What do you really get for a $350 desk top engraver ?

andypugh wrote:

I think we would need a better macro photo to tel if that was a fatigue failure or not, but it leads me to be rather suspicious of the motor-to-shaft alignment and mounting. Is there any type of flex-coupling there?

I couldn't make out any beach marks on the surface. A macro would be interesting, as would pulling the bearing shield/seal and checking for signs of heat or abnormal conditions.

Wouldn't misalignment between motor and driven component tend to break outboard of the bearing?  That's assuming the bearing didn't come with lots of negative preload from the factory, of course.

andypugh wrote:

The moment is greater further from the shaft end, and a ball bearing isn't particulary stiff against torsion. Also the step that the bearing butts against, if machined poorly, might be a stress raiser. 

Why do I get the feeling this is not the first time you've looked at failed (or suspect) industrial components?  Spend any time involved with, or doing, NDT/NDE?

I think Andy is on the right path. I can't think of anything other than shaft misalignment that would cause a stepper motor shaft to snap like that.