You can actually use a similar technique to 'drill' a variety of other shapes, such as hexagons.... even timing pulleys. Try searching 'rotary broach' on YouTube.
The triangular drill for drilling square holes has been around for a long time. It relies on a floating chuck to allow the drill to wander around the hole while still being rotated.
Geometery is shown well on this page - http://www.maa.org/mathtourist/matht..._08_31_09.html
You can do more than square holes, but it's not really that commonly used, as rotary broaching is a more versatile technique.
Slight difference in the way they work.
rotary broaching can only work to about 2X the diameter of the tool in depth.
the cutting in the video can go far longer and stay vertical.
It's based on the Watts drill that was patented in 1918 but known about long before that.
http://www.youtube.com/watch?NR=1&v=JeIi04JoAooJohn S -
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