BA, BSA

British Association screw thread series

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In 1884 the British Association for the Advancement of Science adopted a thread form and series, primarily for use in precision equipment. It was inspired by one used in the Swiss watch and clock industry, and was formerly sometimes called the Swiss Small Screw Thread System; but is now just referred to as the BA series. Like the Whitworth thread, it has a rounded root and crest, but the included angle is 47.5° and the radius was originally 2/11 (0.1818…) times the pitch, but at some later point was changed to 0.18083. The gauge numbers and pitch are related, roughly, by pitch = 0.9 to the power of the gauge number. 

thread form in silhouette

Despite the date of origin and the British sponsorship, the BA is a metric series. Not only is it based on the meter, but the thread frequency is specified in terms of round numbers in the pitch sequence, with threads per meter allowed to fall where it may.

The British Association thread played a role similar to that of the ASME series in the United States, that is, it filled in sizes below a quarter-inch. The sizes most used were the even-numbered ones between 0 and 10, inclusive. The very small sizes, from 17 up, were rarely used. The British Standards Institution (BSI) discouraged the use of #0 BA in favor of the 732″ BSF.

The BSI recommended this series replace the BSW and BSF series for all screws smaller than ¼ inch, and deprecated the use of the odd-numbered gauges. The ¼ inch BSF was retained and the 0 gauge BA discontinued (the two were too easily confused). 

In 1966 the BSI declared the BA screw thread obsolete; its place was taken by ISO screw threads.

Gauge
Number
Major
diameter
(mm)
Threads
per inch
(approx)
Pitch
(mm)
Tap
drill
25 0.25 363 0.070
24 0.29 317 0.080
23 0.33 282 0.090
22 0.37 254 0.098
21 0.42 231 0.11
20 0.48 212 0.12 80
19 0.54 181 0.14 78
18 0.62 169 0.15 76
17 0.70 149 0.17 75
16 0.79 134 0.1900 74
15 0.90 121 0.2100 72
14 1.0 110 0.2300 70
13 1.2 102 0.2500 65
12 1.3 90.7 0.2800 61
11 1.5 82.0 0.3100 56
10 1.7 72.6 0.3500 54
9 1.9 65.1 0.3900 53
8 2.2 59.1 0.4300 49
7 2.5 52.9 0.4800 46
6 2.8 47.9 0.5300 43
5 3.2 43.0 0.5900 37
4 3.6 38.5 0.6600 31
3 4.1 34.8 0.7300 29
2 4.7 31.4 0.8100 23
1 5.3 28.2 0.9000 17
0 6.0 25.4 1.0000 10

sources

Randall C. Brooks.
Standard screw threads for scientific instruments. Part II: The British Association Screw Gauge.
History and Technology, vol. 6, pages 45-59 (1988).

British Standard 93-1919 (formerly CL 7271). Screw Threads, British Association, with Tolerances for sizes No. 0 to 15 B.A.. An addendum was issued in August 1940.

British Standard 93:1951. 

The first report of the British Association for the Advancement of Science committee on small screw threads.

The second report.

resources

In the United States, BA screws, helicoils, and taps and dies are available from www.britishfasteners.com

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