wrote in message
...
I have a broket 1/4-20 tap in some 1/2 in steel plate.
(Last hole of 8, of course)

I can access both sides of the tap, the tap is a 4 flute tap
and has a dimple in the end
(Looks like it was setup to drill it out.)

Assuming that you've tried to gently puch it out by using a small punch on
the flutes and failed. Only a diamond drill will work on a typical tap, but
that is in itself very chancy. A tap extractor (hah!) might work if you're
lucky. Realize that the point of a tap extractor is to save the work, not
the tap extractor. They break more than half the time. If you try that,
be sure to lubricate well and to work back and forth, very slowly and
gently. Maybe 100 times or more.
In a big shop with great tooling it would be done with an EDM
machine.

The simplest way (what I would do) is to punch it out. Do this in three
steps. First use a very small cold chisel and attack whatever is sticking
out on the broken end. You might be able to chip part of one or more
flutes that way and thereby make it easier to remove with a tap extractor.
This works about half the time. Next (if that failes) use a punch with a
slight hollow so that it can fit exactly over the center of the point of the
tap. Then whack it good and hard. Very rarely, you'll be able extract it
then. More likely, with a few good whacks you'll drive the tap out, leaving
a worthless thread and over-sized hole behind. And that's why Helicoils
were invented.

I made myself a 12" cast iron lathe face plate with about 120 3/8" x
16 holes tapped in it. Had to use the above procedure with Helicoils for
three holes.

Boris

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