Some folks are confused by which marks to use on the threading dial on their
(assumed Imperial) lathe. Of course, you're always safe restarting on the
same dial mark on which you started but that means you'll spend a lot of time
'waiting for your number to come up'.
It's pretty easy to think through. Most dials have four numbered marks
labeled, unsurprisingly, 1-4. Between these marks are smaller, unnumbered
[Aside: A consequence of the four numbered mark dial is the fact that the
gear that meshes with the feed screw will have a number of teeth which is four
times the tpi of the feed screw, i.e., an 8 tpi lead screw will have a 32
tooth gear on its thread dial. This is worth knowing if you intend to build a
threading dial for a lathe that lacks one. Since there's no load on this
gear, a perfect match to the helix angle of the lead screw is not needed. Any
old gear with the right number of teeth can be pressed into service.]
The numbered marks almost always correspond to a carriage movement of one
inch. (However, you should confirm this by actual measurement on your lathe.)
By deduction, the unnumbered lines must then correspond to a carriage movement
of one-half inch.
Now, suppose I'm cutting an even-numbered thread (e.g., 32 tpi). If I move the
carriage by one-half inch, the tool will reenter the thread perfectly. If I
move the carriage one inch, the tool will also reenter. Therefore, I can
reengage the half-nuts on ANY line on the threading dial.
If I'm cutting an odd-numbered thread (e.g., 13 tpi), the tool will not reenter
the thread if I move one-half inch - I have to move one inch. Therefore, I
must reengage the half-nuts on ANY NUMBERED LINE on the dial. (If I started
the thread on an unnumbered line, I would then reengage on unnumbered lines but
that is generally too difficult to remember in the 'heat' of thread cutting and
should be avoided.)
If I'm cutting a half-fractional thread (e.g., 11-1/2 tpi), the tool will only
reenter the thread every two inches. Therefore I must reengage the half-nuts
on ONLY THE ODD NUMBERS ON THE DIAL (1 and 3) or on the even numbers (2 and
4). It's a good idea to decide on one of these options and always stick with
it. I always start the thread on 1 and then use either 1 or 3 to reengage.
Rarely, a lathe can cut a 1/4 thread (e.g., 1-1/4 tpi). This thread will only
repeat every four inches so, in cutting such a thread, one would reengage the
half-nuts on the same mark one used to start the thread.