In such a dual-column *machine, of Super Shop scale, might the tubular
bed ways be slotted full length allowing access to a nicely shielded
lead screw, without them springing open with an increase in diameter?
It's a question of residual stress in the steel.

The residual stresses you could probably compensate for. It's the
reduced stiffness under load that I'd be worried about. Under
bending, I'd expect the slot to open and close as loads are applied.
If there are twisting loads, then either side of the slot may move in
opposite directions axially, giving much less resistance to that sort
of load. Take something like a paper towel core, and expriment with
bending and twisting with and without a slot up the side, I think you
will find the differences are significant, even if it retains it's
shape (which is what I think your biggest worry is with the residual
stress).

I would not be supprised if to get an equivalent stiffness with a
slot, that you'll have to double or more the wall thickness of an
unslotted column. And of course, any deflections in the columns may
also cause sliding assemblies to jam at the most inopportune times...
--Glenn Lyford