question about ways vs. shafting

Hello,
I'm relatively new to this forum and am laying out ideas for building my own machine (CNC mill). My first thought in designing this machine was to support travel of all axes on steel shafting, with beaings & pillow blocks on the necessary parts. As I look at the construction of larger CNC's, I see that they are all constructed by means of ways (I believe this is the term for the 'dovetailed' slide) rather than the shaft/bearing arrangement I have just described. Which method is preferable, from an accuracy standpoint?
I am inclined to believe the latter must be, as it is used on the big machines and I can't imagine economics being a factor in those large expensive things. Please correct me if I am wrong, and if anyone would care to give me a brief explanation of this I would greatly appreciate it.

Thanks,
Rick


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