Screws are used for many applications in the workplace and at home. When choosing screws for a particular job, the correct diameter and length must be selected. Incorrectly sized fasteners can cause the material to deform, break or work itself loose. When determining which screw to use, consider the thickness of the material and whether the head of the screw will need to be countersunk. Lastly, the screw type must be compatible with the intended job and material, as well as the screw head style.
In the US, the thread dimensions of inch-thread hardware are specified using a system called Unified Thread Standard (UTS). Screws that follow this system are designated by their gauge and length, rather than their metric equivalents, so they can be interchanged across countries. The gauge of a screw is the diameter of its shank, while the length is the distance from the bottom of the screw head to its tip.
The majority of screws are single-start threadforms. This means that every axial turn of the screw’s body advances the lead by the width of one ridge. This leads to a very smooth and precise threadform, which is easy to use in tools and machines.
The thread numbering system is based on a roughly logarithmic series, so that increasing the screw number by one size approximately doubles its tensile strength. A screw’s diameter is given by the formula: Diameter = (Screw Number X.013″) +.060″ (for sizes larger than 0). Screws smaller than size #0 are supplied in the Unified Miniature thread series, and their diameters are calculated by the formula: Diameter = (Screw Size X.005″) +.003″ (for sizes 00 and 004). screw diameters