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Funny you should mention driving the ganty from one side ,.. I tried
this and noticed that it was more efective to drive it down the sides
or single down the middle. What one has to keep in mind is the lever
action. Driving down the center halves these forces as opposed to a
single drive at one side. Another problem with single
sided drive is with the twisting of the ganty uprights. It is doubled
for the same size mill. Any of these problems can be overcome though
,..by overbuilding. I like unconventional thinking and may use the
same concept with my next mill. I would like to hear some more about
your idea of a single sided drive.

Frank ,.... CAD CAM FEA backround

For the purposes of this discussion I will declare the length of the table
as X and the length of the gantry as Y.

To make the single side drive work we must abandon the idea of a gantry as

beam and think of it as a truss (triangle). Think of it as a right

where the long leg is the Y axis and the short leg is the X axis.

The short leg if the triangle is roughly 50% of the long leg. Thus, for a
60X120 table the distance between trucks that run the X axis rail is about
30-36 inches. As such, the in-plane force (neglecting acceleration) on

X axis trucks is about twice the tool load - not much.

The principle advantage of this method lies in the rigidity of the truss.
Bending is no longer an issue. Deflection in the XY plane is avoided

resorting to adding bulk. Les Watts' router base is built this way. It
seems incredibly stiff though it is only 2X2 box tubing.

The one sided approach allows you to invest your ballscrew budget in one
good piece instead of two lesser units. The drive side rail should be
beefier than the opposite rail. No problem. A disadvantage is that the
drive side rail must be about three feet longer than the intended travel -
still better than with a moving table though.

One of our list members (was it Bob Campbell?) has built a few tables this
way and was happy with the results.

Doug