Have you thought about double drilling. I frequently do
this.
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+1, double drill that sucker to get a better starting condition for the reamer. Also, all other suggestions are good.
In alum, I generally run reamers at 300 RPM and 3-5 IPM on anything under .25, and a little slower for anything bigger.
I have also found that you can change the dia by a few tenths by adjusting the speeds/feeds; Keeping the reamer in the hole longer with more revolutions will make it bigger, etc.
'Rekd