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Magnetized mill spindle generating static electricity??

davesvo

Aluminum
Joined
Apr 1, 2013
Location
Pottstown
Ok guys I have a weird one. Just purchaed a used Hurco VM20I (2020 year machine) Got it installed and up and running fine, everything is in great shape spindle is quiet, machine has about 2400 spindle hours on it. 12k spindle with chiller. When I was setting up the first job (aluminum casting) I noticed a spark when the tool first engaged the material, small spark, almost not noticable and i did'nt think much of it untill i saw it again (running first passes with coolant off so i can see what's going on). So I started taking notice and sure enough everytime the tool engaged the material there was a small spark, just like a static electricty spark you get from dragging your feet on the carpet. So I went over the whole machine checked all power and ground connections, checked everything on the electrical side, nothing wrong. There is no measurable voltage anywhere it shouldnt be. Then through talking with one of our engeneers trying to understand whats going on we decided to see if the spindle was magnetized and sure enough it is. The rotating part of the spindle and the housing are magentized, the rotating part more than the housing, the conclusion is it is generating static charge due to the magnetism. Has anyone seen this before? How would the spindle get magnetized? previous shop used it for aluminum and some titanium. Any Ideas on how to demag the spindle? When the spindle is rotating above 1000 rpm it has measurable ac voltage between the rotating part and the housing, about 3.5V AC.
 
I avoid using magnetic bases/holders on my machines. Just chips wanting to stick is bad enough.

Cutting/machining causes a voltage. There was an outfit that would measure that voltage and cause a reverse voltage to reduce cutting tool wear.
 
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Magnetism doesn't generate a static charge. It can induce current, but I doubt that is enough to make a spark.

When the bearings get up to speed the balls float on a layer of oil, and the ball bearing becomes nonconductive. Static can build up from friction. A lot of motors have a brush on the shaft to ground it to the housing, as static will destroy the bearings as it discharges across them. See if you can find a parts diagram and look for something like that. If the brush is worn, damaged, or oily it will not do its job.

3.5VAC across a 10Mohm multimeter doesn't really surprise me, as a result of induced current and static from friction.
 
Magnetism doesn't generate a static charge. It can induce current, but I doubt that is enough to make a spark.

When the bearings get up to speed the balls float on a layer of oil, and the ball bearing becomes nonconductive. Static can build up from friction. A lot of motors have a brush on the shaft to ground it to the housing, as static will destroy the bearings as it discharges across them. See if you can find a parts diagram and look for something like that. If the brush is worn, damaged, or oily it will not do its job.

3.5VAC across a 10Mohm multimeter doesn't really surprise me, as a result of induced current and static from friction.
Hurco says to install some kind of brass brush on the spindle, they don't have one or supply one and the machine never had one. This is a belt drive spindle so rubber belt between the motor and spindle. The spindle bearings are ceramic and I am guessing the cage is non conductive also, there is no continuity between the spindle housing and shaft when the spindle is off. even if the static is created by friction I feel the more pressing problem is the magnetism, I have concerns that steel chips will find there way stuck to the spindle housing or in the spindle taper, it's magnetic enough to hold on to a paperclip so enough to collect small chips. Hurco has never seen a spindle get magnetized. What's really odd is we have 6 other Hurco mills and 2 more on the way and have never had this problem with any other machine.
 
Just for reference here's a video of the sparking that happens, I have tried it on multiple pieces of material, this is aluminum with a 3/4" uncoated em.
 
Hurco says to install some kind of brass brush on the spindle, they don't have one or supply one and the machine never had one. This is a belt drive spindle so rubber belt between the motor and spindle. The spindle bearings are ceramic and I am guessing the cage is non conductive also, there is no continuity between the spindle housing and shaft when the spindle is off. even if the static is created by friction I feel the more pressing problem is the magnetism, I have concerns that steel chips will find there way stuck to the spindle housing or in the spindle taper, it's magnetic enough to hold on to a paperclip so enough to collect small chips. Hurco has never seen a spindle get magnetized. What's really odd is we have 6 other Hurco mills and 2 more on the way and have never had this problem with any other machine.
That definitely sounds like an answer Hurco would give to somebody. "Try this, we don't have a part number because its not standard."
 
I guess if it was me...I'd install a shaft grounding brush. I doubt it's coming from 'magnetism' but rather from a poorly grounded machine. The grounding brush would be a sort of Band-Aid but would directly address the issue. The other choice would be to attempt to resolve the issue by ensuring the machine itself is properly grounded between ALL of it's various pieces and the external ground. In other words, you might be adding 10 or 15 ground straps across the machines parts that are otherwise bolted together.

