I've tried milling a few boards with my CNC machines just for fun. Using scrap copper boards. In all of them, the precision required for a clean trace was not there. Either the table was not square, the track not straight, or something. When having to control depth to 0.001" of an inch, having a straight and level system is critical. Even positioning the bit manually introduces too much error. The solution is AutoLevelling. While I still have a way to go on quality construction, here is a video showing how the AutoLevelling enhancement to a great Eagle add on called pcb-gcode works. Enjoy!