A request for information on your mill or lathe

[rgsparber]

I would like to learn the electrical resistance between the spindle and the machine's body of people's mills and lathes using a common ohmmeter. Take a reading, turn the spindle by hand a few degrees, and take a second reading.

If you could supply the make, model, and resistance readings, I would be much obliged.

Thanks,

Rick

[nova_robotics]

Here you go good buddy. Measured with a cheapo low ohm meter and verified against a semi-okay multimeter/oscilloscope.

Standard Modern 11" (Manual Lathe)
0.66 - 1.31 ohms from chuck to ways

Hardinge VMC600ii (CNC 3 axis mill)
Strange readings, assuming it's due to lubrication floating the spindle when it's turned
Let it settle for a minute: 0.97 ohms
Spin it slowly and let it settle over a few seconds 80 ohms
Moving slowly by hand >1k

Chiron FZ 08 KS Magnum (CNC 5/7 axis VMC)
Spindle on XYZB head: OPEN (meter goes >1M, tried lots of things, couldn't get any reading)
Lathe spindle 10k to over 300k. It looks like this is actually open, but there's some capacitance coupling the spindle to ground which changes as the spindle rotates.

Craftex CX600 (Small Manual Mill)
0.09 - 1.2 ohms from spindle to table

Shark HD520 (CNC Router Table)
Spindle is OPEN to table
Spindle is 29 - 77 ohms to Z axis rails

[rgsparber]

Thank you so much for the invaluable information!

Rick

[nova_robotics]

Thank you so much for the invaluable information!

Rick

Whatcha doin'? Making a touch probe?

[odd one]

Atlas 12" late 60's to early 70's WELL WORN had been in a production facility.
Spindle to ways .4 - 7.7 ohms High and low readings were repeatable at the same places on the chuck after several revolutions.

Smithy Midas 1220XL probably less that 2 hrs. on machine
.4 - .7 ohms Lathe chuck to ways

[Toolmaker51]

Bet this turns out interesting. Two variations that come to mind immediately are direct spindles vs driven spindles, and the bearing packs. A lightweight drill press gets away with just two. A tough mill often has three or four. Other machines are completely isolated.
Don't know the exact term, but there is an issue where stray current erodes contact areas of bearing components and races. IIRC, VFD rated motors have additional frame grounding to bypass the bearings.

[nova_robotics]

Don't know the exact term, but there is an issue where stray current erodes contact areas of bearing components and races. IIRC, VFD rated motors have additional frame grounding to bypass the bearings.

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.

[Frank S]

Why to the smart guys always come up with questions for things that I haven't given a second thought about in my 55+ years of working around lathes and mills or many other rotational powered equipment. I have always made sure all of my equipment have the motors well grounded to the frames of the machines, and the machines well earthed to the building. But have never even given a single thought to the possibility of there being current no matter how minute even if nothing but static being transmitted separately through the spindles to the work piece or the bed of the machine.
Almost all of mu machines will be belt driven between the motor and spindles the exceptions being some direct gear driven things like saws or hand drills.
I have a matched pair of Simson meters which were used for checking HVAC equipment and my trusty Fluke multimeter, If I remember it maybe tomorrow I will run some checks on a few pieces of my equipment, if for nothing else you have now raised my curiosity

[nova_robotics]

It's pretty interesting stuff. I never thought it was a problem until I heard of a bunch of satellites failing because of static discharge across the bearings in their reaction wheels. The bearings were dying during times of high solar activity. It makes sense that the electronics would be damaged during solar storms, but why would bearings care? Static discharge.

https://blog.est-aegis.com/in-space-...-bearings-fail

https://support.newgatesimms.com/ele...uctive-grease/

[nova_robotics]

Bet this turns out interesting. Two variations that come to mind immediately are direct spindles vs driven spindles, and the bearing packs. A lightweight drill press gets away with just two. A tough mill often has three or four. Other machines are completely isolated.
Don't know the exact term, but there is an issue where stray current erodes contact areas of bearing components and races. IIRC, VFD rated motors have additional frame grounding to bypass the bearings.

OH! New thought! This actually makes the case to turn flood coolant on when machining plastic, for the purpose of reducing bearing wear. Flood coolant will do little for the part, but it will provide a conductive path to dissipate static. I'll have to think about that one. You're right, this is interesting.