Tolerance/fit question

[Peter Fairbrother]

I am turning a ballscrew end down, to fit a 6mm dia bearing. The bearing
will be ABEC7, which has a hole tolerance of 0 to -4 microns.

So far, so good. But how big should I make the shaft?

It is 6.000 mm now (yes, to +/-1 micron :o), but normally I'd turn it
down a bit more; I don't want to have to heat the bearing to get it
on/off, a sort of medium interference fit will do.


I have been googling, and there are tables of tolerances, and pages with
recommended fits, but I haven't seen one page which actually recommends
an actual size or even what a normal type of fit is in terms of
tolerances - maybe my google-fu, or perhaps my engineering-fu, or both,
is weak today.


Thanks,

Peter Fairbrother

[Mark Rand]

Post by Peter Fairbrother
I am turning a ballscrew end down, to fit a 6mm dia bearing. The bearing
will be ABEC7, which has a hole tolerance of 0 to -4 microns.
So far, so good. But how big should I make the shaft?
It is 6.000 mm now (yes, to +/-1 micron :o), but normally I'd turn it
down a bit more; I don't want to have to heat the bearing to get it
on/off, a sort of medium interference fit will do.
I have been googling, and there are tables of tolerances, and pages with
recommended fits, but I haven't seen one page which actually recommends
an actual size or even what a normal type of fit is in terms of
tolerances - maybe my google-fu, or perhaps my engineering-fu, or both,
is weak today.
Thanks,
Peter Fairbrother

You don't want to reduce it below 6.000mm. Doing so brings the risk of a
slightly sloppy fit. With the bearing tolerance and the measured shaft size,
the worst case would be an exact fit with 50°C differential temperature. It'll
press on at that sort of fit, but will be a hand (clean handkerchief for
insulation to keep bearing warm :) fit at 100°C No bearings will be damaged at
that temperature.

regards

Mark Rand

[Peter Fairbrother]

Post by Mark Rand

Post by Peter Fairbrother
I am turning a ballscrew end down, to fit a 6mm dia bearing. The bearing
will be ABEC7, which has a hole tolerance of 0 to -4 microns.
So far, so good. But how big should I make the shaft?
It is 6.000 mm now (yes, to +/-1 micron :o), but normally I'd turn it
down a bit more; I don't want to have to heat the bearing to get it
on/off, a sort of medium interference fit will do.
I have been googling, and there are tables of tolerances, and pages with
recommended fits, but I haven't seen one page which actually recommends
an actual size or even what a normal type of fit is in terms of
tolerances - maybe my google-fu, or perhaps my engineering-fu, or both,
is weak today.
Thanks,
Peter Fairbrother

You don't want to reduce it below 6.000mm. Doing so brings the risk of a
slightly sloppy fit. With the bearing tolerance and the measured shaft size,
the worst case would be an exact fit with 50°C differential temperature. It'll
press on at that sort of fit, but will be a hand (clean handkerchief for
insulation to keep bearing warm :) fit at 100°C No bearings will be damaged at
that temperature.

OK, I'll go with that for now, just round off the end edge and cut the
slot for the circlip. Thanks.

I was a bit worried, it took me hours to get the ballscrew chucked with
sufficiently concentricity and [parallelness of the axes of rotation and
ballscrew, whatever that's called], and I didn't want to take it out of
the lathe and disturb the setup.

On reflection that worry was a bit pointless, as I am using carbide
paper for the final finishing-to-size, and I could just rechuck it
any-old-how and hold the paper in my fingers ..

ah well, thanks anyway.

-- Peter Fairbrother

Post by Mark Rand
regards
Mark Rand