Does the load of the tool and the Z-assembly hang only on the coupler and the Z-stepper motor’s bearing?
Wouldn’t it be advantageous to use the bearing in the top motor mount to take the load? Two nuts should do the job.
Maybe an additional lid to press the outer bearing ring into its bed? Does anything speak against that?
I understnad that the 608s are basically radial-bearings, but they can take some axial load too.
Pure axial bearings start at 10mm inner diameter.
Ryan’s kit comes with a 608 bearing for just the purpose you mention. There are no nuts, but the bearing sits in the z motor mount and the rigid coupler rests on it, taking the load off the stepper.
Thanks, Karl. Did not realize that, having seen a picture somewhere where a small gap was left between coupler and bearing. I’ll give that a try. So, for the opposite direction, the influence of gravity is taken as being high enough? Otherwise, the nutlock wuld also need some sort of lock for the downward motion, I guess. Kind of safety zone…
Yup the coupler should rest on that inner bearing in the z motor mount.
As strange as is seems when you are milling the bit actually wants to pull itself through the material so , yes, all I was really concerned with was that one direction (same as gravity).
Imagine what would happen if gravity would work in the other direction… 
Thanks for the explanation. And yes, the pulling force of normal end mills is known. Less in mills with straight cutting edges, and there are even end mills with a thread that pushes the chippings downwards.
BTW, I made the coupler, the nutlock, and the IE universal tool mount from PETG - that’s another class of definition, hardness (yet elastic) and impact resistance, compared to PLA.
Cheers,
U.