Peter - I think I have worked the dimensions so that it can do spiral turning. I was concerned about the gantry height because of flexing, so have kept it down, but the working position of the lathe when in 4th axis mode can be at router bed height or slightly below or even slightly above. It isn't fixed yet and could even depend on the work piece. I am aiming for a Z travel of not less than 150mm, and the maximum radius of the lathe in the picture is 120mm. OK, this will not allow fancy manoeuvres like turning a wooden crankshaft where the bit has to cross to the opposite side of the centre point, but I think I can live with that.

Tim - Yes I am bothered about flexing. My original ideas had the X-Sides high up like Mechmates, but I couldn't get the dimensions to work properly. I might just try that again. I figured that I could put in a false bed to raise it's height for flat work so at least the Z wasn't reaching down. I am also a bit concerned about my own construction capabilities and have gone for a method which worked great for me on my manual router table. There I took two 18mm sheets of MDF and found their curvature by laying them against each other, which actually turned out to be tiny. I then glued them together with opposing curvature so that they are now my best reference plane. Lots of clamps were involved! I intend to make the table surface this way with two standard size sheets uncut, so I then reference from that hoping it is pretty true being "flat enough" and all four edges are factory originals.

I too have seen a rise and fall bed. It used supported round rail mounted vertically as the guides, but didn't have a lathe on it. And it was huge This is what gave me the idea. I don't think there is anything original in my drawing. It's all taken from various other designs. The gantry height and the Z are very little different from existing designs but maybe the Y height between rails needs to be bigger to give the Z more stability.