The Unified National thread system are divided into two primary categories: Unified National Coarse (UNC) and Unified National Fine (UNF). It is important to distinguish between UNC and UNF threads; choosing the wrong type can lead to a bolt that strip or one that loosens due to vibration. UNC threads is characterized by coarse threads, whereas UNF threads contains fine threads.
UNC threads have fewer thread per inch (TPI) than UNF threads, which contains more threads per inch. UNC threads are used in general construction and for everyday task. The threads are more forgiving and can bite deeper into soft material.
Difference Between UNC and UNF Threads
Additionally, UNC threads are faster to assemble and are more resistant to stripping. UNF threads are better for high vibrations. UNF threads have more contact point between threads, which prevents them from loosening in machine or aerospace applications.
Additionally, UNF threads allow for precise torque to being applied. The size of the bolt will determine the number of threads per inch for both UNC and UNF threads. For instance, a half inch UNC bolt will have 13 threads per inch, while a half inch UNF bolt will have 20 threads per inch.
For smaller bolts, such as a #2 screw, there will also be a difference between UNC and UNF threads. A #2 UNC screw will have 56 threads per inch, while a #2 UNF screw will have 64 threads per inch. Use UNC threads for structural steel application that require more clamping power.
Use UNF threads when you need to preserve more material under tension. When selecting the type of thread for you’re bolt, make sure that the bolt and the nut are of the same thread type. If you try to thread a UNC bolt into a UNF nut or vice versa, the threads will not mate correct.
Always hand start the bolt to ensure that the threads are properly align. If the bolt feels difficult to turn after two rotations, back it out and ensure that the thread type is correct. There is also thread classes that determine how the bolt fits into the nut.
Class 1 threads are used for loosely fitted bolts that need to be quickly release. Class 3 threads are for bolts that must have a tight fit in instruments that cannot allow for any play. Most hardware stores’ products will be Class 2 threads to provide a balance between ease of assembly and holding power.
External threads will have an “A” next to the bolt size, while internal threads will have a “B.”
Be careful when using fine threads to avoid over-torquing the bolt; this can stretch the slim root of UNF threads, especially on aluminum. Make sure that there is enough engagement between the bolt and the material; there should be at least one full diameter in engagement. Use calipers to measure the bolts diameter, pitch combs to determine the number of threads per inch, and gauges to ensure that the threads will fit correct.
Using the proper type of thread and the correct tool will result in a successful threading process.
