The thread in the UNEF thread table is an American thread standard with a particularly small pitch. The abbreviation UNEF stands for Unified National Extra Fine. Together with the UNC and UNF threads, the UNEF thread forms a trio of American imperial threads with a 60° flank angle. The individual thread standards differ only in the pitch. The values presented in the UNEF thread table are defined and monitored by the American National Standards Institute (ANSI). The associated standard is ANSI B1.1. The UNEF thread is mainly used in the USA, but also in Australia, Great Britain and Scandinavia.
The American extra-fine thread is characterized by an extremely small pitch, as you can see in the UNEF thread table. This further enhances the well-known properties of a fine thread, such as good self-locking and a shallow thread depth. The UNEF thread can therefore always be used for screw connections when a small screw-in depth or a thin-walled component (usually a pipe) make this necessary. However, as the extreme fine thread is also very sensitive due to the extreme values (as shown in the UNEF thread table), it is only used when a coarser thread cannot be used.
In the UNEF thread table, you can see a special feature in the thread designation that also occurs with the UNC and UNF threads: Thread sizes below 0.250 inches are assigned a fixed number by the standard. In the table for the UNEF thread, however, this only applies to a single thread size. The thread with an outside diameter of 0.216 inches is designated as No. 12. For all other thread sizes in the UNEF thread table, the abbreviation UNEF is followed by the outside diameter in inches and the number of threads per inch. To make it easier for you to work with the UNEF thread table, we have also listed the outside diameter in mm and the pitch P in mm in addition to the inches. This saves you the tedious and error-prone conversion.