Simple foam cutter for Ryan "allted" Zellars' Mostly Printed CNC (http://www.thingiverse.com/thing:724999). Cut foam planes and figures from $tree foamboard or 1/4" fanfold foam insulation board. Uses DC motor salvaged from inkjet printer or battery-powered drill/screwdriver, .025" music-wire needle, small ball-bearing with 3mm bore, and sports-ball inflation needle for guide. Vary motor voltage to get good cut action -- 10-15 strokes/perforations per mm cleanly cuts paper and foam. Form needle loop around drill bit shank to be just under bearing OD. Mount bearing with 3x10mm screw in off-center hole, put small washer between inner race and plastic wheel, and lightly groove outer race with Dremel and cutoff wheel to retain wire needle loop. Dress the needle point with a fishhook sharpener to remove burrs and barbs. Needle exposure below end of guide should be material thickness plus 2-3 mm. Works well at feedrate of 1000 mm/min and cutter speed of 10000 rpm (strokes/min).
Originally inspired from "A Foam Cutting Machine" by Tom McGuire -- http://makeict.org/wiki/Foam_Cutting_Machine
Foam cutter in action --
UPDATE: I have created a version 2 foam-cutter that uses a smaller, higher-rpm motor (27mm X 38mm body and 2.3mm shaft). This resulted in improved cutting action especially when using a piano-wire (rather than Mig-wire) needle. The smaller motor lacks the torque of the larger however so am still cutting at 24-30 in/min feedrate... it tends to bog/drag a bit with attempts to increase the feedrate so far.
UPDATE 11/29: Version 3 has gone to more powerful motor from a cheap battery-powered drill that had been discarded. The motor holding fixture has also been adjusted to make mounting the completed cutter head assembly easier.
UPDATE 12/03: Improved flywheel/eccentric replaces old "shaft_eccentrics". Holes around periphery allow short 3mm screws to be placed strategically to balance the ball-bearing on the other side. Ran very smoothly to near 10000 rpm in configuration shown.
Too much vibration?
If vibration is an issue, check balance of "shaft_eccentric" with a RC prop balancer... or, stick a thin, loose shaft/wire through the center hole, with all your hardware attached, and find the heavy side. On all mine the bearing side was lightest, so I played with longer screws, screw-types, and additional washers to add weight and minimize the vibration. My motor had "Johnson" written on it and became clearly legible while running when the vibration had been sufficiently reduced.
While trying to increase speed and quality of cut, I found the MIG-wire needle tended to bend and spoil the cut. A .025" piano- or music-wire needle greatly improved cutting... it was straighter and stiffer and cut far better than the similarly-sized MIG-wire I had been using.