Well, answer the question: WHAT is fundamentally wrong with the idea? Explain, please.
Rob
like a shroud
Not just a luddite, but also a poor reader! Sigh, I had such high expectations for the generous man.
I was attempting to stay out of this …
I was intrigued by the problem and before Terry’s infamous post sat down in a spare moment and with a little geometry, vectors and a hint of calculus (oh no there is the dreaded word - don’t worry I’m not going scare anyone with details, I fear that those who would need convincing the most would not follow in any case)
Needless to say I came up with the same answer as Terry’s friend - though I can present it in slightly different way:
The inward force applied by a thread is 2pi times ( or just over 6x) the tension in the thread. Note it does not matter what the diameter of the tenon is, so a larger tenon is under the the same force but less pressure. In any case a 700g weight force will apply about 4.4kg ( that is 4400g) of inward force. That is per wrap, and can in fact be more force than the breaking strain of the thread. Go figure, remember not all physics is common sense unless your sense is uncommon!
Lets consider something that most can wrap their brains around, an apple or perhaps that should be ‘their teeth around’ (I have taught high school physics…) An average apple (or close enough to an average apple) has a mass of 100g, or a weight of about 1N ( N stands for Newton, a unit of force - so 1 newton = weight of 1 apple) which for our non metric readers is about 4 ounces. If we pull on our thread with the force of 1N then the thread is going to have a tension of 1 N and apply a force of about 6 N inwards.
Now if there are about 2000 wraps (150 x 13 = 1950 but who’s counting) that is 12000 N of force applied inwards which converts to over 2500 lbs or well over 1 ton.
The resulting pressure would be easy to figure out and is left as an exercise for the reader…
So go easy on pulling the thread when wrapping a tenon!
Isn’t wood amazing stuff!
sorry for the long post, but had to do my bit…
Well, that at least is an experiment which should be easy to do. I have an apple here. I may give it a go this evening. I should think a ton would be enough to completely pulverise an apple …
Casey’s tenons have a surface area of about 3 sq in.
That a hard-tugged bagpipe stock binding can still be loose and jem can set a tenon aside mid-wind suggests that there is nothing like that amount of tension stored in the thread.
However, because most thread does not stretch much before breaking there will be large forces a soon as the wood swells on wetting. The thread is not going to break.
Question: Is the thread to provide something that (like cork) gives a bit to allow a push fit that won’t bind when the wood is wet or is it an attempt to stop the tenon swelling as it takes up humidity?
http://www.youtube.com/watch?v=y_vBJra3MOw
Here I am thread wrapping an egg. I get about 139 wraps in before I run out of thread I’ll try for more tomorrow =)
Bang, zoom. Nice work - thanks.
Rob
Thanks indeed, Highwood, some clarity is starting to dawn at last! You have I think confirmed my interpretation that the formula gives the total pressure exerted by one full turn of thread… But I am still unclear as to whether that means it exerts that much force at all points along its length, so the point pressure or pressure total on any part-circumference/arc would be the same as the total for one circuit, or whether the pressure on a given subdivision would be a proportional division of the circuit total. ??? Also, what would the opposed crushing force across one selected diameter be? I realise that the latter’s action would be affected by all the other possible diameters, the arch-strengh of any and all possible arcs of the circumference interacting to equally distribute and resist the load (assuming uniformity of tube material and consistency of wall dimensions!).
The force is 6 x the tension exerted by one whole wrap - so the pressure is that force divided by the contact area of that length of thread (one wrap). If there multiple layers the forces will add - multiple windings (not layered) the pressure remains constant.
The fact that it works is testament to the relative eveness of the force applied, the strength of circular things and that amazing stuff wood.
@george I don’t think anybody is saying that one can’t wrap things without crushing them - whether they are flute tenons, eggs, or balsa wood - just that it is possible to apply an amazing amount of force with a thin thread and just maybe if one is aware of this fact they will not ‘strangle their flute’.
Eggs, as many are probably aware, are quite strong but brittle - not sure what your demo is meant to prove beyond possibly showing what an amazing structure the egg is, or that if you wrap carefully with not much tension all is well with the world - how much tension are you using?
Jem, the error in your thinking is the concept of total pressure. There is just pressure, force acting on an area. And the pressure is relatively even. There is no pressure on sections, which you could add together. There is just an adding of forces due to adding of wraps.
If the tenon is grooved there would be areas of higher pressure inside the grooves, compared to the groove ridges.
If the tenon has a tendency to go oval, there would be a higher force acting on the sections with the larger diameter, compared to the sections with the smaller diameter, and given enough force it may prevent the tenon to go oval, or pull an oval tenon back into circular shape. But I imagine that would require a very tight binding (= high tensile force).
Thanks Hans. I was getting there - you’ve supplied the necessary clarification.
I presume from all this that doing any experiments on flat pieces of timber would not be particularly relevant as the strength of a flute tenon is at least as much to do with its cylindrical structure as with its material composition. And re: another point I mentioned, of course even well selected, perfectly prepared and ideally worked wood is not totally uniform - it has grain, so the cellular structures will affect its properties - this is why wooden tubes tend to go oval even if only to a minute degree.
