British Association screw threads

British Association screw threads, or BA screw threads, are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.

They are unusual in that they were probably the most "scientific" design of screw, starting with 0BA at 6.0mm diameter and 1.0mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size. They then spoiled this by rounding to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch. This anticipated worldwide metrication by about a century. The design was first proposed by the British Association in 1884 with a thread angle and depth based on the Swiss Thury thread, it was adopted by the Association in 1903.

The Thury thread was different in that it went both positive and negative all the way up to a size of -20 which was 75.2mm diameter by 8.23mm pitch. The Thury numbers were rounded to three significant figures. The Thury thread form had the crests rounded at 1/6p and the roots rounded at 1/5p so the thread angle was close to 47.5 degrees but not exactly. This was simplified in the BA thread definition by defining the thread angle to be 47.5 degrees exactly and the thread form to be symmetrical with a depth of 0.6p.

The British Standards Institution recommends the use of BA sizes in place of the smaller British Standard Whitworth (BSW) and British Standard Fine (BSF) thread screws (those below 1/4"). Generally, the 0BA size was dropped in place of 7/32" BSF in assemblies that included larger fasteners, however, in smaller equipment that was primarily electronic/electrical the 0BA size would typically be used in place of the BSF or BSW screw where it was the largest size required.

BA threads are still used in some precision instruments, such as optics and moving-coil meters, relays etc. A 2BA thread is used to connect the head of a dart to its shaft — as such, it is one of the few common uses of this thread in North America.

2BA threads are commonly found in the old imperial BESA (British Engineering Standards Association) conduit boxes in the UK which are still in use (but not installed) today.

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