature a cross-shaped head that slots into a similarly shaped screw head. These screwdrivers are usually able to produce more torque than flathead screwdrivers, though they do have a tendency to round out the screw head if the wrong size driver is inserted. The blades of this type of driver can be elongated, but the longer they get, the wider the tip of the driver tends to get. This means a longer bladed driver will not fit into a smaller screw head, and a smaller bladed head may round out a larger screw head. The size must be chosen carefully to avoid damage to the screw.
Other types of screwdrivers include hex head drivers, torx drivers, and Robertson drivers. Hex head drivers feature a six-sided head that is intended for use with a hex screw. Torx drivers feature a star shape, which allows for more torque transfer and longer head life. Robertson drivers feature square heads and must be used with screws that feature square heads as well. This design allows for more torque transfer and a wider range of driver designs for various applications. Robertson screws are not commonly used in most countries, but they are less expensive to produce and are therefore less expensive for the consumer.
Don't forget offset screwdrivers, ratcheting, stubby, magnetic, spiral, screw-starting, torque-limited, among others. Also, Phillips is designed to have one of the least amounts of torque, which is why there are Frearson, Posidriv and Supadriv.