A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force to the head. The most common uses of screws are to hold objects together and there a…A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force to the head. The most common uses of screws are to hold objects together and there are many forms for a variety of materials. Screws might be inserted into holes in assembled parts or a screw may form its own thread. The difference between a screw and a bolt is that the latter is designed to be tightened or released by torquing a nut. The screw head on one end has a milled slot that commonly requires a tool to transfer the twisting force. Common tools for driving screws include screwdrivers, wrenches, coins and hex keys. The head is usually larger than the body, which provide a bearing surface and keeps the screw from being driven deeper than its length; an exception being the set screw. The cylindrical portion of the screw from the underside of the head to the tip is called the shank; it may be fully or partially threaded with the distance between each thread called the pitch. Most screws are tightened by clockwise rotation, which is called a right-hand thread. Screws with a left-hand thread are used in exceptional cases, such as where the screw will be subject to counterclockwise torque, which would tend to loosen a right-hand screw. For this reason, the left-side pedal of a bicycle has a left-hand thread. The screw mechanism is one of the six classical simple machines defined by Renaissance scientists. Fasteners had become widespread involving concepts such as dowels and pins, wedging, mortises and tenons, dovetails, nailing, forge welding, and many kinds of binding with cord made of leather or fiber, using many kinds of knots. The screw was one of the last of the simple machines to be invented. It first appeared in Mesopotamia during the Neo-Assyrian period BC, and then later appeared in Ancient Egypt and Ancient Greece where it was described by the Greek mathematician Archytas of Tarentum. By the 1st century BC, wooden screws were …