Can I a spin stepper motor by hand without damaging the driver

[A Fresh Lad]

Hi I'm making an electronic lathe lead screw and have a Nema 34 and a DM860S driver to run it, would I be able to still turn the lead screw by hand using the handle (my lathe is a smithy AT300 with the handle connected straight to the lead screw) without risk of damaging the driver. I don't wank to use the engagement lever to disengage it because first, I already cut it off and second, due to my lathes whole lead screw and power feed configuration it has no threading dial so every time I finish a cutting pass I'll have to shut off the lathe, wait for it to slow down (if it doesn't just crash into my chuck), reverse and repeat, which sounds like a pain. So my plan is to have the motor permanently connected, have encoders on both the spindle and lead screw and enable and disable the motor when threading and do some software magic to re sync the lead screw to the spindle. Also on a side note my Nema 34 is stiff to just hand turn (yes the phases aren't shorted), I was wondering if that's normal or not and if I can how to reduce it so it's easier to spin the lead screw by hand.
Thanks

 

[Lo-Fi]

Why not just modify your firmware so you drive the leadscrew with the motor all the time and forget the hand crank? You could use a little joystick for manual control, and power feed is just a very tiny pitch screw cutting operation when it comes down to it. I'm assuming you're making some kind of home brew solution and can hack the code, though?
Also, yes: steppers are not easy to spin, particularly when "in circuit". If you do need manual control, you're probably best off having some kind of multi pole switch to disconnect it from the drivers. Even better, a dog clutch to mechanically disengage it:

 

[A Fresh Lad]

Why not just modify your firmware so you drive the leadscrew with the motor all the time and forget the hand crank? You could use a little joystick for manual control, and power feed is just a very tiny pitch screw cutting operation when it comes down to it. I'm assuming you're making some kind of home brew solution and can hack the code, though?

Good idea maybe I could even put an encoder on a handle wheel and use that or just use joysticks but if there were a way to just use manual control as well that'll be nice as I can have feed back from the cutting if that even means anything

 

[Lo-Fi]

I know what you mean, it's quite nice to be able to feel the tool pressure. You may find that the resistance from the stepper removes much of it, though, and the "cogging" may give you some cut quality issues. I'd look at either a dog clutch to mechanically disengage or full electronic control.

Really cool project!

 

[A Fresh Lad]

Thanks
I guess you are right about the stepper removing most of the feedback, so maybe the only option is to electronically control it as a clutch is just too much complexity. However the motor driver does have a enable disable command which I can use to disable it but I don't know if it could break by me spinning the motor. Also is the resistance of the motor with nothing connected normal

 

[matthewsx]

I have not experienced any problems turning mine by hand when setting up my mill/drill project. I doubt you can spin that motor fast enough to develop any significant current and even if you did the driver should be tough enough to handle it. Not a professional opinion, just personal experience with a NEMA 23 motor and a TB6600 driver.

John

 

[Lo-Fi]

Yeah, a stepper is a permanent magnet motor, so some interaction between coils is expected when you move the spindle. Most 3D printers use the "motor off" function of the drivers, which let's the outputs float and cuts the holding drive. I've not had an issue moving mine around by hand, though it will generate enough voltage to power up the board with a moderate move!

 

[AlanB]

Most stepper drivers are protected at least some against back EMF (especially when power is applied but the driver is disabled), we had some brands that were susceptible to damage but the majority didn't seem to suffer. It is also a function of the motor speed, turning it slowly is safer. A pair of series connected back to back zener diodes across each winding rated for higher voltage than the power supply but lower voltage than the output transistors should protect it.

 

[homebrewed]

The CNC'd Sherline mill we had at work came with a warning on manually operating the axes when the motors are connected to the drivers. If doing manual operations they strongly suggested disconnecting the steppers to avoid damaging the drivers due to induced EMF. This was easy because the system was configured with DIN connectors to the steppers. Their driver had LEDs to indicate activity, and it didn't take much to get them to light up if you manually cranked the stepper. Sort of a warning signal perhaps?

I'd suggest looking at the owner's manual for the stepper driver you have, to see if they have any caveats regarding this. Or contact them, if you can. Better to KNOW than guess....

 

[macardoso]

You can backdrive a stepper and it will generate power to the drive. The cheap drives I've used tend to "power on" and try to lock the rotor when you get them spinning fast enough. An enable circuit would probably nix that. I've never damaged a drive doing this, but it just doesn't seem right. Electrically disconnecting the motor from the drive would be the best option, however don't connect or disconnect the motor while the drive is on or you will blow up the drive.