Stainless Steel Screws: Understanding the Unified Thread Standard

There are numerous ways of fastening metals together from simple nails to banding, strapping and even welding. Each method has its own advantages and so is suitable for particular applications. One of the most common and easily recognisable fastenings is the screw, which has a history that dates back a lot further than you might think. 

The Oxford English Dictionary defines a screw as 'a pin with a raised helical thread running round it and a slotted head, used to join things together by being rotated so that it pierces a material and is held tightly in place'. The screw is considered to be one of the six classical simple machines, along with the lever, wheel and axle, inclined plane, pulley and wedge. 

Made from wood, the screw first arose in Mesopotamia as far back as 911 BCE and appeared in Ancient Greece in around 428 BCE. By the beginning of the first century BCE, wooden screws were being widely used as a pressing device for olives (to produce olive oil) and grapes (to produce wine).

Prior to the 15th century CE, screws made from metal and used for fastening purposes were practically unknown. Even after this, they took a long time to become popular. It was not until the late 18th century, from 1760 onward, that the development of machine tools for mass production of steel screws saw them become a common sight.    

As the usage of screws grew around the world, measurement standards were introduced, leading to the establishment of the Unified Thread Standard (UTS). This is today used by countries all over the world but there are some regional differences in how the standard is referred to, especially in the USA.

Unlike the majority of other countries in the world, the USA still uses imperial measurements for many applications. Though this does not affect the use of the UTS in terms of the number used to refer to a particular screw, it does mean that the length and thread diameter of that screw is referred to with an imperial unit, namely the inch. 

For example, the No.8 screw has a diameter of 4.2mm, which is around 0.06 of an inch. To obtain a screw of this diameter, one can simply ask for a No.8 screw. The length of the screw must also be specified. To obtain a screw of 4.2mm diameter and 12.7mm in length (half an inch), it is common to ask for a No.8 x ½” screw. To make using UTS designations easier, you can find a helpful chart on our website.

Thanks to its strength, cost-effectiveness and robust resistance to corrosion, stainless steel is one of the most commonly used materials for making screws. The most popular sizes are used for joining sheet metal, particularly in cladding and jacketing applications for tanks and ducting. These sizes, which account for around 90% of demand, are 4.2mm and 4.8mm in diameter (No.8 and No.10 respectively) in lengths of 13mm, 19mm and 25mm (0.51, 0.75 and 0.98 inches respectively).   

Stainless steel screws are most often made from metal grades 304/A2 and 316/A4 though there is growing demand for those made using Duplex stainless steel 318L/31803 as they are stronger and offer increased corrosion resistance. BS Stainless is an expert supplier of a comprehensive range of stainless steel screws of all types and grades so please browse our website for full details. Please get in touch with our technical team if you have any questions or need expert advice.