Any servo experts here?

[Jester]

I'm planning to add CNC capability to the cross-slide of a lathe. I want to maintain the ability to use the cross-slide manually. From past experience using a stepper is not a good idea because even with the stepper disabled, the magnetic poles in a stepper don't allow it to turn smoothly. I have some ac servo motors and when disabled they spin freely.

I have never used a dc servo, not sure if they are free-wheeling like an ac servo or stepper like when disabled?

When rotated manually I would imagine a back emf will be generated, however any manual rotation will be quite slow because it will be done by hand. Do I need to be concerned with that back emf effecting the servo drive?

[jpanhalt]

Coreless hobby servos are easily moved when not powered.  I also have an integrated servo from Stepperonline (iSV57T(S)).  It moves freely when not powered with just a little cogging feel.  Clough42 used a similar servo for his ELS YT series.  I don't know about other industrial servos.

Edit: As for generated voltage, I suspect any DC motor will do that.  A DPDT switch will solve the problem.  Shorting the terminals will make it into a brake, but at slow speed, that probably won't cause much resistance..

[H.O]

Yes, spinning the motor shaft will generate voltage.
On a standard, cheap DC motor you will feel cogging but an actual PMDC servo motor has very little cogging and is smooth and easy to spin by hand.

The output stage of the servo drive is, most of the time, an H-bridge built with MOSFETs. These MOSFETs contains diodes which basicallly forms a bridge rectifier with the motor at its AC terminals and the servo drives power supply at the DC terminal.

This bascially means that the voltage generated by the motor gets rectified and placed across the drives power supply. If you spin the motor fast enough it's possible that voltage generated is high enough to actually power up the drive (if the drives logic supply is derived from the motor supply).

Easiest way is propably to have the drive powered up but with the output stage disabled (if the drive has an enable input). That way the DC terminals on that "bridge rectifies" sits at whatever voltage the power supply delivers and there's likely no way you can spin the motor fast enough to get any current flow. Basically the motor terminals are "open".