• Wood: The threads on wood screws usually are coarse and deep to help them grab the wood.
Wood screws are self-starting and have a pointed end, while metal screws have flat bottoms and are often mated to holes that have been previously threaded or tapped.
• Machine: Machine screws have finer threads than wood screws. They are designed to be used in conjunction with a nut or tapped hole.
• Sheet metal: These screws usually are short and have coarse threads that are designed to grab onto relatively thin sheet metal.
• High-low: These screws have two sets of threads with alternating heights. High-low screws are specifically designed for certain plastics and
other low-density materials.
• Self-tapping: Self-tapping or thread-forming screws feature threads that are designed to tap their own holes.
These work well in softer materials such as wood and plastic but are not suitable for harder materials.
A screw with lugs or weld projections on the top or underside of the head to facilitate attaching the screw to a metal part by resistance welding.
Allen screws also are commonly known by two other names: hex screws and socket screws.
The most distinct part about an Allen screw is the hexagonal hole in the head that can only be loosened and tightened with an Allen/hex/socket wrench or key.
The various types of Allen screws are the hex-cap screw, socket-cap screw (ridged and smooth),
the socket flat-head screw, the socket button-head screw and the socket-set screw.