Effects of thread wrapping, Series 2:




An experiment was devised to test whether tenon thread wrapping increases or decreases the likelihood of flute bores going oval.  The experiment showed that the wrapping, in conjunction with humidity change, produces both compression and ovality, probably confirming that these two outcomes are linked.  It also confirmed the observation that such ovality is not necessarily aligned on the radial/tangential axes as might reasonably be extrapolated from solid-timber experience.  The experiment also demonstrated the dramatic power of serial strangulation and perhaps gives us our first real warning sign that a flute is succumbing to pressure.  The experiment will be of interest to those who felt the original test tenon experiment needed a control. 


In our last experiment, we made a test flute tube, wrapped its two tenons with thread and subjected it to accelerated climatic changes.  It demonstrated the longitudinal distortion we were interested in exploring, but also showed a significant amount of ovalling, that is to say the tube no longer remained perfectly round as made, but assumed a slightly elliptical shape.  I thought it might be important to look at that more closely, which is the purpose of the experiment detailed below.

The Experiment

I decided to make up two short tubes, as identical as possible, wrap one with thread but not the other, and then submit them to the usual harrowing environmental conditions that abound in my lab.

Casting around the workshop, I found just the scrap piece I needed.  It was an off-cut of the African wood Mopane, which had been obtained well seasoned in 2002, and was therefore guaranteed to be very well seasoned by now.  Mopane was attractive for this experiment as it is highly figured, making it easy to determine which is the radial face and which the tangential.  My only concern was that it appears a waxy wood, and I wondered if that might make it slow on the uptake and release of moisture.  As it turns out, that doesn't seem to be the case.

In order to make two samples as similar as possible, I resolved to cut them adjacently from the same piece.  I chucked the piece and, using a boring bar, bored the end out to 19mm (3/4") diameter.  I then turned the outside down to 21mm, leaving a wall thickness of 1mm.  Before separating the two samples with an extremely fine bandsaw blade, I ran an ink line down the inside on one of the tangential faces, so I could always realign the faces.  The bold grain pattern assured that I could always realign them end to end.  The two pieces ended up 14mm long (just over 1/2").

I measured the radial and tangential diameters of each, and then, as intended, wrapped one of them with 100 turns of thin (~0.1mm) sewing thread.

The theory

Before relating what happened, it would be good to rehearse what we might expect to happen, given the usual cabinetmakers' experience of wood under varying climatic conditions.  Wood, as you know, grows on trees, and trees are generally round.  The very heart of these dense trees is rarely sound, and further, trying to dry this slow-drying wood "in the round" usually meets with serious splitting.  So, logs are intentionally split or cut lengthwise as soon as possible after harvesting.  That means the blanks for our flutes come from one side or the other of the split, and therefore of the centre. 

We see that when we look at the end grain of a flute blank.  The growth rings run across from one side to the other, usually in an obvious curve, as these trees are pretty small.  Once we've turned the blank round and smoothed the outside, we can easily identify two types of faces.  The two tangential faces have broad "cathedral spire" markings, while the radial faces have fine straight lines, being the ends of the growth rings.

The usual "rule of thumb" is that the radial face moves much less with change in temperature than the tangential face, with 2:1 being the usually quoted ratio.  That's why guitar and harpsichord makers carefully select "quarter sawn" timber (the fine radial pattern) for their sound-boards.  It's not uncommon for a metre-wide (3') harpsichord soundboard  to vary over 6mm (1/4") in width between dry and wet weather.  Cut in plank (tangential) mode that would be a staggering 1/2" or 13mm!

So, as our test samples suffer alternate dehydration and humidification in the lab, we can reasonably expect them to go oval, first one way, then the other.  To keep track of their movements, I assigned them compass bearings.  I marked the tangential faces N & S, and the radial faces E & W.  The interesting thing for me is what do they do when they are allowed to return to normal intermediate relative humidity.

A possible complication

Before we delve into the results, I should just warn you of a little complication.  The cabinetmakers' theory I outlined above would have us expect that the greatest movement would be observed on the tangential face.  But I have observed over the years many flutes that appear to have ovalled along an axis diagonal to the radial and tangential axes.  So, I'm going to be alert to circularity in general, and follow any leads.

The outcome

The graph below sets out what happened.  At the beginning, I followed the two axes (radial and tangential) of each tube.  The thin lines are for the bare tube, the thick for the wrapped.

