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
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Neillarson wrote:

I have asked this question of Darrell Feltmate also and I am sure that
I will get good info from him, but I thought I would see what other
sources there are.

I want to make some of my own tools. I have found an easy source for
the actual bits, but I am still looking for the tool shafts. Any help
would be great.

Another question, I have not yet seen any of these athat are home
made, but, I can't help but wonder at how the shaft stock is drilled
for the bits. The bits are square stock and the shafts are round. Got
any ideas of how to get a square hole in a round bar?Thanks

Neil Larson