When I heard about the square hole drill I assumed it was
a joke, I have drilled triangular holes in sheet metal but
a square hole drill sounded like it should be kept in the
same draw as the glass hammer.
The first time I bought drills at Drill Service[DS] in Horley,
I got one of their drill catalouge's. They have more types
of drill than you can shake a stick at. Next time I went there
I asked to see a 1 thou drill, they were out of stock but
they showed me a 2 thou drill, what can I say, it was very
small. I found the polygon hole drill quite fascinating it
was so simple, why hadn't I heard of it before. Well that
was in 1982 and I still have only seen them in the DS catalouge.
This system for drilling polygon holes was introduced in
1891, it can be carried out in a variety of machines: drill
press lathe etc.
Figure 1. Complete setup for polygon hole
drilling in drill press.
The drill has one less flute than the number of sides to
the polygon. the drill is held in a floating chuck allowing
radial movement whilst held axialy aligned. One cutter edge
is lodged in a corner and the next corner sweeps around untill
it lodges in a corner, this is the cutting action.
Figure 2. Square hole drill and form plate.