This will probably be a useful thread then for both of us ptjw as I am now looking at a fair few different ways of driving it I want to stay away from leadscrews really have been looking at rack and pinion but that seems even more expensive than leadscrews. I have a theory of using a timing belt that self meshes and running it through a couple of pulleys and over a stepper. This should then mesh against the fixed side which is attached to the ali so instead of 4' of timing belt to sag and keep tight it is running against itself which in my head means it will be a 3" part of the belt that is free the rest is self supported. I have sketchup and illustrator open so let me draw it then it will make more sense :)