DRILL BIT FOR SQUARE HOLES

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.

 - The Free Thinker, 2021