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Introduction
In order to test the
possibility that a thread wrap could damage a tenon, I decided to make
up an under-strength test tenon, wind it firmly and subject it to some
rigorous climatic change.
It has to be understood that
this is an experiment "in extremis". The process I used is a form
of
artificial
aging. Such an experiment is designed to bring results in as
short a timescale as possible. It is therefore not designed to
mirror reality accurately. It will be enough for now to prove
possibility, and to learn anything else we can from it.
As can be seen from the first
column, Just Made, the bore of the tenon was 18mm, and the outside
diameter 20mm. There is an 18mm wide trough for thread lapping
with a floor diameter 19.6mm, but of course we'll lose track of that
once the tenon is thread-wrapped. The full width of the tenon
being 30mm, this left shoulders of around 6mm wide on both sides of the
thread trough.
In the Just Wrapped column,
we can see that a wrapping of 0.5mm depth was added, producing a wrap of
20.6mm diameter. The thread I used was the first to come to hand,
a left-over of a domestic sewing project. The manufacturer
advises: "Rasant - the functional polyester/cotton core spun thread.
Rasant is a functional sewing thread with a wide variety of uses. The
perfect synthesis of polyester core and cotton covering makes Rasant
outstandingly efficient, not only in the sewing process, but in the seam
as well."
Because the test tenon was
small and a bit fiddly to hold, the thread was wrapped on firmly, but
not overly tight. It was also not wound on as neatly as had been
intended, which would have achieved a stronger wrap.
Immediately, a small
reduction of the bore under the wrap was noted. This is to be
expected, as mentioned above, as the tenon will be compressed until the
force exerted by the thread is balanced by the restorative force
produced by the distorted wood. The fact that we can measure
compression immediately after the thread has gone on does however
confirm that we are dealing with a significant force. I left it in
this condition overnight to make sure that balance has been reached
before proceeding.
Effect of Weather
The next day, there was
little change, so it was felt appropriate to start the "Effects of
Weather" phase. The tenon was subjected to an environment at 25%
RH overnight. Naturally, the timber shrunk, as can be seen in all
curves.
The tenon was then subjected
to a very humid atmosphere by suspending it over, but clear of, a wet
sponge, in a closed plastic
ice-cream container. My
laboratory hygrometer reads an impressive 99.9% RH in there. Note
that, in After Humid1, all the wood swelled, excepting that bound by the
thread, which continued to shrink slightly.
Subsequent drying and
humidification cycles have presented an increasingly interesting
pattern. The bore under the wrap (pink) has continued to collapse
in diameter. But the ends of the tenon have started to increase
in diameter, both outside and in. This dramatically enhances the
"hourglass" shape which was a very visible feature of the damaged
flutes. And it is consistent with a previously unexplained issue
with the damaged cocus flute - all three tenons, even when their thread
wrapping has been removed, jam noticeably as they enter their sockets.
I was concerned that I
mightn't be giving the drying phase enough time for it to equilibrate,
so, after the third drying, I gave it some extra time. As you can
see, not much more happened in the More Drying phase. It was felt
at this time, it was appropriate to let the tenon then equilibrate to
local atmospheric conditions, the results of which you can see in the
"After Airing" column. Not shown there is the length, which has
returned to the original 30mm. It had increased to as much as
30.3mm in humidifying cycles, and reduced to 29.8 in drying. The
fact that it is back to normal suggests we have equilibrated. This
pattern was to be repeated throughout the tests.
Summarising the Effects of
Weather phase, we can see that:
-
the diameter at the
middle of the wrap on the outside (aqua) has dropped by 0.3mm
-
the outside diameter of
the ends has increased by nearly 0.4mm,
-
the bore diameter under
the wrap has dropped by 0.8mm,
-
the bore diameter at the
ends has increased by just over 0.5mm
Effects of Playing phase
With clear trends in the
weather established, I think it's now time to try emulating the effects
of playing.
I simulated an hour's
practice, by lightly stuffing the tenon with a damp rag for an hour.
I felt it may not have any different effect from the humidifying we did
in Effects of Weather, but it seemed possible that only humidifying from
the inside could produce a different result.
Certainly, the first hour
"playing" illustrates that a wet bore is a much faster way of getting
water into the tenon than a moist environment! Not really
surprising, I guess.
In order to keep things
moving, I popped it back in the dry environment for an hour or so, then
let it air on the bench. As you can see in the final After Airing,
there hasn't been much change since the airing before playing.
Like most things, change is faster at first and slows as time
progresses. There probably would be more change if we kept this
up, but I think we've reached the point where we can draw some useful
conclusions.
Comparison of bore
distortions
I thought it might be helpful
now to compare the distortion we had introduced into the test tenon with
the kinds of distortions I had found on the two "strangled" flutes.
Not directly comparable of course, as the flutes had tapered bores,
while the test tenon had a cylindrical bore. So I corrected the
test tenon figures to introduce a taper, based on my guess as to what
the cocuswood flute taper might have been originally. So, in the
graph below, we see:
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The boxwood strangled
flute, in yellow,
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The cocuswood strangled
flute, in pink, with my guess as to what it might have been
originally in pink dashes,
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The "corrected" data from
the test tenon, in the purple trace.
You can see that the test
tenon distortion lies between the two other well-strangled flute traces.
Even uncorrected, it pretty much followed the pink solid trace, whereas
it should have been (if it hadn't been compressed) horizontal along the
18mm diameter line!
Note that, had our test tenon
been part of a real flute body, the distortion from the middle of the
tenon onwards (X between 15mm and 30mm) would have been less, as the
more rigid body on that side would have helped resist the distortion.
Although we would have seen some compression in the body. In order
not to confuse the image, I've chosen to map only the first half of the
tenon, which is analogous to the other flutes. I would expect to
see the purple trace following a path approximately halfway between the
pink and yellow traces.
Conclusions so far....
Our Test Tenon has confirmed
the mechanism by which thread distortion takes place, producing results
similar to what we found in the two strangled flutes.
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