BA threads. Why the tpi?

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Delayed response, due to having misplaced my reference book:

MichaelG

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The following are the forms of threads adopted in the Swiss system :

(a) For screws made by the screw-plate, and intended for use in wrought-iron or brass :

1st. The depth of the thread is 3/5 the length of pitch.

2nd. The top of the ridge is formed by a circular arc, the radius of which equals 1/6 of the pitch.

3rd. The space between two consecutive ridges is formed, at the side next the core, by a circular arc, the radius of which equals 1/5 of the pitch.

4th. To complete the generating figure of the thread, the two circular arcs, exterior and interior, are joined by straight lines tangential to the arcs, and which consequently form between them an angle of about 47 1/2 deg. The space between two ridges is therefore a little greater than the solid ridge.

5th. The positive generating figure of the thread should be considered as being the result of the figure above defined, slightly modified by the work of compression which occurs in threading with a screw plate. It is thus that the circular arcs are changed into parabolic curves more suitable than the circular arc itself.

(b) For screws made by means of the lathe, and intended for use indifferently in steel, in wrought-iron, in brass, and in cast-iron the outline of M. Steinlen, of Mulhouse, is recommended. […]

Screws and Screw-Making

The Britannia Company, Colchester. 1891

Michael: Thanks for the notes. Later this evening I might work out the exact flank angle. Of course the next question is why on earth they chose different radii for the crest and root. Ah well, you know what they say about committees; may be the BA thread system is the equivalent of a camel.

Andrew