Current through mill spindle. WTF?

At new $employer I've spent the past couple weeks doing light hard-milling on 420 stainless on a new (month old) VMC. When making
real light finish passes (.001 - .002 DOC with TIALN coated soild carbide end mills) I noticed what appears to be tiny little blue sparks at the cutting edges of the tool when engaged with the material.
I pointed this out to my bossdude and he said they've saw it on the new mill, and an older mill of same model and make. They've got themselves convinced it's a normal reaction between the coated tools and materials. They get the same thing on graphite (EDM electrodes) on the other mill, I've been told.
If it's normal, how come I've never seen any such thing anywhere besides an EDM machine in my 27+ years of doing this stuff? Late yesterday afternoon I was doing some 3D contouring on the new mill and "sneaking up" to some existing surfaces I had to blend to, and when the tool was a couple thou away from from the existing surfaces, I got arcing in the gap between the tool and the workpiece.
At this point I'm now convinced there's current traveling through the spindle while it's turning. No matter how small the current may be, methinks it definitely is not right, and can't be good for the machine itself.
Anybody ever seen anything like this? I'm probably going to need to be armed with some convincing arguments to get a tech to address the problem, and I know fuck all about electricity except that enough of it had a tendency to be painful. :-/
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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 Dragon wrote:

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snipped-for-privacy@excite.com wrote:

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. <g>

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.
http://www.milltronics.net
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 fuck 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.
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Black Dragon wrote: <<Snip>>

Cool dry air is great for building up static. Is the air system grounded anywhere? Should an air system be grounded? Is this question going to- no, I won't even go there.
<<Snip>>

Try Parameters/Tool/Edit, and the length offsets should appear in the left column, and the diameters will be in the right column. As you tab through the numbers with the arrow buttons, the current offset for each one will appear next to itself. You can then type an equation and look at it next to the original number before pressing Enter. Is it a Centurion6 control?
Later,
Charlie
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Black Dragon wrote:

Did you ever get a chance to try incremental 3D programming on the Fadal with the 18i control?
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Joe788 wrote:
[...]

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

Ceramic bearings would do that.
John
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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.
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Anthony

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Anthony wrote:

This sounds plausible. They had some major problems with another machine last week that I think was at least partially power related. Thanks.
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BD, Every time I see someone mention the earth ground as a source of machine problems my hair stands on end! If a machine is 'fixed' by grounding it the only thing you can be absolutely sure of is that it is still wired wrong! It is NORMAL for the grounding circuit in building to have current running through it. The purpose of the earth ground is NOT to drain off this leakage current. The purpose is to ensure that the voltage difference between the earth and all the conductive paths in the building is so low it is not dangerous. If you read how grounding is to be done according to the NEC you'll see that it is not about creating the mythical "clean" ground, it actually calls for creating as many ground loops as possible. The reason for this is that leakage currents will always be present. Most of the time these are in no way dangerous, except to sensitive electronics connected to the mythical clean ground by idiots. What is dangerous is when you have huge currents, thousands of amps flowing, like when a motor shorts out. Then the huge currents follow every path in the grounding circuit. Because the current travels everywhere there is no place where the voltage difference is great enough to be hazardous to you touching metal. Of course for the sensitive electronics connected to this "clean" ground, you might as well try welding on the circuit boards!
I do think the sparks are potentially a problem, unless they are static in nature. Static electricity has a very high voltage, but very low current. It can zap electronics because they are typically sensitive to high voltages even if very little current is present. On the other hand the 1-1/2 volt that you measured could be very damaging to bearings. Consider a spot welder. The typical spot welder works by using a transformer to convert a high voltage power source at low current into a low voltage (1 to 5 volts or so) at very high currents (thousands of amps) You see how two pieces of metal get welded together by that current, now imagine what it could do to bearings. So I think further troubleshooting could be very worthwhile.
Gary H. Lucas
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<snip>
Yup, but when that earth ground is no longer present, or degraded, then that leakage current tries to go anywhere it can find the least resistance to earth, not where, or how you wish it to to be directed (by creating a path of least resistance via the earth ground). I've seen poor grounding cause all kinds of ills, especially in a logic-controlled device.
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Anthony

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Anthony, Again I'll say what I have said before. If removing or installing a ground affects the way a machine operates then the machine is wired WRONG! Properly wired systems do NOT change operation due to grounding. The grounding is only for electrical safety. When you drive a ground rod next to a machine you have in fact compromised the electrical safety of the whole building, not just this one machine. Electrical inspectors ought to be on the lookout for this kind of "fix" and order the offending machines shut down until the proper repairs are made and the ground rod is removed.
Gary H. Lucas
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On Sun, 13 Aug 2006 23:38:18 GMT, "Gary H. Lucas"

Gary,
There are times in some area's of the states where ground rods solve problems.
As long as the ground rod shares a common connection to the building ground it poses no safety hazard.
It is not something to do unless you understand your "Specific: buildings ground and what the oem has to say.
Supplemental ground rods are permitted under the National Electrical Code (NEC), but they should only be installed, if at all, after a careful analysis of the building's entire electrical, grounding and lightning protection systems.
(http://copper.org/applications/electrical/pq/casestudy/grounding.html ) (http://www.mmsonline.com/articles/0302bp4.html) (http://www.ewh.ieee.org/soc/pes/cleveland/Papers/cnc-isa-icps.pdf#search ='grounding%20rodsmachine%20tools')
I know your an expert on the subject, maybe you can read the links and present some comments as to where they are wrong.
Regards ' Regards Daveb
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On Sun, 13 Aug 2006 23:38:18 GMT, "Gary H. Lucas"

Driving a supplemental ground rod is a practice that many in the industry either don't recognize or don't support. The NEC doesn't consider the practice because it's not a matter of life safety. Manufacturers of electronic equipment like computers and electronic motor drives don't subscribe to the theory because it's unnecessary. And IEEE standard 1100 (The Emerald book), which focuses on powering and grounding sensitive electronic equipment, doesn't recommend it. Instead it recommends a single-point ground, from individual electronic cabinets, be individually bonded to a local ground grid.
Some CNC machine manufacturers recommend, or even require, the addition of a supplemental ground rod at the CNC machine. (CNC Machine Manufacturers Speak Out on page 24). In most cases, this is an 8-foot copper rod driven next to the machine, often through the concrete floor, and bonded to the machine's ground plate. The NEC does permit such an installation, as long as the ground rod is bonded to the building electrode grounding system. However, you must realize that this installation may still invite stray ground currents.
(http://ecmweb.com/mag/electric_avoiding_pitfalls_powering/index.html )
Best
Daveb
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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
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Gary H. Lucas wrote:

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
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john wrote:

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.
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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
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Gary H. Lucas wrote:

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. <g>

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.
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