Which brand and model of lathe are we talking about?
I think that the dash means any number between the two given,
with some lathes having numbers in the range 1 through 4, and others in
the range 1 through 8. (I've also seen some with the numbers only being
1 and 2.)
There is typically a '|' between the numbers, and for threads
which are an even multiple of the pitch of the leadscrew, you can close
the half-nuts on any one of those.
The "one or another" are typically numbers half-way around the
dial, e.g. for a 1-4 range, *only* even, or *only* odd.
IIRC, (without going down to verify it), my 12x24" Clausing has
1-4 as the numbers.
To be sure, with your particular lathe, set up for the threads
in question, set the tool depth to only put a fine scratch in the
surface (using the '1' number), and then try various combinations to see
which ones won't add a second scratch between the lines of the first
Start with odd numbers (since you first used a '1'), then try
the even numbers, and if things are still fine, try any line, with or
without a number.
It helps to know what the pitch of your leadscrew is, so you can
tell which threads are an integer multiple of the leadscrew pitch.
Typically X-1/2 type threads will be only the same number.
Metric threads on an imperial leadscrew are "Forget about the
half nuts and the threading dial -- keep them engaged until your thread
is done", running the lathe backwards to get to the position for cutting
the next pass. Same with imperial threads on a metric leadscrew. (This
presumes that you have the necessary gears to set up for cross-system
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Actually, the indicator doesn't indicate what you think it does. What it
indicates is the phase of the shaft so that yo can keep the correct phase of
the cutter position relative to the cuts that you have made before.
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