Can't see enough in your picture, but newer lathes typically have charts for threads per inch, metric pitch, module threads, and diametral pitch.
http://www.wisegeek.com/what-is-diametral-pitch.htm I often see people talking about "diametrical" pitch, but I do not think there is any such thing...
from Wiki (
https://en.wikipedia.org/wiki/Gear): Module or modulus, m
Since it is impractical to calculate circular pitch with
irrational numbers, mechanical engineers usually use a scaling factor that replaces it with a regular value instead. This is known as the
module or
modulus of the wheel and is simply defined as
m = p / π {\displaystyle m=p/\pi } where m is the module and p the circular pitch. The
units of module are customarily
millimeters; an
English Module is sometimes used with the units of
inches. When the diametral pitch, DP, is in English units,
m = 25.4 / D P {\displaystyle m=25.4/DP} in conventional metric units.
The distance between the two axis becomes
a = m ( z 1 + z 2 ) / 2 {\displaystyle a=m(z_{1}+z_{2})/2} where a is the axis distance, z1 and z2 are the number of cogs (teeth) for each of the two wheels (gears). These numbers (or at least one of them) is often chosen among
primes to create an even contact between every cog of both wheels, and thereby avoid unnecessary wear. and damage. An even uniform gear wear is achieved by ensuring the tooth counts of the two gears meshing together are
relatively prime to each other; this occurs when the
greatest common divisor (GCD) of each gear tooth count equals 1, e.g. GCD(16,25)=1; If a 1:1 gear ratio is desired a relatively prime gear may be inserted in between the two gears; this maintains the 1:1 ratio but reverses the gear direction; a second relatively prime gear could also be inserted to restore the original rotational direction while maintaining uniform wear with all 4 gears in this case. Mechanic engineers at least in continental Europe use the module instead of circular pitch. The module, just like the circular pitch, can be used for all types of cogs, not just
evolvent based straight cogs.
(Bob) Have I helped or hindered? 8^)
You may never have a need to use this information, and all lathes with the proper change gears can cut them, but, lacking the charts, you would have to figure it out for your lathe. My advice is to simply ignore everything except threads per inch and metric pitch, even perhaps cover them so you do not use them by mistake (I did that once.)
