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

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I would say that holding torque is better than what I'd expect to get from a 180W servo.
You could probably improve it through tuning, but 180W is never going to have lots of holding torque.

I did play with the parameters and did some tuning, but perhaps it can be improved even more. What I wish I could do is to prohibit the driver from going into low current mode, because I think that's what is the biggest problem. When the motor is on holding current, the driver reduces the current even more very fast after stop, which is OK if you are using the motor as a spindle motor, but not if you want to use it as a rotational axis, where the motor is mostly on holding. Of course, there are motors with dedicated electromagnetic brakes, but that might not be necessary if the current settings would allow full current all the time when the motor is stopped, as long as the driver is enabled. So maybe I missed one setting or maybe the PID regulator can be tuned even better.

Anyway, I have seen some YT video where the same issue was fixed by tuning, but those motors all had V6 firmware, and this is V5. I was in contact with the manufacturer and they said sorry, you need a V6 motor, which is why I complained to the seller, who in the end gave me full refund because he could not ship the motor he advertised. I am happy this way, but feel sorry for those who wont get a reasonable compensation, if the problem is due to the PID in the motor and requires parameters which don't exist in the V5.

A quick search tells me a JMC 180W 3000RPM servo has 0.6Nm and 1.5Nm peak torque, so even through a 3:1 reduction, that's only 4.5Nm peak.
You can put that kind of torque through a decent screwdriver with a bit effort, so on a 4" chuck, it's not going to take much effort to move things.

Yes, that was the way I was thinking also, I mean, 0.6Nm x 3 should work fine, but in the worst case, I can even change it back to 6:1, so the total holding torque should be fine, but what I noticed now that it seems like a sudden quick pressure can start the rocking, while a slow increase can not, so I think the problem with the PID must be fixed for this to work as I want it. Otherwise I can not use it. I don't know how other people manage with this servo, but it is my understanding that this is a very popular servo in the DIY segment of CNC builders.

What I also find strange is that I can NEVER, not once, force maximum current consumption, not even for a very short time. Because I think that is not good, since in that case it can not be called 180W at all. The maximum I could ever measure was below 2A. In my opinion, the driver should increase the current until maximum is reached when I try to twist the shaft when on holding, but long before maximum current, the motor throws an error and just release the tension all together, instead of increasing the current and applying more holding. So perhaps, the motor is a totally different one, while the label says it's a 180W but in reality, maybe it's just a 100W motor.

Anyway, I want to finish my "lathe" first, before spending too much time on the seemingly weak holding torque.