Sorry Kwok if my facts were not exact, but the HobbyCNC controller is still a cheaper alternative.

I do speak from experience as I have two of their boards that have worked wonderfully on bigger machines (2'x4' at over 600ipm) for many years without a problem. Stepper force calculations rely on alot of factors (volts, amps, speed,,,) so you must have the torque curves which compare these variables. But, oz-in as used a force measurement is the same whether it is bipolar or unipolar. Its just that steppers wired unipolar put out less torque than the same stepper wired bipolar. A 100 oz-in rated unipolar puts out the same torque as one rated 100 oz-in bipolar. Its just that the 100 oz-in unipolar might put out 150 oz-in if it could be wired bipolar. Here is a good primer on steppers:
http://www.cs.uiowa.edu/~jones/step/
Dave has sold tons of boards over many years and is one of the commonly used sources of controllers in the much wider hobby cnc world. I hope he can add to the wonderful stuff you all are doing here.
Sorry if I confused anyone.

 

Darn, I was going to buy aswell, untill he coped a tude, so sensitive.