Whilst out tramping the mountains the other afternoon and pondering the best way to do a z-axis, I had a weird idea . Instead of the usual way of having a high gantry and dangling the spindle down from this on the z-axis mechanism, why not mount the spindle straight to the X carriage (I'm viewing the machine normal to the gantry when talking of X and Y) and move the whole gantry up and down? Yes, it will require more bits but perhaps could yield more rigidity at the lower extremes of Z (tool close to the bed - which is probably where it'll be used most of the time). Any "racking" problems should be avoidable using closed-loop Z motors I think.
Does anyone know of any machines that use this approach?