Engineering Tip – UN Threads

To any fellow engineers or machinists out there… I would like to share with you a fact I just discovered. UN thread pitch diameter tolerances are based on the Length of Engagement (LOE) of the thread. The default values you see in tables such as in the Machinery’s Handbook only apply to the standard ranges of LOE. For those who don’t know UN stands for Unified Inch Screw Threads.

See below for an excerpt from ASME B1.1 that details the standard LOEs.

The pitch diameter tolerances specified in Table 2 for
all classes of the UNC and UNF series are based on a
length of engagement equal to the basic major (nominal)
diameter and are applicable for lengths of engagement
from 5 pitches to up to 1.5 diameters. For the 4-UN,
6-UN, and 8-UN series, the pitch diameter tolerances
specified for Classes 2A, 2B, 3A, and 3B are based on a
length of engagement equal to the basic major (nominal)
diameter and are applicable for lengths of engagement
up to 1.5 diameters.
The pitch diameter tolerances specified in Table 2 for
all classes of the UNEF, 12-UN, 16-UN, 20-UN, 28-UN,
and 32-UN series are based on a length of engagement
of 9 pitches and are applicable for lengths of engagement
from 5 to 15 pitches.
The pitch diameter tolerances specified in Table D-1
of Nonmandatory Appendix D for all classes of the UNS
series are based on a length of engagement of 9 pitches
and are applicable for lengths of engagement from 5 to
15 pitches.

I posted this because no other engineer I have talked to knew this either!

Also a tip about when to use which designation UN, UNC, UNF etc…

UNC (coarse), UNF (fine) and UNEF (extra fine) are to be used when the given nominal diameter and pitch are listed in the tables as one of the standard pitch designations above.
UN is to be used when the pitch is 4, 6, 8, 12, 16, 20, 28, and 32 and not listed as one of the above designations for the given diameter. UN stands for constant pitch series. I think the constant means that a designer has decided to use a specific pitch across several diameters for engineering purposes. (constant across sizes)
-For a non-standard diameter or a diameter-pitch combination not listed as one of the other pitch designations it should be called UNS.

If you come across a drawing without the U ie NC-4 or something that means that it is made to the outdated American National screw threads standard.

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