OK finally got past rails and bearings level design and onto screws.

The total weight of my x and y axis is only 155Kg minus the weight of the screws and motors.

If I could realistically get 200 N/um from the bearings I would go for 35mm screws. But I think I have more to gain realistically by taking advantage of the low weight to stiffness ratio of this design on the milling plane and use 20 or 25mm screws instead.

Using a gantry design of this stiffness and you must be looking at having to move at least 500kg on your x and z axis.

Also this design lives and dies on the X axis length, All measurements are derived from that. And it is almost impossible to change or upgrade. In this case the inner diameter of the torus, so I wont be making accurate drawings of anything until I have the X-axis components in my hand.

I am now going to focus on assembly methods until I have the bearings and screws in hand, take some drawings to the engineers, I know that milling a 1.6m x 1.6m plate is a lot more expensive than if it is less than 500mm, I could split the end plates into sections and save myself a lot of money.