Sure it won't flex if you're only milling foam. The Mantis I linked to costs about $150 in hardware and motors and $100 in electronics (using, for, example, San

Sure it won't flex if you're only milling foam. The Mantis I linked to costs about $150 in hardware and motors and $100 in electronics (using, for, example, Sanguinololu from the reprap project and pololu stepper drivers) . It can mill PCBs and harder woods. The problem with your setup is that your gantry is held on by two pieces of plywood. The gantry itself is decently rigid, as is the base, but the linkage between them is subject to flexing. The two wood plates are not rigid, they will bend like a springboard. What you need to do to stabilize them is to add a vertical support to each side that is at a 90 degree angle to the existing one. Wood can handle compression better that deflection, so adding a support like that (which of course must be rigidly attached to the sliding part below) would prevent it from flexing and give you a much more rigid machine. The gantry axis itself is stabilized by the slides, though it will deflect somewhat around the middle. The vertical axis seems okay.