Our friend Kiko is a bit of a luddite. He asked me if I could make an adapter for his bit brace so he could use 1/4" hex screwdriver bits. Being lazy I checked to see if such adapters are available and indeed they are. Lee Valley tools has them for $8.50. So Kiko ordered it. I decided to try making one anyway.
There isn't a standard for bit brace tang dimensions as far as I can tell. Easiest way to measure the taper was with my vernier protractor. All the bits I measured have different tapers.
Here's another way of measuring.
It seemed that 10 degrees was a good average so I selected a 5 degree angle to support the work.
Then I drilled out some 1/2" tool steel to be a press fit on the hex circle of a magnetic bit holder.
And reamed it for a slip fit on the outside diameter of the holder for about 1/4".
Then I pressed the holder into the steel.
I knew it wouldn't run true so I checked it against an indicator and whacked it to get it within about .005" which is way more precise than needed.
I then turned a taper at the other end.
I set up the mill with the angle clamped in the vise.
Then put a 5C collet block in the vise against a stop.
It was a simple matter of milling a flat, turning the block 90 degrees and milling another flat.
About halfway done.
I then turned down the shank so it was smaller in diameter than the tapered section.
And it works.
I figured that since I had the mill and lathe set up I'd make another adapter.
Turning the other end 3/8"
I used a die to make 3/8"-24 threads. The tailstock chuck keeps things square
And a drill chuck threads right on.
And I have a nice adapter.
I gave them to Kiko and he gave me a wooden spoon.