at the moment I'm still going to go for the 80x40 10mm slot kjn aluminium, partly as all the other extrusion I have is 10mm slot so have the jigs and extras to go with it all.

with the 10mm slot I'm leaning towards the 15 or 20mm rails, 12mm doesn't give much to sit on the aluminium.

I have drawn my gantry at 40x80 also but may go for 40x40 on that.

the vertical axis I was thinking would be belt driven however I want to link the two sides together which may be easier if they were screws

I think maybe a lego mock up is neededClick image for larger version.  Name: 003.jpg  Views: 524  Size: 32.2 KB  ID: 22784Click image for larger version.  Name: 002.jpg  Views: 534  Size: 66.8 KB  ID: 22785