Current through mill spindle. WTF?

BD,

Been hard milling and HSM machining for some time now. I've seen this once or twice in all my years but it was done while dry machining and using only air - very light finish cuts. I always use oil/mist. However during HSM of graphite this is more common. I would lean towards that something is wrong. Yes, perhaps its because of the "right conditions" but I would suggest having it checked out. This is interesting so please keep us all updated. btw, care to let us all know the machine?

gary

Black Drag> At new $employer I've spent the past couple weeks doing light

Did you ever get a chance to try incremental 3D programming on the Fadal with the 18i control?

If you think there is current going through the spindle check it. That's a simple way to find out. I don't see how it could unless the headstock was completely insulated from the bed.

Black Dragon wrote in news: snipped-for-privacy@bdhi.local:

It could be static electricity, but I'm kinda leaning toward a bad ground somewhere. It could be the building ground. As dry as it has been this summer on the East coast, it could be that the ground around the electrode is too dry for efficient grounding.

BD, Electric motors have had bearing problems for years due to stray currents passing through them. 3 phase electric motors work by inducing currents to flow in the armature, so leakage is always a real possibility. If this mill has direct drive spindle that could be the source. This problem should be easy to diagnose. just hold a voltmeter probe lightly against the running spindle and the other end to the table. Anything that has enough energy to produce a spark will show up on the meter. Spindles today are also using ceramic bearings. This could make the problem worse because to bearing don't conduct so the cutting tool would have to.

If the spindle is belt drive, and the air is dry, then the belt and pulleys can work like a Van De Graff generator and build up a large static charge on the spindle. I've seen this problem on various kinds of rotating machinery.

Gary H. Lucas

IF the belt is charging the spindle, spray it with some antistatic spray from an electronics supply place.

A spark indicates electric current flow. This is not a good thing for the bearings since the flow is also through the bearings. If you are seeing sparks I bet if you touch the smooth surface of the running spindle you will feel them too.

Does the machine have ceramic bearings?

John

That's weird--BUT...(and as as someone else has already suggested)...concievably this could only could happen if there's ceramic bearings.

I say it's probably really nothing to worry your head about.

Thing to do, (I spose) is to see if you get any kinda "jolt" after the spindle has stopped, when releasing the tool manually--IF so, suggest then always run the atc in order to cycle in a different tool beforehand.....

Then next time the bearings get replaced, specify or even mix for yourself some small amount of graphite or other conductive substance into the grease.

Fwiw, one of our cats simply loves it--vigorously pet his fur...then touch his nose....

Bzzzzttt !!!

We don't have oil mist, yet, only air. The compressed air is well dried and the shop has been bottled up with the A/C on making the air in it dry too.

I've cut A LOT of graphite on a Roku-Roku and the only time I saw sparks is when I hit a screw holding the graphite in its 3R tooling.

After reading this entire thread, I'm still leaning towards the 'something is wrong'.

It's a Milltronics VM17. Why a VM17 isn't listed on thier web site I have no idea.

The machine itself seems pretty decent. I don't like the control because it's PC based, and because it's missing important to me features such as being able to increment tool and work offsets. To change an offset, you have to do the math yourself and enter a *new* value. A dyslexic moment will f*ck something up and we all get them from time to time. Also, I've seen people here on amc complain about all the soft key pushing one has to do to run a Fanuc control, well, the Milltronics piece has Fanuc beat by having at least twice as many buttons to push to accomplish the same tasks.

[...]

No I didn't, and I no longer work for the company that bought it.

This sounds plausible. They had some major problems with another machine last week that I think was at least partially power related. Thanks.

I did check with a voltmeter but knowing little about electricity I was unsure of what I saw. When touching a probe to the spindle housing and the other to the table with the spindle turning I didn't get a reading. When touching a probe to the rotating tool holder itself and the table I got a reading of about 1.7 volts IIRC. The tool was spinning at about 4K at the time. Can the act of probing a rotating tool generate any current?

The (12K rpm) spindle, I'm told, does have ceramic bearings. I haven't time to research the machine, too busy using it. The first time they have a Milltronics tech in he's going to get bombarded with questions.

Intersting. I didn't know it when I made my original post on the subject, but apparently this is exactly the kind of info I was looking for. :-)~ Thanks.

Ceramic bearings would do that.

John

======================= I note that you are a new hire. I will pass on a bit of advise that my Father passed on to me, and which I ignored for some time to my cost. "No body likes a wise-ass."

Having got that out of the way, see below:

googled on >bearing failure "induced current"< and got c. 17,000 hits

see

had to view html version at http://64.233.161.104/search?q=cache:AzfIA4ZZ3vYJ: let us know what you find & what happens -- inquiring minds want to know.....

Unka George (George McDuffee)

...and at the end of the fight is a tombstone white with the name of the late deceased, and the epitaph drear: ?A Fool lies here, who tried to hustle the East.?

Rudyard Kipling The Naulahka, ch. 5, heading (1892).

Yes, it has ceramic bearings I'm told. But I'm not sure how the spindle is driven. The mill does have a high and low spindle range so I'm assuming it's not direct drive. Probably belts in there instead of a gearbox. Only Gene Haas was stupid enough to but gear boxes in low powered / inexpensive machines. Easy enough to find out and easy enough, I see, to diagnose if there are in fact belts whether or not they're the source of the current. Thanks.

Where there's a spark there's likely to be erosion if I'm not mistaken, correct? Where's the current going? Is it finding its way down through the ball nuts, into the screws, and out the servos to who knows where, and it there enough to do any damage to those components in the long run?

Those are some of the things that started going through my head when I first saw the sparks.

I do believe there's justified reason for some concern here.

If the spindle is directly driven by the motor with no belts, you have leakage in the motor winding. A check with a good megger will quickly prove that out.

Perhaps, too, that the ceramic bearings themselves are generating the voltage. ( Piezo electric principle).

John

Bla

The high/low range may be done with changing the pole wiring on the motor, putting the pole pieces in parallel or in series.

A gearbox makes up somewhat for an underpowered machine.

John

Convert it into an EDM machine :)

Unless you can determine where the voltage is coming from you cannot determine if it will damage anything.

You did say that you had another older machine that has been running for quite a while that was producing sparks too. If that machine is still running fine, I would not be too concerned about it, only interested as to why it is making sparks.

John