Rigid tapping, pretty much, works.
By the way, forgot to mention. I have three aluminum plates that are
approximately 12" wide, 30" long, 3/4" thick, and have handles on
them.
I wanted to drill a rectangular set of tapped holes in it, probably
1/4-20 holes on every inch of a 10x24" area, so it would be 240 holes.
I have a theory that then I could sell the plates for big bucks, as
layout/fixturing plates.
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