Re: [CAD_CAM_EDM_DRO]spindle plans (was VFD Selection)

Hi Les:
Good points all.
With regard to ER collet extension shafts, they are normally 0.0005"
undersize from nominal, just like a cutter shank.
(at least all the ones I own are!!)
This means a slip fit on a 1.000" bore, and no way to retain the inner race
in relation to the shaft except for Loctite.
There's not much to a shaft like that...I'd make one up with just a lathe,
no problem.
I can turn and hone to tenths with no effort; I just can't do it at high
production rates with a typical jobbing shop.
Hardest part is probably measuring it accurately, but turning it between
centers, and honing the journals to size with a ring-lap is duck soup, even
on a lightweight lathe.
Cheers

Marcus

----- Original Message -----
From: "Leslie M. Watts" <leswatts@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, April 15, 2004 7:04 AM
Subject: RE: [CAD_CAM_EDM_DRO]spindle plans (was VFD Selection)


> Hi Dave
>
> Good question...why are most bearings pressed on (making $#@! removal
> hassles)
> when the locknut should hold them in place?
>
> There are a couple reasons the bearing manufacturers specify this fit.
> (and I always use their engineering recommendations; this is a mature
> technology and they know what they're doing)
>
> The first is that in operation a radial temperature gradient can develop.
> This gradient can make the bearing grow relative to the
> shaft. Radial forces then could overcome the clamping pressure of the
> nut and cause a loss of precision. A press fit preloads the system to
> prevent this. Note that in some applications just the opposite happens.
>
> A second reason for the press fit is to insure that the shaft diameter
> fully and stiffly locates the bearing. If it is a looser fit
> inaccuracies in the retaining nut and its threads can cause the
> bearing to cock or distort.
>
> Now these press fits are not much...tiny amounts. They get even
> tinier with increasing bearing precision. Lets have a look at
> fit tables for say... an ABEC-7 25mm bore bearing with shaft rotating:
>
> shaft fit:
> 0.0000 to 0.0004" tight
>
> Housing fit:
> 0.0000 to 0.00055 loose
>
> This is not much! Generally just warming the bearing with a light bulb
> is enough to allow it to slip on.
>
>
> A comment about the ER collet shaft extension:
> This is an interesting idea that could have some merit.
> I do see some potential problems though.
>
> 1) It is probably hardened. How would a home shop machinist
> do things like cut threads and keyways? If it is hardened
> or casehardened it would likely warp with much machining
> done to it requiring finish grinding.
>
> 2) If say, a 1.0000" nominal bore bearing were fitted
> to a 1.0000" ER extension shaft to avoid machining the
> shaft getting in the fit range would be hit or miss.
> Might get lucky. I aim for the Median of the fit tables usually.
> This would be a 0.0002 press. A line to line fit (light press)
> is the limit. Sometimes one can fudge the fit tables a little-
> but a machine tool spindle is not a good place to do so.
>
> Now perhaps an ER extension isn't very hard in the
> shaft area and perhaps it is pretty stress free. I don't
> know. If that is the case it could be a pretty good way
> to go!
>
> Oh..ps... for your question on top end speed... I had mentioned
> about 10krpm. This is to allow grease lubrication. With oil
> mist it could be much higher. I know routers use grease at 20k+...
> but the bearings don't last long!
>
> Les
>
> Leslie M.Watts
> L M Watts Furniture
> Tiger Georgia USA
> (706) 212-0242
> http://www.lmwatts.com
> Engineering page:
> http://www.lmwatts.com/shop.html
> CNC surplus for sale:
> http://www.lmwatts.com/forsale.html
> CNC carved signs:
> http://www.lmwatts.com/signwp.html
>
>
>
>
> -----Original Message-----
> From: David A. Frantz [mailto:wizard@...]
> Sent: Thursday, April 15, 2004 8:04 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO]spindle plans (was VFD Selection)
>
>
> Hi Leslie;
>
> Thanks for your posting. I do have a couple of questions though.
>
> Why would you want to have the bearings pressed fitted to the spindle?
> This seems like an unusual approach if you have a lock nut securing the
> bearings inner races to the spindle. I've seen many a spindle built
> with tapered roller bearings that had a press fit of the inner race, in
> fact it is almost universal. On the otherhand it does not seem to be a
> universal arraingement for angular contact bearings.
>
> When I'm thinking about turning a ER collet extension into a spindle the
> thought was to put the angular contact bearings near the collet end, add
> a sleeve and then a ball bearing at the other end. All retained via a
> lock nut threaded onto the end. Seems simple in my mind, with the
> only machining requirements being the thread on the end.
>
> What bothers me is the machining requirements for the outer housing.
> At this time I'm of the opinion that getting the proper precision with
> home shop tooling will be difficult.
>
> I suppose that for low speed operation this would be fairly straight
> forward. Getting good life out of a high speed spindle is another
> issue. The question is at what point do you move away form preloaded
> angluar conact bearign and go to externally loaded designs. The few
> high speed spindles I've disassembled in the distance past where very
> complex. What would you estimate to be the top end speed for the
> arriangement you describe?
>
> Short of finding a spindle cartridge cheap at an auction some place I do
> not see an alternative that can be purchased. Prefabricated spindles
> are awfully expensive.
>
> Thanks
> dave
>
>
>
>
>
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