A lot of the drawings I have show BA screw sizes. My natural inclination
would be to convert to the nearest metric size. Are there any practical
reasons for investing in BA taps & ties in addition to my metric ones?
Only the fact that there are a greater number of sizes available in the
With metric, from 6mm down the standards are only 6, 5, 4, 3.5, 3, 2.5,
2, 1.6, 1.2 and 1 (and 3.5, 2, 1.6, 1.2 and 1 are not easily available)
whereas the BA series (shown as their metric nominal diameters) are 6,
5.3, 4.7, 4.1, 3.6, 3.2, 2.8, 2.5, 2.2, 1.9, 1.7, 1.5, 1.3, 1.2, 1, 0.9
and 0.79 (again 1.2, 1, 0.9 and 0.79 are not readily available).
In other words 5 metric sizes compared to 13 BA.
The other thing to consider is are the materials on your drawings that
threading metric or imperial. eg 5BA is the common size used on
1/8 rod, 7BA on
3/32 and 2BA on 3/16.
If you were using these imperial sizes from the drawings the nearest
thread would be a bit slack or tight. If you start changing
threads to metric
then you will need to alter the off the shelf stock
sizes, which will entail
buying metric fasteners, metric reamers, etc.
BA threads in the standard sizes like 8,6,4 and 2 are generally available, and
model engineering places still keep the odd sizes.
Longer-term they will die out through lack of use. We still keep some sizes at
the factory as transformer laminations are still coming through with BA cleance
We did a bulk buy of 2BA CH HD recently, and have previously bought bulk lots of
4BA as well.
The 3BA screws that I asked about recently, turned up this morning, many thanks
to Colin Docherty!
Peter & Rita Forbes