Effects of Thread Wrapping series:



What happens when you play a country song backwards?  According to the popular song, it reverses all the sad things that usually happen to you in country music.  Your truck engine springs back into life, your dog doesn't die, and even your wife comes back to you.  Powerful magic indeed!  If we reverse what we did to our strangled tenon, could we reverse some of the damage?  Can we use the boxwood test tenon to work out possible cures for strangulation in real flutes?  We need to know, so...

I measured it once more, to ensure no further changes over the last few days.  Nothing worth reporting.  I then pulled the thread off.  For the record, it was 20 metres long.  (Easier to measure coming off than going on!).  That is substantially less than I had taken off the cocuswood strangled flute. 

I remeasured the tenon, no immediate change other than the bore under the thread increased in diameter to 17.45 as the remainder of the thread tension came off.  Originally 18mm, it's still pretty squashed. 

We can also now pick up the diameter of the bottom of the thread trough that we lost track of when it was wrapped (purple trace).  As we can now see, it had started at 19.6mm, been crushed down to 19mm.

The first test is to simply humidify it.  It seems too hopeful that it will obediently spring back into place, but let's see.  Back into the ice-cream container....

After rehumidifying overnight (humid 3 column), we can see that all the diameters have increased (as expected).  Indeed the crushed bore under the thread wrap (pink) and the crushed bottom of thread pack (purple) are now well over their original uncrushed sizes.  So, what will happen when they dry out?

Oooh, that's promising!  After the final airing, on 28 January, we can see that the bore under where the thread wrap went is back to pretty well normal, as is the outside diameter at the bottom of the thread trough.  So this does suggest that removing the thread wrap and then humidity cycling might be helpful in attempting to deal with a collapsed tenon.

Surprisingly, it's the free end(s) of the tenon that have ended up still wrong, and oversize at that.  Now who would have thought?  But I think I can see the basis of a way of dealing with that too.  If one were to dry the tenon first to make it all undersize, wrap some thread around the free end(s) to constrain them and then humidify and dry, one could probably get that back to size too.  If wrapping thread isn't the answer (might be too hard to predict the end size), then maybe installing a delrin ring which can be popped off or turned off later.

The tenon straightjacket

OK, it's not enough to propose a solution like that, is it?  We should test it, so I have.  I turned up a Delrin "straightjacket" to constrain the outside of the test tenon to the size I wanted, the original 19.6mm in the trough, with 20mm ends.  A moment's thought will show I can't get such a thing on and off a tenon while it's in one piece - the smaller diameter section intended to constrain the bottom of the thread trough will not pass over the end shoulder.  I slit the straightjacket down one side, and put a pair of partial slits along the outside at 120 rotations - in other words I turned it into a collet.  I was now able to expand it enough to slip it over the test tenon, by inserting a screwdriver in the slot and twisting sideways.  I clamped the outside until the outside diameter was the same as before I slit it. 

At this point, the test tenon was very damp, and so enlarged.  A day or so later, I removed the straightjacket and measured.  Yay, even better!  This table summarises the results:

Condition: Originally Strangled Steamed Constrained
End bore 18 18.6 18.5 18.2
Mid Bore 18 17.6 18.1 18
End OD 20 20.6 20.2 20.1
Mid OD 19.6 19 19.6 19.6
Worst case error Reference +0.6 / -0.6mm 0.5 0.2mm

You can see the worse case error has dropped from +0.6 / -0.6mm when strangled, to 0.5mm when steamed and aired, and then to only 0.2mm after constraining, a six-fold improvement!  And we could probably do better still by making the straightjacket a little undersize.  Those who bend wood for musical purposes find they need to "overbend" a little to end up where they want to be.  The wood springs back a little.  We can expect this to happen to us too.

The proof of the pudding...

... is traditionally located in the eating.  So, can we really cure a real flute from the distortions of the past, by the same processes that got us there?  I resolved to find out using the cocuswood strangled flute.  I opted for the approach used by violin, guitar and harpsichord makers, suitably adapted for flutes, but there would be many possible approaches.  This method offers minimum water uptake and so speedy recovery. 

I turned up a brass plug following the bore shape I guessed at for the cocuswood strangled flute.  Holding the tenon in boiling water, and then pushing the heated plug into it produced steam, which infuses into and softens the wood.  Each time I did it, I could push the plug a little further, until it finally reached the point I wanted.  I let the wood cool and dry with the plug in place, to prevent it sneaking back when my back was turned.  (If you want to try this yourself on an affected flute, be careful to constrain the outside of the tip of the tenon from expanding too far, as the thin wood of the tenon is at considerable risk of splitting from the wedging action of the plug.  Paradoxically, wrapping some thread around the tip as mentioned above might be just the answer!)

You can see the result in this next graph, with the steamed cocuswood flute in green.  Comparing my previous guess (pink dashed) with what we have now, it looks like I might have overdone it at the tenon tip, but that's easily fixed if necessary with the straightjacket approach.  But looking at the steeper bore taper down around the 60-70mm mark, I'm also wondering if this approach might not have reached far enough into the thicker wood just beyond the tenon, circa 30-40mm.  A longer, slower soak might do that, combined with a tapered plug on the inside, and a constraining ring on the outside.  But the flute has to be far better off now than with the constriction in its throat seen on the pink curve!  I decided to let its performance tell me if we're done yet.

A Happy Ending

The strangled cocuswood flute is now back together, and working very nicely.  It is impossible to say how important fixing the choke in the top tenon was, as I also found lots of other little things that warranted attention along the way.  This is normal with old flutes.  They might have one or two dead-obvious things wrong with them, which is usually why they were sent in for attention.  But it's often the accumulated effect of a dozen or more little unnoticed problems that continue to pull an old flute's performance down.  It's like layers on an onion - they can seem to go on forever.  But if you want to give the flute back its original joie-de-vivre, every one of them has to be detected and ruthlessly eradicated.  Bore strangulation is now a new one on the agenda.

Flute, heal thyself [Luke 4:23]

(See, it's true.  Even the devil can misquote scripture!  Chapter and verse.) 

Our experience with the test tenon might even bring us some hope for a self cure for strangled flutes.  Imagine you have a flute with bore compression.  You remove the thread-band tourniquet and replace it with something that won't restrict expansion, say cork.  You put the flute back in service, meaning it gets subjected to wide humidity cycling but now without constraint.  If our test tenon's recuperative experience is any guide, slowly the damage might be undone.  At least you know it won't be getting any worse!

If that were to happen, we might notice that the corked tenon became tighter over a period than it had been when first installed.  That would be a good sign, suggesting the tenon was recuperating, but should also be regarded as a warning that excessive force could soon be applied to the socket.  It would be important to ease any such tightening, until equilibrium was once more restored.

Wrapping it up (get it?)

Puns aside, I don't think there's much point in attempting much more on the test tenon, as we're now getting into finer points, which is likely to depend on matters like how compressed a real tenon is, what timber it's made from, how thick it is, how long the damage was sustained over, how long since the damage was sustained, and so on.  But we can certainly thank our test tenon for confirming how this compression comes about, and offering us some hope that it can be reversed or at least ameliorated.

I'm very happy with the in-extremis test procedure adopted.  In just over a week, we were able to simulate what might have taken years under normal circumstances, and yet end up with entirely plausible results.  And discover an unexpected side effect.  And explore a possible road to recovery.  Rewarding!

And to apply our new-found knowledge successfully to a real-life patient and effect a cure. Priceless!

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  Created: 23 January 2011; last updated 20 February 2011.