The problem is that once you start adding up the cost of the components to make a capable CNC mill, you get really close to the price of existing ones. Ball screws, good linear rails, etc. are no joke. Big heavy castings (cast iron or polymer concrete) are no joke. The most "disruptive" maker-style machine is the Tormach and it's not well liked.
Lots of open source designs that can route steel, like PrintNC [1] and OpenBuilds [2]. If all you need is Aluminium, then you can make do with something like the RS-CNC32 [3] or MultiBot [4].
Not parent, but I’d consider 0.001” over 10” to be totally acceptable for home and hobby use and 0.00025” over 20” to be acceptable for general production use (“great” isn’t really meaningful, but “makes parts to spec” is good enough).
That's the specification of a measuring instrument. Commercial VMCs don't hold 0.00025” over 20" even without being loaded. A slight temperature change will throw that off.
What you say is correct, except that a hobbyist can source parts from Ebay and the like. Which you could never do for a production machine. That can bring the cost WAY down.
