CNC machine
I have 5 stepper motors to start with. I don't think they are very strong, but I should be able to get a functioning model built.
The head assembly should be interchangeable. In the long run I would like to have a mill-head, a plotter (pen) and a laser cutter. Concrete extruder may be interesting like this.
http://www.instructables.com/id/Make-Your-Own-DIY-CNC/
Goal is to make the first one simple, but able to handle 2x4 plywood. Then I can cut pieces for later versions.
V-Groove bearings are pretty cheap (can be). I should be able to build something very smooth with a few of those and some 90 angle metal.
To do a really nice one would require 2020 V-Rail, but that can be a little pricey.