Screws come in a wide range of types and sizes for an almost endless number of construction tasks. Choosing the wrong screw for the job can split wood or damage the structural integrity of an entire structure. There are three main measurements that you need to know when deciding which screw to use: screw gauge, length and threads per inch (TPI).
The first number in a screw size is the screw’s gauge, or diameter, which measures the width of the part of the screw that will pass through the hole into which it is being screwed. The next number is the screw’s length, which indicates how far the screw will be driven into the material. For instance, a Senco Duraspin #8 x 1-1/4″ washer-head timber screw has a head diameter of 0.17″ and a shaft length of 1.25″.
A screw’s thread diameter is measured by the distance between the screw’s teeth. You can find the thread diameter by looking at a sample of the screw, or you can use a special tool called a thread gauge that has numerous strips of different threads cut into them. Screws are usually sorted into categories of looseness or tightness, with size 1 screws having the loosest threads and size 5 having the tightest. Some screws also have a tolerance class specified after the screw size, such as LH or right-handed. This is important if you are using a left-handed screw, as you will need to use the threads in reverse.
Lastly, you need to know how thick the shank of the screw will be. The shank is the long section of the screw beneath the head, and it can be threaded or unthreaded, depending on the type of screw. The diameter of the unthreaded portion of a screw is called the root diameter or minor diameter. The major diameter is equal to twice the root diameter for screws that have a round head and will protrude from the surface, but the ratio may be off if the screw has a flat head that will sit flush against the surface.
In addition to these basic measurements, a UTS (United States Thread Standard) screw chart may also include other information such as the thread standard and tolerance class. Our extensive screw chart includes these details as well, making it easy to figure out exactly which screw to use for a particular job. We’ve even included a conversion table from inch decimal to screw size, so you can use your digital calipers or drill bits to match a specific screw. You can access this useful chart by clicking here. You can also download a free printable version of the screw chart by clicking here. If you have any questions about this article or about our screw charts, please contact us. We’re always happy to help… 1/4 screw diameter
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