Choosing the right screw size is crucial to a job well done. Using the wrong screw for a project can lead to failure, or worse, damage the material you’re working on.
There are three fundamental qualities to consider when selecting a screw, its gauge, length and threads. Learn more about these measurements so you can select the correct fastener for your next project.
Length
The length of a screw is the distance from beneath the head to the tip. It is a factor in how far the screw will be driven into a surface. Some screws, like truss-, round-, and hex-head screws, do not extend all the way into the surface, but only into the material below it. These are known as non-countersinking screws.
In some applications, it is important that the point of a screw penetrates the material before the threads engage to avoid over-heating and breaking. The point may also have a sharp or blunt tip.
Choosing the correct screw size and type for a particular job can be difficult to determine with all the information and specs that are available. This can be especially true for woodworking projects where plans, instructions, and specs reference specific kinds of screws all the time. But what do all those numbers and letters mean? This guide will help explain how screw sizing is determined.
Diameter
The diameter of a screw measures the largest width along its length. This is important because it determines the load capacity and material type that can be used for a particular project.
To measure a screw’s diameter, you need to look at both the minor and major threads. The major thread diameter is the largest distance between two consecutive crests on one side of the threads, and the minor thread diameter is the smallest distance between those same crests on the other side.
Another important factor in determining screw size is the thread pitch. Thread pitch is the distance from a point on one screw thread to the same point on the next thread in the same direction. To measure thread pitch, you can use a screw pitch gauge or a pitch calculator. Screws and bolts that mate with a nut (like nuts, washers, and hex keys) have diameters denominated in actual inches or fractions of an inch, followed by a number of threads per inch (TPI). Other screws (and hex nuts and nutsetters) are denominated by industry Numeric Sizes preceded by a # sign.
Threads
The threads on a screw are what make it possible to fasten two pieces of hardware together. Often, they can determine how tight or loose the connection will be. Standardized thread dimensions have evolved since the early nineteenth century to facilitate compatibility between different manufacturers and products. In the United States, for example, there are both imperial and metric screw thread standards.
The most common screw thread is the UTS (Unified Thread System) series, which has a 60deg angle and a V-shaped profile. It is used in screws and bolts, and can be inserted into tapped holes.
Other popular threads include the Whitworth, BA and DIN systems. These are all based on the ISO metric system, which is slowly displacing older systems in continental Europe and the United States. The preferred metric sizes are designated by diameter and pitch, which is the distance between the threads. The latter is also called the lead, and can be determined by using a pitch gauge or other form size comparison tool.
Gauge
Depending on how precise your screw gauge is, there will be some level of error involved. This can be a positive or negative error. The smallest measurement that your screw gauge can accurately measure is called the least count. A screw gauge with a higher least count will be more accurate than one with a lower least count.
When you use a screw gauge, make sure that it is clean and in good working condition. Gently close the jaws of the screw gauge around the object to be measured and apply pressure until there is slight friction. Then, look at the main scale and circular scale readings to determine the screw size.
For example, let’s say you have a screw gauge with a least count of 0.01 mm. Upon rotation, you find that the main scale reading is 2 mm and the circular scale reads 40. Then, you will need to determine its total reading.1/8 in to mm