Metric, UNC, UNF and More: Thread Types forFasteners
Metric thread is the most widely used today, and can otherwise be referred to as ‘ISO Metric’ or ‘M’. Both coarse and fine thread fasteners are available, and can be supplied in a wide range of materials and sizes – with a number of popular DIN standards.
Do you need fine or coarse metric thread? Fine metric threads are more susceptible to galling, need longer thread engagements and are less suitable for high-speed assembly. They are more easily tapped into hard materials though, requiring less torque. They also have less tendency to loosen, are stronger, size for size, than a coarse thread and allow for finer adjustments because of their smaller pitch.
Coarse threads are larger pitch and easier to obtain than fine thread fasteners, and they are specified for the majority of applications.
Metric threads come in a variety of strengths, in ‘grades’ ranging from 4.8, to 8.8, 10.9 and 12.9.
UNC stands for ‘Unified Coarse’ and is the most commonly used thread type in the United States of America. UNC fixings are often used within the agricultural industry because of their strong coarse threads.
The thread is deeper and more generic than that of a fine thread and allows for easy removal, whilst they also have a higher tolerance for manufacturing and plating.
UNF stands for ‘Unified Fine’, which is also a popular thread in the United States. UNF fixings are often used in the automotive and aerospace industries industry because of the requirement for tight, fine threads. UNF fasteners are typically made from steel or stainless steel fasteners. Features include better torque locking and load carrying ability than UNC, with a more specific fit and tighter tolerance.
BA are also referred to as ‘British Association’ and are an old-style British thread with numbered diameters. When we say it’s old-style, this thread has been around since 1884 and was standardised in 1903!. Whilst still found, mainly in electrical fittings and small screws, it is sl