Hi Tom - The long axis with a gantry can be driven on small machines with one drive (screw or belt) under the table via a saddle. On large machines its usual to drive the long axis with two motors, one either side. If the design is functioning then one or two is fine. If you have seen a machine in which only one side of the long axis is driven this is most likely for a machine like a plotter or laser that does not have any contact force at the "tool". These would not work for routing work. Peter