Broken Tap Disintegration (“Tap
Burner”)
a
Overview:
One day I broke a #10-24 tap in some steel flat stock. I had removed broken drill bits before by using an acid etch, but I had never removed a broken tap. So I decided to make a broken tap remover for small taps.
This document presents a conceptual sketch for a broken tap disintegrator that is intended for small broken taps, specifically 10-24 thread, or thereabouts. It uses a longitudinally vibrating 1/16” solid tungsten electrode which is powered with about 5 to 10 amps @ 12 volts (AC preferred). The tiny cross section may require multiple passes even on a 10-24 tap. In the initial test, about 15 minutes was required to penetrate a broken 10-24 tap in a 3/16” steel plate.
The vibration generator comes from a broken ACE Hardware brand “Professional Electric Engraver”. The driven electrode is electrically isolated from the disintegrator metal case, which in turn is safety grounded via the AC power cord. The electrode is connected via a simple alligator clip positioned on the vibrating electrode below the nylon guide block. The power source was a 12v battery charger in parallel with a small 3Ahr battery. AWG #14 stranded wire was used for the connections. During use, a drill press quill is used to slowly feed the disintegrator with the vibrating electrode into the work. The work has DI water flowing across (or through) the active area to remove particles and cool the electrode.
The pictures shown below are of a test version, not the final unit.
Note: Drawings have not
been rechecked for
accuracy, missing dimensions, etc.
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The tap disintegrator is chucked in a
drill press for rigidity and depth control. (The drill press is OFF.) DI water flows across the broken tap
inside a foam dam. Alligator clips are used for the electrical
connection to a 12 VDC, 10 amp supply. The vibrator coil connects to a
wall outlet (preferably one with a GFI) A 1/8" ID drain tube proved to be too
small (had too much capillary resistance) and was removed. The disintegrator burned a fairly clean,
round hole through the tap. Remains of the tap were broken out with a
screw extractor (which was otherwise unable to extract the tap). Then
the threads were cleaned up with a new tap. The results were
satisfactory. (if they
had not been, I would have welded the hole shut and then redrilled and retapped it.) I was surprised that the screw extractor
was very scratched up during this operation. I did not know it at the
time, but Electrical Discharge Machining (EDM) produces a very thin
layer (a few ten-thousandths) of extremely hard, brittle material. "Superficial hardness
tests have been performed on as-cut EDM surfaces and it is often found
to be equivalent to a 70 Rc
hardness or higher. This explains why tools shaped by EDM generally
will wear an extraordinarily long time in comparison to tools made of
conventional heat treated steel" (Heat Treatment, Selection,
and Applicaton of Tool
Steels, William E. Bryson (2009) ). See also http://www.cladtechnologies.com/articles/hardness%20vs.%20wear/hardness.htm |
| An Aside: A bit of trivia about hardening thin objects by induction hardening. | |
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Close-up of
#10-24 broken tap with center burned out by tap disintegrator using
1/16" tungsten electrode, 12 volts DC @10 amps. DI water flows across
the tap, top to bottom in photo. The water is confined by a foam dam
with a drain outlet at the bottom of the picture. |
Same close-up
but backlighted. Hole is fairly clean and round. Tap remains have not
yet been broken out. |
Hole after clean up. |
