34 http://www.model-engineer.co.uk Model Engineers’ Workshop
cut off from the smaller drill.
I had grave doubts about the heating
eff ects of gas welding (all I have available)
on the state of heat treatment in the drills
so I made up a brazed test piece from
another SDS drill in another section of bar
of the same size to evaluate those eff ects
on the drive fi tting.
There was a little soft ening of hardness
which I thought might be benefi cial in this
case. As a crude test, I hammered the test
piece about in the vice to see if side strength
had been aff ected, but that too also seemed
satisfactory within the limitations of the
test. Certainly, there was no sign of a brittle
nature, which is at variance with what took
place later on the job.
These joints are shown here side by side
before brazing in photo 2. Note the fi led
out extension to the helical dust clearance
slots on the drill body. I imagined they
would be essential to prevent a choke when
the drill was buried out of sight and reach
within the depths of the wall. That would
as a minimum be an embarrassing disaster.
For the brazing I used Sifb ronze to make
all the joints in the tool; braze was fed in
through the side holes in the joints to get
well into the sockets.
Photograph 3 is a rather fi ne posed
fashion picture of me in my overalls
delivering the fruits of my labour. It does
at least give a good idea of the size of the
thing.
The drilling experience
The plan of attack by the team was to use
another 1 metre long drill of 25mm diameter
to make an initial hole and when that was at
full depth, swap over to my modifi ed drill to
extend it onwards, using the hole as a guide
to steer it.
Progress was very encouraging at fi rst
until about four feet into the wall when the
drive fi tting in the drill chuck broke off with
all the signs of brittle failure.
Urgent work took place in my workshop
to try and salvage the job and fi nish in what
was left of the day which led to us crudely
milling three fl ats around the drive end.
Because of the length involved it needed a
pretty hairy set up on my horizontal mill that
involved hand bracing at the remote end.
Crude as this was, it did at least give the
circumferential locations for an angle grinder
to deepen and lengthen them to allow
gripping by a three jaw SDS drive drill chuck.
This approach failed miserably on the
job as hammer action cannot be used
successfully in that way since all the
hammer action is dissipated within the
chuck itself. Forward progress was now
measurable in microns, if at all.
A new attempt was made on Day 2,
having now brazed in the cut off drive end
from a well used 16mm diameter drill, this
time using a length of the full diameter
of the drill. It was joined to the bar with a
sleeve and is shown here in photo 4.
Without knowing any better, I hoped
this would improve the control of heat
decay back into the coupling. The original
attempt had used the old dodge of a raw
potato stuck on the end of the coupling, amethod I had been doubtful about in this
case anyway.
Despite all the care and attention this
new attempt failed within two minutes of
use before brittle failure again took place at
the same spot as before on the coupling
By now the electrical leam was beginning
to think that the Good Lord had taken
exception to this assault on His church,
either that or they had a closet heathen in
their midst!Better success with a new
approach
Fortunately, I was able to take advantage
of a supply delay in the lighting units that
were coming from Germany so there was
time to try again in somewhat less of a rush.
This time a 1 metre long 12mm diameter drill
had been bought to be used as a pilot from
the existing hole, we would then drill back
from the outside aft er breakthrough and
bring the fi nal hole up to 25mm diameter
throughout.
By now I was getting rather concerned
about the reductions in the length of the
black bar from cutting off the failures, it
would take too long to get a fresh length
in time for the job so I tried to remove the
25mm drill from the end of the bar.
Even with the braze dribbling out of
the fi tting it would not let go and what I
thought was rotation was actually necking
of the small end within the hole. The bar
also bent during this battle.
I could have saved the gas and cut it off
instead which is what I had to do but with
an extension to remove the bend in the bar.
This time even more care with the joint
preparation measurements was taken with
to preserve both the length of the working
end and, hopefully, leave enough of the
drive end to contain the condition of temper.
With fi ngers crossed as well, it was cut
through at about a quarter of its length.
I had to trust that gave enough length
back from the 12mm drilling tip to the
coupling to prevent fouling before
breakthrough by the coupling within the
wall that would getting close to the mouth
of the smaller diameter hole. There was a
very defi nite element of “muck or nettles”
creeping in by this stage!3
Delivery4 5
The new joint on attempt 2 A new joint on attempt 3