If you’re new to building or working with fasteners, you may be overwhelmed by the many different types of screw sizes. The labels on the outside of a box of screws can be hard to read, and the numbers and letters used in a screw size chart might confuse you. This article aims to clarify some of the basics of a screw size chart and how they work, so you can find the right screw for your next project.
There are two systems of measurement for screw threads – the standard Unified Thread System (UTS) used in the United States, and the metric system used elsewhere in the world. The UTS system uses inch-based measurements, and has several different thread series, from coarse to fine. Screws with a coarse thread are labeled UNC, while those with a fine thread are labeled UNF.
Screws are also classified by their length, which is usually shown as the first number in a screw size callout. The second number is the screw’s diameter, and the third number is the screw’s pitch. A screw’s diameter is the distance between the peaks of its threads, and the pitch is the number of threads per inch.
A screw’s length is important because it determines how deep the screw will go into your project’s material. This is a critical factor in how well a screw will hold, and can affect the strength of your joints or fasteners. A screw’s length can also affect how easy it is to screw in and remove the head of a screw.
When reading a screw size chart, the first number in the callout is the screw’s diameter. The second number is the screw’s pitch, and the third number is the screw’s nominal length. The nominal length is not the actual length of the screw, but a shortening that’s usually rounded up to the nearest millimeter to avoid unwieldy decimals.
The screw size chart will also often include a tolerance class number, which is a code that indicates how tight the screw fits into its host material. Screws with a higher tolerance class fit looser, while those with a lower class fit tighter. The chart might also have a LH symbol, which is used to indicate that the screw is left-handed.
In addition to the three basic measurements, a screw size chart will usually have some other useful information. For example, a screw size chart might specify whether the screw is coarse or fine, and the prevailing thread standard. Screws with a finer thread are more resistant to damage, and are often used in more delicate projects.
A screw size chart will also usually include a list of symbols that identify the different fasteners in it. For example, a screw with a square or hex head will have an icon that shows the shape of the head. This helps assemblers ensure that the correct type of head is being used in a specific application. screw size chart