Workshop Hints
BA Threads
The British Horological Institute has archived and edited the following
from e-mails sent to the Clock/Clockers
mailing lists on the Internet. The information here does not necessarily
indicate a method approved by the BHI, we are only publishing this digest
so that others can decide for themselves whether the methods listed below
will suit them.
From: Tony Harrison-Smith, Randolph Lee, Eliot Isaacs FBHI
Way back in the 1890's, the British Association for Advancement of Science (BA)
realised there were no English screw threads standards for small electrical and scientific
equipment, so they proposed the BA system loosely based on the Thury threads already in
use in Europe (hence the metric sizes). They quickly caught on and rapidly became the
standard in the UK. They are good news - thread angle of 47-1/2 degrees (good for holding
in thin work like clock wheels); rounded tips and crests (less power needed to cut, less
risk of tearing threads, less wear on the tools); enormous range of sizes (0 - 26BA).
Before you rush out and order a full set, a few useful tips: Hardly anyone ever uses
the odd numbers - most engineers get by with 2, 4, 6 & 8 BA. Watchmakers tend to add
10, 12 & 14 BA. If you stick with the even numbers, then the tapping drill for one
size becomes the clearance drill for the next smaller. And, if you still use number drills
instead of the newfangled Napoleonic upstarts, you can use the 'rule of 4' i.e.,
2BA Tap size 24
4BA Clearance size 24 Tap size 34
6BA Clearance size 34 Tap size 44
8 BA Clearance size 44 Tap size 54
Not exactly optimum sizes, but close enough for most everyday use, and saves having to
look up tables each time.
BA Taps and dies are readily available from 'The Tap & Die Co' 445 West Green Road,
London N15 3PL, UK. Tel +44 (0)181 888 1865 or Fax +44 (0)181 888 4613. They carry massive
stocks of these taps and dies as well as other cutting tools and are highly recommended
Thread |
TPI |
Major Dia. |
Pitch |
Tapping Drill |
0 |
25.4 |
6.0mm |
1.00mm |
No. 5 |
5.1mm |
1 |
28.2 |
5.3mm |
0.9mm |
No. 14 |
4.5mm |
2 |
31.3 |
4.7mm |
0.81mm |
No. 22 |
4.0mm |
3 |
34.8 |
4.1mm |
0.73mm |
No. 29 |
3.45mm |
4 |
38.5 |
3.60mm |
0.66mm |
No. 31 |
3.00mm |
5 |
43.1 |
3.20mm |
0.59mm |
No. 36 |
2.65mm |
6 |
47.9 |
2.8mm |
0.53mm |
No. 41 |
2.30mm |
7 |
52.9 |
2.5mm |
0.48mm |
No. 45 |
2.05mm |
8 |
59.1 |
2.2mm |
0.43mm |
No. 49 |
1.80mm |
9 |
65.1 |
1.9mm |
0.39mm |
No. 52 |
1.55mm |
10 |
72.6 |
1.7mm |
0.35mm |
No. 54 |
1.40mm |
11 |
81.9 |
1.5mm |
0.31mm |
3/64" |
1.20mm |
12 |
90.7 |
1.3mm |
0.28mm |
No. 57 |
1.05mm |
13 |
102.0 |
1.2mm |
0.25mm |
N0. 60 |
0.975mm |
14 |
110.0 |
1.0mm |
0.23mm |
No. 66 |
0.775mm |
|