Another strangled flute

[paddler]

One possibility that does not seem to have been discussed seriously is that thread is less compressible than cork. If tightly wrapped it fills all the available space between socket and tenon and so when the flute is assembled the pressure of the socket on the thread is transferred to the tenon whenever the thread is not the right thickness. Even if the thread is the right thickness it may not allow as much expansion of the tenon as the humidity rises. Obviously it will be somewhat compressible, so there are tolerances, but these may not be as large as with cork, which may be more compressible.

For these reasons, wrapping the thread too tightly may not directly cause the strangulation due to crushing pressure from the wrap, but may indirectly cause it due to filling all the available space between tenon and socket and hence reducing overall compressibility and tolerance for expansion. The thread may not appear crushed afterward because it may be able to rebound after compression. In its compressed state it may be less compressible than the wood of the tenon, so when the tolerances are gone the wood suffers. Within limits the wood can rebound too, but if you exceed these it will remain crushed.

As for the question of whether the breaking force of the thread multiplies with the number of wraps, it does, but so does the surface area over which the force is applied. Whether 1.5lbs is a damaging force or not depends on the surface area over which it is applied. Try balancing a 1.5lb weight on a needle whose point is on your skin. I bet it hurts! On the other hand, its not clear to me that hanging a 1.5 lb weight on your flute using one thread is going to damage it.

Jon

[Terry McGee]

As for the question of whether the breaking force of the thread multiplies with the number of wraps, it does, but so does the surface area over which the force is applied.

Yeah, that's probably a useful way to look at it, Jon. So let's rejig the maths. I'll stay in the Imperial system, as that probably is more familiar to more readers.

So, one thread breaks at 1.5lbs. Let's assume the worst for the moment - our strangler pulled as tight as he/she dared. There are at least 150 threads along and 13 threads deep, so the overall force is 1.5 x 150 x 13 = 2925lbs. I can't remember how many lbs Americans consider make a ton, but it's not that many. The average American male weighs 180 lbs, so that's like coming back from the bar at the session and finding 16.25 blokes finely balanced on the top tenon of your flute. Reasonable cause to be grumpy, even in the Metric system.

Now the tenon trough is 15mm long and about 21mm in diameter - or, in Imperial, 0.6" and 0.827". The area of such a cylinder (oh, would that it were still a cylinder!) is PI x D x L = 22/7 x 0.827 x 0.6 = 1.56 sq inches. So the pressure being exerted on the wood under the wrap would be the force divided by the area = 2925 / 1.56 = 1875 lbs/sq in (or psi). (Can someone check these maths - these are scary numbers!)

Trouble is, now I'm struggling to find a meaningful pressure to compare it with. Your car tyres are at about 30 psi, but this is 60 times more. Suggestions, anyone?

So, even if our strangler only pulled to a tenth of thread breaking strain, the wrap force would be equivalent to your average married couple standing on the tenon!

Terry

[jemtheflute]

I don't doubt your maths, Terry, but I do doubt that the proper underlying concepts or reality of actual forces are represented by them. I suspect that there are significant reductions or losses from your hypothetical conditions. I don't pretend to know the physics involved, but surely the fact that the tube is (sub) cylindrical and that thread is being laid around it affects the surface force it exerts? I doubt the tension applied to each individual length of thread as it is laid on translates to an equal down-force upon the surface below any particular point. There are probably formulae for this... marine or climbing, rope- or wire-making and -using specialist info? The tension is linear to the thread. Amateurishly, one might even suppose that, wrapped around a cylinder, the crushing force exerted by the thread towards the tube-centre would be double - from opposite sides on a diameter! It probably is so, but not in a 1:1 ratio with the thread tension.

Would it help to think of/make some comparison to your average cotton reel or other thread bobbins? Old style (soft)wood ones or modern (hollow) plastic ones..... they have thread uniformly wrapped around them at a moderately tight, defined tension in precise layers. Even ones that have been stored for years don't tend to show signs of constriction! Cylinders are incredibly strong structures (in whatever material) against lateral and crushing strains. Think of the drum of a winch and the strain of a heavy load on a high-tension cable running on it - if over-loaded there is usually more risk of the cable snapping or the winch coming off its mounts than of the drum itself breaking or collapsing. (OK, I know that's not a direct analogy by any means, but it isn't totally irrelevant in trying to eye-up the conceptuals....)

In any case, when I lap a tenon with thread, I do it by hand, not on any kind of jig where it might be possible (quite difficult, I'd think) to set up a fixed tension in the thread between the flute tube and the source reel and to lay it on in exact neighbouring passes with mathematical precision - something the combings would prevent in the first couple of layers anyway. I may pull my thread fairly taught (but nowhere near its breaking strain) as I lay it on, but every time I pause for whatever reason - to turn the tube, to free new thread, to blow my nose, to re-lay a few passes I'm not happy with..... the tension will be released at least somewhat - and will never be constant anyway.

[benhall.1]

I don't disagree with Terry's conclusion that poorly wrapped thread could damage tenons.

However, there are severe flaws with the logic behind his maths.

Firstly, the tension of an individual thread: the maximum pressure it can exert is 1.5 lbs (accepting that that is, in fact, the case). But, unless renewed pressure is put on the thread, for most of the time, it won't be at that much pressure, because the wood will never be in quite the same state as wehn you first put the thread on - if the wood expands, the thread will break, and if the wood contracts, even a tiny amount, there will be no pressure at all.

