British Association screw threads, orBA screw threads, are a set of smallscrew 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 6.0 mm diameter and 1.0 mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size. They then rounded to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch. This anticipated worldwidemetrication by about a century. The design was first proposed by theBritish Science Association in 1884[1][2] with athread angle and depth based on the SwissThury thread,[3] 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 75.2 mm diameter by 8.23 mm pitch (p). The Thury numbers were rounded to three significant figures. The Thury thread form had the crests rounded at1/6p and the roots rounded at1/5p so the thread angle was close to 47.5° but not exactly. This was simplified in the BA thread definition by defining the thread angle to be 47.5° exactly and the thread form to be symmetrical with a depth of3/5p.
TheBritish Standards Institution recommends the use of BA sizes in favour of the smallerBritish Standard Whitworth (BSW) andBritish Standard Fine (BSF) thread screws (those below1⁄4 inch).[4]: 4 Generally, the 0BA size was dropped in place of7⁄32 inch 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 metal barrel of adart to its shaft — as such, it is one of the few common uses of this thread in North America.
In the UK, 2BA threads are commonly found in the old imperialBritish Engineering Standards Association (BESA)conduit boxes, and 4BA threads in light switch and socket back boxes which are still in use (but not installed) today.
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 soK-BA has a pitch ofp = 0.9K[4]: 12 rounded to two significant figures in mm. The major diameter is given by 6p1.2,[4]: 12 rounded to two significant figures in mm and the hex head size (across the flats) is 1.75 times the major diameter.
BA sizes are specified by the followingBritish Standards:
The angle of the thread is 47.5°[4]: 2 and the depth of thread is 0.6 times the pitch with rounded tops and bottoms. Thus, the shortening at the crest and root is given by 0.26817p with a radius of 0.18083p at both crest and root.[4]: 2 The 1884 report specifies a radius of2/11p ≈ 0.181818p but this was an approximation to the correct value.
The pitch diameter of the thread is simplyOD − 0.6p and the minor diameter isOD − 1.2p.
The thread angle is different from that used by Whitworth (55°), US Unified threads (60°) and ISO Metric (60°) so BA fasteners are not properly interchangeable with Whitworth or metric ones even when the pitch and diameter are similar enough that they can be screwed together (e.g., although 0BA appears similar to M6×1mm, the male and female surfaces do not mate properly for bearing a load). Standard BA sizes are defined between 0BA and 22BA. The even sizes are much more common and the very small sizes are not used very often and other standards tend to be used (e.g., Norm Horlogere Suisse).
| BA | Thread pitch | Outside diameter | Threads density | Outside diameter | Spanner | |
|---|---|---|---|---|---|---|
| (mm) | (mm) | (tpi) | (in) | (mm) | (in) | |
| 0 | 1.00 | 6.00 | 25.40 | 0.2362 | 10.5000 | 0.4134 |
| 1 | 0.90 | 5.30 | 28.22 | 0.2087 | 9.2750 | 0.3652 |
| 2 | 0.81 | 4.70 | 31.36 | 0.1850 | 8.2250 | 0.3238 |
| 3 | 0.73 | 4.10 | 34.79 | 0.1614 | 7.1750 | 0.2825 |
| 4 | 0.66 | 3.60 | 38.48 | 0.1417 | 6.3000 | 0.2480 |
| 5 | 0.59 | 3.20 | 43.05 | 0.1260 | 5.6000 | 0.2205 |
| 6 | 0.53 | 2.80 | 47.92 | 0.1102 | 4.9000 | 0.1929 |
| 7 | 0.48 | 2.50 | 52.92 | 0.0984 | 4.3750 | 0.1722 |
| 8 | 0.43 | 2.20 | 59.07 | 0.0866 | 3.8500 | 0.1516 |
| 9 | 0.39 | 1.90 | 65.13 | 0.0748 | 3.3250 | 0.1309 |
| 10 | 0.35 | 1.70 | 72.57 | 0.0669 | 2.9750 | 0.1171 |
| ⋮ | ||||||
| 25 | 0.072 | 0.25 | 352.78 | 0.0098 | 0.4375 | 0.0172 |