Vibration, mass and massive arguments

[JAZZCNC]

Is it better for the machine to just sit on the surface, or be bolted down to the surface, or sit over the surface on small feet?

Very much depends on the machine.? If small lighter machine that's going to rapid around fast then you'll want it bolted down to stop it walking off and even heavier machines can and will do this thru inertia when changing direction.! I always prefer bolting down has it keeps things that bit more ridged and safe.

Also, I seem to recall a few years ago coming across someone using a levelling compound in their project to try and get a perfectly flat and level surface, though it might have been specifically for the surface where the rails would be mounted. Has anyone any experience with using a pourable levelling compund that would be suitable for the worktop?

Unless special levelling compounds like Epoxy resin then they don't level to anywhere near accurate enough to direct mount rails and not use levelling feet.

Unless you have a very very stiff frame and I mean cast iron stiff then some form of levelling method will be required to keep or bring rails parallel so it's always a good idea to start with a level surface and if your wanting accurate parallelism of rails then some form levelling will be needed.!