Screw Sizes – How to Read a Screw Size Chart

Screws are used in woodworking projects to connect parts together or provide support for heavier materials. Choosing the correct screw for your project is important to ensure proper installation and avoid damaging the workpiece or structure. Screws are typically labeled with a gauge, length and threads per inch (TPI) designation. These three measurements help you select the correct screw for a specific application. However, these markings can be difficult to understand when you’re first encountering them – whether in a box of loose screws or on the packaging of your favorite brand.

For most woodworkers, understanding these measurements is an essential skill for successful woodworking projects. The wrong screw could cause the workpiece to split or affect the structural integrity of a project, making it important to get it right the first time. The good news is that screw sizing is relatively simple once you break it down to its basic elements.

The first number on a screw size chart indicates the diameter of the shaft or threads, often referred to as the “gauge.” Screw sizes go from 0 through 16, with higher numbers designating larger shafts. A screw’s diameter size also correlates to its head-bore or shank-hole dimension, which defines the smooth portion of the shaft above the threads. Screws designed for light applications such as finishing or molding typically feature smaller heads, while screw types intended for heavy applications like decking usually feature larger, hex-style heads.

A screw’s length is a measure of the distance from one end of the shaft to where it would rest on a surface. Screws with longer shafts require more torque to drive them into a workpiece, while shorter screws may not be able to anchor properly. For woodworking projects, selecting a screw that is about half the length of the material to which you are fastening it will give you the most versatility and the least damage to the workpiece or structure.

When reading a screw-size chart, the second number listed is the number of threads per inch on the screw shaft. Screws with a coarse series have more threads than those with a fine series. The threads on coarse screws are closer together, while those on fine screws have more space between them.

Screws sold with imperial system measurements tend to list the gauge and shaft length separately, while those with metric system measurements typically include the gauge and thread count on their packaging. For example, a screw might be labelled as “10-35 x 2” on the packaging, with the ten representing the gauge and the 35 indicating the number of threads per inch. metric to standard

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