Last year I was stripping down an old A3 sized HP 2500C inject printer for parts. Once I got to the base chassis, I realised the thing was sturdy enough to form the base of a CNC machine.
5 days later, I had the first working version of it, completely making it up as I went along, using a mix of standard hardware and supplies from the local hardware shop, some parts of eBay and basic tools in the garage.
I didn't want to massively modify the general appearance of the printer (get all 'Frankenstein' on it) so I built it to take a dremel flexishaft handle mounted to the Z axis to provide the milling/cutting/engraving component. That was fine except for the runout in the bearings making the engraving tool wander all over the place and generally being very inaccurate.
I then built a Mendel90 3D printer using self sourced parts, and from that, printed 'upgrades' to the CNC machine and redesigned the Y/Z axis assembly. The upgrades also included changing to 13mm smooth rod and linear bearings, using a 12,000 rpm 400W 48v air-cooled CNC spindle and slight mods to the milling platform. I used the 3D printer to make bearing mounts, smoothrod mounts, Z axis gantry parts and some cosmetic parts. With careful planning and design, I was able to make a Z axis that fits down a gap of 60 mm and carries a 52mm diameter CNC spindle.
The upshot is a CNC machine that mostly maintains the look of the original inkjet printer (complete with the plastic bodywork) and is quite capable of cutting it's way through 6mm acrylic sheet in one pass (albeit a bit slowly, but I'm not in a rush so who cares)
I created a blog about the build, and that was more for the concept as opposed to a blow-by-blow on how I did it, but I did blog some of the finer details. It's got loads of photos, and in the August/September archive, details of the upgrades and 3D printed parts, and links to videos. If nothing else, I hope it provides some inspiration or introduces some concepts that help you along your CNC journey.