This idea of a printer/mill combo has been discussed plenty of times. These machines operate on completely opposing principles. Those principles directly conflict each other. Trying to compromise and create a middle ground only makes a machine that can do neither. In order to actually build a mill that can be a decent printer, you would need very good mechanical guidance (linear rail) to take the heavy loads from the mass alone that has been created by the need for load bearing. You need to be able to push that heavy load very fast and accurately, which means power steppers and incredibly precise digital monitoring (think servos). Even after that, you will be leaving a lot of each type of machine on the table. At this point, you could have just built two world class separate machines with enough spare cash to build another machine.

 

It's a mechanical issue, not a digital issue.