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

Ian Wright has a page on his web site which lists a fairly comprehensive table of screw threads, which can be downloaded.

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