Secondly, as I understand these things, what matters is not the total pressure along the entire length of the tenon, but the maximum pressure at any given point. So, you can divide Terry's figure by 150 for a start.

Thirdly, Jem's right - the force at which the thread will break is along its length. That does not translate to an equivalent downward force at right-angles to the force applied along the length of the thread.

Fourthly, I don't buy this vertical bit, whereby each turn of the thread is assume to add the same amount of force at right-angles to the force applied to it. I don't even know how that could be measured. It could be theorietically calculated, I suppose, but only provided you assume that the tenon is completely immoveable and inelastic. If it is in any way elastic, as soon as it is compressed at all, the tension on the lower layers of thread would be reduced to nil, so the maximum force is still whatever the amount of force is applied by one thread, which is, because of Jem's point, significantly less than 1.5lbs.

Mind you, if you stuff the thing up with so much thread that you have to force the tenon into the socket, the force you are applying at that time (plus whatever happens when the tenon expands due to moisture) may be enough to wreck the joint.

[david_h]

I think you need to know of the thread is elastic and what happens with change in moisture content over different time periods. If you hang a 0.5 lb weight on it, then add another, then add another, then a third, then take them off one by one does the thread increase in length and then go back to the length it was with the first weight ? If so then it is stretching and storing energy. What if you leave it loaded overnight before unloading. With the load still on does it change length if wetted ? Does it go back to the same length when it dries out ? How about if the thread is loaded when wet and kept wet, or allowed to dry ? Winding under constant tension (if you wanted to !) is fairly easy - do the wrapping with a weight on the end of the thread and guide it backwards and forwards.

Question for Terry: is the tenon wall in the area where it is most strangled thicker or thinner than near the ends of the joint ? Is the the tenon still the right length ?

I am wondering if this is more like strangling the solid wood of a balsa wood model aircraft by crushing the wood fibres as the thread 'digs in' or strangling a piece of rubber tubing by inducing elastic deformation.

[rama]

would you like your tenons pre-shrinkwrapped or do you prefer to shrinkwrap them yourself?

[George]

So, one thread breaks at 1.5lbs. Let's assume the worst for the moment - our strangler pulled as tight as he/she dared. There are at least 150 threads along and 13 threads deep, so the overall force is 1.5 x 150 x 13 = 2925lbs. I can't remember how many lbs Americans consider make a ton, but it's not that many. The average American male weighs 180 lbs, so that's like coming back from the bar at the session and finding 16.25 blokes finely balanced on the top tenon of your flute. Reasonable cause to be grumpy, even in the Metric system.

Visualize your hand wrapping around a tenon and gently squeezing, there aren't 150x13 subdivisions in your hand but that also doesn't change the force being applied.

I remember watching 1st hand highland bagpipe stocks being tied into a bag. This guy took a long bit of dental floss, doubled it up 3 or 4 times, attached each end of that to a hammer's handle. Then securing one handle with his feet began wrapping with the other one as hard as he could maintaining pressure the whole time with the handles. In the end I think there were one or two stocks he had to redo as they were slightly too loose and leaking. Point being it's hard to get a thread wrap to hold a lot of tension.

No way you're getting nearly 1.5tons of pressure from wrapping a thread around a tenon, no way.

You're lucky enough to keep it from spinning in place.

[benhall.1]

For drier climates, try honeydew tenons and for more humid conditions, there are those water-tenons. I think cantaloupe tenons are my favourites though.

[jemtheflute]

For drier climates, try honeydew tenons and for more humid conditions, there are those water-tenons. I think cantaloupe tenons are my favourites though.

Pre-spoof Combine Harvester?

[benhall.1]

You a tractor mate?

[jemtheflute]

Nah, that'd be a bit of a cart before situation. Stick to bikes and roller-skates.

Way to mangel a thread!

[Nanohedron]

Mind you, if you stuff the thing up with so much thread that you have to force the tenon into the socket, the force you are applying at that time (plus whatever happens when the tenon expands due to moisture) may be enough to wreck the joint.

For issues solely of force upon the tenon, the above is what I think makes the most sense. When I wrap a tenon, I'm not thinking about "bondage", as it were; instead I'm thinking from bare tenon surface outward, about building up the lapping until there is adequate contact/friction with the socket, and of a uniform-enough distribution of thread-cushion (for that is how I think of it) to hold the joint together sturdily without wobbling. I don't know about anyone else, but aside from making sure the thread is not going to slip, how tightly I wrap has little to do with getting the job done. Essentially, buildup of the lapping is what I'm after. That is all, aside from the need to do it without carelessness, without too much thread so as to make for a too-tight fit, and without uneven thicknesses along the wrapping's surface. Indeed, I wouldn't wrap as tightly as I could because the result would be too hard and unyielding; again, keeping the "cushion" idea in mind. Of course, one can wrap too loosely.

If anyone's already brought this up, my apologies.

[jemtheflute]

Fair enough, Nano, though I'm sure you'd agree one is going to apply sufficient tension to the thread to form firm lower layers that upper ones won't cut in to and a surface layer that won't drag during joint dis/assembly.

BTW, I'm not at all suggesting that thread constriction can't be an issue or an agent of tenon deformation.

Ben has seemingly squashed his brain - or something.

[jemtheflute]

You a tractor mate?

Never was any good at pulling.
But I might be a pro-.

[benhall.1]

Before you can really start pulling, you have to practise your ploughing. Lots.