Woodworking Screws – How to Choose the Right One

Screws play a critical role in woodworking projects. From wall framing to furniture-making, they’re responsible for holding materials together and creating a solid connection between the fastener and whatever it’s attached to. However, with so many different screw types available in the hardware store, choosing the right one can be challenging. The type of screw you choose will depend on a number of factors, such as its length, diameter and head size. The correct screw can also depend on the material you’re using, and it’s important to understand how to measure screw thread size, or gauge.

While many people know that a screw’s length is measured in inches, fewer are aware that screw sizing is actually determined by two separate numbers: the gauge and the head size. The diameter of a screw is given by the first number on the gauge, while its head size is based on a ratio of the gauge to the shaft diameter. This formula is not only straightforward, but also useful in estimating the correct drill bit size to use for screw installation.

The numbering system for screw diameters follows the standard UTS (Unified Thread Standards) system and is used in the US, Canada and many parts of the world. A screw’s thread pitch, or number of threads per inch (TPI), is typically listed right after the gauge. A coarse screw will have a higher TPI, while a fine screw has a lower TPI.

When choosing a screw, the general rule of thumb is that it should be long enough to penetrate at least half of the thickness of the bottom material, or about 3/4 of an inch in the case of a 2 x 4. Wood screws are usually classified both by their length and diameter, called gauge, with larger numbers indicating thicker shafts. Utility or galvanized steel wood screws are typically used in framing carpentry and outdoor woodworking, while stainless-steel or chrome-plated steel wood screws have thicker bodies and are used for more precise woodworking and interior projects.

Screws are also classified by their head style and material. Slotted screws have a simple head with a hole that accepts a wrench or screwdriver, while cross-head screws have a star-shaped or round heads with a flat top. Phillips and square-drive screws are another common screw head type.

A screw’s head size and gauge are based on a simple relationship: the head diameter, or “head bore,” is approximately twice the screw’s gauge. While this isn’t always true, it’s a good guide to follow when building a woodworking project.

Screw sizing can be confusing, and it’s crucial to know how to measure screw threads correctly. To ensure you’re getting the most accurate measurements, always use a precision tape measure to get your diameters, and don’t forget to factor in the thickness of the bottom material. With this information, you’ll be able to select the perfect screw for any project. The right screw is essential to a successful outcome and will help you avoid costly mistakes. #4 screw diameter