A Dumb Idea? Stepper Motors As Power Feeds.

Hi Jim,

I found myself here thinking about a similar solution, but i am out of my normal zone. How would this setup handle manual turning of the leadscrew handle? I assume one wouldnt have to overcome the holding torque as power would be off the motor, but arent there issues with turning over steppers and it driving a voltage back towards its drive? Is this somehow accounted for here? Apologies if that is a stupid question, as i mentioned i'm (as yet) out of my depth on this.

Thanks,
Kevin

There no stupid questions at Hobby Machinist:) and this is a really good question.

The best way would be to simply unplug the motor (between the motor and the drive) when using the machine manually. This could also be done with a 4 pole switch or a relay. It is possible to fry the drive by turning the motor, but it is unlikely that you could turn it fast enough by hand to do that.
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I have had a similar feed situation on a small russian lathe. I purhased a 12V wipermotor from a car scrapyard, built a simple two speed tumbler gearbox and connected the whole lot headstock end to the leadscrew. A PWM drive drives the wipermotor and there is loads of torque, even and deadslow speed. I used a pc powersupply as a powersource and car relays to control forward/reverse and jog. A pot on the pwm drive regulates speed

The 2speed tumbler box has a neutral position so the leadscrew can still be operated manually
 
Try to find motors with shafts on both ends. This should allow you to fabricate a handle to manually operate the mill when the motors are not powered.

Steve
 
There no stupid questions at Hobby Machinist:) and this is a really good question.

The best way would be to simply unplug the motor (between the motor and the drive) when using the machine manually. This could also be done with a 4 pole switch or a relay. It is possible to fry the drive by turning the motor, but it is unlikely that you could turn it fast enough by hand to do that.
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Jim, thanks for that. I will keep that in min if/when i get round to this job. As others have mentioned, i like the idea of steppers as it is a step (Geddit?!) in the direction of a cnc conversion at a later point. Im working on an old (but tight) Beaver mk2 knee mill) Am i right in saying that some drives now have some protection built in for this too? I will take your word for it, i just noticed your signature, seems i caught the right guy to ask! ;)

Coincidentally, just looking at the map, i only drove within 10 minutes of your office last summer, was over from Ireland working down in Corvallis and spent some time around Portland and Seattle. Small world!

Kev
 
Normally to get the back EMF protection for a drive you would install a regen clamp. This dumps the regen power across a resistor, and protects the drive. This would be rare on a small CNC machine axis drive, more commonly used where the load can over run the motor, like an elevator or very large Diesel-electric earth moving equipment. Also use in the braking circuit in VFDs.

If you ever come this way again, let me know. I'm always happy to meet the members here.
 
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