On Mon, 16 Aug 2010 23:28:56 -0400, Joseph Gwinn
wrote:

In article ,
Ned Simmons wrote:

On Mon, 16 Aug 2010 09:55:27 -0400, Joseph Gwinn
wrote:



For the record, the two outer races ("cups") are each Timken 19268; and
thetwo
assemblies of inner races ("cones"), rollers, and cages are each Timken
19150.
The stated ID of a 19150 cone is 1.5000".

The quill shaft is about 0.0011" larger than the stated cone ID, so it was a
light press fit. As others have noted , this cripples the running preload
adjustment by turning a ring nut on the spindle, and press fits are too hard
to get exactly correct.

Is there a standard solution? I'm tempted to polish the spindle shaft down
until the (new?) bearings are a (hand) push fit. Has anyone done this?

That's near the high end of the recommended shaft diameter, which is
.0007 to .0012 (+18 to +30 microns) over nominal. The ID of a class 0
precision cone is nominal +13/+0 microns, with a resulting 5 to 30
micron interference. Personally, I wouldn't mess with the bearing
seats. Even if you can maintain cylindricity, you're risking fretting
of the seat if the fit is not tight enough.

D.C.Morrison says that the bearings should be a "tap fit" on the spindle, not a
press fit, and endorsed my plan to polish the quill down a bit to achieve a tap
fit.

I dunno who DC Morrison is so can't decide whether their opinion
should carry more weight than that of Timken's engineers in this case.
"Tap fit" is a rather subjective way to characterize a bearing fit,
depending on who's doing the tapping and the size of the tapper, but I
assume it means a transitional fit with minimal interference. The ID
of your cones can vary by 5 tenths, so a tap fit on one bearing may
result in excessive clearance on another.



Be aware that the specs on standard tapered rollers are very loose
compared to inexpensive ball bearings, and precision class bearings
are quite expensive. Expect to pay around $270 for each cup/cone ass'y
in class 0 vs. $45 for a standard bearing.

What class is needed for a MVI anyway?

Class TIR
4 .0020
2 .0015
3 .0003
0 .00015
00 .000075

The runout of Class 0 roller bearings is approximately the same as the
ABEC 7 angular contact bearings typically found in machine tool
spindles.



BTW, the first thousand or so J-head Bridgeports used tapered rollers
on the spindle; the subsequent couple hundred thousand have angular
contact bearings.

Burke used taper roller bearings for the standard R8 spindle, and ball bearings
for the high-speed spindle. Taper roller bearings are more robust than ball
bearings under shock loads (like interrupted cuts).

True, but BP spindle bearings will take a lot of abuse if they're
lubricated properly and kept clean. Tapered rollers will carry a
substantially heavier load than angular contact bearings in a given
volume. But there are tradeoffs, such as added complexity, expense,
and less tolerance of contamination.

--
Ned Simmons