Screw Sizes and Thread Sizes

When choosing the right screws for your project, you must consider several factors: head-size, length and thread-size. Using the wrong type can cause your fasteners to bend or break. It’s also important that the screw fits the material you’re screwing into properly – using the wrong length or gauge could result in a poor anchor point, causing your screws to loosen over time.

Screws are sized by gauge (diameter) and length, with the length factor usually being listed after the diameter on the packaging. The number of threads per inch (TPI) is also important, as it indicates the tightness and strength of a screw’s threads. To determine a screw’s TPI, count the number of thread peaks along a one-inch length of the screw. For example, a screw labeled #10×36 has a diameter of 36 threads per inch.

Tighter and finer threads are designed for specific materials, such as drywall. Using coarse-thread wood screws on drywall can damage the surface of the sheetrock, so specialized drywall screws with fine threads are available to prevent tearing.

Screws are created in a variety of ways using subtractive and additive techniques, with the most common subtractive methods being grinding and milling. Regardless of the production method, screw threads must be standardized in order to ensure reliable interchangeability between different brands and manufacturers’ hardware. During the late 19th century, engineers developed a series of standard inch screw threads known as USS and SAE to replace earlier variations. More recently, the International System of Measurement (ISO) and metric threading have begun to displace these older inch sizes. 1/4″ to mm

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