We've actually dedicated an old Pentium-4 computer to its control. The parallel port runs to the stepper motor controller (a Gecko G540) to control all four stepper motors. There are two on the X (one on each side driving an acme leadscrew) and on both the Y and Z. Theoretical accuracy is actually limited by the precision of the leadscrew ("precision acme leadscrew", off of mcmaster) which put us at around 7 thousandths of an inch over a foot.
We're using EMC2 software to do actual movement control, which runs industry standard g-code. See here for a sample of what it actually cuts:
http://d5robotics.org/Content/View/942/No_Title
On top is the aluminum plate we had cut at our sponsor's place on a waterjet. On bottom is an engraving of the same plate into a piece of MDF. It's capable of cutting through but would have had to do multiple passes (we didn't set it up for that in this case, just testing accuracy). If you hold the aluminum plate on top of the MDF, they match up exactly.
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D5 Robotics, Team 1293: Programmer, CAD'er, Mechanical, Electrical... I've made my rounds.
Events: 2006-2009 Palmetto Regional
Website: http://d5robotics.org/