Effects of Thread Wrapping series:

A Test Tenon




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:

  • The boxwood strangled flute, in yellow,

  • The cocuswood strangled flute, in pink, with my guess as to what it might have been originally in pink dashes,

  • 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. 

On to Recovery, or

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