Some questions to JAZZCNC or any other closed loop stepper experts

[JAZZCNC]

There is a lot to be considered and since I have no experinece with closed loop steppers or servo, I really don't know why a weaker servo is more expensive than a stronger closed loop stepper.

The servo at first might appear to be weaker, however, the stepper torque rating is when the motor is stood still, the moment it starts turning the torque drops away, and as the rpm's rise it drops dramatically to the point where at say 1500rpm you will have very little torque left. A servo motor rating is continuous across the RPM so at the rated speed you will have the motor's full torque rating, also they have a peak rating which can be between 3 x and 5x the rated torque for a number of seconds.
Often this is more than enough to accelerate an axis up to speed before dropping back to the rated torque which again is often enough to drive the axis and then peak again to stop it.
Then if you apply a ratio like what you will be doing, let's say 6:1, then your 0.6Nm becomes 3.6Nm at the output, and let us say the peak of 3x the rated then you have nearly 11Nm of torque for short bursts of time.

Now I'm not saying this motor is the answer to your question but if you need speed and torque then you really do need a servo, DC or AC.
The servos tend to be more money because they are so much more sophisticated in terms of electronics and what can be done with them.

I had a closer look at this one:

It is very similar to the one recommended by Jazz, bur it is a Stepperonline version. There is though one difference which immediately is obvious, that is the the Stepperonline version has no enable signal. It is not clear to me when the motor is enabled and when not.

It does, however, this must be done using the RS232 port. Not ideal I know but this one of the advanced features a Servo provides among others. Though I do think it's silly not providing a dedicated enable pin.

It is also not very clear to me if this is a 3-phase servo or not. The one in Dean's link looks like a 3-phase motor.

The link I gave wasn't a recommendation for that motor but more to show the type of motor, that is a DC Servo motor with integrated drive. After Muzzer's comment, I looked again and agree with him that it does read like it's a Stepper but says it's a DC Servo, these two things conflict with each other because while both uses DC current they are internally structured very differently, so I wouldn't trust the advert description on this.

But there is another question... that is about the torque. Both Dean's, and the one here have considerably less torque than 4Nm... so would a closed loop stepper with 4Nm be equal to these two...? I mean, high rpm and smooth, quiet running is one thing and is very nice, but if it is at the expense of less torque then I don't see the benefits. I need this motor to be able to hold the chuck when I use it as a rotary axis, not just rotating fast. So what's the secret? Can these servos be used as ordinary rotation axis steppers also, and can they hold the position when used on the CNC mill?

Well without knowing the torque you require then it's nearly impossible to answer, but my gut feeling is that you will need a little more than 0,6Nm and the answer to me is to use either an AC or DC servo with a higher Nm rating and with a brake.
This will give you both higher RPM you would like and the brake will hold the motor when at standstill. If this was an industrial 4th axis it would have a separate brake to lock the rotation.

Obviously, this would be more expensive than your stepper setup but it's the only way you will get higher RPM's and torque with reliable results without getting silly about it.
End of the day a typical 4th axis isn't a lathe, it's not designed for high rpm's but for positional indexing. If you want Lathe performance with accurate indexing then you will need to dig deep as it gets expensive very quickly to do it right.
(Look for Simpson36 on youtube if you want an idea of how complex it can get.)