Glenn Ashmore wrote in message news:2Idub.6403$0K4.2832@lakeread04...
This is not directly boat building related but I thought it might come
in handy for a very frustrating situation that you may encounter. I was
tapping some holes for #6 screws this weekend in a piece of aluminum
that I had spend some time machining. I was on the 8th of 8 holes when
the tap broke off below the surface.

To keep it boat related the aluminum was for a very high tech piece of
boating equipment. A pielter effect thermoelectric beer coozy. :-)

A little research and I find that a hot solution of alum will desolve
the tap without effecting the aluminum. I pulled out the old propane
fish cooker/steambox fire and an aluminum pan. Boiled a gallon of water
and added alum until it quit desolving. Dropped in the part and turned
the fire down real low. Six hours later the tap was mush and the
aluminum was bright and shiny.

Like I said, just something to file away in the back of your mind for
that frustrating situation. It works for brass and bronze too.

You can buy or make a little tool that extends 3 'fingers' into the
tap grooves and will (sometimes) allow you to back out the tap
fragment. It sometimes helps a lot to first pour a little acid down
in the hole to loosen things up. Lubricants serve the same function,
but don't work nearly as well.
If the hole is a through-hole, here's another method that nearly
always works: simply press the fragment out. This will tear some
grooves in the wall of the hole, but there is quite often sufficient
metal remaining for adequate threads, and the results won't leave any
trace of your folly to hurt your pride.
Paul Mathews