The sizing of screws and bolts can be confusing. Machine screws follow the Unified Screw Thread System (UTS) and are sized in inches, while bolts usually follow the ISO metric screw thread system and are measured in millimeters.
To determine the size of a machine screw or bolt, you can use a thread gauge that looks like a spark plug feeler gauge. The chart below lists the equivalant diameter-threads per inch measurements in both systems.
Diameter
The diameter of a machine screw is its outermost dimension. It’s measured from below the head of the screw to the tip. This is the major dimension and determines the tensile stress area. A major diameter that’s too small can cause a screw to snap in use, so it’s important to get the correct size for your application.
The threads on a machine screw are usually right-handed, which means that turning the screw clockwise tightens it. However, some applications require left-handed threads. For these situations, there are special types of machine screws that have left-handed threads.
The best way to measure a machine screw’s diameter is with calipers. To do this, place the jaws of the calipers against both sides of the screw’s shaft and read the measurement from the dial. This method is much more accurate than using a tape measure or ruler.
Pitch
In order to choose the right machine screw, you must know its diameter and thread pitch. Screws of the same diameter with a different pitch will not fit into each other and may damage the tapped hole. A screw’s diameter is indicated by the first number in its callout, while its thread pitch is given by a second number.
In a nutshell, the thread pitch is the distance between two adjacent thread peaks. You can find out your screw’s pitch by measuring the distance with a caliper, which is far easier than trying to count the minute spacing of individual threads.
In the US, a screw’s diameter and pitch are both measured in inches. International machine screws use metric measurements and are identified with the letter M followed by a number. The first number is the major diameter and the second indicates the screw’s thread pitch (threads per inch). Both are determined by the standard Unified Thread Standard (UTS).
Length
There are a couple of different length measurements that you need to know for machine screws. One is the length of the screw from its head, and the other is the total length of the screw.
It’s important to understand these differences, especially if you plan on using your machine screws for fastening purposes. If you use a drill to shorten your screws, you might damage the threads, which can affect their tensile strength and torque loads.
There are a few different types of machine screws that can be used for a variety of applications. Most of these are designed to be threaded into uniformly-threaded nuts or holes that already existed in the parts they’re intended to fasten. Some of these screws have hex heads, while others have flat heads. Additionally, these screws can have coarse or fine threads, which are better suited for certain projects than others. They’re also categorized by their stress areas, which help us determine whether or not they’ll be suitable for our application.
Stress Area
Using this chart, you can calculate the stress area of a screw by multiplying the head diameter (in imperial gauge) times two. It’s a useful tool for ensuring the proper torque is applied to each screw without damaging the threads or the surrounding material.
The second number of a machine screw callout indicates the threads per inch (TPI). For example, a screw with a number of #3-48 would have a major diameter of 1/4” and 48 threads per inch. This is known as a coarse thread configuration and is less resilient to stress than a finer-toothed configuration.
While the outside diameter, TPI, and length are standard measurements for most fasteners, other types of bolts have unique characteristics that require their own measurement callouts. For instance, hex nuts, coupling nuts and jam nuts have different shapes that require their own unique numbers. This chart makes it easier to keep track of the various variations in fastener sizes.machine screw size chart
