Essence: A type of external thread fastener, whose head features an internal hexagonal socket (hexagonal recess), and requires an internal hexagonal wrench (Allen wrench) for driving (tightening or loosening).
Main Components: * Head: Usually cylindrical, with a flat or slightly rounded top. At the center of the head is an internal hexagonal socket, whose size (such as the width across flats) matches the driving wrench. The bottom of the head typically has a bearing surface (flat washer face or no washer face). * Shank: The part connecting the head and the thread. It can be fully threaded (with threads throughout the entire shank) or partially threaded (part smooth shank + part threaded). The diameter of the smooth shank section is usually slightly smaller than the nominal diameter of the thread. * Thread: A helical structure manufactured according to specific standards (such as ISO metric threads, UN imperial threads), designed to mesh with nuts or internal threaded holes. The thread section has an end chamfer to facilitate insertion.