It can be calculated, and the probable best reference would be
_Machinery's Handbook_ -- but for metric threads, no single gear will do
the job. The threading dials on metric lathes have a sliding gear
cluster connected to the dial, and the threading chart tells you which
gear to use for each thread pitch -- so it is a bit more complex than it
at first seems.
Aside from that, you either have to cut each gear with sloped
teeth to match the lead angle of the leadscrew, or make a mounting for
the dial holder which holds the axis of the gear cluster parallel to the
lead angle (and thus tilted somewhat relative to vertical on the lathe).
If you want an example, and have access to the old South Bend
_How to Run a Lathe_ book, turn to the section on metric threading, that
is page 85 (in my 1944 edition (edition 50)), and you will see the
drawing of the gear cluster on the bottom of the page. For whatever
pitch leadscrew that one has, the gears are 26, 28, 30, and 33 teeth.
The dial itself is also more complex, with lots of marks with different
letters from A through D, and the threading chart shows which letters
may be used with which gear step for which pitch. There is not a clear
enough close-up of the dial to make it totaly clear just what the
division is, but it looks somewhat irregular.
So -- this is a bit more involved a project than it would be for
an imperial leadscrew and imperial threads.
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