Unified National Extra Fine (U.N.E.F) threads are characterized by which feature?

• A. Has an external thread controlled root radius.
• B. Uses a 60-degree thread angle.
• C. Designed only for pipe threads.
• D. Has the same root radius as UNC.

Answer: (A)

Unified National Extra Fine uses a specific approach to the shape of the thread roots for external threads. The key feature is that the external threads have a controlled root radius, meaning the bottom of the thread groove is formed with a defined curved radius rather than a sharp corner. This rounding reduces stress concentrations, improves fatigue strength, and helps the threads engage smoothly, which is especially important for extra-fine pitches where the thread depth is small and the root is more vulnerable. While a 60-degree thread angle is common to all Unified threads, that property isn’t unique to UNEF. Pipe threads and matching with UNC aren’t defining traits of UNEF. So the defining characteristic here is the external thread with a controlled root radius.