Its for registration . A machined reference edge to butt up against makes getting your rails level far easier than without.
Ah but what if the edges which are use to butt up against are not parallel? It does indeed make assembly quicker, but the assumption is that whatever is used to mill the edges is near perfect.

I'd love to see you bend a hardened steel rail that's fixed every 80mm lol
Not planning to:) I was simply referring to m_c post regarding shock loads and possible movement of the rails if not fixed against an edge. Surely this will not be a problem when milling MDF:)

1. It all depends on what machine you use to mill the datum, so I'm fortunate enough to have a VMC at hand. (when i use this method i get a parallelism of 2um)
Lucky b*stard!;)

The reason for asking is purely to understand where the edge idea comes from, I do like the ease of assembly, when butting the rails against the premachined edges, but worried about parallelism.

Ad, how do you mount the carriages? similar approach against an edge with a side push plate via a block bolt?

Regards,
dsc.