What’s a screw thread?

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Screws have grooves along the shaft to create a secure bond with materials. The screw thread can differentiate screw types and determine where to use them. Screw size and pitch are important characteristics to consider. Screw thread sizes are measured using UTS in the US and ISO worldwide.

A screw is a type of fastener used to join wood, metal, and composite materials. While nails have a smooth shaft, screws have a series of grooves, or threads, along the length of the shaft. These threads bite into a material during installation, helping to create a secure bond. The screw thread can be used to differentiate screw types from each other and also to determine where the screw should be used.

The screw thread on each unit can cover the entire stem or just part of it. In general, a fully threaded screw is designed for metal, while a partially threaded one is designed for wood. The screw thread can be right-handed or left-handed and is determined by the direction installers need to turn the screw during installation. Most are right-handed and need to be turned clockwise, but some have left-handed threads for special applications.

There are two basic characteristics to consider when comparing screw threads. The first is the screw size, or outside diameter, which measures the diameter around the outside of the threads. This measurement determines what hole size the screw can be used with. The second feature to consider is pitch, which measures the number of threads per inch or millimeter. Screws with a large number of threads are said to have a “fine” pitch, while those with few threads have a “coarse” pitch.

Screw thread sizes are measured using two basic systems around the world. In the United States and Canada, screw thread sizes are provided using Unified Thread Standards (UTS). These sizes are listed as a series of two numbers, with the first showing the nominal diameter and the second showing the pitch in threads per inch. For example, a UTS measurement of “3/8″ x 16” means the screw measures three-eighths of an inch around the outside of the threads and has 16 threads per inch.

Most people around the world rely on the screw thread sizes created by the International Organization for Standards (ISO). ISO screw measurements always start with the letter “M” followed by two numbers. The first of these numbers shows the outside diameter and the second shows the pitch. For example, a screw labeled M10 x 1.5 measures 10 millimeters around the outside of the threads and has a pitch of 10 threads per millimeter. Many woodworking shops and hardware stores provide information on sizing screws using both of these systems or provide a chart so people can easily switch between the two systems.

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