How to Select Screw Sizes

Screws are used to fasten wood, metal and other materials, and a good screw can make the difference between a successful project and one that falls apart. Screws are sized by several factors, including driver type (flathead or Phillips), length, shank diameter and threads per inch. Standardized screw sizing helps prevent improper screws from being used, and it ensures that screws are compatible with the material into which they’re being screwed. The most important factors in selecting the right screw for a particular job are its gauge and length.

The first number listed on screw size charts, known as the major diameter, indicates the width of the screw shaft. The number ranges from 0 to 16, with increased numbers corresponding to larger screw diameters. The second number, indicated by a hyphen, is the number of threads per inch of the screw shaft. Threads are the gaps between adjacent turns of the screw. The numbering system for screw threads is different from the major diameter numbers, and it includes several series of different thread counts, ranging from coarse to fine. The thread count is also sometimes indicated by a letter, A or B, following the hyphen. These variations are called tolerance classes, and they describe how tightly the screw fits into the nut or hole. For example, a class 1 screw has a looser fit than a class 3 screw.

Once you know the major diameter of a screw, you can choose its length by counting the number of threads in a one-inch section of the screw’s shaft. You can also find the screw’s length by measuring from where the head of the screw will rest on a surface to where it will protrude. Screws with heads that can be pressed into the surface are often specified as having a head height in millimeters.

Another key factor in choosing the right screw is the type of material into which you are working. You’ll want to select a screw that is as long as the thickness of the material, but you’ll also need to consider its weight. Too short, and the screw won’t anchor properly; too long, and it could bend under the pressure of the material.

Once you’ve selected the screw length and type, you’ll need to choose a driver type. Flathead, square drive and Phillips screwdrivers are most common, but some screws may have a special head design that requires the use of a tool with a specific tip shape. Some screws have self-tapping tips, while others are designed with hex heads to accommodate power drills. If you need to screw into a surface with a finished edge, you can use screws with countersunk heads, which have rounded heads that allow the screw to sit below the surface. 5/16 to mm

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