Oh dear – splinters or something worse in CNC

[Iain DownsParticipant @iaindowns78295]

recently I've been ignoring my poor CNC conversion and using the handles as I was working on some stuff that needed fiddling with rather than know accuracy.

Yesterday, I got the PC out and prepared to make some accurate holes. First, of course, a dry run!

This was not a success. With lots of grunting and growling, the beds moved erratically.

My first thought was that I'd tightened up the jibs a bit much and that I would have to do this bit by hand.

BUT. When I came to turn by hand I got some horrid grunting and grinding and the handles moved erratically! This did not seem like good news.

When I had a closer look, here is what I found.

X Motor

X Motor rear

Y motor close up

This did not seem good.

However, I've taken the motors off and inside and pulled off the splinters (generated by cleaning up the end of a 20mm square steel bar with a carbide rod) and the motors still turn (by hand) judderingly (though perhaps less so0).

Before I try and open the motors, I thought I 'd seek advice. 'Whatever you do, don't open the motors….' most likely!

The other thing is to understand what's happened here and how I can prevent it.

The CNC bit has run nicely for a while, then a few weeks hand turning and now, graunch .

Iain

[Iain DownsParticipant @iaindowns78295]

Motors clogged?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Iain Downs on 14/11/2016 18:14:38:

The other thing is to understand what's happened here and how I can prevent it.

.

That looks nasty

Putting the motor in an enclosure, with a shaft seal in the box-wall, might be a good move.

MichaelG.

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http://www.skf.com/uk/products/seals/industrial-seals/power-transmission-seals/radial-shaft-seals/index.html

Edited By Michael Gilligan on 14/11/2016 18:50:53

[John RuddParticipant @johnrudd16576]

The motors look like steppers? ( pardon my ignorance of cnc…)

If they are, then the swarf is attracted to the magnets in the motors…so, you could arrange a magnet upstream of the coupling to attract/ capture any ferrous swarf and add the seal arrangement suggested by MichaelG….

[Martin WParticipant @martinw]

Iain

You say that the stepper motor rotates but it judders when turned by hand. All/most stepper motors won't spin freely and have a cogging effect that can be felt when turning the shaft by hand and no power applied. This is due to the permanent magnet poles passing and being attracted to the winding/stator poles. While each step is small the effect of breaking the attraction of the poles can feel like running your finger on a smooth wet surface where it grabs randomly and you have to apply more force to move and you get a slip stick or a juddering motion.

I think the best way to test the motors, if you can, is to power them up and drive them slowly to see if they run smoothly without any problems. Hope this helps, not particularly clear I realise, and I hope that I am not teaching my gran how to suck eggs. Really depends on the level of judder but this will be affected by the size of motor.

Martin

[Ian PParticipant @ianp]

Nearly all motors contain magnets or magnetism.

I would say that nearly all motors have some arrangement to keep dirt out of the bearings (even if its only a close fitting hole).

I dont think I have ever seen a motor where swarf such as you show could get to a bearing (unless the bearing had its own seal).

I know that with some devices containing strong magnets its advised not to dismantle the magnetic 'circuit' but it might not be be relevant nowadays as magnets are much more permanent than of yesteryear.

Ian P

[Iain DownsParticipant @iaindowns78295]

The splinters are mildly magnetic – they are attracted to my pnicers more than the motors. However it could be that the motors grabbed them when they started running with fields being generated.

I would not have expected a stepper motor to have such poor shields that swarf could get in. Perhaps I'm wrong on this.

I'm aware of the cogging as each pole is passed when turning by hand. This is different. There is a distinct catch at certain points in the rotation. I've only played with my X motor so far. The catch location seems to move around a little bit so it does sound like some muck inside.

I've had these motors running on a few jobs, most notably in my crop circles thread where I've been experimenting with various settings and mill tunings to get the best finish.

One thing which may be relevant is that, before the trimming of the steel I did a log of work on cast iron. I wonder if cast iron dust has got in. But why now and why both motors? and – what to do

I shall attempt to open and inspect (and clean) motor number one and see what comes of it.

Iain

[Neil WyattModerator @neilwyatt]

As long as you don't wreck the bearings (sounds like they may need replacing anyway) it shouldn't be dramatic. The rotor is magnetic and will take the swarf with it, leaving the coils behind.

Should be no danger of damaging the coils.

[Anonymous]

Posted by Ian Phillips on 14/11/2016 19:41:59:

I know that with some devices containing strong magnets its advised not to dismantle the magnetic 'circuit' but it might not be be relevant nowadays as magnets are much more permanent than of yesteryear.

That's probably more because if something slips and the magnetic circuit decides it is going to reassemble itself it'll take your fingers off in the process, if they're in the way.

Andrew

[Iain DownsParticipant @iaindowns78295]

So I've taken it apart, cleaned the turny but and put it back together. It's still not right. What's baffling is that it's erratic. it will turn smoothly in one direction for while (clockwise). It does NOT like turning counter clockwise and once it's done so it can 'catch' turning the other way. When I had it apart, the bearings seemed to turn OK by hand, but perhaps that's because there was no load on them? May be I need to put some load on?

I did find some swarf inside, but in hindsight, I'm not sure my work area was clean (I've gone inside to my computer desk, but of course I took the outside swarf off here so it could be a bit dirty).

