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Confusing Calibrations of British Standard Pipe Gages

Confusing Calibrations of British Standard Pipe Gages

Certification Values NOT Match The Specification Values?
You are likely in the same situation as one of my good customers. She had a new requirement for British Standard Pipe Threads and Screw Thread Gages because of her company’s expansion in to European markets. She knew the sizes, and she purchased the gages she needed. She also purchased ISO 7/1 and ISO 7/2 so she could assure they complied with them while making the products. When she received the gages and compared their certifications with ISO 7/2 she found that there was no seeming logical correlation between the two documents.

What The $%&!*#?
Okay; she expressed her confusion much more politely than that, but it does raise the question of: WHY? We had a conversation with our UK Gage Maker and he provided an explanation.

The Explanation:
See if you follow this. We were discussing the Rc3/8” ISO 7 Gage-1. The story goes back to BS:21/1985 where the nominal screw thread plug major diameter in that standard was: 16.662mm ±0.010mm. In 2000, BS:21 was replaced by ISO:7/2. Nominal sizes and tolerances of gauges became confusing (more so the product customers than to the gauge manufacturers), probably because their drawings still referred to a nominal product major diameter of 16.662mm, whereas the gauge major was identified as: 16.670mm. The Gauge manufacturers decided to leave the nominal diameter the same and to adjust the tolerances (by adding 2 x Tpl = top limit) so that they stayed with the intent of the specification.

Restated with more detail…
We were discussing the Rc3/8” ISO 7 Gage-1 Major Diameter (D) at Gauge Plane. ISO 7/1:2000; Table 1 gives value as: 16.662mm. ISO 7:2000; Table 2 gives value as: 16.662mm, but paragraph 7.1.1.1 says that new plug gage diameter is enlarged by Tpl. Tpl = Tolerance on pitch diameter and wear allowance for Gage-1. This is also stated in Table 8 where it defines the gage major diameter at gage plane as D+Tpl. In this case; the nominal major diameter of 16.662mm is increased by Tpl (0.008mm per Table 9) to 16.670mm. The gage tolerance as stated in Table 8 is +/-Tpl; thus +/-0.008mm. So putting this together the gage certification should read: 16.670mm +/- 0.008mm. But when a customer sees the gage size listed as 16.670mm and his product size listed as 16.662mm; he begins to wonder why the gage made to check his part has a different diameter than would be expected. Logically he calls the gage maker and asks that question. This is where/why the gage makers decided to list the nominal gage size as 16.662mm with a tolerance of +0.016/-0mm.

I hope this clears up the matter if you find a similar situation.

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 documents: BS-21:1985; ISO 7-2:2000; and EN 10226-3:2005; which are a copyrighted documents.
To purchase a copy visit an Authorized Reseller.

Comments:
Original Posting: 12/18/2017
Last Revision: 12/18/2017
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: gageguy@gagecrib.com.

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