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Solid vs. Adjustable Screw Thread Ring Gages

Solid vs. Adjustable Screw Thread Ring Gages

I have been asked many times to explain which is better: Solid or Adjustable Screw Thread Ring Gages; so I will try to tell-all on these pages and see what comments return.

The screw thread consists of many complex features all wrapped helically around a shaft. As screw threads became standardized for interchangeable part mass production it became important to verify manufactured screw threads to see if they met a verified specification. Measuring all the aspects of the screw thread was determined too complex of a task to undertake at the production line. Easier methods of testing the screw thread to assure compliance to design parameters became a necessity; thus screw thread gages were designed.

Screw plug gages are not the topic of this discussion but need to be mentioned briefly. It is likely that the screw thread plug gage was designed before the screw thread ring gage because testing the internal threaded hole is near impossible to accurately directly measure. Compared to Screw Thread Ring Gages; Screw Thread Plug Gages used to test the internal product screw thread are reasonably simple to make because the gage screw thread is on the external surface of the screw thread plug gage where it is possible to directly measure the screw thread plug gage features.

It is logical that the initial design of screw thread ring gages is the solid style ring gage; modeled after the standard product nut. Unlike the standard nut, the screw thread ring gage needed to be made of hardened material so that it did not quickly wear when used on test pieces. The screw thread ring gage also needed to be precision machined to tight tolerances; leaving most of the tolerance to the work piece. Tight tolerances on the screw thread ring gage is cause for much scrap in the manufacturing process of the gage and eventually the amount of tolerance allowed for economic production of screw thread ring gages has been codified into ISO-1502, BS-919 and other industrial specifications.

During the era of the World Wars; when production accuracy was critical as was reduced costs of production and speed of manufacturing; the Adjustable Screw Thread Ring Gage was designed by Mr. Johnson while he was working at Pratt & Whitney in the USA. The new design had many benefits over the status-quo solid ring gage:
 Less scrap in the ring gage manufacturing process speeding gage production and reducing gage cost.
 Lower cost to make special size thread gages.
 Tighter gage tolerances allowing more manufacturing tolerance for the end product thus reducing the cost of the end product.
 Longer life of the screw thread ring gage because the user can readjust the adjustable style ring gage back into tolerance as the gage wears.
 The ability to inexpensively rework a worn/galled/damaged ring gage to renew its usefulness.
Because of these benefits the design was quickly adopted across the USA. The design is known as AGD (American Gage Design); or Adjustable.

As it stands today both styles of thread ring gage are in use around the world. The use of the adjustable style thread ring gage is concentrated in the USA with the rest of the world still favoring the older less accurate solid style screw thread ring gage. One would think; with the benefits of the Adjustable style thread ring gage helping turn the tides of the World Wars (OK, only a little bit); the rest of the world would clamor for this new and improved measurement tool. This seems not to be the case.

The cost of the gages is about equal for standard screw thread sizes (assuming that you purchase the solid style outside of the USA and the adjustable style from the USA) because the gage tolerance is so much larger for the solid style screw thread ring gages. In other words when you purchase a solid style gage the gage itself uses more of the product part tolerance, increasing the production costs of the end product. For special screw thread sizes the adjustable is often less expensive and some special screw threads are not available in solid style ring gages.

When the USA changed over to the adjustable style screw thread ring gage it was done with the end goal of winning the wars and bringing the soldiers home as soon as possible. Since the wives and girlfriends of the soldiers were working in the factories; acceptance of the adjustable style thread ring gage was rapid. They wanted their men home. Also, since the men who had the experience with the solid style gages were off fighting the war; the women who had limited history of using the solid style thread ring gages accepted the new design for what it was: a good tool. The USA Gage makers all tooled-up to make the adjustable style thread ring gage. The innovation of the adjustable screw thread ring gage did not spread internationally and after the war the rest of the world was still tooled up to make the old solid style thread ring gage. To those manufacturers, there was no reason to invest in the new tooling and the training effort needed to educate their customers in the improved thread ring gage design; as long as they still accepted the old design with no question. They likely even sold against the design so that they did not have to make the investments.

With all of the stated advantages of the adjustable style screw thread ring gage; why are customers still insisting on purchasing and using the obsolete solid style thread ring gage? The reasons I hear are:

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 documents: ANSI/ASME B1.13M and ISO 1502; which are copyrighted documents. To purchase a copy visit an Authorized Reseller.

Original Posting: 3/3/2015
Last Revision: 3/17/2015
Error corrections in or comments about the above data can be sent to: office@gagecrib.com

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