The outer side of the screw head of the inner hexagonal bolt is round, and the middle is a recessed hexagon. The outer hexagonal bolt is the kind of hexagonal bolt with the sides of the hexagonal shape of the more common screw heads.
Hexagon socket bolts are often used in machinery, and mainly have the advantages of easy fastening and disassembly, and not easy to slip corners. Allen wrenches are generally in the shape of a curved square with a 90° turn. The bent end is long and the other is short. When the short end is used to screw the screw, the long end can save a lot of force and can better tighten the screw. The long end is divided into a round head (a hexagonal cylinder is similar to a sphere) and a flat head. The round head can be easily slanted in and disassembled to install some parts that are not convenient for wrenching.
The manufacturing cost of the outer hexagon bolt is much lower than that of the inner hexagon. Its advantage is that the screw head (the position where the wrench is stressed) is thinner than the inner hexagon, and in some places it cannot be replaced by the inner hexagon. In addition, machines with low cost, low power strength, and low precision require much fewer inner hexagon bolts than outer hexagon bolts.
