There is a lot of discussion on whether to use bipolar or unipolar stepper motors for your CNC project. Each type of motor has different characteristics, but when all is said and done, the best choice is usually bipolar stepper motors. This article is a short explanation why bipolar stepper motors are usually the better choice.
These motors typically have 6 wires, where two of those wires are usually connected to ground, also called ‘common’ or common ground. This leaves 4 wires for control. When a voltage is applied to a control wire, current will flow from that wire to ground. When no voltage (0 volts) is applied to a control wire, no current will flow to or from that control wire. These wires are usually called A, ~A, B, ~B. When A is 1, ~A is 0. Similarly with B and ~B. This scheme means that at any given time, at least half the wires in unipolar motors are not activated. This means that if a unipolar and bipolar motor are the same weight, the bipolar motor will almost certainly be stronger. This also means that the unipolar motors are slower, because less torque is being applied. So in short, unipolar motors are heavier, weaker, and slower than bipolar motors. The only real advantage to unipolar motors is that they require less power than bipolar motors. Powerful power supplies can be expensive, so it may be slightly cheaper to buy and use unipolar motors.
These motors usually have 4 wires, but may have 8 wires for very powerful motors. In these motors, current can flow either direction through the coils, and all coils can be activated at the same time. This means bipolar motors can be very strong and fast. Some people point out that this means the control circuitry needs to be more complicated, but stepper motor drivers are cheap and simple to use, so circuit complexity should not be a big factor in your decision. Again, bipolar motors are faster, stronger, more weight efficient, and similar in price to unipolar motors. The only disadvantage is that these motors can require a lot of power, which means you might need a beefy supply if you want to run the motors at a high voltage for faster speeds.
All images in this article are under the GNU Free Document license, based on image by Yegorius.