Quote Originally Posted by dsc View Post
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.
Its for registration . A machined reference edge to butt up against makes getting your rails level far easier than without.

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
I'd love to see you bend a hardened steel rail that's fixed every 80mm lol Rails bending is the least of your worries when you constructing your machine out of only aluminium or steel box section. It ain't the weak link.