Unified Screw Threads
The Definitions of the Unified Thread Standards:
The Unified Thread Standard (UTS) is the Basic American Standards for fastening screw threads as agreed upon by standard bodies of the United States, United Kingdom and Canada. They are complete and integrated system of threads for Fastening Purposes. Their outstanding characteristics are general interchangeability of threads achieved through the standardization of thread form, diameter-pitch combinations, and limits of size.
Unified National Coarse (UNC) Coarse threading that is the preferred thread for general use and is based on inch sizes. UNC is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a larger thread pitch than an equivalent diameter UNF thread, which allowances allow for thicker plantings and coatings.
Unified National Fine (UNF) is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a smaller thread pitch than an equivalent diameter UNC thread. Fine threading is not as strong as coarse threading. However, with their smaller helix angle, they permit closer adjustment accuracy.
· For example: a 3/8-16 (coarse) thread has a 3/8" nominal major diameter and 16 threads per inch, while a 3/8-24 (fine) thread has the same nominal major diameter and 24 threads per inch.
Unified National Extra Fine (UNEF) is a unified thread form with a very fine, small pitch that are typically used on instruments and parts requiring a fine adjustment. These extra fine threads are commonly used on very small screws to make the screw stronger.
Unified Round (UNR) thread form with a rounded root contour, applies only to external threads. (The UN thread form has a flat, or optionally, a rounded root contour.) The majority of fasteners with a Unified thread form have a rounded root contour i.e. are UNR threads. The root radius must be rounded but does not necessarily have to be inspected.
Learn more about Thread Terminology by clicking here.
Learn more about Fasteners, check out our fastener reference guide by clicking here.
