Screws are used for a variety of projects and in countless ways. It is important to use the right size screw for your project or the screw may work itself loose or cause damage to the material.
The key to understanding screw sizes is knowing the three essential measurements: gauge, length and pitch. This article will cover these basics so you can select the correct screws for your projects.
Diameter
Screw sizes are usually described in three figures – first the gauge (also called major diameter), then the threads per inch, and finally the length. The length is normally given in inches or millimeters (mm).
The diameter of a screw or bolt is the distance from one peak to the next, or the diameter of the root of the thread. The threads are helixed – they have a conical spiral shape. The helix angle is the angle made by the pitch of the thread with a plane perpendicular to its axis.
The threads can be coarse or fine. Coarse threads are easier to install but can loosen more easily. Fine threads are more resistant to vibration and are therefore better suited for use in moving components. Some smaller machine screws that don’t need to match with a mating part are denominated with Industry Numeric Sizes preceded by a # and do not have a threads-per-inch designation.
Length
The length of a screw refers to how far it will go into the material you’re working with. Generally speaking, the screw should enter the material at least half its own thickness. If the screw is too short it won’t be able to support the load you need it to and it could also cause damage to the surface of your project.
If you’re using screws that are labeled with measurements in the imperial system they will usually list the gauge first, followed by the length. If they include the thread count it will be between the two numbers, for example 10 x 2” means it is a #10 gauge screw and is 2 inches long.
If you’re using metric screw sizes they will be labeled with the major diameter first, followed by the threads per inch. They will also often include the length of the screw in mm. For example 3 x 16mm would mean it has a 5 mm major diameter and 20 threads per inch.
Pitch
When it comes to screw sizes, the diameter of the shank and thread pitch are two measurements that need to be taken into account. While these measurements may seem similar, they are quite different and they have to be matched in order for the screw and nut to fit together.
The pitch of a screw is the distance from the crest of one thread to the crest of the next thread. This is often measured using a caliper. The lead of a screw is the amount of linear travel that the nut makes in a single screw revolution and it is equal to the pitch multiplied by the number of starts on the screw.
Screws are often specified by the nominal diameter in millimeters followed by the pitch, which is also listed in mm. The preferred combinations of diameter and pitch are set by the corresponding standards organizations. For example, an M6 screw has a nominal diameter of 6 mm and a pitch of 1 mm.
Material
There are a number of different types of screws available for use in various materials. The type of screw you choose will depend on the kind of material you’re working with, and also the strength or holding capacity required. Screws are made from a wide variety of materials, including steels, stainless steel, brass, copper and even plastics.
Screws come in a wide range of heads, from flat to square and Phillips to quadrex. You’ll also find tamper-proof heads and other variations. The head type you choose will depend on how the screw will be driven into the material.
A screw’s size is usually identified by its gauge and the number of threads per inch. It may also be given as a ratio, such as 6-32 x 1 1/2″, which indicates that the screw has a #6 gauge and 32 threads per inch. The next step is to decide on a length. Generally, the screw should enter at least half of the material’s thickness to ensure a strong hold.3/8 lag bolt pilot hole
