Thanks very much for that, Irving.

The gantry members will be two lengths of 2" square x 10g wall 6082-T6 alloy box, with SBR16 fully supported rails screwed to them, so it should be pretty rigid.

I'm thinking of something similar to the layout shown on that Durham Robotics link, with the leadscrew outside the rail, so it should be OK, with luck. The main reason for using this configuration is the reduction in machine height and a possible increase in overall stiffness, as the gantry doesn't need an under-table drive. It also means that I don't need to add the complexity of a double leadscrew arrangement.

Jeremy