I plan to buy a cheap or even second-hand Chinese 3020 or 3040 CNC mill so I looked around to find a way to improve it with parts I already have like Arduino or some simple electronic upgrades. I found two interesting guides:
Upgrading 3020 (and other models) with Arduino, GRBL and CNC Shield V3
Detailed guide by EricAusome can be found at:
John Lauer hacked his 3040 to run with ChiliPeppr and TinyG.
This post will serve as my knowledge base and will be expanded as I gather more CNC hacks.