Fitting Stepper Motor to 2.125" x 7" Small Lathe.

I am refurbishing a small Instrument type lathe from hand long travel to Stepper Motor drive using electronic means..

I have considered to fitting the leadscrew to the Stepper as a direct drive in both longitudinal directions of the toolpost. However thinking back to schooldays 70 odd years ago, I seem to remember that rotating electric motors without application of power, as in this case when using hand feed, can create a back EMF and possible damage to the electronics involved.

I am trying to do away with the complications of fitting a small clutch type device to the leadscrew drive.

I am sure within our fraternity this question may have been answered many times in the past and would welcome any help in solving any problem involved.

My milling machine table is driven by a stepper motor, permanently engaged, and it hasn't suffered any problems from being hand wound

Steppers get very stiff back driving when the power is off. Not so bad if you can disconnect from the driver. If the power is on back driving is very difficult and "ratchety". A clutch, disconnecting the drivers, of arranging a manual encoder, would be best.

Yes it can cause problems to the electronic controller but it really depends on the controller.

I've had an issue with an Arduino driving a stepper via a driver module. When moving the stepper with the Arduino powered off I noticed that the Arduino LEDs would light briefly. On one occasion I managed to corrupt the Arduino memory by doing this. I think the reason is that feeding substandard voltages into the output pins can bypass the 'brownout' protection on the memory refresh.

Higher end control systems use opto isolators between the sensitive control electronics and the nasty noisy electromechanical bits which should prevent issues like the one above... at a cost of course.

In terms of resistance to movement, stepper controllers typically apply a holding current when the motors are not moving because in many applications you want the motor to hold position even if there is some force trying to move it. The software can turn this off (maybe via a manual command or after a timeout period) and the steppers then become quite easy to turn by hand.

A friend of mine has suggested using steppers with a shaft on both ends - one end coupled to the leadscrew and the other with a handwheel on it... but bear in mind all of the above if you go down this route.

If you set up a system that can use a manual pulse generator (mpg) you will not need to manually turn the stepper motor. Just use the mpg to drive the stepper under hand control. I have seen comments from someone who left handwheels on a CNC modded mill using Mach3 who expected to want to use them. After a short time he found he wasn't using them and they were a safety risk so he removed them.

Martin C

Don't worry about it. All modern stepper electronic will not have a problem. The coil driver FETS/Transistors are all sized to suit coil back EMF amplitudes that occur during coil switching, and all have protection diodes to clamp said energy. You need to wind the stepper quite fast to generate such voltage levels, and even if that voltage got past all the protection diodes ( and the reverse diodes intrinsic to the FET), the base electronics of the stepper driver are all fed from voltage regulators, which would protect the electronics anyway.

John Haine says:

Steppers get very stiff back driving when the power is off. Not so bad if you can disconnect from the driver. If the power is on back driving is very difficult and "ratchety". A clutch, disconnecting the drivers, of arranging a manual encoder, would be best.

With Stepper driver power OFF, I have not found any impediment presented to the manual rotational effort by such action - this with quite a variety of Chinese stepper controllers, varying from 35vDC to 80vDC max input, and current ratings from 2A to 10A.

I just went to test again on my lathe and shaper - no noticeable resistance at all - the lathe has a 4NM stepper, the shaper X axis has a 16NM stepper running 10amps at 90V peak.

Joe

My stepper drivers have an extra 'layer' with protection diodes but moving the steppers still made the LEDs on the Arduino light up and on one occasion corrupted the memory.

I think the protection prevents damage to the drivers but does not prevent voltages from being fed back into an unpowered Arduino.

The 'damage' was just data but I did need to reflash the boot loader before the Arduino would work again.

I've seen a similar issue on another Arduino - no steppers involved there but the common factor was a stray voltage being fed into the Arduino outputs. On this occasion it was settings held in EEPROM that were corrupted - easier to fix.

So in my experience... back EMF can cause a problem.

It will depend if the drivers share the supply with the Arduino. The protection diodes rectify the induced volts from the stepper as its driven. Though it seem that Joseph hasn't observed a problem I certainly have. Never known it damage the driver though, it's just a nuisance.

Stepper drivers ALWAYS apply a holding current at the current microstep value, though this may be reduced while the motor is stationary. Otherwise the stepper will lose its position.

Much the best approach, as Martin suggests, provide manual pulse generators so you have "electronic" handwheels. Actually I used to have a game controller with one of these used with Mach 3 - you could select which axis it drove. Now replaced with an Xbox hand controller, works fine. I almost never apply cuts manually anyway now.

Is there another route into the Arduino like a shared power supply?

Shameful confession time! My rotary table driver gets 5V to power the Arduino by regulating off the 24V line used to power the stepper motor module. Although it saved me fitting two mains-powered PSUs, it's bad practice because it increases coupling between the motors and the Arduino. The opto-couplers don't protect the power lines and turning the stepper motor with the power off might well spray the Arduino with unstable volts and confuse it. I doubt the Arduino's Brown-out Detector would handle extreme power glitching reliably.

I got away with the gamble but my rotary table setup never rotates the motor as a generator. That said, any sign of Arduino misbehaviour and I would instantly suspect my power supply bodge.

Dave

Yep, as Dave said - sharing the stepper driver power bus with your other electronics, arduino, etc, is really not recommended.

Andy, I suspect your stepper supply also powers the Arduino? ( via regulators, etc..) That also makes the opto-isolation in the stepper driver somewhat useless...

It is NOT recommended to use a common supply for stepper and processor - Voltage spikes become common and destructive - small steppers are probably ok, but still..

John, you correctly mention the holding current, etc - note I did say that the stepper is easily turned by handwheel with the driver power OFF..

Terry, a simple test - fit a handwheel of sorts to the stepper shaft, connect the stepper to your stepper driver, without power, and turn the handwheel. If it turns freely enough, ignore the lack of a problem, and get on with it..

Joe

It's quite possible that the power supply was shared at the time this happened. The setup has had dual supplies and a shared supply at different times. TBH it's a while back and I can't remember.

@Joe - my setup doesn't have opto-isolators, just protection diodes.

My current CNC setup probably does have separate supplies because the Arduino is powered (only) via the USB port and the CNC board and motors have their own supply.

I think we are all agreed that in answer to Terry's original question, yes back EMF can cause a problem but it really depends on how you have arranged your controller and its power supply/supplies.

The holding current behaviour depends on what the software does. I'm currently running Marlin and that applies a holding current but it will time out after a (configurable) period. Marlin also provides a g-code command (M18) to switch off the steppers immediately. Other software likely has something similar.

My own approach is to make my stepper mounts easily removeable. I can put the handles back on in a couple of minutes if I want to use the machine manually (it does happen).