Screw Sizes Explained

Screws come in a wide variety of shapes, sizes and styles with varying thread patterns for different materials. Using the correct type of screw helps to ensure proper holding strength and prevents screws from becoming loose or even breaking due to improper use. Whether you’re working with wood, drywall, sheet metal or concrete, selecting the right screw for each job is essential to the success of your project.

Knowing what kind of screw you need isn’t always easy, especially when plans, instructions or specs refer to specific types and sizes. It’s a common woodworking question that Lee Grindinger of Woodworkers Journal addresses in his article, Screw Sizes Explained. He explains that woodworkers need to consider a number of factors when selecting a screw for a project, such as the material the screw will be used for and the weight it will support. In general, heavy projects require thicker screws, while fine applications demand thinner screws.

The screw gauge is also important to consider. The screw gauge, which is ranked with numbers from 1 to 20, indicates the diameter of the shank part of the screw above the threads. The higher the number, the larger the screw.

In the United States, screw sizes are classified using both the imperial system and the metric system. Most modern hardware is standardized to the ISO metric system, which is slowly displacing traditional US inch-based measurements. However, you may still encounter hardware that is labeled as “USS” or “SAE,” indicating that it uses inch-based measurements.

A screw’s thread is the spiraling pattern that runs around its cylinder. Screw threads are manufactured by subtractive techniques, including cutting (single-point and multi-point); boring; tapping; forming; molding; casting; rolling and lapping. Depending on the manufacturer, screw threads may be coarse or fine, and they can be straight or helical.

While there are many different methods of manufacturing screw threads, all threads must be centered in order to be compatible with other components, such as nuts. A screw that has uneven or misaligned threads will be difficult to install and will likely cause other parts to break.

The head-bore and shank-hole diameters are also important considerations when selecting a screw. You’ll want to select a head-bore that fits the tool you plan to use for driving the screw, and you’ll want to choose a screw with a shank hole that matches the diameter of the hole in which you plan to drive it. For example, a square or torque head screw is a good fit for power drills, while flathead or Phillips heads work well for hand tools. metric to standard

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