Screw Sizes and Types

Screws come in a wide variety of shapes and sizes. Having an understanding of what each type is best suited for helps to ensure that they will fit properly and last as long as possible. The primary features that distinguish each type are head-size, length and threading size. Understanding these enables product engineers and designers to choose the right kind of screws for specific projects. This improves product design and also enables them to make the most of what screws can do for their products.

Choosing the correct screw for any project is crucial to ensure that it will have a good hold in the materials and will not damage them, or work itself loose. Choosing the wrong kind of screw for a job can lead to broken or bent hardware, poor holding power or even structural failure. Getting the right screw for the job will not only ensure that it is strong enough to do its job, but also that it can be used with a range of different tools.

Many woodworkers have questions about what all of the screw sizes mean. Plans, instructions and specs refer to the types and sizes of screws all the time, but it can be confusing to know what each number indicates. A general rule of thumb is that the higher the screw gauge, the larger the diameter, and the lower the gauge number the smaller the diameter. But there are many other factors to take into consideration.

When choosing a screw, the diameter of the shank must match the material into which it will be screwed. Too big and the screw can tear into the material. Too small and the screw will not be able to firmly anchor in the threaded hole. The thread width is also important and must match the width of the material into which it will be screwed. It is also helpful to consider the thickness of the material when selecting a screw, as this can also impact the amount of force required to penetrate and anchor it.

Screw threads are produced in a variety of ways, including traditional subtractive methods such as cutting (single-point, taps and dies); molding; casting [die cast, sand casting, milling]; grinding; and forming. The result of these processes is that there are a multitude of screw thread sizes available for use in a wide variety of applications.

Measuring the thread size of a screw can be done using a digital caliper or with a threaded drill bit, but it’s also possible to convert from US fractions to metric mm. There are tables on the internet that can help with this conversion. Once the correct thread size has been selected, the next step is to select the screw head and length. A variety of head types are available, from flat and square to Phillips and hex heads. Some are more suited to hand-driving while others are designed for use with power tools. Screw length can be varied according to the specific project, as longer screws may be needed for heavier or thicker materials. 5/8 to mm

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