Inch Screw Thread Series as Defined by ANSI/ASME B1.1
Unified Inch Screw Thread Series as Defined by ANSI/ASME B1.1
ANSI/ASME B1.1 was revised in 2003. The revisions pose the risk
of controversy over thread requirements and disputes over part
acceptability. It is suggested that you obtain a copy of the revised
standard and begin implementation of the new standard immediately.
ANSI/ASME B1.1:2003; 1.1 Scope (paraphrased)
ANSI/ASME B1.1 specifies the screw thread form, series, class, allowance,
tolerance and designation for unified screw threads. These threads
may also be known as unified inch screw threads. ANSI/ASME B1.1
is the base document for the UN-series screw threads and ANSI B1.1
covers the UN-series and the UNR-series. Several variations in screw
thread form have been developed as modifications of the unified
screw thread form defined in this standard, but their definitions
will be found in alternate standards. The official version of ANSI/ASME
B1.1:2003 has included several Nonmandatory Appendices that contain
useful information that is supplementary to the sections of ANSI/ASME
Metric Translation of ANSI/ASME B1.1
Nonmandatory Appendix C contains an easy reference metric translation
of ANSI/ASME B1.1.
ANSI/ASME B1.1:2003; 1.2 Unified Screw Thread Standards
The unified screw threads as defined in ANSI/ASME B1.1 are in agreement
with ISO standardization for diameter-pitch combinations, designations,
and tolerances for 60° triangular form inch screw threads. The
unified screw threads as defined in ANSI/ASME B1.1 originated in
1948 and have subsequently superseded the American National screw
threads; the N-series.
ANSI/ASME B1.1:2003; 1.3 UN and UNR Screw Threads (paraphrased)
UNR applies only to external screw threads. The difference between
UNR-series and UN-series screw threads is that the root of the UNR-series
screw threads has a radius, where the root of the UN-series screw
threads may have either a radius or a flat.
ANSI/ASME B1.1:2003; 1.4 Interchangeability (paraphrased)
The UN-series, UNR-series and N-series
screw threads are interchangeable. They have substantially the same
screw thread form, and screw threads of any of the three designations
having the same nominal major diameter and pitch (threads-per-inch)
which makes them mechanically interchangeable. The principal differences
between these three screw thread series relate to the application
of allowances, the variation of tolerance within size, differences
in the amounts of pitch diameter tolerances, and differences in
the base designations.
The UN-series, UNR-series and N-series screw threads are not mechanically
interchangeable with the M-series screw threads.
ANSI/ASME B1.1:2003; 1.5 Designations (paraphrased)
Unified screw thread series is designated by the letter combination
UN in the screw thread designation. There may be additional letters
in the designation. Examples: UN; UNC; UNF; UNEF; UNS; UNR; UNRC;
UNRF; UNREF; UNRS; UNJ; UNJC; UNJF; UNJEF.
In the UN-series the pitch diameter tolerances for the external
screw threads differ from the tolerances for the internal screw
threads. For this reason the letter A is used in the screw thread
designation to denote an external screw thread, and the letter B
in the screw thread designation is used to denote an internal screw
Where the letters: A, B or U do not appear in the screw thread
designation, the threads identified may be the N-series. The N-series
is the outdated American National screw threads. For data related
to the N-series refer back to B1.1:1935.
If you have the N-series designated on your drawing, to avoid confusion
over thread minutia, the drawing should be updated to the current
UN-series designation because the UN-series, UNR-series and N-series
screw threads are mechanically interchangeable.
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; which is a copyrighted document. To purchase a copy visit
an Authorized Reseller.
Original Posting: 6/15/2010
Last Revision: 4/1/2013
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