The round column machines worry me. The $1395 with DRO Griz O758Z has a
dovetail Z axis, rectangular column which will be dramatically stiffer from the
side loads from milling than the round column. R8 spindle, variable speed
on spindle, two down feed power feeds, tapered roller bearings. They are
far stiffer than ball bearings, when adjusted properly. Tapered rollers have
line contact, balls have point contact. Flex under load is dramatically less
for tapered rollers.
Large quill diameter, 2.36" again important in bending. Key to understand
that the bending of a circular beam (quill or circular column) is proportional to
the diameter to the fourth power. Go from say a 1.5" diam to a 2" diam, and it is over
three times as stiff in bending. Go from 3" round (my estimate of the HF 33686, and
the 4" square column (my estimate of Griz 0758Z) and bending stiffness of the
square (assuming solid for both, not true, but gives and index of comparison
for the two section shapes) 4" square will be about 5 times as stiff in bending as 3"
round, given proportional wall thicknesses. Stiffness is pretty important, chattering
happens due to low stiffness.
This structural guy sees stiffness of structure everywhere I look, and it may not
be very obvious how dramatically stiffness changes with dimensions and cross
section shapes on critical machine components.
I haven't looked as closely as you have, no doubt, but I see a lot of important features
on the 0758Z. Worth a look.
Generally, trying to find a square,rect column unit is worth looking for, too. And tapered
roller bearings. Big quill diam, big column dimensions.
Whatever you get, you will make the most of it, and enjoy it.
Bill
Bill