National Standard vs. Unified Inch Standard
N vs. UN
NC vs. UNC
NF vs. UNF
NEF vs. UNEF
NS vs. UNS
The origin of the standardized screw thread began in the mid 1800's
through the efforts of William Sellers in the United States and
Sir Joseph Whitworth in the United Kingdom. Both men in their separate
countries had their screw thread designs standardized. Sellers'
design was known by several names: Sellers; Franklin Institute; US Standard; and ultimately standardized as the American National Standard Series. Whitworth's design was standardized as: Whitworth Series. Discussing
the differences between the two series is not in the scope of this
The “UN” thread form was developed after World War II by representatives of Great Britain, Canada, and the United States of America, to prevent recurrence of the wartime difficulties in supplying fasteners and tools in both British Standard Whitworth and US Standard configurations when and where needed. In 1949, after years of committee meetings between Canada,
England and United States of America the American National Standard
Series was replaced with the Unified Inch Standard Series. In the end there
were three base reasons identified for the change. The first reason was to
provide interchangeability with Canada and United Kingdom. The second reason was to allow for
interchangeability in the growing global marketplace. The third
reason was to correct certain thread production difficulties. The jointly-developed thread form was named the Unified Thread Form. the Unified Thread is also referred to in the B1.1-2003 as Unified Inch Screw Thread. This Unified Inch Screw Thread both superseded the previous British, Canadian and American national standards, and later served as a prototype multi-national thread form standard that was eventually metricified to become the ISO Metric Screw Thread (the M-series).
Some confusion over the UN designation exists. Some people think it means Unified National based on the fact that three national entities came together to unify their thread standards; thus: Unified National Screw Thread Form. By reading the B1.1 this is not supported; rather UN is the abbreviation for UNIFIED. In the B1.1 the thread is called either Unified Inch Screw Thread or Unified Screw Thread with the preference being Unified Inch Screw Thread to indicate the unit of measure used in the screw thread series.
HISTORY OF CHANGE
The American National Standard Series (N-Series) was last defined
in ANSI/ASME B1.1:1935 and has been obsolete since 1949. In ANSI/ASME
B1.1:1949 when the American National Standard Series (N-Series)
was made obsolete the N-Series screw thread data was moved to the
Nonmandatory Appendix. When the standard was revised again as: ANSI/ASME
B1.1:1960 the American National Standard Series was intended to
be forgotten and was removed completely from the standard. ANSI/ASME
B1.1 has since been revised in 1974; 1982, 1989 and 2003.
FED STD HANDBOOK H28 was a predecessor ANSI/ASME B1.1. The data
in FED STD H28 is slowly, revision by revision, being removed and
placed in ANSI/ASME B1 Standards. There seems to be no reason to
maintain two separate yet identical standard systems. As of 2010
the Military has reduced its focus on keeping their standards current
in favor of using nationalized standards. Much of the current FED
STD H28 is more like an index which points you to ANSI/ASME publications.
In the case of the American National Standard Series (N-Series)
screw thread the FED STD H28 was a little behind ANSI/ASME B1.1.
After ANSI/ASME B1.1:1949 was published it took the Military until
the 1957 edition of the Federal
Standard Handbook H28 for the American National Standard Series
screw thread to be removed and placed into FED STD H28 Nonmandatory
Appendix 1. In next revision, which came nine years after ANSI/ASME
B1.1:1960, the 1969 edition
of the Federal Standard Handbook H28 removed the N-Series from
the Nonmandatory Appendix. In Table 1.8a: Designations for UN,
UNJ, N, NR thread series; the N-series was identified as being
superseded by UN series. FED STD H28 has since been revised several
To be clear, the most concise definition of superseded is replaced
with something better. In other words: Stop using that old N-Series.
Get with the times. Start using the current Unified Inch Standard
Series (UN-Series) screw threads.
When significant changes are made in the standards, the old data
gets moved to the Nonmandatory Appendix. The Nonmandatory Appendix
to a standard is not considered part of the standard; it is considered
"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 Nonmandatory Appendix is one revision
away from being eliminated from the standard completely.