The voltage you measure is enough to eventually cause bearing issues...if it were an electric motor that level would not be acceptable...though it is close to being acceptable.

The state of the art in shaft grounding is (for example) offered by Cutsforth...it does not use a brush but a length of copper braided rope which lies around the circumference of the shaft. This method is considered the best as it provides long wear and a lot of grounding surface. I wouldn't necessarily seek to buy one, as they are expensive, but you could make your own.
 

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Hurco says to install some kind of brass brush on the spindle, they don't have one or supply one and the machine never had one. This is a belt drive spindle so rubber belt between the motor and spindle. The spindle bearings are ceramic and I am guessing the cage is non conductive also, there is no continuity between the spindle housing and shaft when the spindle is off. even if the static is created by friction I feel the more pressing problem is the magnetism, I have concerns that steel chips will find there way stuck to the spindle housing or in the spindle taper, it's magnetic enough to hold on to a paperclip so enough to collect small chips. Hurco has never seen a spindle get magnetized. What's really odd is we have 6 other Hurco mills and 2 more on the way and have never had this problem with any other machine.
Any belt driven load becomes a Van De Graff generator, so that tracks. Ceramic bearings are not susceptible to damage from static discharge, typically.

Any chunk of metal can be demagnetized by placing it in an AC coil capable of inducing more magnetism than the metal already has, and then slowly reducing the voltage to zero.

I have heard of belts with a conductive additive. I don't know how available or practical this is, but it would solve your problem.

Generally static electricity issues come about due to low humidity. Is this your only belt driven, ceramic bearing machine?
 
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Any belt driven load becomes a Van De Graff generator, so that tracks. Ceramic bearings are not susceptible to damage from static discharge, typically.

Any chunk of metal can be demagnetized by placing it in an AC coil capable of inducing more magnetism than the metal already has, and then slowly reducing the voltage to zero.

I have heard of belts with a conductive additive. I don't know how available or practical this is, but it would solve your problem.

Generally static electricity issues come about due to low humidity. Is this your only belt driven, ceramic bearing machine?
We have 6 other machines with the same basic spindle/ belt assembly and none of them have ever done this. I am aware of the demagnetizing procedures, but have never tried it, let alone on a expensive piece of hardware. At that point we would have to take the spindle out of the machine anyway. Our worst case scenario is just replace the spindle.
 
I guess if it was me...I'd install a shaft grounding brush. I doubt it's coming from 'magnetism' but rather from a poorly grounded machine. The grounding brush would be a sort of Band-Aid but would directly address the issue. The other choice would be to attempt to resolve the issue by ensuring the machine itself is properly grounded between ALL of it's various pieces and the external ground. In other words, you might be adding 10 or 15 ground straps across the machines parts that are otherwise bolted together.

The voltage you measure is enough to eventually cause bearing issues...if it were an electric motor that level would not be acceptable...though it is close to being acceptable.

The state of the art in shaft grounding is (for example) offered by Cutsforth...it does not use a brush but a length of copper braided rope which lies around the circumference of the shaft. This method is considered the best as it provides long wear and a lot of grounding surface. I wouldn't necessarily seek to buy one, as they are expensive, but you could make your own.
We did go through and add temporary ground straps everywhere from all components and nothing had any effect on the static generated. The machine is properly grounded and we double checked all connections. This is a fairly new machine and in excellent condition. The spindle is insulated by the belt and ceramic bearings and the only place the static charge is being generated is in the spindle.
 
I don't think a spinning spindle will create electricity unless it has something to work against. Are the ceramic bearing running dry, or in oil? Oil will conduct electricity.
 
We have 6 other machines with the same basic spindle/ belt assembly and none of them have ever done this. I am aware of the demagnetizing procedures, but have never tried it, let alone on a expensive piece of hardware. At that point we would have to take the spindle out of the machine anyway. Our worst case scenario is just replace the spindle.
I've done it on a CRT with a coil and variac with good results. Never tried anything else.

For the sake of curiosity, have you measure the spindle to housing resistance on all machines? A megger would be ideal to spot the difference.
 
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I think graphite in oil would conduct electricity? they make rubber tires that conduct electricity. They used those on the liquid hydrogen bubble chambers. You do not want hundreds of gallons of hydrogen igniting from static. Sand blast hoses are conductive so they can be grounded.
I agree, add a grounding brush that touches the spindle. It just has to conduct. a little better then the air gap between endmill and work right before they touch. A motor commutater brush and holder would be fine. There is almost no current tt carry away.
 
