A request for information on your mill or lathe

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Originally Posted by nova_robotics

Do they rely on the spindles discharging static through the workpiece? So for example the bearings in a mill used to machine aluminum will last longer than a similar mill used to machine UHMW?

My router table produces an ungodly amount of static when running. It's basically a Van de Graaff generator. The spindle is 3 phase VFD driven. I'm now questioning whether I should try grounding it or not. I really want to take that spindle apart. Perhaps it could use some conductive grease and a ground strap.

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If you are running CNC, the static must not be getting into it or you would crash. I'm sure your machine body is grounded so you should not get shocks from static discharge by touching it.

Static discharge current could be flowing through the motor's shaft, bearings, and into ground. My guess is that if there are sparks, they occur millions of times a minute. If the motor hasn't failed yet, it is probably fine.

Rick

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Originally Posted by bi11hudson

A strange request.
I have a Craftsman 101.27440 (12X36) and a Grizzly G-1550 (Taiwan 9X19). Both machines have roller bearings in the headstock. I have NE,,VER welded any stock in the Grizzly, and pretty sure the Craftsman never has, the bearings are too "smooth".
I don't have any resistance readings, I strongly disapprove of any device that passes ANY (even micro-amps) current through the spindle bearings. Background: Industrial electrician with 50 plus years experience, Wang Field Engineer, BSEE, and a handful of other accolades.
Bill Hudson

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Running a welding current through a mill or lathe would be criminal. We are talking tens of amps of current at over 20 volts.

The issue is arcing on the surface of ball bearings. These arcs chip microscopic bits of metal out exactly the same as EDM. To get arcing, you must have both current and voltage. All of the papers I've read on this subject say that more than 6V is needed. I couldn't find anything for currents below 1 amp. Of course, this has to happen repeatedly as when a spindling is turning at high RPMs.

In my application, my open circuit voltage is 5VDC and my short circuit current is 100 mA. The spindle is not rotating. Once my circuit connects, the voltage drops to around 0.1VDC, and the current stays at 100 mA. From all that I've read, this is far below the energy level needed to dislodge metal atoms.

We started our careers at about the same time. I mostly designed telephone equipment, spending a few years on circuits that could be hit by lightning. Great fun.

Rick

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Originally Posted by Carnel

Hi Rick,
My readings:
2 digital volt meters

DVM1 Escort 1111A
readings: test cords only: 0.2 Ohm. Lathe (AI, dutch brand, ca 1960) 0.5 Ohms between chuck en ways, but same as between chuck and tool post
drill-mill: (HBM, dutch version of HarbourFreight, ca 2000) 0.4 Ohms between quill and cross table

DVM2 Velleman DVM4200
readings: test cords 0.3 Ohms. Lathe 0.5 Ohms (same conditions as above)
drill-mill: 0.4 Ohms

So at my machines the resistance is very low, in the order of 0.1 or 0.2 Ohms. With a lot of relative uncertainty in the milliohms range.

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These are the lowest readings I've seen so far. I'm guessing that your machines are new enough to have ball bearings and not babbitted bearings. I once measured 0.2 ohms and it turned out to be a bit of swarf bridging the gap between spindle and bearing cover.

I could live with 0.2 ohms but not much lower.

Thanks!

Rick

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Originally Posted by brianr47

SB Heavy 10
0.5 Ohms initially, then 0.4 Ohms after 1 revolution of spindle

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Could you be running babbitted bearings?

Rick

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Originally Posted by Hans Pearson

Interesting thought.
Myford ML7 with plain white metal bearing and unused for 5 days - 6 Ohms
Chuck turned ¼ turn - 3.3 Ohms
Oil drip feeds opened and about 4 drops dispensed form each feed, chuck turned about 3 turns - 2.5 Ohms.
As others said above, one thing that never enters your head, and I have been associated with this old machine for over 50 years.

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Great information!

Thanks,

Rick

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Originally Posted by Frank S

A little disappointed in my Fluke 81438 multimeter. I use it mostly for reading VAC or VDC and HZ hardly ever for any ohm readings if I want of think I need to know the true resistance since it has no zeroing adjustment. I had removed the batteries from both my Simson 260 multimeters since I rarely need to use them I don't want the batteries available today to develop a leak in them.
I checked my LaBlond 17inch lathe today with the flake, "sorry" the Fluke, I hadn't used the lathe in about a week so the first test reading came out to be .8 ohms. but when I put the ends of both leads together instead of getting a reading of o,oo I had a reading of 0.2 ohms Huh? so I removed the red lead and touched the black lead to the red lead socket , o.1ohms, same for the red lead when I touched it. Checked from the chuck to the bed again and the reading was 0.8 ohms. SO I'm going to call that one 0.6 phms allowing for the 0.2 of the leads. I did a 1/4 rotation same reading half rotation no change full rotation still no change.
Since I never start my lathe after it has been sitting unused for a full day or more I filled the oil cups the turned it on for a few seconds the took readings again 8.2ohms I decided to switch it on at a low RPM and hold the probe to the outer edge of the chuck while it was turning and the bed. Nothing no continuity reading at all OK at the highest reading nothing. wondering about whether there was a possibility of the chuck generating voltage I set to DCv at low RPM touching the chuck and the bed nothing. High RPM . 0.001VDC I will get some new batteries for my Simson meters the next time I think about it and check with them

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0.2 ohms for just the meter leads is normal. You are right to just subtract that from your reading. 0.6 ohms is reasonable. I was more surprised that a 1/4 turn had no effect. The jump to 8.2 ohms after adding oil does make sense.

