Screws
Screws are externally-threaded fasteners consisting of a spiral-shaped shaft and a head. The shaft fits into a work piece and is held in place by the head. A screw is technically a metal pin with incised threads and a broad slotted head that can be driven as a fastener by turning with a screwdriver. Screws can be found in various materials including steel, stainless steel, bonze, brass, titanium, zinc, and chromium.
The main application for screws is as threaded fasteners. Threaded industrial fasteners, including screws, contain spiral ridges called threads, which aid in the fastener's attachment. A thread formed on the top of the head is specifically formed to mate with a screwdriver. The two main types of screws, machine and wood screws, are both made of metal. They differ in that machine screws have a constant diameter and join with nuts while wood screws are tapered and grip to the actual wood surface.
The threads on a screw can run right handed or left, tapered, straight, or parallel. Right-hand threads refer to screws that tighten when turned clockwise, while left-hand threads refer to screws that tighten when turned counter-clockwise.