Using the right kind of screws for your project can help ensure that it’s held together correctly and stays that way. This is especially important for products with exposed threads that may need a strong hold to survive.
Screws are typically listed with a gauge, thread count and shaft length in inches. When a screw is labeled, for example, 6 x 32, it means that it has a #6 gauge, 32 threads per inch and is 1 1/2″ long.
Length
One of the most important factors in choosing a screw is its length. For wood screws, the general rule is that the screw should enter at least half of the thickness of the material it’s fastening to. This ensures a strong hold.
The length of a screw is measured from its shaft, which is the part that goes into the material. Screws are generally labeled with two numbers, the first being the screw gauge and the second being the number of threads per inch. For example, a screw labeled as 6 x 32 has a diameter of 1/4″ and has 32 threads per inch.
It can be difficult to choose the correct screw length for a given project, especially when multiple factors are involved such as the type of material and the differences in lateral versus shear forces. However, following some simple guidelines can help to simplify the process and improve the overall quality of your work.
Diameter
For the majority of screw sizes, the first number on a screw chart is the head diameter and the second number is the shank (long section) diameter. The second number may also be referred to as the threads per inch (TPI) or pitch, which is the inverse of the head size (how densely packed the threads are).
Choosing the correct screw size for your project depends on both the materials you work with and the strength needed to hold or anchor your fastener in place. Measuring the diameter of a screw can be complicated, but the process is much simpler with the help of a screw size chart. You can use a screw gauge chart to convert fractional inches (and decimals) to screw sizes and/or a millimeter converter for the most accurate measurement.
Thread Pitch
The distance between two adjacent thread peaks is the thread pitch. For metric screws, this is measured in millimeters (mm), but for American sizes it is usually represented as ‘threads per inch’ (TPI).
Most screw threads are not perfectly sharp — they are ‘truncated’ on the crest and root to yield a final diameter which can be expressed as a fraction of the thread pitch. Nevertheless, there are standards for the form of the thread, its dimensions and tolerances which ensure that mated male and female threads will be a predictable fit, and interchangeable.
When a screw is given a designation such as UNC or UNF, this indicates the type of thread pitch. A coarse thread will have a larger pitch than a fine thread. Generally, screw producers will offer both coarse and fine thread versions of the same major diameter. This can make calculating the exact screw size needed a little tricky if the ‘threads per inch’ measurement is not specified.
Material
Screws are made of a wide variety of materials, depending on the intended application. The most common material is steel because it offers a combination of strength, affordability and longevity. However, stainless steel and other metals are also used because of their superior strength and corrosion resistance.
Wood screws, sheet metal screws and drywall screws are typically made from low-carbon steel while machine screws, roofing screws and high-tensile bolts use higher-grade steel. Brass, aluminum and titanium screws are also available for special applications.
When selecting a screw for your project, consider the materials it will be exposed to and the environment in which it will be used. For example, outdoor projects may require screws that can withstand exposure to moisture and temperature changes, while indoor projects might call for a screw that looks more attractive. Screws are also available with a number of coatings for added protection and strength. These are usually plated with zinc, copper or black oxide for extra durability.metric to standard
