Understanding Thread Standards and Specifications
UNC? UNF? Or UNJ? You’ve likely seen these indicators before on your threaded components or thread gauges. These indicators provide information about what standards the screw threads meet for dimensional size and tolerance, as well as what type of threads they are. A major part of thread checking and quality inspection is ensuring that the threads match the specifications set out by organizations like ASME and SAE. These standards help manufacturers ensure that their fasteners are safe, reliable, and long-lasting in whatever end-use applications they’ll serve.
It is important to note before we jump into the different standards that these are all dimensional requirements, not manufacturing requirements. Any method of manufacturing threads including cutting, tapping, rolling, grinding, etc. can be used to achieve the dimensional requirements laid out in each specification.
A little history on thread standards
The first unified thread codes were developed just after World War II. During the war, America, Canada, and Britain discovered a lack of interchangeability between the parts made in each respective country. Afterwards, they developed inch-based thread standards that are still internationally recognized today, commonly referred to as the Unified (UN) thread series. At the same time, several other organizations adopted the UN thread series for their own unique needs, like ASME for pressure systems and SAE for threaded aerospace components. Over time, the UN standards have been updated and expanded to incorporate different applications, thread classes, and thread sizes. Thanks to these specifications, manufacturers can ensure the compatibility and interchangeability of threaded parts for applications around the world.
The Common Unified Thread Types
UNC
UNC threaded bar and fasteners have coarse threads, which have a wider thread pitch (distance between each row of threading). UNC threads are the lowest strength threads of the major Unified types.
UNF
The UNF series of parts has fine threads, which have a narrower thread pitch than the UNC series. UNF threads are stronger than UNC threads, but take more turns to install.
UNR
UNR is a slightly rounded root radius on the external thread only. It is stronger than UNF, but will still fit a standard UNF nut.
UNJ
The UNJ series was developed specifically for high stress applications. Both the internal and external root radii are modified, and require a special UNJ nut. These types of threads are designed for high fatigue strength and are often required for aerospace applications.
The Most Common Thread Standards and Specifications
ASME B1.1
The American Society of Mechanical Engineers is a globally recognized organization that has developed many codes, standards, certifications, and educational materials to promote collaboration, safety, and continuous education across the world. The ASME B1.1 specification is a standard aimed at the thread form, series, class, allowance, tolerance, and designation of UN screw threads based on inch modules. The standard covers UN, UNR, and UNJ thread series.
SAE AS8879
The Society of Automotive Engineers (SAE) was developed alongside the automotive industry, and has since grown to include standards for both ground-based vehicles and Aerospace applications. AS8879 is an aerospace specification aimed at UNJ screw threads with very specific dimensions. This is the standard for all aerospace threads worldwide.
The standard also lays out the basic UNJ profile and design, the standard thread classes and form tolerances, and the methods used to designate UNJ threads.
About Superior Thread Rolling Company
Superior Thread Rolling has been a supplier of thread rolling and CNC machining services for well over seventy years. If you’re in need of a manufacturer of threaded components, check us out! We’d be happy to help you!