welding steel base or just getting aluminium extrusion

[Neale]

By the same token, the guy bolts his rails straight to the top of the extrusion. No particular reason to believe that this is any straighter than the side of the extrusion - the surface will be influenced by errors in the supporting components, etc. However, errors in work on the machine will be in depth rather than profile accuracy, so perhaps up-and-down errors are less important for a lot of work.

For a router intended for mostly work in wood or plastics, though, will the errors be significant? If you are going to be making sign boards, doing lettering, cutting out stand-alone shapes which do not have tight manufacturing tolerances, is it worth chasing the last micron? I'm not saying you should ignore any errors, but you need a sense of proportion here. If your ballscrew/ballnut/bearing assemblies give, say, 0.05mm backlash, how much better than that do you need to be in the supporting structure?

Steel is different - my own 1.8m support rails dipped by 1.5-2mm in the centre, which is why I used epoxy - but in practice, how accurate are these aluminum extrusions?