Is there somewhere that covers how to fix nod in the Primo toolhead? In surfacing with an 18mm bit I do notice I have a small amount.
Is it the core, ie: is the Z axis not square, or is it the tool mount?
If it’s the core that isn’t plumb, check that your bearings are engaged on all 4 of the core bearing holders, tighten as necessary, check again.
If it’s the spindle not sitting plumb, it’s probably easier to shim it subtly. There is an STL file for a thing to check the spindle for squareness. I think it’s just a sleeve that goes over the bit with a pointer that rotates around at a fixed radius.
I guess I’m wondering how tightening the bearings affects squareness and if it’s documented.
Spindle fixes don’t soundfun 
More likely to introduce error every time I unbolt the front plate on the mount.