It depends on what kind of machine you're building and what
it's intended purpose is.
Mine is right for the design of machines I build. Reason is,
the further the bit is extended, the more the bit can act as
a lever. Leverage is what causes flexing or bit breaking in
And that's why I always suggest use of layers...like MDF...to bring
the work closer to the bottom of the Z axis.
It can work like this. If you write your name with a pencil and hold
the pencil normally, it's easy. But the further you hold the pencil
from the writing end, the harder it gets to write clearly.
It can also depend on lots of other things. Like your choice of rails.
The uprights. How the X rails are oriented. The backing for your
If you want the bearings to carry the weight of the router, it's mount,
whatever fixtures it has, and reach down a long way, then
precision linear rails like Hiwins will handle it all better.
With something like round shafts...that can be the cause of binding
with the extra weight. Again, depending on how those bearings are
oriented....and/or what kind they are.
The further a Z needs to reach down, the more rigid and stable the
design needs to be.
I've built them both ways many times. For the small machines I make
now, the way I do it is the best way for that design....and what it's
supposed to do.