Mill power feed using stepper motor

[Martin ConnellyParticipant @martinconnelly55370]

The red buttons on the pulse board change the voltages on the unused terminals so are not used in the layout shown.

Martin C

[leonidasParticipant @leonidas]

Thank you, Martin.

[BazyleParticipant @bazyle]

Whatever happened to the old favourite – windscreen wiper motor? Nowadays there is an endless supply of battery drills with variable speed but dead batteries that cost more than a new one. Old car battery and charger and you can rig something up maybe?

[Alan Wood 4Participant @alanwood4]

I added steppers to my Myford VMB mill with a simple control box that selected which axis, forward or reverse and speed.  I retained the handwheels so no clutch etc needed.   Controller is a single Arduino electronically switched to the desired axis.   Circuitry, bill of materials and Arduino code included in the write up.

https://altrish.co.uk/2020/11/09/adding-stepper-motors-to-a-myford-vmb-manual-milling-machine/

[Joseph Noci 1]

On Joseph Noci 1 Said:

Posted by John Haine on 06/10/2021 22:04:57:
That can be quite hard, so a clutch is welcome (or you have to switch off the drive which is a pain, or as Duncan suggests use the drive enable signal if it has one) – and you can’t just disconnect the motor as that will probably make the driver emit magic smoke. There are different views on back driving the stepper when switched off – might it blow something in the drive or not?

I guess a small toggle switch in series with the stepper drive enable signal ( they all have this signal..) is just as easy to activate, if not more so, than a lever/pins, etc for a clutch action, and a lot easier to implement mechanically.

The issue of back driving the stepper – I did cover this with an analysis on some previous post on the subject, but the posts get lost in time so quickly..

There is NO down side to backdriving other than if the stepper is a powerful one you may feel a slight notchiness at the handwheel.

All half decent stepper driver will have back-emf diodes at the switching devices, and if FET’s these are inherent in the FET. In addition, the back emf will never exceed what is seen under driven conditions, since the generator action back emf is the same as that seen while the stepper is driven as well – it is a permanent magnet motor/generator..Most stepper drivers are rated from at least 24volts, most 4 to 6 amp drivers at 40 to 50 volts – spin a poweful stepper at 200rpm and see what voltage you get, even off load – you will be hard pressed to reach 50volts rms…And when ‘spinning’ the mill table handwheel, the stepper rpm will never reach 200rpm…

That myth needs to be put to bed..

Joe

Hi Joe,
That is not quite correct. The back EMF of a motor opposes the applied voltage i.e. it is reverse polarity to the applied voltage. When you overdrive a motor the generated voltage is the same polarity as the applied voltage so back EMF diodes or MOSFET body diodes do not conduct to clamp it. In fact for a lot of designs the protective diodes will feed the voltage back into the supply. This can cause overvoltage damage. In particular if the circuit is fed from a integrated circuit voltage regulator. Most of these can (will) be damaged if the output pin (where the motor drive circuit is connected) is at a higher voltage  than the input. Higher means less than a volt (>0.6V typically) more. This may or may not be an issue when the supply is on but if the supply is off the input will be at 0V so it does not take much voltage (shaft speed) to cause damage. It is of course possible to design the driver to withstand this, but there is no certainty that a given driver (especially a no-name modle from the far east) has. On big industrial motors large resistor banks may be required to absorb the excess energy of the motor over-running and convert it to heat. This even applies to AC motors on VFDs or servodrives where the generated voltage is rectified and fed into to th DC “bridge” capacitors. Even with small motor if they have a load with a lot of inertia they will cause a VFD to trip on “bus overvoltage” it the system trys to stop it too quickly.

Robert.

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