The stepper is from sherline, gearhead is from junkyard (hsc electronics). It looks funny (with the crooked screws and exposed shaft) but quite robust. 36x reduction provides more than enough torque (stronger then the default drive for sherline lathe, but I don't think it can self destroy in case of a jam). Maximum spindle speed is over 10 rpm. It has a more than 3 degrees backlash.
I have no idea how to measure linearity error under various loads. In front a test cut is seen.