If CNC is more than you were looking for here are a few sites that just use a bow. Lots of info but the bow is really simple.
you have a cross bar and to legs one leg is fixed at about 90 deg. to the cross bar and the other is atached so it can pivot. make it like an H the top of the H will act as a way to tension the cutting wire on the bottom of the H select a wire and a PS that will give you the temp you need for the foam you are cuting I use a dimmer switch and a 25 amp salvaged xformer. the dimmer limmits the poer going into the xformer hence limiting the power going to the cutting wire. I can go from cold to red hot with this setup.
Hope it helps
http://www.vatsaas.org/rtv/construct...irecutter.aspx
http://www.info-central.org/construction_hotwire.shtml
http://webpages.charter.net/rcfu/Con.../FoamWing.html
http://www.geocities.com/CapeCanaver...ng/coring.html
http://www.rcuniverse.com/forum/Hot_...1178270/tm.htm