Harbor Freight sells a very small air turbine that will take dental
drills. I have had good luck using one of those to get broken taps
out. Not real fast, but it does work on tool steel.

Dan

John wrote:
I'm trying to determine which of the acids we used to use, in the old days,
to disintegrate broken taps or drills. I remember when that was done
occasionally, but don't remember which acid was used. I have a friend who
has a broken drill in cast iron and I thought this might be a possibility
before we start chopping at it with carbide tools and make matters worse. It
might at least loosen it up enough to remove by hand. It's a 1/8" dia. drill
and not in too deep, about 3/16", but it's bound pretty tight.

Thanks
John