Re: Universal gantry idea - here is something I came up with

good points. It looks like the slate will expand about .001 inches
over the four foot length with a +10 degree C change in temp,
according to that website. I am pretty sure steel expands a bit more
than that. Actually, it makes the idea of clamping the rails to the
slate attractive, because the clamping action would perhaps allow for
this expansion.

I took some measurements of the slate, and it does not appear to be
as uniform as I first thought. I would say it's thickness is +/- .01
inches and I found the surface to be slightly concave. Using a
precision ground 1 inch diameter section of 3 foot shafting held
against the slate across the center, I measured a gap in the center
of about .008 inches. Perhaps this is why I can never sink the balls
in the pockets? :-)

Of course all of this can be shimmed out, but what a pain.

Rick