High speed induction motors

I'm musing about trying to boost the maximum spindle speed of my little Novamill CNC machine, currently 5000 rpm, too slow for small cutters in ali and brass for engraving etc. It will probably need a new motor as the maximum speed of the current PM DC motor is 5000 and running a commutator motor faster than that is a bit noisy. Also access for replacing brushes is a pig!

One option might be a high speed induction motor run from a VFD. Standard VFDs seem to go up to 400 Hz which on a 2-pole motor would be 24000 rpm. Interestingly, modern washing machines often use VFDs driving induction motors with this order of top speed, with a big pulley reduction to the drum. As the motors are made for consumer products they are relatively cheap, surplus on eBay for ~30 squids. There's a video on youTube showing one driven off a Huanyang VFD.

Does anyone have any experience of these motors or can point me at data sheets please? It's hard to even see how big they are from the photos available.

By the way, yes, I know that the high-speed spindles that are available from Arc etc use induction motors, but I don't want to do any massive mods to the mill head, and there isn't enough room or bed length/travel to mount a spindle alongside the standard one.

Edited By John Haine on 27/01/2017 09:28:16

Mill spindle bearings? Will they cope?

Regards Ian.

Good question that I'm also investigating.

A router motor.....typically runs at 20-30000rpm.....any good? Adaptable?

Can we have a link or search words please?

Yes, but noisy! Also not sure they will run for 2 - 3 hours at a time, and aren't they usually commutator type?

I'm specifically interested in info on induction motors here - other types I am aware of, thanks.

Youtube link is here

Not any more; they've been discontinued. I bought the last of the lower power spindles, which I think uses steel bearings. The higher power version used ceramic bearings. So far mine has been fine. It runs for hours at 24000rpm without a problem, and is quieter than the stepper motors on the machine axes.

I've used an industrial router motor to drive a test rig for characterising a Formula1 alternator, up to 24000rpm. It was definitely an induction motor. But it was large (3hp), very noisy and very expensive.

Andrew

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Thanks for the link, John

Great potential there !!

MichaelG.

I have a Novamill, I know there is poly V drive from motor to spindle - not sure about pulley sizes though.

From a drawing I have, there isn't much chance to reduce pulley size on spindle, but maybe increase pulley size on existing motor and use a longer belt? (If only using small cutters).

Judging from size of bearings on spindle (7208) I'd be surprised if you can get the speed up to the sort of figures mentioned above. Information on bearing manufacturer's web sites should help identify max speed with grease and with oil lube.

SKF specify 10k rpm for a 7208 bearing. Don't know enough about bearings to know if there are circumstances where higher speeds are permissible but this data is from a good manufacturer...

Presumably if you are trying to run a cutter at those high speeds you'd be better to use a (small) high speed spindle rather than overspeed a large spindle. Apart from the bearings, you can get problems with "whirling shafts" at high speeds. 

Edited By Muzzer on 27/01/2017 11:13:52

Talking of 'whirling shafts' - the Novamill spindle has the bearings near the bottom end, it's quite some distance to the top of the spindle - which is unsupported.

How well spindle (and tool holder) is balanced will also become more important as speed increases.

Old fashioned way might have been to add a 'spindle speeder' - but there isn't much headroom on the Novamill.

John

I believe the Novamill has an R8, BT30 or 35 spindle, if so there was an article in ME 21 April 1989 showing details to build a speed multiplier, with that you would get 5k to 20k from your 5k spindle.

I planned to build one using an ER 11 collet chuck on a 12mm spindle to keep rotating mass as low as possible. You may loose a little height to table but no more than if you use the Coventry quick tool changer chuck.

Picture of drawing attached, if it breaches copyright perhaps Neil will remove it.

Emgee

Yes it's a BT30 spindle. One problem with this sort of gadget is that the machine has little enough headroom as it is. That's why I don't use the Coventry system very much and I'm also trying to work out how to make a very short collet chuck as it's hard to find an er16 chuck with less than ~50 mm projection. I have wondered if one could put a motor on top of the head with a shaft through a BT30 arbor.

If adapting a spindle to run at much higher speed then initially intended consider re-balancing. Also tool holders will have a maximum speed. Bearings are only capable of sustained high speeds with a suitably balanced shaft.

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I suspect that might be the best approach, John

... Do you have a drawing of the spindle?

... We should all get our thinking-caps on

MichaelG.

Drawing RM7_100 from this zipped collection available from Denford shows the arrangement of the spindle for the Novamill.

**LINK**

Apologies - I can't make head nor tail of how to get a meaningful title to appear as the link after submission.

David, I've got those drawings, you wouldn't offhand know which one it is would you please?

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Thanks to a bit of juggling on my iPad ... here is RM7_100 from David's link, as a jpeg

MichaelG.

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Edit: We could roughly scale it from the bearing dimensions, but it would be useful to know the actual diameter of the drawbar.

Edited By Michael Gilligan on 27/01/2017 13:07:57

RM6_100 is also pretty interesting. Shows the spindle in section including pullstud mechanism, which is presumably a build option.

Edited By Muzzer on 27/01/2017 13:06:06

John - see PM, and MichaelG's post above.