In article , B.B.
says...

I drilled undersize by 1/32", as our teacher told us is generally
correct for any hole over 1/2", I used lubricant, (though it's anyone's
guess what was in that bottle) and the material is aluminum.
The cutting tools are all in one cabinet that sees quite a lot of
abuse, so I wouldn't be too surprised if I picked up a tool with a bad
edge or a chip welded on already, though I did try to inspect anything I
got out of there and I am fairly certain my reamer was in good shape
when I used it.

Sounds like you were taking a bit large of a bite with the
first try, and maybe you were running the reamer a bit on the
fast side. Harold's comments about keeping the feed rate
up there (not too slow) are good.

One thing I learned from a toolmaker at work is, you can
tweak the final diameter by adjusting the speed. The faster
you run, with a fast feed rate, the smaller the final hole
is.

As far as centerpunching goes, he and I disagree to some point.
Done accurately and inspected carefully, a centerpunch will
give final hole locations between one and five thou which is
pretty good for rough work.

If you are not doing 'rough work' and want the hole to be
inside a thou location, then drill with almost anything,
and *bore* to within five or ten thou of the final size,
then ream.

You won't get an accurate location that good (+/- 0.001 or better)
unless you bore the hole to start.

Jim


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