4th axis as a "poor man's lathe"

[A_Camera]

There's no black and white definition of AC and DC servo but generally a DC brushless servo uses trapezoidal ("6 step") drive current waveforms and an AC (brushless) servo uses a sinusoidal current. If you drive the motor and look at the phase voltages, so you find an AC servo produces a fairly sinusoidal voltage and a DC servo produces more of a wobbly trapezoidal (rectangular) voltage. A DC brushed motor has just 2 terminals and is driven by a DC current.

The manufacturer calls it AC servo with integrated driver and that's good enough for me. I think they know what they are talking about, even if they are Chinese. JMC is selling all kinds of motors, so they have no reason to give us fake information about this motor.

Must admit, that machine of yours sounds more like a closed loop stepper. There are 2 quick tests you can do to check. A stepper will feel lumpy when you turn it by hand with the motor disconnected, whereas a true servo will run freely. You could also remove the belt to show that the noise is indeed due to the belt. Having said that, the belt shouldn't be making a racket....

Yes, I know the differences between motors, I have many different types and am an engineer, even if I am not designing motor controllers. Of course, I did the obvious tests, the motor when rotated with hands and is switched off, rotates freely with very little resistance, unlike a stepper. It feels totally different and is definitely brushless. I have also opened the driver case, and the motor is definitely a 3 phase motor, since there are three wires going into the motor case and there are of course no brushes. I did not hook up any oscilloscope since I did not feel I had the need to do it. Never the less, I think how sinusoidal the signal would look like depends on the quality of the driver also.

Anyway, I have made a video even without any load, but have had no time to edit and publish it yet. Will do it as soon as I have time. Yes, the motor is quiet on the table without load, so I am perfectly comfortable with believing that it is a real AC servo, just like what JMC is saying. They do actually sell closed loop steppers of the same size and with similar looking integrated driver, but then they call it closed loop stepper, and not a 180W AC servo.