Sounds like you're using a spiral flute cutter and that will dig down at the
edge. Cutting the edge first to about .001' or so less and then cutting the
rest of the hole leaving a little rounded edge is the best thing. Also
using a dtraight flute cutter will tend to keep the mill from digging down.
As to the caps, why not make them thread on? That would be a lot more
classy to me. Even more spectacular would be to have the m snap on and off
with internal threading.

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Why isn't there an Ozone Hole at the NORTH Pole?