"Michael Koblic" wrote in
:

I was looking at a Rong-Fu clone today (RF30). Unlike the dovetail
column mills that I am used to this one has a round column. The head
can be moved up and down and swivelled 360 degrees, however one loses
the X-Y position doing it. The spindle (quill, really) has a maximum
range of movement of only 5".
snip
What am I missing? How are these issues dealt with on these machines?


You aren't missing anything. You have to find a way to re-register after
moving the head on the column. Some people modify the mill to have the
head keyed to the column. I read somewhere (HSM probably) Someone drew a
plumb line on the wall opposite the mill. They had a laser pointer mounted
on the mill head with the laser dot pointed to the line. When the head was
moved the alignment was good as long as the dot remained on the plumb line
on the wall (and the mill level). It's suprisingly accurate if you do the
math.