Re: How to properly check British Standard threads?
One of the first items to obtain would be the actual British Standard specifications for threaded ring and plug gages. The Brits use deferent requirements from what we have here in the USA. The specification will tell you the requirements and how to verify their conformity to the requirements.
British Standard Threads
BSPP = British Standard Pipe Parellel specified in ISO 228.
Neither BSPP nor BSP are authorized designations.
Avoid using BSP because confusion between BSPP and BSPT frequentally occurs. Clearification is mandatoary by standard number to assure the correct thread.
The threads series is nomenclated as: 'G' (example: G1/4)
BS 2779 has been retired and replaced in whole with ISO 228.
ISO 228/1 defines the product threads.
ISO 228/2 defines the gage parameters.
BSPT = British Standard Pipe Taper specified in ISO 7 and BS 21
BSPT is not an authorized designation.
The thread series is nomenclated as: 'R' but with and without additional lower case letters.
R = External Taper Thread
Rc = Internal Taper Thread
Rs = External Parellel Thread
Rp = Internal Parellel Thread
Beware, these parellel threads are not the same as the G-series threads. Always identify the applicable standard when dealing with British Standard Pipe Parellel threads.
The product threads defined in each standard are identical.
The gage systems defined in the two standards differs. BS 21 is less expensive to use because it requires fewer gage members.
A funny side note to this issue arose when a company I worked for decided that to save money (early 1980's) on the procurement of ring and plug gages by having their division in England buy the gages. The shinny new gages arrived from England with calibration certifications and they promptly started rejecting lot after lot of product. After much research it was found that gages manufactured to British Standards had much deferent thread form and tolerances than those manufactured to US Standards.
There is no US gage maker that makes British Standard Pipe Gages to the appropiate standard. The US gage makers do not have the capability to accuratelly make the thread profile. Because of this they make a modified form to the thread form. They truncate the radisus from the gage and expect the end user to use alternate measurement methods to assure the thread form is accurate. Gages made in USA should be marked: 'Truncated' or: 'Modified'. Eather way is just a method of saying that the gaqes do not meet the requirements of the designated standard. Use caution when purchasing cutting tools for the same reason. Always specify 'Full Form' for gages and cutting tools to assure that the requirements of the standard are being met in your finished products.
Here in USA, way back to WW I, we have been modifying the British Standard Pipe thread form. The rest of the world uses the full form version of the thread, as specified by the standards. We now are making more for export. If your customer is not in USA, they are expecting the threads to be made to the standard, not to the USA bastardized version of the standard.
The R-series is designed to be a dry seal thread. The root and crest radii are intregal to make the dry seal connection.