Since the load direction is constant relative to the non-rotating
inner race, the fit isn't very critical. A close slip fit is fine. If
you go by the bearing books the shaft should be 4.996/4.988mm.
Easiest thing, if you're attaching the bearings to the sides of flat
fingers, is to not worry about the fit and just fasten them with M5
socket heads and a shim under the inner race so the outer doesn't rub.
A little fussier would be to use 5mm precision shoulder screws.
I only use ball bearings on rough stock in the steady rest, and only
then in order to put a center in the stock so that I can support it to
machine a good surface for pads. Besides the chips being rolled into
the stock by ball bearings the bearings will also reduce the diameter
a bit where they are rolling, leaving a depressed band on the work.
For steady rest work I have a drip oiler mounted to the top of the
steady so there is always a cushion of oil between the pads and the
work. This prevents marking of the shaft, makes things run cooler, and
won't cause chips to be pressed into the surface of the work causing
While others have answered your fit question, you might want to
consider one of the problems with ball bearings on the steady rest
fingers. (Or more so on follower rest fingers.) That is that the fingers
will simply sweep chips aside, but ball bearings risk rolling chips into
the workpiece, spoiling the finish -- or even moving it momentarily off
center, thus spoiling the finish by causing the cutting tool to dig
deeper or back off.
Just a consideration -- so make it easy to disable the bearings
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