selecting linear rails ?

Hi all,

I'm working up the design of a small milling table. And want to move
the milling head up and down on a couple THK or similar type rails and
blocks.

Looking at the forces, there appear to be 5 basic forces listed. the
simple compression and it's reverse would be just carrying a load with
the rails mounted on the bottom and slides on top, or rails mounted on
top and the slides and load hanging.

The other three forces are more of a twisting force.

Looking at the ratings, the load compression forces are very high, and
depending on the rails and blocks, the twisting forces can be just a
small fraction.

using 2 rails and 2 blocks per rail would seem to eliminate most of
the twisting forces. Is this correct ? can I just ignore those when
using a dual rail/dual block (quad block?) setup ?

Can anyone offer a link to an introduction of how to select rails ?

also, if one has a milling cutter 12 inches off the rails and is
cutting, how much force would the side of the cutter see ? This
force would offer one of the values in the calculation needed to
select the rails.

Dave