looks to be something similar to a hard metal carbide drill & some
I have tried similar things in the past with mixed results. What I
have used & have had the best luck with (other than a EDM) is a used
4fl carb endmill...eat away the center of the tap & then break the
flutes out of the threads.
This might sound weird, but it works and fast. I can get a 1/4 tap out in
2-3 mintes no fuss, no picking, or almost no picking.
This requires a dekel cutter grinder for my example.
grap a piece of carbide or the back of a carbide endmill, etc... and grind
it down to the tap drill size. The turn the cutter grinder on 45 degrees.
Now using the 3 slots in the collet as a guide, grind 3 sides till they meet
at a pont. Very simple cutter. No cutting edge.
Now at the bridgeport mill, have the quill in one hand and an air hose in
the other, running say 1000 rpm. Bring the cutter into contact with the
broken tap gently, then give it some semi light pressure for a few seconds,
then pull real hard on the quill for a few seconds, raise it up and air
repeat real fast.
The cutter melts its way thru. Each peck will get you .010-.020 deep. The
cutter won't break, its just a 3 sided nothing.
The air blast gets the chips out.
I'd get one if I were you and keep it unused in your toolbox as a
sample on how to hand grind the same type of tip on the shank of a
broken off carbide end mill.
You might have to regrind it more than once to get through a long
broken tap. The end can get "mushed" pretty quick if you push them.
Try to start off with the broken tap being as "flat" as possible (chip
away any pieces sticking up with a punch).
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