Desktop CNC Milling Machine  
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My 4-axis desktop CNC machine. There is an X, Y, Z axis and a rotary table. This machine is actually a manual machine from a company called Sherline that has been retrofitted with stepper motors. You can go to their site to find companies that make CNC retrofits for these machines. My first major project built using this machine was a custom 6 legged robot. 
   
The milling machine X-Y base. There is around 8.5" of usable x-axis travel and 5" of usable y-axis travel on the stock machine. I am currently working on a modification that will extend the usable y-axis travel to 8".  Project completed! link here
   
The z-axis has around 7" of usable travel. There is also a manual rotary attachment that allows the axis to be tilted.
   
This is the spindle with a variable speed controller and DC motor.
   
The rotary table has a 4" diameter. It is mounted on a tilt plate that allows a 0 to 90 degrees tilt.
   
The stepper motor control box connects to the parallel port. It's able to control 3 unipolar stepper motors. The electronics are very straight forward. Power comes in from a huge AC transformer. The AC is then rectified into DC with power diodes and a big electrolytic bulk capacitor. This voltage is used to power the stepper motors which have a current limiting resistor connected to the common of each stepper motor. 5 volts is derived from a 78L05 5V regulator. The motor windings are switched using ULN2803A Darlingtion transistor driver chips. These driver chips then interface to some 74LS08 logic chips before connecting to the parallel port. There's also a fan and on/off switch. 

The control box came as part of a CNC retrofit kit. For those who don't know, the electronics described above use cheap components and a very simple design. In my opinion, it gets a score of 2 out of 5 in terms of sophistication and robustness. If could do it over again, I would have bought a better CNC retrofit from a different company like below. 

When buying a retrofit, you want to look for bipolar stepper motors instead of unipolar stepper motors. Bipolar motors are faster and give you better torque at high speeds. The control electronics are more complicated but it's worth the additional cost. You'll realize the extra speed is important when it takes 30 minutes to make a simple part just because your unipolar stepper motors are too slow. Also, you want to buy a control box with some type of smart controller that handles the stepper motor timing. This will allow better control under a non-realtime operating system like Windows. I would recommend the CNC retrofit control system from Flashcut CNC.

I plan on doing a quick modification to this design by adding a 4th axis control and a microcontroller for handling the stepper motor timing. Details will be provided soon.