multi-axis lathe project- polar or unipolar?


here is an example of a project to build a router to mill model ships and plane parts.  Above is a picture of the 3 step motor used, how can you tell if the motor is bipolar or unipolar; which 3-axis control driver must be used.

The 6 wires suggest that it is a unipolar stepper. However, you can reliably determine the phase winding type with just an ohmmeter (or VOM, or multimeter, etc).

First create a wire table like this:

1. Red
2. Yellow
3. Blue
4. Orange
5. Black
6. White

Starting at one end of the connector (assume Red), connect one ohmmeter lead to the first (Red) wire. Connect the other  ohmmeter lead to the second (Yellow) wire. If the measured resistance result is not an open circuit draw a resistor from the first wire color to the second wire color and label the resistance value. Continue moving the second ohmmeter lead to successive connector pins (wire colors) and drawing and labeling resistors between the measured wire colors until you have measured between the first and last pin.

Now move the first ohmmeter lead to the second wire color (Yellow) and follow the above method for remaining wire colors
(Blue through White). Continue doing this procedure until you have measured the resistances between all of the colors and have completed updating the wire table with resistors and their values.

A bipolar motor will have only two wire colors connected together with a finite resistance per phase winding. A unipolar motor will
have 3 colors connected together with finite resistances per phase winding and the center tap for the power supply will have the same resistance to two other colors.

Since the name plate on your motor shows that the phase winding resistance is around 2.3 ohms, I expect that you would see about
2.3 ohms between between the center tap color and two other colors for each phase (assuming that this is a unipolar motor). Likewise
I expect that you would see around 4.7 ohms between the two other colors when not connected to the center tap color.

On a bipolar motor you would see about 2.3 ohms between only two colors per phase, with no other colors being connected within the
motor. You wiring table result should make the distinction quite obvious.


My project on the multi-axis lathe 12×23″ made a giant advance today !

I successfully tested the servo motors (aka treadmill motor) with the 10k count encoders … and they seem to work fine. This is after I have spent 4 days and 4 tries getting the encoder properly lined up and centered.

I also have the z-axis extension working, and a working motor mount fabricated for the z-axis. I also found a great fan/cooler for the treadmill motor. As I have no previous experience with servos, this was my biggest issue, that now seem to be resolved.

I finally disassembled the motor, turned the little shaft end down a bit, and fitted an extra brass shaft of the right size. It works fine, and now has very little eccentricity or wobble, some 0.01´s of mm. This was necessary as the little shaft is not straight, ie its tapered,
and is too short, and the shaft must be well centered, according to us digital.

I had a concern with taking apart permanent magnet motors, but there seem to be no problem, so far.

The centering and runout is more of a concern for me due to the high encoder count. I have run it at 25kHz kernel speed, and 48 V DC, and the results were acceptable by guesstimate-tuning. Its stiff, and the hiss/jitter can be tuned out. The speed is slow, obviously, due to the 25 kHz, but the SS will fix that.


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