PROCESS OF CHANGE
When changes to the standard are made, it is the intent of the Standards
Committee to encourage manufacturers to move away from the old standard
and work to the new standard data. It is intended that all new designs
be made in accordance to the new standard data. It is understood
that it is not cost effective to change all old drawings to reflect
the new standard data, but it is expected that the old screw data
be translated into the new standard data. Years are given for the
conversion to be made so that it does not adversely affect manufactures
production costs. Manufacturers are told to translate the obsolete
requirements on existing drawings and to now work to the new requirements.
For the purposes of this discussion (there are some exceptions)
the American National Standard Series translates into the Unified
Inch Standard Series screw thread as follows:
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
Class 2 changes to 2A for external threads or 2B for internal
Class 3 changes to 3A for external threads or 3B for internal
Class 4 obsolete. Use 3A for external threads or 3B for internal
Class 5 is still used for interference fit threads
SUMMARY OF DIFFERENCES
There are obvious changes between the N-Series and the UN-Series.
First, the thread nomenclature was changed from N to UN. This was
necessary to designate the internationalization of the Unified Inch
Standard Series screw thread. Second, the male and female threads
have received individual alpha designations: A = Male; B = Female.
Third, the pitch diameters of the threads were adjusted. The class
2 thread pitch diameter adjustment allows for an allowance between
the male and female threads. In other cases the pitch diameter were
changed to remove tolerance issues which made the threads nearly
impossible to manufacture and gage. Within the now obsolete N-Series
some product pitch diameter tolerances were practically absorbed
by the combined tool and gage tolerances leaving little working
tolerance for the product manufacturer. Finally there were several
other minor changes made to the general thread form of the end product
conform to manufacturing realities and some benign changes were
made relating to the major and minor diameters.
What did not change was interchangeability with previous versions
of the screw thread. The UN-Series is fully backward compatible
with the N-Series. This is codified in ANSI/ASME where the authors
of the standard, in several places, remind the reader that the N-Series
is mechanically interchangeable with the UN-series. The first sentence
of the Foreword to B1.1:2003 is their strongest move to eliminate
the N-series screw threads once and for all.
It reads: "This standard is the outgrowth of and supersedes
previous editions that were published as B1-1924, B1.1-1935, B1.1-1949,
B1.1-1960, B1.1-1974, B1.1-1982 and B1.1-1989."
This statement is strong because it has been printed in the now
current standard. It informs all who make 60 degree inch screw threads
that all previous versions of the standard have been replaced. By
replacing all previous versions of the standard, the N-series screw
thread as defined in B1.1-1935, has been replaced. This statement
could be used as authorization to deviate from a drawing. When the
drawing indicates the N-series screw thread, it is known that the
thread is specified in B1.1-1935, thus it is superseded by B1.1-2003
and the UN-Series is to be used.
CALL TO ACTION
Hello! It has been over half a century and high quality ISO registered
companies are still making screw threads to the long obsolete B1.1:1935.
The standard has been revised six times, and still people insist
on using the 1935 version of the standard! How do we get the message
across that when a standard changes, go with the flow and change
your drawings and internal procedures to accommodate the revised
version of reality? It was the intention of the standards committee
that the American National Standard Series screw thread be replaced
with the Unified Inch Standard Series screw thread in all cases.
The threads made to the Unified Inch Standard Series are designed
to screw together with the now obsolete American National Standard
Series screw threads. Without exception: Drawings should be updated
to reflect the current standard. The class-of-fit requirements for
the obsolete American National Standard Series screw thread can
be translated to the current Unified Inch Standard Series. This
is something that all engineers need to address. Make drawing changes
at the detail level of product designs. If the drawing change process
is too daunting, issue a blanket engineering change order dictating
that 60 degree inch series screw threads will be made to the most
current version of B1.1. Begin an education process aimed at third-party
government inspectors to train them to understand and work to the
now current version of B1.1.
Is Change Really That Difficult?
Well here we are in 2012 and I daily get screw thread gage requests
for the N-series thread.
