routercnc - many thanks for your suggestions. It took a while for my brain to get around your notes about the rails on the z-axis but I now I realise what you say and it makes total sense. My original plan was to machine the z box so that when all is put together there is approx 1mm gap between the 3rd rail guides of the z-column and the 'backplate'. I would hold the guides roughly to the 'backplate' with loose screws then inject aluminium epoxy putty into the 1mm gap. I would tighten the screw once the putty had set. This method would I hope would mean I wouldn't need the high tolerance machining that would be otherwise required. Not sure what to do yet so will mull it over.

The order of assembly is also great advice, again some thinking needs to be done.

Re the tensioner - You've caught me being lazy, I will add it in the cad drawing soon but I just wanted feedback before I got into the details. There's quite a big gap between the end plates 80mm.. I plan to have something simple, probably a slot running on the inside end wall of the gantry with a couple of bearings on a screw to tension the belt.



Sven; Many thanks for your input, I have gone to and fro with this thought for a while and I guess what I really need to do is learn the simulation part of the CAD program to get a definitive answer.
I get the idea that unless you build the extra rail perfectly you just end up introducing error and not taking it away but there is this constant niggling at the back of my brain that says when that long Z-column is at full extension, the leverage on two rails will be very high especially in the centre of travel. I can't help thinking 3 rails has to be stronger but that is just a gut feeling and nothing more. It would be very interesting to other peoples thought as well.