The thread shown in the BA thread table is a British thread with a small diameter. It is subject to monitoring by the British Standards Institution (BSI) and is listed there under the BS 93 standard. A special feature of the BA thread is its small dimensions, which can be easily seen in the BA thread table. The history of the BA thread is also quite interesting. It was developed by the British Association in 1884 and has been in use since 1903. As the dimensions in the BA thread table suggest, the main areas of application are in precision mechanics and model making. The BSI recommends the use of the BA thread for thread sizes below 1⁄4 inch instead of the corresponding sizes of British Standard Whitworth (BSW) or British Standard Fine (BSF). Today, BA threads are still used for precision instruments or in older switch boxes. Outside the UK, the BA thread also appears on darts, where it joins the shaft to the point.
The values that you can fortunately read off the BA thread table today are the result of a rather complicated calculation. Based on a thread with a diameter of 6 mm and a pitch of 1 mm, the other dimensions of the BA thread table were determined by always using 0.9 times the pitch of the next larger thread and rounding to two decimal places. It is also worth noting that BA worked with metric values when designing the BA thread table, long before this became the standard. The flank angle of 47.5° also makes the BA thread a special case.
As you can easily see from the BA thread table, the thread designation for the BA thread also works differently to most other threads. Instead of specifying the diameter or pitch, BA threads are simply numbered from 0 to 25, with BA 0 having the largest diameter according to the BA thread table and BA 25 having the smallest diameter at 0.25 mm. Although it is a British thread, the pitch is also given in mm in the BA thread table. However, we have added the corresponding conversion to inches to our BA thread table.