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Unified 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 B1.1.

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 (paraphrased)
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 The unified screw threads as defined in ANSI/ASME B1.1 originated in 1948 and have subsequently superseded the American National screw threads; the N-series.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 thread.

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.

Related Pages
ANSI/ASME B1.1:2003 Changes
Pitch Diameter Changes for 1A and 1B
Pitch Diameter Changes for 2A and 2B
Pitch Diameter Changes for 3A and 3B

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
Error corrections in, or comments about, the above data can be sent to: office@gagecrib.com


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