"PrecisionmachinisT" wrote in message
...

"Karl Townsend" wrote in message
...

You need a good interchange chart to double check but in all likelyhood
the first 2 on your list are simply #206 in Barden and something like
2mm9206 in fafnir for example.

http://www.ntnamerica.com/product-su...rchange-search


The last few digits (IE the "ctc/c7xx" ) part of the bearing are
generally not important for the purposes of a spindle bearing--main
thing
needed being physical size ( ID OD and thinckness ) and actual contact
angle.

Can you tell me how to measure the contact angle on my existing
bearings by chance? here's the chart i got from the above link with
the numbers off the bearing itself (SKF7206 B G)


Sorry no way to measure after the fact

http://www.ntnamerica.com/products/b...id=RBSRA&ctc=B

It gives me the code for contact angle not the actual number


As a practical matter, contacy angle will vary depending upon the ratio of
how much load is to be placed on the end of the spindle as compared to
side loading.

So for machine that is used mostly for drilling you want to take that in
consideration as opposed to if you are going to be millling all of the
time then you want a higher rating in the direction so your bearing set
will more closely approximate a standard ball bearing IE something like 15
degrees


Reason I ask, I found this in a fafnir 3M (25 degree contact angle)

25 or 30 degrees is about middle of the road for a milling machine
although Fadal uses a 15 deg in the lower set and 30 deg in the upper so
YMMV

http://www.ebay.com/itm/FAFNIR-3MM20...em4cfadc db80

Sounds like a really good bearing pair


pinch them together in a vice with a shim on the outer race and jiggle the
inners to measure preload (if any ) if you are unable to obtain specs from
the manufacturer.

But if I my memory of Fafnir's numbering systemthat is correct that is a
matched set that's intended for duplex universal mounting which means
they can be mounted face /face or back /back and either way you will end
up with zero preload.


Thanks for your help, I'm studying this.


No prob, I'll be out for the weekend but I'll check back when I return.

Your typical modern milling spindle has a duplex set that has the inner
races locked into the lower housing...basically its clamped tightly onto the
spindle with a jam nut and then is fitted rigidly into the housing so that
it cannot permanently migrate up nor down by even a tiny amount unless
fairly HUGE pressures are applied.

Its clamped onto the shaft with a jamb nut which by itself would cause a
zero-preload condition...but since the assembly is then pressed up into the
spindle housing against a shoulder into a cavity that is just a *tiny bit
shallower* than the bearings' height......( only shallower by a 1/2 thou or
so, mind you ) but this is how your preload actually comes about...

The upper bearings are different, basically they are intentionally allowed
to float in the axial plane, no attachment to the housing at all other than
some way to keep them from falling clear off if they were do inclined to do
so--reason this is done is so that the shaft lenght can grow due to heating
without having it affect preload ( binding or sloppiness would occur
otherwise ) and also so that undue problems don't arise with trying to
incessantly chase tool length as the spindle warms up which can be a real
PITA especially where you are under cnc control.

Your mileage may vary but the above probably describes the vast majority of
milling spindles out there today with a fairly high degree of accuracy.