Hexagon head screw Hex cap screw and large hexagon bolt Hex bolt are external thread fasteners with hexagonal head, designed to rotate with a wrench. 
According to ASME B18.2.1, hexagonal head screws (Hex cap screw) have smaller tolerances for head height and rod length than general large hexagon bolts (Hex bolt), so ASME B18.2.1 hexagonal screws are suitable for installation where all hexagon bolts can be used, including where large hexagon bolts are too large to be used. 
The head of the inner hexagonal screw is round on the outside, and there is a concave hexagon in the middle, and the hexagonal screw is the kind of hexagonal side of the common more screw head. 
The inner hexagonal screwdriver looks like'7'. It is cut by two sections of hexagonal steel bars and bent into 90 degrees to become an inner hexagonal screw wrench, which is sold in hardware stores. 
The inner hexagon screwdriver is not the so-called "special screwdriver for mobile phone". The screwdriver for mobile phone can be bought at the stalls selling repair tools in the electronic market. 
The inner hexagonal screw is often used in machinery, which mainly has the advantages of easy fastening, disassembly, not easy to slip angle and so on. 
The inner hexagon wrench is usually a 90 °turn, with one end long and one side short. When hitting the screw with the short side, the long side of the hand can save a lot of force and tighten the screw better. 
The long end has a split head (hexagonal cylinder similar to a sphere) and a flat head, which can be easily inclined to disassemble and install some inconvenient parts of the wrench. 
The manufacturing cost of the outer hexagon is much lower than that of the inner hexagon, and its advantage is that the screw (the force position of the wrench) is thinner than the inner hexagon, and in some places it can not be replaced by the inner hexagon. 
In addition, the machines with low cost, low power strength and low precision use much less inner hexagon screws than the outer hexagon. 
Inner hexagonal head screw (Socket cap screw), also known as socket head screw or Allen bolt screw, is a screw with a hexagonal inner hole in the head, which can be tightened or loosened after being inserted into the inner hole with a hexagonal ruler (hex key, Allen wrench or Allen key). 
The most common use of inner hexagon screws is cylindrical head screws, with a head diameter of approximately 1.5 times the main diameter of the thread (1960 series). Other head types include semicircular head screws (Bottom head cap screw) for beautiful surfaces and flat head screws (Countersunk head cap screw) for tapered screw holes. 
The countersunk hole design allows the screw head to rotate without being exposed on the surface of the fixed object, so it is often used in places where the surface is small and the traditional wrench is inconvenient to use.