British Association screw threads

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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. glass grinders

They are unusual in that they were probably the most "scientific" design of screw, with a basis in metric size (the 1 mm pitch and 6 mm 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 worldwide metrication by about a century.

The British Standards Institution 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 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.

BA thread sizes

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 . The major diameter is given by 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).

BA

Outer

Diameter

Threads

per inch

Threads

per mm

Tap drill

0

0.2362 in / 6 mm

25.38

1

5.1 mm / 7 gauge

1

0.2087 in / 5.3 mm

28.25

1.112

4.5 mm / 16 gauge

2

0.1850 in / 4.7 mm

31.35

1.234

4.0 mm / 21 gauge

3

0.1614 in / 4.1 mm

34.84

1.372

3.4 mm

4

0.1417 in / 3.6 mm

38.46

1.514

3.0 mm

5

0.1260 in / 3.2 mm

43.10

1.697

2.65 mm / 37 gauge

6

0.1102 in / 2.8 mm

47.85

1.884

2.3 mm

7

0.0984 in / 2.5 mm

52.91

2.083

2.05 mm / 46 gauge

8

0.0866 in / 2.2 mm

59.17

2.330

1.8 mm

9

0.0748 in / 1.9 mm

64.94

2.557

1.55 mm

10

0.0669 in / 1.7 mm

72.46

2.853

1.4 mm / 54 gauge

11

0.0591 in / 1.5 mm

81.97

3.227

1.2 mm

12

0.0511 in / 1.3 mm

90.91

3.579

1.05 mm

External links

Bolt Science writeup.

Early history of non-metric bolts including BA and Whitworth.

Table of thread dimensions.

Recommended ISO substitutions.

Spanner Jaw Sizes Additional background information and spanner jaw size table.

International Thread Standards

Categories: Drilling and threading | Mechanical standards | ScrewsHidden categories: Articles lacking sources from July 2009 | All articles lacking sources