I was wandering through the web this week and came across a CNC DIY kit for the Arduino. It occurred to me I could make my own using a propeller and improve on the simple synchronous belt system with both stability and finer measurements.
This is the general overview.
Use lead screws for X,Y and Z axis with linear scales and a gear on the output motors shaft to run past a hall sensor for counting ticks. The motor would run through a gear reducer so that a steppers resolution would be greatly increased. The gearing down is important to achieve higher resolution and the lead screw will eliminate some of the problems with using a belt, it will also improve a stepper motors resolution if the motor shaft is used for counting ticks.
For railing, use simple rods with linear ball bearings (two per rail and maybe more), preferable pillow blocks.
Then for the up and down, mount a fixed cut out above the max height with a L shaped bracket to hold the motor for the bit. The L shape is specific because the small part of the L is where it would attach to motor that moves the bracket up and down past the linear scale and the long part of the L pointing down would be where the motor and bit attach. When fully raised the tip of the bit would be above the highest part of the working area.
However, this concept requires 4 motors. One for X, Y and Z axis and one for the bit. Also note that by gearing the motors down from, say 2000 rpm, to a minimum of 50 rpm, it opens up the possibility of carving metal. The lower rpm is especially needed for harder steels. Changing out the bits is up for debate as there are several possibilities to do this, obviously one is to manually change the bits but what fun is that?
A Propeller should be able to handle this without a problem.