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British Association 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, with a basis in metric size (the 1mm pitch and 6mm diameter of 0BA) and with a mathematical relationship between the 'number' (e.g. 2BA with K=2) and the corresponding pitch, major diameter, and then spanner size. Though originally defined in terms of the imperial units then in use in the United Kingdom, this was deliberately set to be exact multiples of the metric system units, rounded to the nearest thousandth of an inch. This anticipated world-wide metrication by about a century.

The British Standards Institute recommended the use of BA sizes in place of the smaller BSW and BSF screws (those below 1/4"). Generally, the 0BA size was dropped in place of 1/4" 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 1/4" 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.

The pitch of 0BA is 1 mm and the pitch of each higher numbered thread is obtained by multiplying the pitch of the lower number by 0.9 so K-BA has a pitch of $p=0.9^K$. The major diameter is given by $6p^\left\{1.2\right\}$ and the spanner jaw size as 1.75 times the major diameter.

The shortening at the crest and root is given by 0.2681683p with a radius of 0.1808346p at both crest and root. The angle of the thread is 47.5 degrees which is different from that used by Whitworth (55 degrees) and ISO Metric (60 degrees) so BA fasteners are never interchangeable with Whitworth/Metric ones even when the pitch and diameter seem similar (except 0BA - 6 mm) . Standard BA sizes are defined between 0BA and 22BA. The very small sizes are not used very often and other standards tend to be used e.g. NHS.

* [http://www.boltscience.com/pages/screw5.htm Bolt Science] writeup.

* [http://www.jag-lovers.org/xk-lovers/library/whitworth_system.html Early history] of non-metric bolts including BA and Whitworth.

* [http://shopswarf.orcon.net.nz/ba.html Table of thread dimensions] .

* [http://www.sizes.com/tools/thread_BA.htm Recommended ISO substitutions] .
* [http://www.baconsdozen.co.uk/tools/spanners.htm#chart Size comparisons Whitworth/BSF/BA/AF and mm]

* [http://www.sat.dundee.ac.uk/~psc/spanner_jaw.html Spanner Jaw Sizes] Additional background information and spanner jaw size table.

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