So as to get something out of this mess, here are some nude photos of a stepper motor…

The topless motor

The top bearing

The turny bit is magnetic and hard to get out of the ferrite case. I improvised a press out of a piece of 1/4 in brass rod.

Not my finest piece of engineering, but it worked.

and the innards

I think the next step would be to either try the bearing rotation under load or to just replace them. To be honest, there's not much else in there.

Iain

[Ian PParticipant @ianp]

Iain

When you are trying to turn the motor are all the wires insulated from each other (and not plugged in to the driver)?

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Iain,

Thanks for posting the photos

If you do replace the bearings [a wise move, I think], it's probably worth getting '2RS' equivalents.

MichaelG.

[Iain DownsParticipant @iaindowns78295]

I've just realised that the bearings are the same size as the 'skateboard' bearings I used to upgrade the X and Y leadscrew action in my mill!

Excitedly I swapped out the front bearing…. No apparent change. I should do both really, but not tonight…

Iain

[John Stevenson 1Participant @johnstevenson1]

Still looks to be turnings in the gap on the rotor. These things are a bitch to clean out and when put back they work but you will have lost some power

If we strip them we press the rotor out into a clean, close fitting steel tube to act as a keeper.

As regards getting crap in there in the first place, buy an oil seal same ID as the shaft, fit it to a thick alloy ring and araldite onto the end cover. Very rare we do this though as when we fit steppers to converted machines the front shaft is always inside a solid machined block and the rear is covered by a 3D printed cover off thing inverse. Never fit hand wheels, use a pendant or keyboard MDI commands.

[Clive HartlandParticipant @clivehartland94829]

Some Blu-tack will pick up small bits of swarf from the inside area, also Platsicine.

Clive

[Iain DownsParticipant @iaindowns78295]

Thanks, all.

I stripped out the first motor again and attempted more cleaning. I think I'd done a pretty good job the first time, but I also went heavy handed on the blu tack (white tack, actually, but tack is tack!).

I replace both bearings (including the one I'd put in yesterday and re-assembled. Spins smooth as you might hope! Well, there is the 'cogging' every 1.8 degrees, but that's to be expected.

Next the other motor which went smoothly and now also turns nicely. I've ordered some oil seals from Lancashire Seals which I will glue on before re-assembling.

What I'm seeing as the cause is not the splinters from the steel I referenced initially, but more the dust from the cast iron slitting and milling. I think that that was on and around the motor (gets everywhere!) and when the motor was actually turned on and thus the other shell magnetised strongly, the dust got pulled into the bearing, despite the metal shields. That fits with the erraticness of jerking as the bearings rode over and moved around the dust.

Well, it's a theory anyway.

Another marvellous learning experience based on breaking something…

Iain

[duncan webster 1Participant @duncanwebster1]

I've got some NEMA 17 motors which seized up because they had been out in the rain. For a few days they would rotate freely if you gave them a bit of humpty to break them loose, but then they seized properly. The manufacturer (actually importer) reckoned that taking them apart would ruin them. Was he just trying to sell me new motors? Or is it worth a go?

[mike TParticipant @miket56243]

Taking stepper motors apart WILL ruin them for sure, the core will loose a lot (but not all) of it's magnetic strength. Not a good idea, unless you know something the manufacturer does not.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by mike T on 16/11/2016 22:29:12:

Taking stepper motors apart WILL ruin them for sure, the core will loose a lot (but not all) of it's magnetic strength. Not a good idea, unless you know something the manufacturer does not.

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Mike,

I don't doubt your word; but it begs a question …

Do you happen to know how the manufacturers build them ?

… do they do the final magnetisation after assembly? … and if so, how?

MichaelG.

[mike TParticipant @miket56243]

As I understand the process: The core is magnetised by a powerful electromagnet, then pushed sideways into a steel keeper and then pushed sideways into the stepper motor during the final assembly. The magnetised core is never without a keeper.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by mike T on 16/11/2016 22:50:03:

As I understand the process: The core is magnetised by a powerful electromagnet, then pushed sideways into a steel keeper and then pushed sideways into the stepper motor during the final assembly. The magnetised core is never without a keeper.

.

Thanks, Mike

… That makes sense; although it would suggest that the process described earlier, by John Stevenson, is reasonably 'safe'.

MichaelG.

[Iain DownsParticipant @iaindowns78295]

This is all a joy and implies a need to buy new motors .

The core (turny bit) and the outside all appear to be iron laminates – much like a transformer core. That give some credence to the theory that they are magnetised before insertion, though it would also be possible that the central part is a super magnet surrounded by the laminations for conduction. Probably not.

What I might try and do is to set the motors back up and effectively turn them on and see what the holding torque is (pull on a balance attached to an arm on the spindle). It currently claims 1.26 Nm holding torque, though I was sure it was a lot more than that.

So a tug of 1.26kg at 10cm should force it to move against the energised magnets.

For interest these are the motors I've got. Whilst recognising the risks of handles as per Mr Stevenson, The CNC ness is only part time and the handles are essential (especially whilst prototyping) hence the dual shaft. They are dismountable..

Iain

[Ian S CParticipant @iansc]

It should be ok to strip a motor if it has rare earth magnets, or even ferrite magnets, the problem of demagnetisation was in the days of either steel, or later Alnico magnets.

If it doesn't go, go for it you might just fix it.

Ian S C

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