The Unified National course threads are the most common types of thread fasteners used. This is because their fit and grip on any surface being fastened is much deeper and more generic as compared to that of a fine fastener thread. You use these threads for manufacturing and plating purposes as they do not require any re-tightening, including cross threading, to assemble.
As their name suggests, the UNF threads have fine threads. They also have better torque compared to the UNC. This type of fastening thread is specifically used for fastening materials carrying a heavy load; for instance, trucks. Similarly, the UNF threads are commonly used in the aerospace industry due to its ability to fasten heavy loads. This is because UNF threads have a more specific fit, tighter tolerances and finer tension adjustments as compared to the UNC thread.
The UNEF threads come in handy when dealing with hard materials that have tapped holes and thin threaded walls. They can also be used in situations where the material being fastened has tapped holes but a thin wall. Unlike the UNF, this type of thread fastener is only used in the aerospace industry due to its height, specific fit and fine tension adjustments.
The UNJ threads are another type of thread fasteners and they may be classified into two types: the UNJC and the UNJF. The UNJC thread is course threaded, while the UNJF is fine threaded. Their classification can also be made depending on their type of root. A UNJC thread has a root that fits in the radius of the nut being used with it, whereas a UNJF has a larger root than the radius of the nut assigned to it. The "J" on the UNJ indicates that the fasteners have threads on their roots.