Thanks for the great data.

Rick

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Originally Posted by rgsparber

If you are running CNC, the static must not be getting into it or you would crash. I'm sure your machine body is grounded so you should not get shocks from static discharge by touching it.

Static discharge current could be flowing through the motor's shaft, bearings, and into ground. My guess is that if there are sparks, they occur millions of times a minute. If the motor hasn't failed yet, it is probably fine.

Rick

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The spindle is isolated on that machine. The table is grounded. So if you lean over and touch the spindle when running, your body general makes contact with the table grounding you out and you get a big shock. If machining plastic you can get a shock probably every 10 seconds off of the spindle motor. It's so bad the chips jump off the table because of the charge.

I have little (0.09?) background regarding electronics, barely passable in AC/DC electricity, somehow I retain fragments needed to make things work and avoid calamity.

You describe how the readings change by rotation and when extending quill. Can't immediately get my head around the second part, the expectation is less surface area of parts operating with clearance should reasonably increase readings.
A belt drive is electric isolation, not perfect static cut off; there is carbon in the rubber belt.
To me, it visualizes as some kind of field exists in the rotor/ armature/ brushes/ and contact via bearings to conductors, compared to sufficient contact for stable readings of quill to main castings, all ferrous metals.
Hmmm. Easier to visualize than put into text.

I worked on very large injection molds, giant steel blocks of 20 to 45 thousand pounds. These are not set on the floor; impossible to pick up before drill/ tap rigging gear, set on wood dunnage. No matter how far spread leads of multi-meter or test light were, readings were same. There is no justifiable such test, just "what if" and "how-in-hell" does a single AA battery do that?

Static is odd, no doubt. Know I've related following observation about one particular belt sander. It would deliver low, continuous but obvious shocks to me while running it. 1" abrasive belt, direct drive, 110v, metal stand in substantial, professional toolroom. Meter tested operative ground conductor, plug and receptacle, zero regarding rubber mounted motor case, period. Cowhide gloves stopped it, a little open belt machine, so minimal risk.
Another shop; under strict ESD protocols, even pure mechanical assembly areas; gowns, wrist straps, shoe covers, bonnets. Every day entering the lab you'd already be suited up, step on the check plate, touch the contact (without, then with) wrist strap connected to ascertain conductivity. Everybody would get low readings; I could do it without the wrist strap, consistently. Due to sensitive product, wore one anyway.
The lab's responsible engineer, an Air Force officer watched me one day, amazed at the phenomena. "How is that possible...?" he said.
I quipped "Well sir, I'm not pole-ish, a positive attitude, drink only ground coffee." (dramatic pause) "and I know how to conduct myself!".
Uproar of the staff, best ever.

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Originally Posted by Toolmaker51

Static is odd, no doubt. Know I've related following observation about one particular belt sander. It would deliver low, continuous but obvious shocks to me while running it. 1" abrasive belt, direct drive, 110v, metal stand in substantial, professional toolroom. Meter tested operative ground conductor, plug and receptacle, zero regarding rubber mounted motor case, period. Cowhide gloves stopped it, a little open belt machine, so minimal risk.

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I too had this exact issue with a cheap 1" belt sander. A shock every 5 seconds or so. If you look at the construction of most belt sanders, they are almost identical in construction to a Van De Graaff generator. A non-conductive belt that is driven between two (or more) different pulleys. Charge is picked up on the belt as it comes in to contact with one pulley, then as the belt travels away the voltage is increased through Van De Graaff voltage multiplication. Some fairly high voltage can be developed this way. So what I did was replace the nylon (?) pulleys with some nice aluminum crowned pulleys I turned down on my lathe. That grounds out the sandpaper just before it gets to the workpiece and your hand. Problem completely solved.

https://en.wikipedia.org/wiki/Van_de_Graaff_generator

Here's the belt sander with the aluminum pulleys, top and bottom. I left the original plastic tensioner pulley because it contacts the belt in a place that doesn't really matter. If you have a problem sander I definitely recommend this upgrade. Technically only the top pulley needed to be changed to achieve the desired effect, but some other things happened to that poor little sander that required some additional surgery.

Attachment 42885

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Originally Posted by Toolmaker51

Another shop; under strict ESD protocols, even pure mechanical assembly areas; gowns, wrist straps, shoe covers, bonnets. Every day entering the lab you'd already be suited up, step on the check plate, touch the contact (without, then with) wrist strap connected to ascertain conductivity. Everybody would get low readings; I could do it without the wrist strap, consistently. Due to sensitive product, wore one anyway.
The lab's responsible engineer, an Air Force officer watched me one day, amazed at the phenomena. "How is that possible...?" he said.
I quipped "Well sir, I'm not pole-ish, a positive attitude, drink only ground coffee." (dramatic pause) "and I know how to conduct myself!".
Uproar of the staff, best ever.

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Nice. Well done, sir. I do a lot of electrical design work in my shop. I compulsively ground myself out on every available piece of metal as I'm working. I don't wear a grounding strap, but I have one hand on a grounded frame most of the time anyway. I'm basically ESD grounding OCD at this point.

Logan model 1957 11” lathe, spindle to ways, .3 ohms very consistently
Washington RF-25 pattern mill/drill, .8-1.3 and in between, changes with almost every move of the spindle. Spindle to table.

Good luck with the project, Rick!