August 2019 35
Long Drill
The whole assembly was laid up
for alignment in angle iron as in the
arrangements used in the other
constructions and the new joints were
brazed together. Photograph 5 shows
the re-purposed joint from the 16mm
attempt with the 12mm drill fitted instead,
it is certainly not a prize winning piece of
work but from the experience gained in
trying to break a joint it was clearly plenty
strong enough.
Day 3 dawned and as it was a Saturday
the boss himself took charge to try and
salvage some lost time. I was pretty well
keyed up as well as this approach with the
smaller diameter drill was really the last
shot in the locker without starting over and
running into criticism for the added delay.
You can imagine my feelings then when
he phoned to say “Well-----” paused
dramatically for what seemed a long time,
before announcing that breathough had
happened in under half an hour. It was all
over!
Photograph 6 shows the breakthrough
which came out directly into a shallow
trench outside the church wall. The lights
themselves would not look out of place on
a warship, they are massively made and
really heavy.
Photograph 7 shows them looking just
as solid and unmoving as the Easter Island
statues and about as pretty! The church
authorities however love them.
With buried cables as well and fresh plant
growth to hide the runs the installation
should stand up well to defeating the metal
thieves in our part of the country.
Postscript
Photograph 8 shows the fi rst of the
failed joints in close up with clear signs of
a rapid brittle fracture. The second failure,
at exactly the same point, was not so
easy to diagnose as there had also been
rotary polishing on the broken faces. It
had happened just as suddenly, straight
through the coupling without any deviation
and displayed all the other hallmarks of
brittle failure.
I did fi le hardness tests on both of these
broken sections and yet more on my earlier
brazed test piece as well as on a new SDS
drill without fi nding signifi cant diff erences
between them. Clearly hardness, or any
lack of it, was not a factor here. So what
other possibles existed?
The speed of failure in only two
minutes working of drill 2 was clearly an
extreme reaction to severe stress and
I can only consider that some form of
complex harmonic standing wave effect
was taking place.
The tooling as a whole would be
expected to contain multiple resonant
frequencies when in use which could have
combined and focussed on that point in
the set up as perhaps an obscure effect
of damping within the hole acting on the
combined length of the tool.
The two failures were in identical
positions and the only thing that had
changed in the tooling geometry between
them was a rather small difference at the
drive coupling end.
It is compelling to make the comparison
here with wine glasses being made
to shatter from induced sound waves
resonating within them.
I find it significant that the only major
change to the tooling geometry was with
the rebuild using the smaller diameter
drill which gave the final success; it would
most certainly have had big affects on
sound paths within the tool as a whole.
The new combination raced through
the remaining 19 inches of stone without
any trouble at all.
All this is conjecture on my part of
course but I don’t like unexplained failures
that are uncannily similar to each other; it
is even less satisfactory to just write them
off as “just one of those things”.
What has though been a positiveoutcome has been the knowledge I have
gained of the really surprising strength of
a virtually empty brazed joint, there was
no yieding in it at all at red heat. Given
another such job in the future I will be
much better prepared for it and tackle it
in a different way.
I was given website directions from a
completely unrelated but interested party
to direct me to the Silverline range of SDS
drills who market, amongst others, 24mm
diameter drills at 1.5 metres working
length and at what looked to me like
suspiciously low prices..
One of these would still have been
200mm short of penetration in this case
but it might do for a less demanding job.
On learning this and exploring the Bosch
professional website to see what other
drills they might have, I found it was
very badly let down by the quite useless
search facility.
I gave up trying to wade through the
thousands of products displayed.
I do know from a brief glimpse of one
page that they do much longer drills but
couldn’t note the details in time before
contact was lost entirely.
I then tried the Heller website but
couldn’t get beyond the message telling
me the site couldn’t be reached and to
please check spelling or try blah, blah –
life is short enough and clearly my server
had also had enough as well!
Sadly, the eating of humble pie later at
the tool hire place was rather uneventful
and it took the shine a little off the
triumph; perhaps the choking was too
well concealed to be evident! ■6 7
8
Breakthrough “Bomb proof” lights
A clear view of the fracture face