But wait! My calibration laboratory uses direct measurement to
calibrate my AGD-Style Adjustable Thread Ring Gages whenever they
do not have a Threaded Master Set
Plug Gage that matches the specific size of my AGD-Style Adjustable
Thread Ring Gage. They say they are authorized to do this according
to ANSI B1.2.
Ask your calibration laboratory where in ANSI B1.2 they find their
authorization. They will reply that they are referring to ANSI B1.2;
Appendix A; page 163; section A3; THREAD RING GAGE INSPECTION with
emphasis on paragraph A3.1.3. Once you take a minute to read paragraph
ANSI B1.2; A3.1.3 you will find that it does not match the measurement
method used by the calibration laboratory. Usually the calibration
laboratory brags that they used their high-tech coordinate measuring
machine (CMM).
A further reading of ANSI B1.2; Appendix A brings to light more
problems:
1. Any discussion about Appendix A prescribing an authorized procedure
is mute. The Appendix A title page; page 163; states: (This Appendix
is not a part of the American National Standard Gages and Gaging
for Unified Inch Screw Threads, ANSI/ASME B1.2-1983 and is included
for information purposes only.) In other words, Appendix A discusses
some possible alternate methods of measuring thread ring gages,
but still says to do it right use the method prescribed in the body
of the standard at: ANSI B1.2; section 5 and ANSI B1.2; table 12.
2. ANSI B1.2; paragraph A3.1.2 states: "The measured pitch
diameter on rings fitted to a setting plug may be … larger
than the measured pitch diameter on the plug because the pitch diameter
equivalents from permissible pitch, lead, and flank angle tolerances
on matched plug and ring cause some unavoidable discrepancy."
In a conversation with a member of the B1 committee, he confirmed
this paragraph was correct but explained that the expected error
was understated and could be much larger.
3. ANSI B1.2; paragraph A3.4: Helix
Offset Measurement on Adjustable Thread Ring Gages cautions
that this feature of the AGD Adjustable Thread Ring Gage needs to
be checked. I do not know of a calibration laboratory that checks
this feature. I do not know of a method that this feature can be
brought back into alignment without a Threaded Master Setting Plug
Gage. Using a Threaded Master Setting Plug Gage eliminates concern
about the Helix Offset.
The only prescribed method of calibration for AGD-style adjustable
thread ring gages is threaded master setting plug gages. Per ANSI
B1.2-1983; Reaffirmed 2001; page 40; paragraph 5.1.1: "Adjustable
… thread ring gages must be set to the applicable … setting
plugs." This is reinforced in ANSI B1.2; Table 12; CALIBRATION
REQUIREMENTS AND STANDARDS FOR X TOLERANCES THREAD GAGES ……;
Item 1; Thread rings (ANSI B47.1); sub-items 1.1 and 1.2; column:
Setting gages and Standards. If the calibration laboratory did not
follow the ANSI B1.2-prescribed method for checking the AGD-style
adjustable thread ring gage, I would consider the calibration inaccurate
and void.
Let me tell you a story:
Once upon a time we sold a set of AGD-style adjustable thread ring
gages to a customer without set plugs. The customer used the gages
and had them calibrated every year through a calibration lab which
used direct measurement as the calibration method. Over the years
the gages required some adjustments which the calibration lab did
as a standard business practice.
After three years customer discovered that the usage of the gage
was increasing to the point that they required a second set of the
AGD-style adjustable thread ring gages. A new set was ordered, manufactured,
and shipped to the customer.
Upon receipt the customer called and told me that the new AGD-style
adjustable thread ring gages were bad because they did not fit the
parts which were passed by the existing "calibrated" AGD-style
adjustable thread ring gages. The new gages would not even start
on the parts. Knowing that given the choice of old gages or new
gages being bad, the most obvious choice is that the old were the
bad ones, but they were certified. We were most confused.
All the gages were sent to the gage maker for evaluation. The new
gages fit on threaded master setting plug gages just fine, as expected
because they were just made to fit them. The old gages would not
even start on the calibrated set plugs. The gages were opened up
and set to the set plugs. Once they were fit to the set plugs they
felt just fine with the correct amount of drag. They did not require
any repair.
The determination was that the old gages, when being calibrated
and adjusted without set plugs gained a small helix angle offset
error. Just enough that their reading of the parts was not within
the product requirements. The gages were passing bad parts even
though they were "certified" by an A2LA, ISO 17025 registered,
calibration laboratory. The data the calibration laboratory provided
was accurate as it was read from their CMM, but the gage was not
in tolerance when all gage parameters were considered.
The customer purchased a set plug. Since then there have been no
more problems with gages being out-of-tolerance. Everyone lived
happily ever after.
Rules-of-thumb for threaded master setting plug gages:
1. You should own the set plug gages when your calibration laboratory
does not own the proper threaded master setting plug gages which
match the AGD-style threaded ring gages being ordered.
2. Always get set plug gages for special size, large diameter,
fine pitch, ring gages. If you or your calibration laboratory own
the threaded master setting plug gages, they should be supplied
to the gage maker to be used in the manufacturing process of the
AGD-style threaded ring gages.
3. Always make sure that one set of threaded master setting plug
gages is available where multiple copies of same size AGD-style
adjustable thread ring gages will be used. Especially if one set
is yours and the other set is your customer's. Set both sets to
the same set plug. It does not matter who owns the set plug, either
you or your customer. You do not want two sets of set plugs.
4. Always get set plugs when the part material is in the stainless
steel or aluminum families.
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 B1.2; which is a copyrighted documents. To purchase copies visit an Authorized Reseller.
Comments:
Original Posting: 5/2/2008
Last Revision: 4/3/2013
Error corrections in, or comments about, the above data can be sent to: office@gagecrib.com
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