Metal fastening screw what classification?

Fasteners-screws can be divided into the following three: 1, the common thread (ISO general purpose metric screw threads) 2, unified threads (inch ISO general purpose screw threads) 3, Whitworth thread (United Kingdom inch thread) Three universal fastening threads, they can meet the needs of most applications. Prioritize the order of three threads common thread, unifying thread, and Whitworth threads. General purpose screw threads for the first thread, but it is not everything, in order to compensate for the limitation of its use, the common thread is the basis and evolution of small threads, transitions, and interference fit screw thread, hot-dip galvanized threaded and ultra fine pitch thread, this kind of threads belong to the family of general purpose screw threads.