Reuleaux triangle - you can drill a square hole
A Reuleaux triangle is a shape formed from the intersection of three circular disks, each having its center on the boundary of the other two. It is a curve of constant width, the simplest and best known such curve other than the circle itself.
It turns out "almost" square hole. The square is obtained with slightly rounded edges that need to process with a file, and, nevertheless, is really a square hole can be drilled.
First cutter to make a square hole drilling succeeded Harry Watts in 1917, an English engineer working in the United States.
The video shows how the drill and make the square holes:
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