Screws are, at their basic level, an inclined plane wrapped around a small cylinder. Due to modern manufacturing methods, these amazing building blocks for machinery can be produced incredibly cheaply, and allow for a quick and strong way to attach things together. They come in many different shapes and sizes, designed for specific applications.
Machine screws can be used in precision machinery, and are defined by major diameter as well as threads per inch. Metric screws are specified with the letter “M,” the diameter, then the thread pitch. So “M6 × 1” would be a 6mm diameter screw with a thread pitch (spacing between threads) of 1mm. Unified National threads, also known as “English” or “Imperial,” are defined by the screw number — 10 for example, signifying a .190″ diameter — and threads per inch. So a 10-32 screw is a #10 (.190″) diameter, and has 32 threads per inch. Larger English screws are known by their fractional size. For example, a ¼-20 screw would be ¼” in diameter and has 20 threads per inch.
If you’re working with wood or sheet metal, self-threading screws form the material as needed, allowing you not to worry about the thread pitch quite as much. Simply drill a pilot hole into the material, and screw your fastener in. One disadvantage of this type of fastener is that it can damage your material, especially if it needs to be removed and reinstalled on a regular basis.
Screw Head Types
Screws come with different types of heads, including the flat head (-) and Phillips head (+), as well as hex head (shaped like a hexagon), and Torx head (shaped like a six-pointed star). Though those head types are quite common, there are many other variations, including some that are meant to resist attempts at removal. A good screwdriver set with a variety of interchangeable tips is extremely useful for creatively voiding your warranty!
Screws, though interesting in themselves, aren’t much use without something to attach to. In the case of self-threading screws, the screw, when inserted into your chosen material, forms it into the correct shape. For machine screws, you can use a nut of the correct size, or you can form female threads into your material using a tap set. To use a tap, drill the appropriate pilot hole, then twist the tap into the hole. This can be extremely useful to minimize parts, and when you’d rather not put a screw entirely through both pieces of material.