I explain to the requestor:
1. The N-series thread is obsolete.
2. The N-series screw thread was replaced in 1949 with the UN-series.
3. The NC-2 thread call-out should now read UNC-2A.
4. The N-series is fully mechanically interchangeable with the UN-series.
5. The difference is only 0.0001 to 0.0002 of an inch on the pitch diameter.
After all that explaining the requestor replies that:
1. The print requires the N-series and he does not dare deviate from the drawing.
2. The drawings are not under his control and it takes an act of congress to get a drawing change.
3. The third-party inspector does not understand the finer points of subtle change over time in the screw thread standard so if it says NC-2 on the drawing, the gage had better read NC-2 or his part will get rejected.
4. If the pitch diameter stated on the drawing is 0.0002 of an inch different from the pitch diameter on his gage the government inspector will reject his part.
After hearing the rebuttal from the requestor; I tell him that the government has come to his rescue.
GOVERNMENT TO THE RESCUE?
To save taxpayers money the government has decided not to change all the drawings. Instead they have placed instructions within FED-STD-H28/2B to direct product manufacturers to use the UN-series instead of the N-series and how to make the preferred conversion from N to UN.
It says in FED-STD-H28/2B; Page 4; Paragraph 5.2.1: 5.2.5 Replacements for obsolete American National thread classes. When threads specified with the obsolete American National thread classes are to be replaced by unified threads, the following guidelines are provided:
a. American National class 1 coarse thread sizes (NC-1) is approximately equivalent to Unified class 1A/1B series. Class 1 fine thread series (NF-1) is approximately equivalent dimensionally to Unified class 2A/2B series. Standard Unified series threads should be considered prior to approval of replacement by non-standard threads.
b. American National class 2 coarse thread series (NC-2), 8 thread series (9N-2), 12 thread series (12N-2), 16 thread series (16N-2), and extra fine series (NEF-2) are most nearly equivalent to Unified series UNC-2A/2B, 8UN-2A/2B, 12UN-2A/2B, 16UN-2A/2B and UNEF-2A/2B, respectively. Class 2 fine thread series (NF-2) is approximately equivalent dimensionally to Unified class 3A/3B series, but the use of class 2A/2B series should be considered prior to approval of replacement by class 3A/3B.
c. American National class 3 series NC-3, NF-3, NEF-3, 8N-3, 12N-3 and 16N-3 are most nearly equivalent to Unified class 3 series UNC-3A/3B, UNF-3A/3B, UNEF-3A/3B, 8UN-3A/3B, 12UN-3A/3B, and 16UN-3A/3B, respectively.
d. There is no Unified thread class equivalent to the old American National class 4 which required selective fit of parts due to the possibility of interference.
NOTE TO INSPECTORS
The US Government through FED-STD-H28/2B is directing manufacturers to use the current ANSI/ASME B1.1 UN-series screw threads and classes-of-fit instead of the OBSOLETE N-series which is specified on many old US Government part drawings. This conversion is outlined at FED-STD-H28/2B; page 4; paragraph 5.2.1. Please read the above information and consider the data presented.
The standards change over time to improve. Revision of standards
requires years of debate to reach the final consensus. Consideration
is given to functional properties as well as product interchangeability
with former versions of the standard. Compliance to current standard
version is less expensive because of the standardization of tools
and gages. Compliance to the superseded version is more expensive
because all the tooling has now become special made just for that
job. If this product is for a government project using the obsolete
standard is a waste of our tax dollars. If this inspection is for
a private industry project the extra cost reduces the competitiveness
of the end product in the market place which may schedule the product
for free market failure.
As always, obtain approval or consensus from your customer. This
data is provided for general information only. The intention is
to provide accurate information; regardless; errors may exist in
the supplied information. If accuracy is critical, base your final
decisions on the data provided in the root document: ANSI/ASME B1.1:2003.
ANSI/ASME B1.1:2003 is a copyrighted document. To purchase a copy
visit an Authorized Reseller.
Original Posting: 6/15/2003
Last Revision: 7/31/2014
Error corrections in, or comments about, the above data can be sent