Thinking in terms of a mini lathe, anyone wanting to make just small parts can cut threads using Taps and Dies, unless they want something that is a "Special".
Taps and Dies for non standard threads can be obtained. Tracy Tools carry a wide range.
If you think that the larger sizes are going to overload the motor, make a Mandrel Handle and use that to rotate the chuck by hand.
Also extremely useful when working up to a shoulder, or in a blind hole, to avoid spoiled work or broken taps..
In which case the changewheels will more likely be used to provide different feed rates.
The finest feed will be ,obtained, using 20: 80/20 :80.
With a 1.5 mm Leadscrew this will give a feed of 0.09375 mm / rev.
With a 16 tpi Leadsrew this will give a feed of 0.00391" per rev.
Howard