Screws come in a variety of shapes, sizes and materials to meet the needs of almost any construction task. They’re used for everything from woodworking to metal roof installations and are an important tool in every tradesperson’s toolbox. But selecting the correct screws for a job can be challenging. The wrong size can cause problems like splitting or damaging the material into which it is installed.
To help avoid mistakes, it is crucial to understand the three basic measurements that define a screw: gauge, threads per inch and shaft length in inches. This will allow you to select the proper drill bit and ensure that your screw will fit properly into whatever surface it is being driven into.
The gauge of a screw is determined by the diameter of its head, which may be smooth or have a design like square, Phillips or hex. Screws with larger heads are usually designed for heavier materials, while smaller-headed screws have more versatility. For example, a #8-gauge screw is commonly used in home woodworking and will have a head approximately 2/3 of its overall diameter. A #8-gauge screw will also have 32 threads per inch, which is the standard for coarse screws. There are two main standard screw-size numbering systems, the imperial system and the metric system. The imperial-system screws are often labeled with a gauge first, followed by the length in inches. The metric system usually lists the screw’s diameter first, then lists the threads per inch (TPI) in a column.
Once you know the screw’s gauge and TPI, you can determine what kind of screwdriver to use by its shaft length. Screws with shorter shafts can be driven into the surface with a power screwdriver, while longer screws will need a manual screwdriver. Screws with long shafts are typically designed to reach a deeper point in the material into which they are being screwed, and can even be used to anchor the structure.
A final consideration is the head shape of a screw. Screws have a wide range of heads, from flat countersunk or oval to square and hex. The type of head you choose will have a significant impact on how easy it is to drive the screw into your project.
Once you know the basic screw measurement numbers, it becomes much easier to navigate the countless choices in any hardware store or online. There are hundreds of different types of screws, each with a unique set of properties that make it suitable for a particular application. The right selection can prevent costly mistakes and ensure that your project is completed with high-quality fasteners. This article was written by Lee Grindinger and was published in the April 2014 issue of “Woodworker’s Journal” magazine. 5/8 to mm