Screw Head Types
Hex Socket Head (Allen® key)
Hex or hexagonal screws have a six sided parallel hole in the centre of the screw.
Hex head screws are perhaps most frequently used in Engineering, however, since they are quite cheap to produce, they are increasingly found in self assembly furniture.
There are two forms of driver for this kind of screw; Hex key or screwdriver. Depending on where you live in the world the Hex key is also known as an Allen®, Inbus, Zeta or Unbrako key.
The screwdriver is simply a standard screwdriver handle with a hexagon tip.
In the development of hex drives (which require absolutely straight access to the screw head) there are hex keys which have a ball drive on one end. This is designed to allow the tool to be used at an angle off-axis to the screw (see picture on the right).
Although this solves the problem of access to the screw the thinning of the shaft to create the ball shape means that it's weaker than those without this feature, so less torque can be applied.
The driver also makes point contact with the screw head rather than the line contact seen in the straight versions. This can increase the chances of stripping the head, particularly on smaller screws.