Re: [CAD_CAM_EDM_DRO] Re: GEM-500PCN bedmill

Doug is correct on the bearing grade thing. The specs are different.

The difference is between the american ABEC and
ISO/DIN.

It often confuses me when manufacturers just give a number
without mentioning the spec type.

From the SKF catalog:

ABEC-1 = ISO class 0
ABEC-3 = ISO class 6
ABEC-5 = ISO class 5
ABEC-7 = ISO class 4

(hope I got that right)

Electric motor bearings are usually ABEC-1/ISO 0
While the lowest precision class, these bearings are good to tenths in
smaller sizes.

Many ballscrew and lead screw end bearings are ABEC-1/ISO 0 as well.

Spindles these days are usually ABEC-7/ISO4 or better.
Many distributors do not carry the intermediate grades anymore- they go
right from ABEC-1 to ABEC -7 or 9.
This might be a good thing because with some manufacturers
as most ABEC-1 bearings could be ABEC-3 or better. The distribution might
be centered near there. I suspect ABEC-1
and ABEC-7 are about 3 sigma in the spread. I can't prove this though... my
data is not statistically significant. But it would explain the high cost of
ABEC--7+... perhaps only 3%
of the production is that good.

Some spindles are made with preloaded ABEC-1/ISO 0
(the lowest class) and the tapers etc are machined in place
after the bearings are mounted. These are usually referred to as
"cartridges"-- when worn the whole assembly is tossed
because the bearings cannot be replaced without remachining it. But you can
get extreme accuracy with lower grade bearings. The grade has nothing to do
with "slop" in a preloaded bearing.

They higher accuracy grades usually have higher speed ratings as well. Their
price can be ten times that of an ABEC-1/ISO 0. Extremely close shaft and
housing fits must be
performed to use the greater accuracy of these bearings.
They are also aligned circumferentially with a high spot mark.

For ballscrew nuts radial runout is less an issue but internal
clearance is. In both ballnut and shaft bearings these are designated by a c
prefix. It goes from c1 to c5 usually and
unmarked bearings are typically c3.
More typically important is pitch accuracy (e300) and preload
amount and variation with ballscrew assemblies. Preloaded
ballscrews have no internal clearance and as such do not use the c__
specification from what I see.

Inch size and Tapered roller bearings have a different classification
system... I won't go into that because I don't know it well. I have to
really dig to spec them.

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: <dougrasmussen@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, May 25, 2001 12:14 AM
Subject: [CAD_CAM_EDM_DRO] Re: GEM-500PCN bedmill - CNC ready


> Ballendo & Smoke,
>
> I think the specs said grade 5 "ball screws" not bearings. I don't
> know what that means, is there a system for rating the relative
> accuracy of ball screws?
>
> Incidently, I once had to get some precision bearings that were made
> in Germany. As I recall the grading system was the opposite of
> ours, with the lower numbers meaning higher precision.
>
> Doug