Screws come in many sizes, and knowing the right ones to use for a particular job is essential to keeping your work on schedule and your project looking its best. The first thing to understand about wood screws is the way they’re sized; they are referred to by two distinct numbers. The first number, called the gauge, is the diameter; a higher number means a larger screw. The second number, called the head size, is the diameter of the screw’s head. The bigger the head, the more pressure it can take, and the easier it is to drive.
Wood screws are available in a wide range of lengths as well. Generally speaking, the longest wood screw is used for heavy framing carpentry, while shorter screws are more common for cabinetmaking and other light-duty projects. There are also special purpose wood screws designed for attaching trim and molding or specialized applications like concrete and brick. When it comes to choosing the correct length for a specific application, you’ll want to make sure that your screw penetrates into the thinner piece of lumber by about 2/3 the thickness of the board. This will prevent the screw from protruding too much into the other side of the board.
Depending on the type of screw you’re using, you’ll see various supplementary information included on your package or on the callout on the box itself. These may include tolerance class numbers, the LH symbol if you’re using left-handed screws, or the head bearing surface radius or chamfer. The bearing surface is the portion of the screw that contacts the head, and it should be rounded or knurled as necessary to ensure proper head grip.
A screw size chart is a handy tool to have on hand when selecting the right fastener for any project. This chart displays the equivalency of inches (fractions and decimals) and millimeters for various screw sizes, as well as the number of threads per inch and shaft length. The table also indicates which of the screw thread standards is being used; coarse is designated as UNC, while fine is referred to as UNF.
The chart can also be useful in determining the right screw length for your project. The chart gives the minimum and maximum lengths a screw can be while still meeting its required diameter and thread pitch. It’s also helpful in determining the head and shank bearing surfaces, which must be smooth and free of burrs prior to plating. These surfaces must be concentric with the basic screw diameter, D, within 1 percent, and should have a surface roughness not to exceed 125 micro-inches on the body and fillet or head bearing surfaces, respectively. #12 screw diameter