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Controlled Root Radius on Internal Screw Threads

Controlled Root Radius on Internal Screw Threads

There has been a quiet move toward modifying the J-series screw thread. As far as I can see it was started at Boeing in 1987 and has spread to Parker in 1994. Boeing (BACD2086); Parker (BMF5163) and Dowty (PS158) have published internal company engineering standards covering the same screw thread modification. Other performance-critical companies are adopting this modification of the J-series screw thread which adds a controlled root radius to the female screw thread.

The modification to the J-series screw thread is precisely defined as the addition of a 0.072p to 0.096p controlled root radius on the internal screw thread and the associated 0.0715p reduction in maximum major diameter of the external screw thread to clear the female screw thread controlled root radius. This screw thread design modification of adding a large root radius into internal screw threads subject to fatigue loading is intended to reduce stress related failures.

BACD-2086; BMF-5163 and PS-158 all lay-out the same thread requirements as summarized below.

This modification is made to class-of-fit 3A male screw threads and class-of-fit 3B female screw threads in the UN-series with the J-series modification. In technical language; this modification is made to screw threads manufactured in accordance to ANSI/ASME SAE-AS8879 (formerly MIL-S-8879).

Internal screw threads have gained a controlled root radius equal to 0.072p to 0.096p.

The maximum major diameter of the external screw thread has been reduced by 0.0715p in order to clear the new internal root radius.

This design is not as secret as some would hope. Many companies who do contract work for these two companies are using the standards as are subcontractors and tool makers. I know of at least one other company who is incorporating the design into their own products with no reference to any base document. I am confident that many others are doing the same thing.  As of this writing I do not know of any other companies that have published this manufacturing standard under their own title/number.

BACD2086; BMF5163 and PS158 are protected by copyright so I can not publish them on the web, but if BACD-2086; BMF-5163 and PS-158 were combined they would make a very complete document. BACD2086 Figure 1 is a very complete graphic representation of the modified thread. The BMF5163 tables appear to be a complete recalculation of the tables in MIL-S-8879. At some point I expect that this will be added to the J-series engineering standard, currently ANSI/ASME SAE-AS8879, screw thread requirements.

BACD-2086; BMF-5163 and PS-158 do not directly addresses the modifications required to the screw thread gages. The only modification to the screw thread gauges, beyond what is required for the J-series screw thread, is the major diameter of the GO Work Plug Gage is reduced to clear the new controlled root radius on the female screw thread. All other gauges are as required for the J-series screw thread.

This has been afoot in the screw thread world since the initial release of BACD2086 in 1987. This is considered a new thing in an alternate reality where change takes decades to complete. What happens is that some company determines that a deviation to the existing standard is beneficial. That company tests the change and implements the change into their products. As is the nature of business and new ideas, others learn about the change and as they see positive results, they implement the same change into their products. Eventually so many people are making their own internal standards that someone requests the standards board to implement the change into the national or international standard. Decades later after many committee meetings the new national or international standard is codified and published for public use.

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 as listed above. These documents are all proprietary to the identified companies and to obtain a copy from them I expect that you must be doing work directly or indirectly for them. Good luck.

Original Posting: 10/1/2010
Last Revision: 1/2/2015
If you are aware of any other engineering standards which promote this same design concept, or if you have some old documentation which will make this history more complete and you are willing to share; please email the document(s) to me at: office@gagecrib.com.

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