Daniel,
Yes the rotor core is made in one setup on the mill. The blank is turned onthe lathe to prepare the bottom diameter and that is used to hold it for all the other operations.
The whole core is roughed with a larger cutter and then finished with a 2mm slot drill. even the holes are drilled using the slot drill, the backlash on the mill is good enough to make even 2.5 or 3mm holes acceptable.
I haven't really generated 4th axis GCode. The GCode is "per side" in subroutines and I just rotate to the correct side, then call the subroutine, then rotate again and then call a different subroutine etc.
The spindle is not through bored. The housing is made from aluminium plate. I thought about making the spindle "universal" but decided to dedicate this to horizontal. It is in quite a nice position. Even at the extents of travel the chuck of the 4th axis only just touches the main spindle, so it is fairly "safe" and does not get in the way for any other operations, I can leave it on and don't need to set it up, it just sits there ready for use on a different machine offset.
The belt I used is Syncroflex T5 (5mm pitch) I have not tried to measure the backlash, there certainly doesn't "feel" to be any but I am sure there is some.
Thanks
Simon