N vs. UN
NC vs. UNC
NF vs. UNF
NEF vs. UNEF
NS vs. UNS

SUMMARY
The American National Standard has been obsolete since 1949 and replaced with the Unified National Standard. In all cases, the threads made to the Unified National Standard are designed to screw together with the obsolete American National Standard. Without exception, drawings should be updated to reflect the current standard. The class-of-fit requirements for the obsolete American National Standard can be translated to the current Unified National Standard. As always, obtain approval from your customer.

HISTORY
The origin of this thread series began in the mid 1800's through the efforts of William Sellers in the United States and Sir Joseph Whitworth in the United Kingdom. In 1949 the American National Standard was replaced with the Unified National Standard for three reasons. The first reason was to provide interchangeability with Canada and United Kingdom during the World Wars. The second reason to allow for interchangeability in the growing global marketplace. The third reason was to correct certain thread production difficulties. The change was implemented gradually.

The history of the transition from the American National Standard screw thread to the Unified National Standard screw thread is documented in the old copies of Federal Standard Handbook H-28 and in what we now call ANSI/ASME B1.1

The transition began in 1949 with the competition of the unification work and publication of the standards.

In the 1957 edition of the Federal Standard Handbook H28 the American National Standard screw thread was removed from the standard and placed into the Appendix 1. When significant changes are made in the standards, the old data gets moved to the Appendix. The Appendix to a standard is not considered part of the standard; it is considered for information only. This is a hint to all using the standard that this changed item should be abandoned in new designs and if changes are made to older designs they should be upgraded to the new data. Being moved to the Appendix is one step away from being eliminated from the standard completely.

In the 1969 edition of the Federal Standard Handbook H28, in Table 1.8a: Designations for UN, UNJ, N, NR thread series; the N-series was identified as being superceded by UN series. To be clear, the most concise definition of superseded is replaced. In other words: Stop using that old series; get with the times and start using the current Unified National Standard screw threads. The American National Standard screw thread data was still left in the Appendix, but that was only because so many people did not abandon the series. It is still the intent of the B1 Committee to completely obsolete the old standard.
In the most current version of the standard which is not known as ANSI/ASME B1.1 there is no mention of American National Standard screw threads, but the old standard is still present on many drawings.

When this change was new, like in the early 1959's, thread makers were told to translate the obsolete American National Standard thread requirements on existing drawings to the Unified National Standard using the comparable class-of-fit. There was resistance to change because of the existing inventory of gages. Thread makers were told to use their existing gages until they needed to be replaced and then replace them with the Unified National Standard gages.

TRANSLATION
N changes to UN
NS changes to UNS
NC changes to UNC
NF changes to UNF
NEF changes to UNEF
Class 1 changes to 1A for external threads or 1B for internal threads
Class 2 changes to 2A for external threads or 2B for internal threads
Class 3 changes to 3A for external threads or 3B for internal threads
Class 4 obsolete
Class 5 is still used for interference threads

SUMMARY OF DIFFERENCES
Several changes were made that were specific to nomenclature. Minor changes were made to the general thread form of the end product conform to manufacturing realities. Some benign changes were made relating to the major and minor diameters. Changes were also made to pitch diameters to remove tolerance issues which made the threads nearly impossible to manufacture and gage. Under the obsolete American National Standard the product tolerances were practically absorbed by the combined tool and gage tolerances, leaving little working tolerance in manufacture.

COMMENTS
Refer comments/corrections to gageguy@gagecrib.com.
Revised 3/21/2008