Are gantrys (gantries?) driven on only one side? if so, is 'crabbing' (twisting) a ever a problem? (say, 30# gantry covering a 4' span)

would it be better to run a shaft across to the other side so the stepper is turning two gears, with a rack on both sides?

thanks,
-anthony

Gantry mills can 'Crab" or "rack", not only during basic movement, but much worse during cutting. This is the primary reason I avoid such designs. It is much easier to have a machine that looks like a gantry, but the X axis moves on ways. This design makes the system as a whole much more rigid.

However, for a gantry system, the approach you suggested is doable, but you have to allow for clearance from one side of the machine to the other for the shaft. The other approach, depending on the controller software, is to use two motors, one on each side that are "slaved" together to act as one. While a shaft and gearing might seem cheaper, it may be cheaper to use two motors.

Hope this helps a little bit.
Chris

If your'e using it for plasma or waterjet cutting (which is the forum this was originally posted in ) where there are no cutting forces, you can make a triangulated gantry which shouldn't be able to rack or twist.