Bob and others,
I should have been more specific on why you should use an end mill instead
of a drill. The pointed end of a drill makes is self centering. The drill
will automatically follow the existing hole. Fixturing the part may help
but the typical drill press has enough slop so that you cannot counteract
the self centering force.
End mills, on the other hand, have a flat end. They do not have a strong
self centering tendency although if you are trying to recenter an oblong
hole you would want to take it slow and try to use the shortest end mill
possible because the unequal side forces may still cause it to move
sideways a bit. To do this you need a stiff, tight machine that won't
allow the end mill to move. That is a milling machine. I have seen people
use a lathe too by chucking the end mill in the lathe and fixturing the
part to the tool post carriage using a right angle block. Lathes are
cheaper so they are more likely to be in a home workshop than a decent
vertical mill.
Good luck,
Bill Eastman
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