Good description here of all of the lingo:
BA
BSF
BSW
AF
Etc
Convert Whitworth,BSF,BA,AF sizes to metric.Wrench,socket,nut and bolt sizes explained.
www.baconsdozen.co.uk
Hopefully it’s not too different!
BSW = British Standard Whitworth aka “Whitworth”. Coarse threads optimised for iron. Similar to UNC / ANC.
BSF = British Standard Fine. Same threadform as BSW but finer so optimised for steel. Similar to UNF / ANF.
BA = British Association. Small sized threads optimised for instrument work. Similar role to UNC / UNF “numbered” series, but the low numbers are the biggest.
AF (or A/F) = Across Flats. A designation for a wrench using the American (SAE) or Unified system, where the size shown is the size “Across Flats” rather than the thread size.
The website linked is useful to a degree, but there are some errors / omissions.
I’ve never found these charts showing metric equivalents of BS sizes any use whatsoever. Most metric sizes don’t quite fit, or are simply too big. Use the correct size.
The BSW / BSF sizes shown are correct as regards pre war fasteners, but post war the BSF sizes were used for both BSF and BSW (and 2/4 BA were substituted for the smallest sizes).
When the U.K. adopted the Unified system, the “numbered” sizes were never adopted, so BA sizes are found much later on British equipment, and in fact are still used in some areas.
BA sizes are actually metric threads. O BA is 6mm (x 1mm pitch) and each subsequent size is 90% of the previous. This is actually far more sensible than sizing in fixed increments, where, say 1mm, might not be enough of a jump in larger sizes, and too much in the smaller.
I’m quite happy using ISO metric fasteners as substitutes for Imperial sizes (either BS or Unified). 1/4, 5/16 and 3/8 can be switched for 6,8 and 10mm quite easily. However, I find the ISO metric very cumbersome in the smaller sizes so generally still use BA. The screws are better quality too.