CNC/FDM Combo printer

Hello everyone,

I am building a new unit. Will be my 3rd printer. Second one I've fabricated myself. This time I am looking to make the toolhead fully interchangeable with a router for a 3 axis CNC.

Basically looking for any advice anyone has on this matter, and whether a stock reprap firmware can control the CNC if I can power the toolhead another way (relay from a 24v supply controlled by the arduino board)

Also the whole frame is going to be welded steel maybe aluminum ( depending on what local suppliers have at what prices). I am going for 18"*18"*12" with the bed moving on the Z axis to make enclosing the device and using a heated camber simpler. Also would steel reinforced belts be recommended for such an application or should I use either lead screws or ball screws?

Also I am a software developer... C isn't my sharpest language but I can find my way around the firmware and have some experience modifying the Merlin firmware.

Any tips, and advice is much appreciated and I will certainly share the entire design and any firmware tweaks to make it work with the community!

Thanks in advance.

I'd go for two different systems in one frame:Ballscrews for the router and belts for the printhead.
I'm currently building a similar thing. A CNC mill with laser engraver. They share the same frame, but I'll park the unused head/spindle at the far end of the table instead of replacing them.
The spindle gantry run on NEMA23 with lead screws at the outside, while I'll build a NEMA17 CoreXY system inside the frame to run the laser. Controller is a Duet-WiFi with some external stepper driver modules ( Pololu compatible ).

The biggest concern I have about milling and printing in the same room is: the dust and dirt from routing goes practically everywhere. Not a good starting point regarding 1st layer adhesion.
Milling and engraving is less of a problem. ( I hope )