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Originally Posted by Andrew Blair
And, since the gear is hardened.....
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To ream a hardened object, Carbide is your only choice.
However, if you drill it under size, you can just lap the hole to fit. Use fine sandpaper on a dowel, by hand or with a drill.
But, I think more importantly you need to think more about the tolerances you're dealing with. 5 ten-thousandths is really tight, exceptional for most jobs. You're in the realm where the temperature of the workpiece is important, ten degrees warmer and the hole's too big.
If you're trying to get an interference fit, you can be off by a whole lot more than a thousandth, just chill the shaft and heat the gear, press them together and you're done.
There are tables of standard 'fits', how undersize you should make a hole for a certain typr of fit - loose, even, tight, very tight - look that up (or ask a lathe machinist) and machine to that.
Good luck,
Don