Feature
The thread is clear, the screw mouth is deep, strong and durable, not easy to fall out.
Flat head screws, also called set screws, are usually not bolts with threaded rather than tapered screw grooves.
Hexagon head flat head screw with cup-pointed socket. Usually used to prevent pulleys, gears or other parts from rotating relative to the shaft.
These headless screws have a hex socket in which a hex wrench is inserted to tighten and loosen the screw.