Hex bolts have hexagonal heads and machine threads for use with a nut or in a tapped hole. Also known as hex cap screws or machine bolts.
a set screw is a type of screw generally used to secure an object within or against another object, normally not using a nut. The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless (also called blind), meaning that the screw is fully threaded and has no head projecting past the major diameter of the screw thread. If a set screw has a head, the thread will extend all the way to the head (whereas a bolt might have an unthreaded shank between the head and thread). A blind set screw (known in the uk as a grub screw, quite possibly from its figurative resemblance to a soil-dwelling grub) is almost always driven with an internal-wrenching drive, such as a hex socket (allen), star (torx), square socket (robertson), or slot. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts compression or clamping force rnthrough the bottom tip that projects through the hole.

Hex bolts have hexagonal heads and machine threads for use with a nut or in a tapped hole. Also known as hex cap screws or machine bolts.
a set screw is a type of screw generally used to secure an object within or against another object, normally not using a nut. The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless (also called blind), meaning that the screw is fully threaded and has no head projecting past the major diameter of the screw thread. If a set screw has a head, the thread will extend all the way to the head (whereas a bolt might have an unthreaded shank between the head and thread). A blind set screw (known in the uk as a grub screw, quite possibly from its figurative resemblance to a soil-dwelling grub) is almost always driven with an internal-wrenching drive, such as a hex socket (allen), star (torx), square socket (robertson), or slot. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts compression or clamping force rnthrough the bottom tip that projects through the hole.