Your Z axis box/router assembly will be heavy if it is steel. If you mount the box tube to the Y axis slider assembly and mount the router to another slider assembly that is attached to rails on the fixed box tube you can significantly lower the weight that the stepper or servo has to lift. This will also reduce the tendency for the Z axis to back load the Z lead screw and drop down after turning off the power. A brake is used on some machines because of this. Adding dead weight is not always a good feature.