VMC spindle speeds/bearings

In a previous post of mine here, the subject came up on older VMC's spindle speeds. Most from the 1980's were limited to 3500-4000 RPM.
Modern VMC's easily run 6000-8000, but I have heard the spindle design is supposedly different to allow cooling of the bearings by oil or coolant circulation.

It occurs to me that some modern CNC lathe spindles are designed to run similar speeds, and I have seen the headstock designs first hand. No 'extra' provision like coolant or oil circulation is incorporated to cool the bearings.

At what RPM does 'additional' cooling come into play? I want to run a VMC spindle orig. designed for 3500 RPM at 5000-6000 RPM.
Fitting modern bearings is an option if required.
Incorporating 'coolant circulation' is not.

dan k

Is your mill spindle a simple grease packed bearing? What kind of duty cycle are you anticipating for it? You can always try it out and monitor the temperature rise of the bearings. So long as the top bearing does not get too hot and lose its grease pack, or worse, make the quill stick in the housing, you would be okay, and fortunate. Ask me how I know

By comparison, lathes generally have a splash or pumped oil lubrication system which does serve as a heat transfer medium. Often the sump is fairly large and carries some heat quite a distance away from the spindle. The higher speed VMC's also use a splash or pumped lube system, with cooling.

OK - dumb question - why can't you add a coolant system? If not oil cooled, how about air cooled (with a vortex tube, and oil mist injection)?

Just a thought...

Yes Murray, grease packed bearings.
Hardinge uses greased packed bearings in their Conquest CNC lathes, and they run to 6000 rpm.
I'm just wondering if a Lathe can do it, why not a
mill spindle?

dan k

Dan,
The difficulty I had with my old knee mill, with a quill on the Z axis, was the heat expansion of the upper bearing caused the quill to begin to stick and seize in the head.

Its all about heat, if you run it fast and don't get it too hot, then its fine. Lathes may run a shorter duty cycle than a mill, too, and not be running for hours non-stop, which is conceivable with a mill cutting a mold.

You have to wonder what keeps the grease up there in that upper bearing. Too much heat, and I suspect the bearing could melt most of the grease out.

Murray,
Do you recall if your spindle ran moderately warm
or cool in the 3000-3500 RPM range?

dan k

Well Dan, it ran really hot with the old bearings, like 195°F

When I replaced them with the Cerbec hybrid bearings it was much better, around 120°F when running continuous at 4000rpm for about 4 hours.

Actually, a small airline with just a tiny little nozzle pointed at the quill would help cool quite effectively, at least at the lower end. If a guy were to plumb in some kind of air flow to blow air through the inside of the head above the quill housing, it would likely be better for the top bearing, as well.

When I replaced the spindle bearings, I used the Cerbec bearings both top and bottom. The ceramic ball is relatively unaffected by heat, and super precision run cooler anyways. Expensive touch though, I think about $1800 Cdn for the two pair.

The original top bearing was a double row angular contact. I modified the spindle a little bit, and this included putting a large diameter slinger washer beneath the top bearing. This washer ran with a very close fit to the inside of the quill, but the idea was that the grease from the top bearing could not as easily run down and out of the top bearing.

I did quite a bit of heavy duty steel milling with this spindle last year. It has stood up well, and I was relieved because I had heard rumors that the ceramic balls were not maybe tough enough. I don't know how they'd do in an actual crash, though.

Dan, I think they must be doing something different thats not real obvious to run the speeds they do today. My old Monarch VMC which is a quill type mill from about 1980, uses air/oil mist and also has an air circuit that increases bearing preload at lower speeds and decreases it at high speeds. They went to this extent for a max speed of just 3500, so there's gotta be something the machine makers are doing different now to run the speeds they do with grease lubed bearings.

Thanks for the comments guys......

With the current bearings in the machine, the spindle doesn't get much above room temp when running @ 3000 for several hours straight.

I think it would be interesting to run this past
a MRC or ********* (strange company name)tecnical guy and see what they have to offer.


dan k

Actually, a small airline with just a tiny little nozzle pointed at the quill would help cool quite effectively, at least at the lower end. If a guy were to plumb in some kind of air flow to blow air through the inside of the head above the quill housing, it would likely be better for the top bearing, as well.

Click to expand...

That's interesting, sounds like it "doesn't take much"....

dk

dkmc, Barden is now handling all the machine tool applications for *********, so they would be the best place to contact about it.

I ran your question past one of the application engineers here that used to work on the Barden side and he said if the older machine used an ABEC 5 for instance and you go to a 7 or 9 then you can get a higher speed from the bearing due to the better tolerances. Without knowing the spindle design, it is not possible to tell how high you can run the spindle without experiencing problems. Several factors determine the speed limit, preload, type of bearing, type of installation, etc. Email me if you want me to put you into contact with one of the Barden engineers.

Mike

Mike,
I haven't got the spindle out of the machine yet, nor do I have the machine retrofit with new CNC control and VFD..... yet
But, when I get to that point, I will want to talk to a bearing rep about this project.

I will try & figure out just what the spec is on the current bearings.

Thanks very much!

dan k