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