In article , Ryan says...

I have a task which requires boring four 4" holes in .750 plate aluminum,

Holesaw. Get the best bi-metal blade you can, mount it up
in a bridgeport, and use WD-40 for the cutting fluid.
Retract feed frequently to blow out chips. You can
cut from both sides, just pick up the pilot hole from
the first side.

Jim

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