If you’ve spent any time in a woodshop, you’ve likely seen or heard of many different kinds of screws. You might even have a few in your drawer, but maybe you’re not sure what they all mean or how to choose the right ones for your projects. Luckily, it’s fairly easy to determine what a screw means by looking at the head-bore size and shank-hole size.
These measurements indicate how much force a screw can support perpendicularly to its body, as well as how far it can be driven into the material without breaking or pulling out. Screws also have shear strength, which is how much lateral force they can hold before the threaded portion of the screw breaks off or becomes damaged.
Screws are available in various lengths, which can also affect how much force they can carry. A general rule of thumb is to select a screw that’s at least half as long as the thickness of the material it will be going into. Using a screw that’s too short will leave the fastener hanging in the air and risk damage to the opposite side of the material.
In the United States, most of our screws have a standard designation that indicates their major diameter and screw thread pitch. These dimensions are determined by the screw’s governing standards, which are based on a system that was originally developed in the 1840s by Joseph Whitworth. Later on, a more simplified version was created by William Sellers that is still used today, called US fractional. 3/4 to mm