Hi Prometheus
Home shop square hole broaching is done quite easily.
Drill a hole the same size or slightly larger (like 1/32")
Grind the end of a same size tool bit square, and position and strike
the bid with a brass hammer, knock the bid back out with a brass rod,
and use a square file to remove the bur in the hole, don't try to
drive the bit all the way through, just repeat the short amount of
broaching, and filing the burr, if you use cutting oil it will help
with the broaching, but clean the oil out before filing.

Have fun and take care
Leo Van Der Loo


On Sep 11, 6:12 am, Prometheus wrote:
On Mon, 10 Sep 2007 16:36:50 -0500, spaco
wrote:



Lots of good comments on this thread, so far. Broaching is a good idea
for this application, but the broaches you probably saw require a
through hole and need to be pushed through the round hole with something
like an arbor press.
Here's my suggestion: Go ahead and drill the hole in the end of
the hot-or cold rolled mild steel bar..... But drill it about 1/64 or
1/32" smaller than the diagonal of the bit you are going to use. (I
assume you are going to use square HSS tool bits, or carbide brazed onto
square tool bits). Oil the hole. Take a brand new unsharpened tool bit
with a squared-up end and drive it into the drilled hole a little way.
Use vise grips to pull it out, pick out the shavings, then drive it a
little deeper, etc.. You are broaching the hole. You don't need to
totally square the hole, you are just broaching the edges to help hold
the tool bit in place. You'd still use set screws. Maybe a bit of
overkill, but this method positively locates the tool bit, since it
would be hard to drill a hole that fits the bit perfectly. This way,
any vibration created won't chew up the periphery of the round hole.

Pete Stanaitis
-----------------

Wouldn't be a terrible idea to put a piece of unhardened steel between
the bit and the hammer if you go this route, either. If the tool bit
chips, those sharp little bits of metal can sting you pretty good- and
they seem to have artery-sensing abilities. I managed to get the
artery in my thumb once by hitting two hardened steel surfaces
together, and while that wasn't all that serious, it could be a real
problem if it strikes you elsewhere.