Screws can be used in a variety of materials and for many different applications. This is why it is important to understand how screws are sized and what each number means on the package. The first number, the gauge or diameter, refers to how large or small a screw is. The second number, the threads per inch or TPI, measures the number of thread peaks in one inch of shaft. This determines how tightly the screw fits in a given material, and allows you to select the right drill bit for the job. Finally, the third number, the length in inches or millimeters, refers to the overall length of the screw from the head to the shank.
Screw sizing is determined by several factors, including the driver type (flat, Phillips, square, etc), length, shank diameter and threads per inch. Knowing these specifications will help you choose the proper screw for your project and avoid unnecessary frustration and delays.
There are two standard systems of screw sizing, imperial and metric. Imperial units use a length and diameter system, while metric uses a thread count and pitch system. Imperial and metric values are not interchangeable, so it is important to know which system you are using when selecting a fastener.
In the US, screw sizes are based on both a major diameter and a number of threads per inch on the screw’s shaft. The first number is called the gauge, and it usually consists of two or three digits. The second number is the threads per inch, and it is normally listed as a decimal or fraction. For example, a screw with a gauge of 6-32 has 32 threads per inch and is an inch and a half long.
The tolerance class is often noted right after the threads per inch or TPI, and it indicates how tight or loose the screw will fit in a given material. Screws are rated from class 1 to 5, with 5 being the tightest. Screws for heavy or thick materials are usually marked with a higher class, while fine woodworking projects often call for lower class screw sizes.
The final number on a screw is the shaft length in inches or millimeters. This is important to know because it will determine the maximum driving torque that can be applied, as well as how far into the material a screw will extend. Screws that are too short will not anchor, while those that are too long will risk breaking or damaging the opposite side of the material. The recommended length for a screw depends on the application, but in general, it should be at least half the thickness of the material that will hold the screw. This ensures that the screw will be firmly anchored and will not pull out over time. It is also a good idea to select a screw that will be longer than the actual hole to avoid exposing unfinished or damaged wood. 1/4″ to mm