On Day 1, I made the tubes, and as you can imagine, they measured the same.  I then wrapped one of them, and we can see a small degree of compression set in immediately, as shown in the second column, Wrapped.  I then put the two tubes in the Desiccator, at about 15%RH. 

On Day 2, you can see that both dimensions of both tubes have shrunk to the same degree.  The tubes then went into the humidity chamber at over 90%RH. 

Day 3, we see that the bare tube (thin lines) has expanded well above its original dimensions.  There is some evidence that the radial axis has become bigger than the tangential axis, which seems contrary to expectations. 

But, the real story is being told by the wrapped tube.  The tube tried to expand, but that expansion is totally cut off at the original diameter by the thread wrapping.  Great, you might applaud!  The wonderful thread has halved the pesky expansion of the timber in damp conditions.  Bravo!  But keep reading ....

Remembering my suspicions about the waxy-looking Mopane, I wanted to make sure that this was about as far as humidification would go, so I left the pieces in the chamber for two more days.  Not much further action, so 1 day per event seems like a reasonable speed to take things from here on.  So, now, into the desiccator again.

Day 6, after drying, shows that the two tubes are still free to shrink, although interestingly, the lapped tube has shrunk a little more than the bare tube.  Back into the humidifier!

On Day 7, I started to notice something quite interesting.  The bare tube had expanded back up to about where it was on the previous humidification cycle, and it remained surprisingly round (both axes the same).  So much for the quoted 2:1 ratio!  But the wrapped tube had stopped expanding below its original diameter.  Further, although its two main axes were still the same as each other, I could easily detect ovality with the callipers.  But it wasn't along the main axes, but diagonally to them!

So, I resolved to start logging these diagonal dimensions on the wrapped tube also.  You'll see they start on Day 7, in thick pink and thick navy, marked with the compass bearings NW-SE and NE-SW.  And you can see why I could hardly ignore them - they are 0.3mm different!  Time to go back into the desiccator...

Day 8, and both tubes have shrunk again under the action of the dry air in the desiccator.  The bare tube back to its normal dry dimensions, the wrapped tube somewhat smaller.  The four axes of the wrapped tube were back closer together, i.e. the degree of ovality had faded.  What was also interesting is that the wrap on the wrapped tube was now quite loose, and could be easily slid en masse around the tube.  Wow!

I thought this was about the time to find out what the outcome would be if we let them both equilibrate to atmosphere.  As you can see, on Day 9, the outcome was unspectacular.  The bare tube went back pretty much to original dimensions, while the wrapped tube was compressed, but not oval.  Drat, was this whole experiment a waste of time?

Serial Strangler Needed!

About then, a little voice said to me, maybe we need to try some serial strangulation?  But was this the right time?  Of course by this time, the wrap was tight again.  Then the penny dropped.  You don't need to tighten the wrap on your thread-wrapped flute in normal weather.  It only comes loose in dry weather, and that's when you do it!  We need some drier weather.  Back into the drier!

I resolved at this time to also start measuring and logging the diagonal dimensions of the bare tube, even though I hadn't detected any significant ovality.  You can see the thin extra pair of lines starting Day 9.  I've replicated the graph below so you can more easily follow the action.

Day 10, the tube is dry and the wrapping loose.  Once more the bare tube descends to its usual dry dimensions, once more the wrapped tube shrinks even smaller.  And ovality is now becoming noticeable even in the dry condition.  I take the wrap off and rewind it on, before popping both tubes into the humidifier.

Day 11, after 24 hours of humidification, a fascinating picture emerges.  The bare tube has moved up to its wet weather dimensions, with a very minor amount of ovality showing.  But the wrapped tube has gone bananas.  Only one diagonal, NW-SE, has even approached the original dimension.  The tangential axis has tried but been arrested 0.2mm lower.  The radial ditto, except 0.3mm lower.  But the other diagonal, NE-SW, is now a full half millimetre less than original, and all this when the bare tube is 0.4 to 0.5mm engorged!  Bare tube ovality 0.1mm, wrapped tube ovality nearly 5 times greater!

OK, definitely time to let these tubes air and see where they end up.  To save time, I give them both a short burst in the desiccator until the bare one was close to original dimensions, then let them both air on the bench.  I wait a few days to make sure they have landed.

Day 14, and the bare ring is now back to just over the original size.  The weather had been dry at the start of the experiment but we've had 105mm (4") of rain over the last few days, so that probably explains the slight but negligible size increase.  Ovality negligible at about 0.05mm. 

