With this adapter you can transform your 3D printer into a CNC: it allows you to install a 300W engraving DC spindle (52mm dia) onto a base plate that has 50mm center-to-center M4 holes. These CNC spindles are common on eBay and typically come with an ER11 collet and weigh around 800 grams (a bit more than two extruders).
The design might require modifications to fit your printer as I've built this originally for my Delta bot which is custom build and I did not have an opportunity to test on specific printer models, if you do and it works, please report back. Remember to limit the Z axis travel to fit the spindle.
See also: http://www.thingiverse.com/thing:296799 for a Dremel variant of this.
Additional parts needed:
3x 30mm M3 screws and nuts for clamping the spindle
2x M4 screws with nuts - height will depend on your printer, typically 10-16mm
I printed mine out of PLA, with 0.2mm layer height and 50% infill. Seems to be plenty strong.
I enjoy designing minimalist-functional 3D objects.