NOTICE:
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.
What is the big controversy?
The last time the screw thread standard was revised this drastically
was B1.1-1949. That was the time that the N-series was replaced
with the UN-series. Two basic things in B1.1-1949 changed: First
the pitch diameters of the threads were adjusted by 0.0001 to 0.0002
of an inch. Second, the thread nomenclature was changed from N to
UN.
That does not seem like much. Right?
Well here we are in 2010 and I daily get screw thread gage requests
for the N-series thread.
I explain to the requestor:
The N-series thread is obsolete.
The N-series screw thread was replaced in 1949 with the UN-series.
The NC-2 thread call-out should now read UNC-2A.
The N-series is fully mechanically interchangeable with the
UN-series.
The difference is only 0.0001 to 0.0002 of an inch on the pitch
diameter.
After all that explaining the requestor replies that:
The print requires the N-series and he does not dare deviate
from the drawing.
The drawings are not under his control and it takes an act of
congress to get a drawing change.
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.
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.
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?
So what changed in the B1.1 this time that will cause so much
controversy?
Revision to some values in Table 2:
This is a seemingly innocuous statement, but this is where the pitch
diameters change by 0.0001 to 0.0002 of an inch … again.
The values in Table 2 were recalculated using the new rounding
procedures specified in ANSI/ASME B1.30-1992. This did not affect
only the pitch diameters, but also several other unified inch screw
thread dimensions throughout Table 2. The pitch diameters are the
most visible of the numbers that changed. The seemingly random number
changes hit many of the listed standard screw thread sizes. Full
listings of the old values are recorded in Table E-1 found in Nonmandatory
Appendix E.
Adding to the controversy is the fact that ANSI/ASME B1.2, the
standard that defines the unified inch screw thread gages, has not
been revised and still lists the old pitch diameters. Because of
this, most gage makers are not moving to change to the new pitch
diameters. If you insist on the ANSI/ASME B1.1 pitch diameters,
you will in many cases be charged extra for a special gage.
Paragraph 8.2.1 (paraphrased)
Limits of size for the majority of the standard unified inch screw
threads are shown in Tables 2 and E-1. Until ASNI/ASME B1.1 is revised
to require the calculated and rounded values per ANSI/ASME B1.30
as shown in Table 2, both these and the values in Table E-1 will
be equally acceptable.
Paragraph 8.2.1 will be important in any debates over specific
thread sizes in a dispute, but ANSI/ASME B1.2 paragraph 2.1.1 has
been ineffective in stopping gage disputes. I expect that paragraph
8.2.1 will be equally as ineffective in stopping thread size disputes.
The customer is always right. I suggest verifying the specifics
before manufacturing product, and pushing toward ANSI/ASME B1.1-2003
when ever economical.
I hope we are not we looking at another 60 years of controversy.
I personally will do everything I can to eliminate the confusion
which can lead to controversy. 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.
ANSI/ASME B1.1:2003 is the Foreward:
One of the most important parts
of ANSI/ASME B1.1:2003 is the Foreward. It outlines the changes
in ANSI/ASME B1.1:2003.
B1.1 Supersedes all previous versions:
The authors of the standard are doing their part to eliminate the
N-series screw threads. Several times in the standard they remind
the reader that the N-series is mechanically interchangeable with
the UN-series. The first sentence in the Foreward 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, 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.
Table 3B has been moved:
The Table 3B has been renamed Table D-1 and has been moved to Nonmandatory
Appendix D. This is a smart move to encourage standardization. It
removes certain screw thread combinations from the select status
of Standard Special. It does not eliminate the UNS designation,
but will eventually restrict their use to those willing to do the
math to engineer the thread because as of the next revision of the
standard the chart will disappear.
Other Changes:
There are several other changes in the standard,
as would be expected. To get the complete details, buy a copy and
read the Foreward.
Disclaimer
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.
Comments:
Original Posting: 6/15/2003
Last Revision: 5/13/2013
Error corrections in, or comments about, the above data can be sent
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