If you’re like most woodworkers, you use plans, instructions and specs that refer to a specific kind of screw and size all the time. But what do those numbers mean? That’s the question Lee Grindinger, editor of The Woodworker’s Journal, sets out to answer in this article.
Screws have three basic measurements — gauge, thread count and shaft length. A screw’s diameter is called its major diameter, and its thread count is the number of threads that pass over a given distance of the screw’s major diameter. The shaft length is how far into the material a screw extends. Screws sold with imperial system measurements on their packaging usually list the gauge first and the length next. They may also include the thread count between the two numbers, like a 10-35 x 2 inch screw.
In the metric system, screws are measured in millimeters. They’re often listed with the major diameter (mm) and thread pitch (the distance between a point on one screw thread to the same point on the next). To find the proper size screw for your project, measure the screw’s major diameter with a ruler or tape measure from one end to the other.
Screws come in a wide variety of heads, including flathead, oval countersunk head, and phillips head. Some have self-tapping tips and others are predrilled so that you don’t need to create the hole before screwing it in place. The type of screw you choose depends in part on the materials you’re working with and how much weight the screw will support. 3/8 in to mm