Converting highwood’s total figure to a pressure gives an idea of what the outer layers if the tenon is suffering at right angles to the surface - that may relate in some way to the ‘squashing it in under a press test’. You can decrease outer diameter by ‘flattening’ the wood and making it thinner, but in Terry’s example the inner diameter of the tenon is also decreasing so the wood is being compressed around the ring, at least on the inside. I think the equation for hoop stress (look it up) is relevant to both tenon and the band of windings - but for the latter one then has to work back to the tension on an individual thread, if that is what you are interested in, using the thread diameter . (Highwood’s calculation starts from what you know you are doing to wrap the thread rather than where you are at when you have done it)
If a tenon is being strangled do people add thread to keep the joint tight or do they start again with new thread ?
So, after at least three independent confirmations by experts that the total amount of pressure exerted by thread as applied to a tenon is not limited to 1 x the breaking strength of the thread, can we all agree that adding more thread at tension multiplies the total tension applied to the tenon? That will allow us to dispense with the various versions of the following:
…and move on to trying to measure how much force is applied in a practical test. I’m working on getting some professional assistance to do just that.
Rob
I had the string snap on me the 1st take and was using about the same pressure I’d use wrapping a tenon. I think it’d be hard to wrap without any tension and keep it neatly tucked together, but that’s not what I did. It was as tight as I can make it without wrapping the string over my hand and pulling hard after each loop or taking much longer on each individual wrap and hurting my fingers.
Also did a wrap with a pint glass of a few hundred loops over about the same width as I did on the egg and again to a plastic cup. The glass didn’t shatter and the plastic cup didn’t crack or deform. Are they amazing structures as well?
I know an egg is a strong structure, but I also figured the tenon would be much stronger and the egg might explode if I was really putting huge amounts of pressure on it.
I am not convinced, with natural fibre thread at least, that the tension used in winding stays there for long. For me the importance of the calculation is that when the wood gets damp and expands the wrapping is not going to break and may not stretch much.
I wonder how much pressure it takes to force a slab of wet flute wood back to the thickness it was when dry.
This is starting to remind me of the old arguments about the age of the Earth. The so-called Catastrophists tried to ascribe all of the Earth’s features to singular, catastrophic events that could have taken place over a short period of time. Gradualists like Charles Lyell correctly pointed out that ordinary forces, acting over long periods of time, can be responsible for making just about any kind of geographic feature you could name.
Trying to smash eggs and pint glasses doesn’t begin to address the sort of change-over-time that Terry has noticed in improperly-wrapped flute tenons. At best it’s an attempt to prove or disprove the physics of thread tension, but all it proves is that, in one particular trial, the egg/glass didn’t smash.
For a real-world trial, we need an instrument of measurement. Working on that.
Rob
Yup, not that I ever doubted that, I just wanted clearly to understand the correct physics of the basic one turn/one layer first.
Hear, hear. My thought too - good man Rob. Starting with: 1) what is a par-for-the-course thread application tension - what do folk who do it mechanically - use a bobbin jig and their lath - set their tension at? (If they think about it - I’m guessing most would just go for a light brake, non-specific tension, on the source bobbin to prevent it running free and over-running etc.); and 2) what kind of average tension is achieved by hand-winders like myself?
@ George - I may only just have begun to get a handle on this myself, but you are misunderstanding the physics involved… and yes, a pint glass is also an “amazing structure” in the same sense - it is a cylinder or close thereto - as it tapers, just as an egg is an awkward shape, doing an effective snug or tight whipping may be tricky given a tendency for the thread to slip off - not a problem on a true cylinder… Crushing anything round is mechanically very difficult, especially if you apply pressure evenly all around the circumference. Do you understand how an arch bears a load? A ring or cylinder is like a never-ending arch, transferring and sharing any load around itself. Its strength is limited by properties of the material it is made from and its own dimensions (diameter, wall thickness) in relation to those.
@ david_h - I don’t know, but I think the tension in the laid-on thread will, depending on the nature of the particular thread, give a little initially as things settle and the thread fibres draw apart a bit in response to the initial tension, but it won’t lose tension entirely or it wouldn’t hold in place - and very old lappings wouldn’t as they sometimes do spring apart when cut across. I agree that wet wood of course swells and may well push itself against restrictive lapping, at least temporarily increasing the pressure between the two materials - but then, most fibres will also react to wetting and drying, so we have quite a complex system at work.
I was wrong about it not multiplying and said I was wrong in theory a few days ago, it was after Terry made his first post with the formula.
0.5lb of pressure warping a tenon would still be a silly argument. The reason I put “in theory” and said I didn’t think that’s what’s happening with our wraps is that I’m not seeing what I’d expect to with over a ton of pressure applied on objects.
Also, I’m touched that you’re going back and reading my posts, truly.
At the end of the day measuring the actual force applied by a wrap would be super informative. Especially after it’s had a chance to compress/stretch/settle and has gotten wet & dry a few times. Please do that!
Hey, what are mates for? 
Rob