isn't there also a thread called "Witworth"
Yes, but it's not in common use, also it was a fairly coarse thread. Without wanting to take over this thread, considering the nature of the last few posts, I think some people need to understand that threads just ain't threads, there's a damned sight more to them and the following is only scratching the surface............................:
Joseph Whitworth proposed the first standardisation(sp) of screw threads in 1841 but it wasn't until 1860 that his 55 deg pitch and set TPI per diameter was accepted by industry in Britain and known as British Standard Whitworth (BSW). In 1864, in the US, William Sellers proposed a 60 deg thread with various TPI based on diameter, these proposals went on to become American Standard Coarse (NC) and Fine Series (NF), sometimes refered to as National Coarse and National Fine or UNC and UNF, the "U" signifying "United" or "Unified".
At the same time, in Europe there were moves to standardise the different thread series in use there, the 60 deg Metric thread was the eventual result.
Additionally, there was the British Fine (BF) thread and for thread sizes below 1/4", there was the British Association Thread (BA) which was a small fine thread based on a 47.5 deg thread. It was as far back as 1948 that the UK, US and Canada agreed to use the Unified Thread (UNC and UNF) as the standard for countries use inch units of measure,and in 1965, the British Standards Associaction recommended that BSW, BF and BA be made redundant with the ISO Metric thread as 1st choice replacement and the ISO Unified series as 2nd choice.
What is not easily seen, is the fact that there are frequently differences in the profile of threads, some can have flat crowns, the radius of the crown and root (the valley) can be different, etc., etc. Add to all of this the existence of specialised machining threads where even for a relatively large diameter, an extremely fine thread is used............I suspect this may be what you guys are encountering at the moment.
