[BobM3]

What is the difference between the two?

[Al_The_Man]

In the very general sense, the servo motor provides precise positioning control by use of a feedback element, for example encoder or resolver, the spindle motor is velocity controlled.
They both can consist of various technologies, i.e. DC brushed, AC Sinusoidal (synchronous) and Brushless DC, the spindle motor can also be a common AC induction motor type.
In some cases the spindle motor can be made to accurately position by means of feedback to the controller for functions such as orientation for tool changing and can even be done with induction motor technology, usually 3 phase type.
In the case of the servo motor, PID feedback loop allows interpolated moves between axis.
Al.

[BobM3]

Thanks Al - I ask because I'm interested in converting an old gear hobber to CNC. I would need electronic gearing for that. In thinking about electronic gearing - where you try to sync two spindles, I figured there was no difference between synchronizing spindles and synchronizing two servo motors (X and Y axis). From what you say though, it sounds like synchronizing spindles would require a bit more position control then normally would be there.

[Al_The_Man]

If thinking of electronic gearing you may have to use servo motors for the spindles.
Controllers such as Galil will gear two or more motors off of a slave motor at whatever gearing you wish.
Al.

[BobM3]

Ah, that's interesting. For a gear hobber I really don't need high speed for either of the two spindles. I'll look into using servo motors instead of spindle motors for the two "spindles" (hob drive and work piece drive).