OK, there are many parts to a CNC machine. You have the computer and some software that runs on it, which is called the controller. You also have the stepper motors (or servos...a different kind of motor) which make everything move.
There's also a black box between the two, that actually turns the computer signals into the electrical signals needed to make the motors move. That's the part I'm talking about.
Needless to say, since steppers are used in everything from floppy drives to telescope positioning systems, to xerox machines, there are a lot of chips that are made to control them, and consequently make them move. (You can't just hook it to a battery like a regular motor)
Now...there are 2 kinds of stepper motors. One kind has 4 wires coming out of it, and is called bipolar. The other has more wires (usually 6, but can be 8 or more) and is called unipolar. The difference in these motors, is how many different ways there are to apply power to the magnetic coils inside of them, that make the motor turn.
Think about it this way. You have a coil of wire, with 2 ends. You hook up the + side of the battery to one side, and the - to the other side. This is how it hooks up inside a bipolar motor. If you solder a third lead on, right at the halfway point on the wire, you'd essentially have 2 coils...this is the way a unipolar motor's coils look. The wire in the middle is called the center tap. you hook up the + side of the battery to it, and the - side to one of the other ends.
Now...unipolar motors are easier to control, and are commonly used in consumer electronics -- I got my motors out of some old (and I mean OLD) LASER printers from Hewlett-Packard. The huge old LaserJets.
Bipolar motors offer certain advantages, and are actually simpler in terms of construction. And, by wiring up unipolar motors in a certain way, they can be used as bipolar motors. The down side is that, traditionally, it takes more control circuitry to make them work.
I've managed to find single chips that will drive either kind of stepper motor. And, by applying voltage to more than one winding, the chips can actually make them take a half a step at a time (or as little as 1/8 step...that's the 3 bits...2 raised to the 3rd power = 8) instead of a whole step. Which means that my 200 step per revolution motor could be made, functionally, into a 1600 step per revolution motor.
Why would I want to do this? Finer detail on my work. My intention is to make clock parts -- certain gears in a clock need to be very precisely made, or it'll stop ticking.
Does this help explain things, at least a little bit?
-- Chuck Knight  |