I know
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has been discussed here several times, but reading about the improvised plastic extruder on a toy-scale 3D motion frame made me wonder: If you've already got a 3D CNC milling machine, couldn't you just use an extruder "tool" to do 3D printing? Chuck it in, load up some pellets, fire up the software, and away you go? Seems like the expensive parts of such a system are already there in your milling machine.