Screws are used to fasten a multitude of materials. They are available in a wide range of sizes and types, suited to many different tasks. Choosing the right size screw for a particular material is important to ensure that the screw can be used safely and securely. The wrong size screw can bend, break or work itself loose over time. It is also important to select the correct type of screw for a specific project as some screws are designed for multipurpose use, while others are specifically adapted to wood, drywall, sheet metal or concrete.
For those unfamiliar with screw measurements, it can be difficult to know how to choose the correct screw for a task. There are three essential measurements that every tradesperson should be familiar with: screw gauge, length and threads per inch (TPI). By understanding these measurements, it is easier to choose the correct screw for any task, whether repairing furniture or building an industrial-sized structure.
There are two main systems of measurement for screw threads: the imperial system and the metric system. The imperial system is more commonly used in the United States, but a growing number of manufacturers are turning to the metric system for ease of production and consistency across products. Both the metric and imperial systems have a standard set of measurements, but there are some differences between the two.
In general, imperial screw sizes are measured by their major diameter and pitch. The major diameter is the largest diameter of the screw thread, and the pitch is the distance between adjacent threads. There are several different ways to create screw threads, and the number of pitches is generally determined by how much more difficult it is to cut the head of the screw than the shank.
The threads on a screw can be created by various processes, including traditional subtractive methods such as cutting (single-pointing, taps and dies, milling, etc.), molding, casting, forming and rolling; newer additive techniques like injection molding; or by combinations of these. Each method of thread creation produces a slightly different screw, and each type has its own benefits and drawbacks.
When choosing a screw, the first number listed on the packaging is the screw gauge. This refers to the diameter of the screw’s thread, and is usually given in fractional inches. For example, a #8 screw has a major diameter of 1/8 inch. Screws with a larger major diameter are designated as larger gauge sizes, while smaller major diameters are labeled as lower gauge sizes.
Screws sold in the United States typically list their gauge size and shaft length on the packaging. In addition, some screw packaging may include the thread count, if available, between the head and shaft diameters. For example, a 3-32 screw has a thread count of 32 threads per inch, which is double the normal thread count for that kind of screw. This is because most woodworkers prefer to use a more secure grip in the hole than would be possible with a smoother head, so they add threads. screw size chart