Ever wanted to drill a square hole? Usually the best answer is to
drill four small round holes in the corners of the square and cut
between them, finishing up with a file to square out the corners if
required. It's not quick. When working with wood, another option is
to drill a round hole of the same diameter in the middle of the
intended location for the square hole, then chisel out the edges.
Routers and milling machines can be other options, but are not
always practical. Often an equivalent to the drill for round holes
would be preferable.
One existing solution is the mortising machine, which automates the
sequence of drilling a round hole and then chiselling out the sides
of a square in wood. There are also attachments for a regular drill
press that allow it to work that way.
However that's limited to wood, and the machine/attachment as well
as sets of the mortising bits that "drill" the square holes are
expensive. My idea is for a mechanical drill "bit" that fits into a
pre-drilled round hole and drills a range of differently sized
square holes of larger diameter than that round hole.
Its construction is similar to existing drill "bits" designed for
cutting round holes of adjustable size, able to work on a range of
materials including metals. Into the hole (and the drill chuck) is
inserted a shaft and near the end of that shaft protrudes from the
side a hardened square metal shaft with cutting faces on two sides.
This cutting bit is able to move laterally in and out of the main
shaft. The angle of the two cutting faces is opposite, so when
cutting the surface one pulls the bit inwards and the other pulls
it outwards from the main shaft. This movement is limited by a ring
around the perimeter of the main shaft and mounted on a bearing,
this ring is held stationary relative to the rotation of the shaft
by an indexing arm that rises up and presses against the body of
the drill.
The bottom edge of the ring contacts a sloped surface on a side of
the cutting bit that is not in contact with the material being cut,
and restricts the depth to which it can be extended or retracted
from the shaft due to the lateral force corresponding to the active
cutting face. The contacting surface of the ring is shaped to work
as a cam, so it has four triangular points which control the
extension of the cutting bit so that it cuts the four corners of a
square. At the point where a certain extension or retraction of the
cutting bit is reached, two of the opposing square edges of the
cutting bit's shaft that secure it against rotation in the main
shaft have been rounded, so it is able to rotate and the other
cutting face that imparts an opposite lateral force is used (some
mechanism, probably in combination with the ring, needs to prevent
it immediately rotating back again). This occours at the peak of
each triangle in the corners of the square.
The result is that a square is drilled around the perimeter of the
prior round hole, down to whatever desired depth (allowing for
manual clearing of swarf for deep holes). Differences in the size
of the square hole may be accomodated by different rings and
possibly different cutting bits, within limits determined by the
size of the main shaft.
As an alternative to the ring for controlling the extension of the
cutting bit from the side of the main shaft, the cavity within the
main shaft that it moves within could be sealed, and the entrance
and escape of air could be mechanically controlled to determine the
speed at which the bit extends and retracts. However that approach
is probably impractical due to mechanical complexity (working at
high speed) and need for calibration.
Overall I believe this could be the basis of a practical
general-purpose drill bit for square holes. Furthermore the same
approach might be applied to cutting other shapes simply by
adjusting the parameters of the ring and cutting bit. However there
would be some significant difficulty to its manufacture, requiring
various machining processes which are probably very expensive at
low to medium volumes of manufacture. Even if I was able to extend
my own machining abilities far enough to make a prototype, I
wouldn't really know where to start with producing these things
commercially. I also have another drill idea with many similar
features but possibly a more attainable mechanical construction.