I think GregSY and Strostkovy are on the right track: magnetism is a red herring and electrically connecting the rotating part of the spindle to the stationary part is probably the solution.

But first measure resistance between rotor and stator of spindle while spindle's stationary (just to avoid an accident) like Strostkovy recommended. If the resistance is low, like less than 1 MegOhm, then the sparking will be much more mysterious.

There's no need for a megger or anything fancy. Any old ohmeter will do the trick.
 
I have a flat plate demagnetizer. Put a piece of steel on the plate, turn it on. The steel hums and vibrates. Do not turn the demag off, but remove the steel from the demag by a couple of feet. Then turn the demag off. The large air gap does the same thing as turning down the voltage at a much lower cost. I would try such a demag process on the end of the spindle.
 
Having read the thread so far, I don't think magnetic fields are the answer, especially with ceramic balls in the spindle bearings.

I think that the spindle needs to be grounded, but almost anything cobbled together would work. Like a graphite brush riding on the center of rotation away from the cutting end. Or a brass point. Verify the grounding with an ordinary ohmmeter.
 
Lots of good ideas. I did some research and there are several solutions available to add a brush to the spindle to dissipate the static charge. That leaves the magnetism to deal with. Short of taking the spindle out and sending it to someone who specializes in demagnetizing anyone have any ideas? Should I drag my bench plate demagnetizer across the spindle face..? any concerns of damaging any other sensors etc. by doing something like that? Or pull the spindle out and make a coil demagnetizer and run it over the spindle?
 
What are the actual problems created by either the magnetism or the static electricity?

If you're using water-based coolant that will neutralize the static because it's conductive. If not then I guess there's some theoretical danger of igniting some flammable vapor, of interfering with an electronic probe during a tool change, or of someone jerking their hand because they get shocked.

Are you worried about ferrous dust being attracted to the spindle?

If it's a matter of weird rather than something that will cost you actual money I'd be tempted to leave well enough alone.
 
What are the actual problems created by either the magnetism or the static electricity?

If you're using water-based coolant that will neutralize the static because it's conductive. If not then I guess there's some theoretical danger of igniting some flammable vapor, of interfering with an electronic probe during a tool change, or of someone jerking their hand because they get shocked.

Are you worried about ferrous dust being attracted to the spindle?

If it's a matter of weird rather than something that will cost you actual money I'd be tempted to leave well enough alone.
Yes water based coolant, and yes it dissipates the charge when the coolant is on, but we do run some steel dry with air blast and at some microscopic level with this much sparking i am concerned with erosion of the cutting edge. Also we run some plastics, as you know they have there own static problems and adding to it with this spindle will probably make it worse, plastic chips stuck to the spindle due to static could cause plastic in the taper. As to the magnetism, it is strong enough that a paperclip sticks to the spindle nose, so yes chips have the potential to stick to the nose and possibly get in the taper on a tool change, that could ruin tool holders, spindle taper and at the least ruin parts due to tools cutting off size. So I am at this point probably more concerned with the magnetism as the static charge has a readily available solution with a brush.
I would love to just leave it, but there are enough scenarios that would cause problems that I need to come up with some solutions to correct this issue.
 
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i am concerned with erosion of the cutting edge.

Highly unlikely since that requires some actual power. But, you can easily test this by running the cutter near some bright aluminum for a while so that it sparks and then see if there's any mark where it was sparking. I doubt that there will be.

Regardless, since the fix is easy I don't blame you for going after it. An alternative to a brush is a sharp metal point that is just almost touching the rotating part of the spindle. Like the end of some wire that's been cut with nippers. That will have the advantage of not wearing out and not generating particulate that could get into bearings.

Joe Gwinn's idea of a brush at the center of rotation to minimize SFM is also pretty slick.

About the magnetism, that's pretty amazing that it's strong enough to make a paper clip stick.

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Should I drag my bench plate demagnetizer across the spindle face..?

Maybe worth a try, but really depends on the specifics of how the spindle is magnetized. You'll probably need to pull out the spindle cartridge to fix this because it's currently magnetically shielded by the casting that it's mounted in.

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any concerns of damaging any other sensors

Yes if there's a tachometer that relies on a permanent magnet you could end up demagnetizing that.

Like you suggested, pulling the spindle cartridge and putting it in a homemade coil demagnetizer is probably the way to go.
 








 
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