I've been offline a long time, but I know there are knowledgeable machinists in the forum.

I have several 316 Stainless Steel castings where the hole for the shaft needs to be very accurately located on the centerline to prevent imbalance. This is an impeller that spins at 1200 RPM. The shaft hole has been bored on a lathe to
.4981". The nominal hole size is supposed to be .4987 +/- .00025". The shaft which is supposed to insert into this hole is .4985".

I have a .4990" cobalt tipped, straight flute reamer.

Question:

If I don't get the reamer to bore position dead co-axial, will the reamer change the hole POSITION?

HOLE SIZE:
I understand that the reamer is built to -.0000 +.0002", so in theory, I'd be taking up to .4990 + .0002 = .4992 Reamer Size
.4992 -.4895 = .0007" clearance on the shaft to hole.

But in removing that material, will the reamer follow the path of least resistance and stay dead co-axial to the bore location?

Any variance off the impeller center axis cauese imbalance.

Thanks

Brian

hone em.

A reamer will folow the parent hole.....if it's properly tapered.
Honing allows you to "creep up" on the required size....but you're gonna be there a while.

I agree with the hone. You only need to hone 6 tenths of a thousandth to hit nominal, and the hone will keep the bore centered in the correct location.

Well, when I am reaming the hole I like to.......

Either way will work, but I see no reason the reamer won't work. The reamer will follow as long as you don't dwell it in the hole or drastically cock it away from centerline. Just line it up the best you can (test indicator would be the most accurate), plunge it in at a quick, but steady rate, and retract at the same rate. This ought to give you the results you're looking for.
Mark D.