I think the point that both Robin and John are making in their own inimitable ways is that accuracy = rigidity and rigidity = mass, and damped mass at that. You haven't said what accuracy you want/expect... a few spec's would be a good start. It is quite feasible to make a reasonably accurate router/gantry style machine that can do 0.05mm or better accuracy (NB accuracy is NOT the same as resolution) in engineering plastics and aluminium... especially if the depth of cut (DOC) is small (.1- .2mm) It is another thing entirely (by a factor of 3 or more) to do the same thing in free-cutting mild steel, and another jump up in stainless steel...