Screws are among the most commonly used fasteners and they are used in a wide variety of applications, from light duty assembly to critical fastening of structural parts on an aircraft. When considering the right screw for any given application, there are several important product specifications to consider. The first is screw type (machine, self-tapping, set, etc.). For example, self-tapping screws create their own holes, while thumb screws are designed to be twisted by hand, and socket-style screws are made for use with wrenches.
Choose the material; it will often match the project’s material. Metals and alloys tend to be more durable, heat-resistant, and strong, but some can be susceptible to corrosion, while plastics are nonmagnetic and tend to be more chemical resistant.
Then consider drive type (Phillips, hex, tamper-resistant one-way, etc). For example, some screws have slotted drives designed for use with screwdrivers, while others have socket-style drives designed for use with wrenches. Next comes head type (flat, pan, cheese, fillister, etc.). For example, a cheese-head screw has a thick head that sits on the surface of your object, while a flat-head screw is countersunk and flush with your object’s surface.
The next considerations have to do with size. What system of measurement to use: metric or inches? The length of different types of screws (flat vs. cheese, for example) are measured in different ways. Flat head screws are measured from the top of their head to the point, while cheese heads are measured from the bottom of the head to the point. The screw's size name indicates the external diameter of the threaded portion of the screw followed by the threads per inch for standard screws and millimeters per thread for metric screws. Standard screws are measured in inches and metric screws are measured in millimeters.
The final consideration is thread style. For example, coarse threads are more common and best for use in low strength materials, but fine threads are better when working with harder materials.