unified screw thread
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unified screw thread[′yü·nə‚fīd ′skrü ‚thred]
Three series of threads: coarse (UNC), fine (UNF), and extra fine (UNEF); a ¼-inch-diameter (0.006-millimeter) thread in the UNC series has 20 threads per inch, while in the UNF series it has 28.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.