Few things I still don't get:

1. mounting against an edge assumes the edge is perfect, otherwise things go wonky. Surely there will be errors when machining the edge every single time.

2. with no side plates pressing the rail against the edge, I don't really see the point of the edge. Surely if the rail moves under load, it will move anyway. Sure the edge will spot the movement towards the edge, but the other way is still unsupported

3. the manual suggests simply bolting the rails down (ie. no edges) if there's no side space, but shows the carriages supported and mounted against edges. Again point 1 comes into play.

Regards,
dsc.