But it's been all downhill for the wrapped ring.  The average of the four axes is now 0.375mm smaller than originally, with ovality about 0.31mm.  Again, the maximum ovality is centred on the diagonal axes rather than the orthogonal axes predicted by the theory.  And all this within 2 weeks, with fewer than five humidity cycles.

So, what's going on here?  How could this possibly happen? 

Probably because of ....


Ooh, now there's a new word to bandy around at the session.  But what could it mean? 

Anisotropy is just the opposite of isotropy, if that helps!  And isotropy means equal in all directions (from the Greek iso = equal and tropos = direction).  So anisotropy simply refers to something having qualities that are directionally dependant. 

The something here is wood, specifically our wrapped tube.  Wood has three dimensions of interest to us.  The length, the radial and the tangential (to the growth rings) planes in wood all exhibit quite different qualities.

And the quality we're probably finding unequal is stiffness.  The wood in one horizontal plane (as the tree stands) is less stiff than the wood in the other horizontal plane.  So, when force is applied equally around our tenon (by the thread wrap), one plane is better able to handle it than the other, so the tenon goes ovoid.

But why diagonal?

That, I will concede, is not so easy to explain.  I can think of several possible factors:

  • the wood in the diagonal planes of a thin tube is the least stiff

  • the curvature of the growth rings favours one diagonal over the other

  • the direction of the wrap puts more stress on one diagonal

A more colloquial explanation might be that when an irresistible force meets an immovable object, things usually go pear-shaped!  What is actually going on is perhaps one for the material scientists and of little import to us; what concerns us is the fact that just one case of serial strangulation has caused our test ring to compress suddenly and go elliptical!

Myth Busted!

Note that this busts a claim long put forward by proponents of thread wrapping - that the tight wrap of thread is somehow beneficial in keeping the tenon round.  In fact, the tight wrap of thread exploits weakness in the wood to make it oval!

Is ovality bad?

Not in reasonably mild cases.  If taken to extreme, it could cause air leakage around the tenon, difficulty rotating the flute sections, etc.  It shouldn't have acoustical ramifications, unlike the bore compression which has to have acoustic ramifications.

Is ovality good?

It's sometimes argued that ovality is beneficial to the operation of a flute.  This probably arises from a claim that making the vertical axis of the head deeper assists with the breath "getting around the corner".  I'm not aware that this claim has ever been tested, and even if so, it would seem to only apply in the head region.  Clearly for the tuning slide and sockets to work, the tubing further down the flute needs to be round by that point.

Even if ovality of the head were proven to noticeably improve the performance, the maker would want control of the amount and direction, not to leave it up to forces beyond our control.

Is ovality inevitable?

I don't know, and we shouldn't extrapolate that far from this initial experiment.  But it was certainly interesting that, in our previous experiment compressing both ends of a tube, the end that compressed the most also ovalled the most.  We now need to observe strangled and unstrangled flutes to see if ovality is more common in the strangled case.

An anisotropic speculation

We saw above that thread wrapping a tenon leads to ovalling of the tenon and adjacent body, as the tenon wrap forces exploit anisotropic stiffness in the timber.  Could other forces, e.g. a tight metal ring, do the same?  My guess is probably, but probably to a lesser extent.  One would expect the metal ring (assuming it was round when it went on) would have some finite stiffness of its own, unlike the thread wrap.  It's not a question this study is concerned with, as the Potter flute doesn't have metal rings.  It does have ivory rings, although these are not normally installed under pressure.  But the question is much complicated by the nature of ivory, which almost assuredly will have anisotropic qualities of its own.  Depending on how the ring is installed, these might line up and reinforce or partially negate the wood's tendencies.  And timber and ivory will have different rates of expansion vs humidity.  I think we just might leave that complexity for another day ....


The experiment demonstrates that thread wrapping is certainly capable of reducing the normal movement of wood with changes in the weather, but at a price.  It causes bore compression and it sends the wood oval.  More dramatically, the experiment illustrates the frightening power of serial strangulation.  A single change of thread, carried out at the logical time - when dry weather makes the thread loose - was enough to convert compression to strangulation.  The need for rethreading probably gives us our first clear warning sign that a flute is succumbing to pressure.

The experiment will also be a confidence boost to those who felt the original test tenon experiment should have been accompanied by a "control" tenon.  The bare tenon came through the experiment unscathed, despite being cycled over a greater range of dimensions in a short time frame.  It was the wrapped tenon that suffered, clearly proof that the wrap was the game-changer.

Back to McGee-flutes Index page...

  Created 28 June 2011