Choosing Screw Sizes and Threads

Screws are measured in three basic ways – their gauge, threads per inch and length. On packaging, you will see the first number listed as the diameter and the second as the length.

Selecting the correct screw size is an important part of any job. The wrong screw can split wood or damage your project’s structural integrity.

Choosing the correct length for a screw is one of the most important factors in ensuring it’s safe to use. Getting this wrong can lead to a variety of problems, from sloppy construction to premature failure of the fastener.

The length of a screw or bolt is typically measured from the flat part of the head (or tip in the case of non-countersunk screws) to the blunt end or end of the shank. This applies to hex-head, pan-head, button-head, round-head, and truss-head screws. Screws with oval heads, however, are an exception to this rule.

Sometimes the length component of a screw or bolt is abbreviated, as in a ‘M’ and a number – for example M4. This doesn’t apply to coach screws (or lag screws), which are usually defined by their diameter and a ‘x’, indicating their length in millimeters. This convention can be confusing, but it is worth remembering that when buying such fasteners the length of a screw should be listed first and its diameter second.

The diameter of a screw is the width beneath its threads, or root. It’s a little more difficult to measure than length, but it’s important because it affects the load capacity of the screw. For example, if the root of a screw is too small it will not be able to support a large amount of weight.

In the US, screw sizes are usually defined by their major diameter and number of threads per inch (TPI). To find the proper size, you can use a caliper to measure the distance between adjacent peaks of a single thread, or refer to a chart that matches screw gauge numbers to fractional decimal measurements.

Metric screws, on the other hand, are listed by their root diameter and shaft length. The latter is the distance from the head to the tip of the shank, which excludes any part of the screw that will protrude above the surface for countersinking screws such as flat-head, bugle-head, or trim-head.

The threads of a screw are not just a way to grip something but also play a major role in load capacity. They are a series of V-shaped notches that wrap around the screw in a wavelike fashion, each thread taking more and more pressure as it advances. This means the bottom of a threaded hole can be loaded up to its tensile strength before anything breaks (assuming no compression loading).

The distance between two thread crests is called pitch, and is usually described by a number that’s an inverse of the screw’s major diameter. This is similar to how the wavelength of a wave describes its frequency. Coarse threads have larger pitch, while fine threads have smaller pitches.

The most common thread standard in the US is the Unified Thread System, and is denoted by a first number in inches or fractions and decimals and a second number in metric units. Engineering Toolbox has a helpful chart that provides the equivalant measurements in both inches and millimeters for each screw gauge size.
Load Capacity

A screw’s load capacity can be impacted by various factors. It can depend on what materials you are using, how long the screw is and whether it is a coarse or fine thread. If you are working with soft materials, then you may need to use a coarse thread in order to secure the material firmly.

Another factor is the type of force that the screw is being strained by. This can be either lateral or withdrawal force. If the force straining the screw is from materials that slide away from each other, then you should utilise a wider screw to maximise the weight support. If the force is pulling the screw out of place, then a shorter screw might be more suitable for your project.

You should also note that the load capacity that is quoted for a screw will not always be reached. A screw can suffer from failure of the nut flange, the nut body or connecting bolts even before reaching its safe load capacity.3/8 inch to mm

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