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Calibration by Direct Measurement

Ring Gage Calibration by Direct Measurement.

Calibration of AGD-Style Adjustable Thread Ring Gages by Direct measurement is not authorized by ANSI B1.2-1983; Gages and Gaging for Unified Inch Screw Threads; Reaffirmed 2001.

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.

Other Related Pages:
Set Plug Usage.
Adjusting Thread Ring Gage Locking Device.
Procedures for Setting Thread Ring Gages with Truncated Setting Plug Gages

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.

Original Posting: 5/2/2008
Last Revision: 4/3/2013
Error corrections in, or comments about, the above data can be sent to: gageguy@gagecrib.com

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