> My recollection is that for ANY thred you can re-engage the
> nut on the
> same point that you were on the dial (eg if you were first
> engaged on 1
> - you can always re-engage on 1).
You got me wondering, so I looked it up. On a lathe with Imperial threads
on the leadscrew, engaging on 1 works for any thread that has an integer
number of threads in 2 (or 4 for some lathes) inches. But metric (and BA,
which is what I was thinking of when I said Whitworth before) threads
typically don't have an integer number of threads in 4 inches, so the
standard Imperial threading dial cannot be used when cutting those threads.
Imagine the lathe stopped, with a long threaded bar between centers. Start
with the half nuts engaged, the threading dial on '1', and the tool bit
exactly in the thread groove. Now imagine moving the carriage to the right
until the dial comes back on '1' and closing the half nuts again. The
carriage has moved some number of inches (2" on my 10" Atlas). If, for
example, your bar has 11.5 tpi, then the tool will be exactly 23 threads
away from the original position and still at a thread groove. (Assuming
you've taken up the slack correctly and so on.) But if, for example, your
bar has 25.4 tpi, the tool will now be 50.8 threads away from the starting
position, and not lined up on the groove.
The rules are different for a metric leadscrew, of course, but similar
limitations apply. From what I found on-line, most metric lathes come with
3 different gears for the thread dial (and still don't cover BA threads).
Here's a neat little device that apparently includes the gears for both
metric & imperial on the same leadscrew
http://www.dgrdesigns.co.uk/metradial.html
But it still doesn't appear to cover 4 BA (38.46 tpi) <g>
-- Randall
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