An electric motor actuated vice

I have been asked about the possibility of making a vice actuated by an electric motor. My (very limited) understanding of 'automatic' vices as used eg in CNC machines is that they are hydraulically or pneumatically actuated, and the clamping pressure is regulated by controlling the pressure in the driving fluid. That's just an assumption - I've no experience and I'd welcome any correction if that's wrong.

It seems to me that it would be difficult to do this with a a direct drive from an electric motor. The only things I can think of are (a) an electronic solution which would somehow set the stalling torque of the motor or (b) a mechanical solution involving a constant torque clutch (maybe like those on cordless drill/drivers). I imagine running a stalled motor for any length of time isn't good, and making a clutch would be - well, possible perhaps.

This is really a sanity check - I'm getting better with this engineering stuff, but still not confident enough to dismiss the idea out of hand without taking advice from m' (more) learned colleagues.

I'd be grateful (difficult as it might be!) if 'why would you want to that'  responders would refrain - I don't know either! It's a 'how' rather than 'why' question.

Happy New Year to y'all, Robin

Edited By Robin Graham on 02/01/2019 00:14:17

Edited By Robin Graham on 02/01/2019 00:28:49

I would opt for Stepper Motor[s], Robin

They are happy 'stalled' and I doubt if you would have any problem with step size when clamping wood.

MichaelG.

Thanks Michael - but how did you know this was about clamping wood?

This project has already taught me a lot, and now I'm motivated to to learn about steppers...

Robin

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Because you mentioned the possibility in your other thread about the wood vice.

However: If your 'customer' has moved on to stiffer materials, then please accept my apologies for being so presumptuous, and start looking at pressure semsors !!

MichaelG.

Edited By Michael Gilligan on 02/01/2019 00:33:22

Emco used to supply a motor driven vice with the 100 series milling machines so another option.

Emgee

Homemade one here  http://forum.retro-rides.org/thread/192636/power-vice-bush-mechanic-style

Edited By Hacksaw on 02/01/2019 07:23:11

If you use a "normal" permanent magnet brushed DC motor this is very easy to do. The torque of the motor is directly proportional to the current so all you have to do is use a constant current supply. As long as the maximum current is less than the maximum continuous current rating of the motor it will sit stalled producing the set torque and automatically take up any slack. You would probably want a geared motor and a ballscrew would be better than a plain nut. Speed is directly proportional to voltage so this can easily be a separate control.

The biggest issue I can see is safety. This is essentially a horizontal press so would need a guard and possibly a two hand operating control. Neither of these are much use for a vice. As the law currently stands you have to be compliant with CE regulations, certainly if using for business or supplying (Inc. giving away) to another party. This means Machinery directive, EMC directive and possibly Low Voltage directive. Note CNC vices operate behind guards.

While your friend may be happy to take the risk, if there was an accident his insurance company may not take the same liberal view and come after you and your house or other assets.

Robert G8RPI.

The unit that Hacksaw linked to shows why safety is a bit moot with this sort of device. The user wanted it to leave his hands free for positioning work in the vice! I'm not sure why this is different from any other machine tool, we know they are dangerous and have to take proper precautions, but there's a line where those preclude the intended operation of the device.

If the vice is powerful enough to cause injury then it needs safety precautions. Just because it was good enough in the past does not mean it's good enough now. Old lathes had no chuck guards or zerovolt releases, new ones do, even the new (and most refurbished) Myfords.
I'm not the HSE or CE police, but as a professional I feel it is important to make sure that anyone asking for advice on this sort of thing are aware of the safety implications. When doing it for someone else it rises to a whole new level. There are too many ambulance chasing lawyers and insurance companies trying to get out of claims or recover costs elsewhere to take chances. The 4X4 community have learnt this the hard way. Helping pull someone out of a snow bank or ditch is all well and good, but if they don't sign a disclaimer beforehand they, or more likely their insurers can come after you for damage actually or allegedly caused during the recovery. Just because Frankenhealy did it does not make it safe, legal or a good idea. Relying on slipping v belts to limit torque is definitely not a good idea.

Robert G8RPI.

