Subject: "drilling" non-round holes (was: square drill bits)

From: [email-masked] (Ian Kirby)
Date: Tue, 21 Mar 1995 08:14:47 +1000

Most of the responses in the thread of square drill bits are close, but
not quite accurate in their ability to create a "shaped" hole.
  There is a way that very accurate holes can be created though.  The main
difference is that a single-point cutting edge is used, not a drill in the
conventional sense.
  Remember a toy called (in Australia, anyway) Spirograph?  Little plastic
gears with various pencil holes in them, running around other plastic
shapes with complementary teeth, and the pencil would draw all sorts of
shapes.  Well, certain configurations could be set up to draw very accurate
square, triangular, penta- and hex shapes.  If the pencil is replaced by a
single-pointy cutting tool, and downfeed applied in a pre-drilled hole, the
required shape can be cut.    
  These tools are called planetary cutter heads, or something similar, and
are available commercially, although very expensive.  They can be run in
ordinary drilling machine spindles, much like tapping heads, where the
outside of the tool is held to prevent rotation, while the spinle drives
the cutting edge shank via its gears.  The corners of the cut shape
frequently have a small radius.
  I cannot put my hands on the reference right now, but they do exist!
Regards, Ian.
-- 
Ian Kirby.                          Ph: +61 42 213353
Dept of Mechanical Engineering,     Fax:+61 42 213101
University of Wollongong.           Email: [email-masked]
Northfields Avenue,
Wollongong. 2522.
NSW Australia