[TTalma]

I want to build a lathe where I can turn the spindle at high speed and also position it accurately for carving etc.

If I connect a stepper to my 3hp spindle and turn it off, then run the spindle at 3000rpm. what will happen to the stepper and or it's driver? If I did this with an AC motor the AC motor would turn into a generator, will the stepper also? If so is there a way to do what I want?

[Al_The_Man]

An AC induction motor or universal motor will not generate, only a P.M. field type.
The stepper motor will also generate, open the feed to the stepper coils if you want to back feed it, although 3krpm is a little high.
A DC clutch would be a better option.
Al.
.

[jfong]

At 3000 rpm, the voltage generated would probably be high enough to damage the attached stepper driver. Do not do this.

[NTLSYS]

Yep, I agree. Use a clutch. On my CNC lathe, you have to stop the spindle, then switch to C-axis mode. A bunch of things happen, like the AC spindle disengaging, and engaging the servo. You can't have both strapped at the same time. I'm sure this is for safety reasons, both for the machine as well as for the operator. If this is a "home-built" lathe, anything can be done, but the question becomes, "is this a good idea?"

Why would you want to drive the stepper at 3k for engraving, unless you are describing an OOPS scenario. In that case, you MAY not damage the stepper motor coils (right away), but the voltages will be high enough to destroy the driver. If the driver is robust enough to withstand that much voltage for any length of time, you'll cook your motor windings. Drivers are usually built using MOSFETS, power transistors or darlingtons. Look up the specs for what you have in the driver. You MAY be able to replace those components with ones that exceed the voltages that come off the steppers at 3k (your example speed). Even if you go that far, it's probably still not a good idea.

[TTalma]

Thanks that's what I figured would happen. My plan is to convert a wood lathe to CNC, I would have 2 motors connected to the spindle. A 3hp drive motor connected to a VFD for speed control, and a stepper connected for positioning.

I would rough out the blank using the drive motor, most of the roughing would be at 800-1600 rpm, but a finishing pass at up to 3000 rpm would be done. Once the blank was finished, then the stepper would take over and do the positioning for the carving etc. The stepper would be low speed stuff so it rotating the drive motor is not an issue.

A clutch sounds like the way to go. Where would I look for a DC clutch? A quick google showed ones for lawn mowers but it looks like the pulley is part of the housing, so I'm not sure how I would connect a timing gear to it?

[Al_The_Man]

I have used Electroid Co in the past, nice and compact.
Electroid | INNOVATIVE ENGINEERING
Al.

[NTLSYS]

Or you could do what the standard 9x20 lathes do, add a belt system with a tension bearing/gear. If you want CNC control over the switch=over, add an actuator, like a solenoid or air piston to manipulate the tension bearing.