If you need to include a safety device [*] have a look at the way they stop electric windows in cars.

... obviously the system will not have 'Type Approval' for use in a vice, but it may well be adaptable.

MichaelG.

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[*] ... Ooops, there's a pun or two lurking in there

May not be quite the same as an electric window needs to stop at any resistance greater than the friction of the channels it slides up but an electric vice would need to stop at a predetermined pressure, but greater than nominal, to hold something tightly.

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Yes, Nick ... I realise that it may not be 'quite the same'

But the control principle [threshold load triggers stop and retract slightly] is still worth a look.

MichaelG.

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https://www.yourmechanic.com/article/how-does-the-power-windows-auto-reverse-safety-feature-work

Edited By Michael Gilligan on 02/01/2019 16:05:36

Sounds like a recipe for making a vice guaranteed to cause maximum frustration!

Neil

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Perhaps I should have put more emphasis on the word 'need'

If you really, really NEED to include a safety device

MichaelG.

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Edit: [for the avoidance of doubt] My suggestion was made to address the issue raised by Robert G8RPI rather than being a personal preference.

Edited By Michael Gilligan on 02/01/2019 16:32:13

Many years ago, as part of a college project, I built a small gripper using a stepper motor to close it and a small elastic band as a 'reset', to open it when the motor was de-energised. It didn't work. My report, written in red, said:

"It was expected that that when the jaws closed the stepper motor would stop, and thereafter 'cog' each time a motor pulse was sent. What was not known, and has not been seen in any stepper motor literature, is that the stepper motor loses all its torque when it cogs. The astonishing result on the gripper is that the jaws fly open to their fullest extent, under the influence of the elastic band! Put simply, the gripper cannot and does not grip. The one consolation is that the mechanical part of the gripper works perfectly."

I've never investigated further, but I wouldn't use a stepper motor for this application.

In terms of torque characteristics, a series-wound DC motor would be better than parallel wound or permanent magnet motors, but would need a current limited supply as Robert Atkinson 2 suggests.

Edited By Georgineer on 02/01/2019 17:31:36

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There is, I suggest, a very significant difference between 'stalled' and 'switched-off'

MichaelG.

Georgineer is correct. When a stepper being driven at speed is stalled it looses virtually all it's torque as the time for the poles to magnetically/mechanically realign is much longer than the time the pole is energised. if ther is an opposing force it has no chance. Before the stall it depends to a large extent on inertia to stay in sync. You have to ramp the speed up and down to get full performance and any resonaces in the system can cause issues. Some stepper systems have intertial dampers (elastomeric mounted balance weights) on their shafts to help with these issues.

Robert G8RPI.

Edited By Robert Atkinson 2 on 02/01/2019 20:10:33

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Perhaps we have a problem with semantics

I know for a fact that I designed a vibration test fixture for part of a weapon system ... this had to be rotated to various angles of 'roll' whilst being vibrated at levels representative of missile flight.

We used a stepper motor and toothed-belt drive, and it behaved perfectly.

Rotate and hold [energised = locked]

... I can say no more.

MichaelG.

Edited By Michael Gilligan on 02/01/2019 20:10:00

Electrically, yes, I agree.

However, on the gripper I built there was no noticeable difference between the 'stalled' condition (which occurred when the gripper closed on an object, thus stalling the motor) and the 'switched-off' condition (which I described as the motor being de-energised).

In both cases the jaws flew wide open under the influence of the elastic band, which astonished me.

George

That is a different situation your system did not stall, it was slowed to a halt. The drive signal was stopped and the motor is still in sync. In the stall situation the rotor is out of sync with the drive signal and magnetic field. Typically the rotor is stopped (or in Georgineer's case moving in the opposite direction) while the magnetic field continues to rotate at speed. To re-sync the field rotational speed must be reduced until the rotor (and load) can re-align and lock together. If you increase the drive speed on a stepper it will accelerate smoothly to a point and then suddenly stall. There are other factors such as the coil inductance and supply voltage (these lower the current available at speed) but primarilly it's loss of magnetic lock that is the issue.

Robert .