A NEMA 8 (20mm) stepper motor integrated with the planetary gearbox (gear reducer) is a good solution to the application that requires high torque or low speed. Frankhumotor provide a lot of NEMA 8 gear reduction stepper motors with different gear ratio and body lengths as below. The speed of NEMA 8 series planetary geearbox stepper motors can be from 0-250rpm within max torque output 0.9Nm. Of course, the output shaft (gearbox shaft) diameter and length (gearbox shaft) can be customzied to meet your machine's parameters.
