Another strangled flute

[Casey Burns]

I'll get around to that experiment later today or tomorrow or the next day. Preoccupied with other matters......

I was curious how strong a tenon was when a point force is applied to it, as opposed to an even force such as applied by thread wrapping compression. So I turned another tenon piece, 3/4" ID, 1.85mm wall thickness, 33mm long. Then weighted it on my scale, pressing against it with a flat piece of steel so that the force was applied in a line. It failed somewhere around 16kg (35 pounds).

The big lesson here is don't leave your flute assembled, and set it on the couch at a session where someone will sit on it. The tenons and sockets will suffer. I have seen this happen too many times.

Casey

[jemtheflute]

Rob, Terry, what is the correct interpretation of that "700 g per unit-length"? Is the breaking strain expressed thus, or simply as a weight applied to ANY length of the thread? I'm still hiding behind being a physics/maths dunce, and yes, I have looked at the basic formula Terry's friend set out.... but I'm not clear how he gets to 700g per cm..... Put another way, if your thread breaks when you hang a 700g weight on it, what is the tension per cm (or any other unit of length? Surely it is the same regardless of the length?

If my befuddled noddle is getting anywhere with this, if the tension in one full circumference-length of thread (2piR) applied T/R inwards radial force, the figure derived from that calculation would be the total radial pressure applied by that circuit of thread around the circumference, not the point pressure? So Terry's 4.4k would be distributed evenly around one thread-line. If I'm right and we stick to the Professor's 1cm radius and a tension of 700g, any given 1/10th of a mm of wood under the thread would be receiving inwards pressure of 44g? (Mathematicians HELP!)

(EDIT: Duh, no. Obviously, if the radial pressure per cm is 700g, then per mm it will be 70g and per 0.1mm it will be 7g??? I got distracted by Terry's gross figure and divided the wrong thing! Or will it? - This is where I'm confused by the "unit length" bit.....)

Even if I'm way off with this, I still don't think we have a full and accurately applicable model in discussion regarding thread pressure... even if that is the/an agent of tenon bore perturbation.

(I agree with George here that using the thread's breaking strain is way over the top, BTW, no matter how you apply the tension while winding.)

Another thought on the middle part of lapping-trough angle...... The trough is the thinnest part of the wood and thus the most susceptible to any disruptive influences - de-and re-hydration, thread pressure, shrinkage from of-its-nature wood cellular structural changes with aging, etc. The ends of the trough usually have shoulders of slightly thicker wood which will act as supports to the parts of the trough adjacent to them, and the body-end of any tenon is further supported by the still thicker wall of the main part of the joint. Therefore, the central area of the trough is the least re-inforced/weakest. It might just be that in some instances - particular pieces of wood - it just does this "by itself" because of its particular structures and the shape the maker cut it to, maybe "helped" by usage factors like moisture..... It also occurs to me that this problem seems to be commonest and most severe in upper-body upper-tenons - the ones with the largest diameter and therefore with reduced cylindrical structural strength - I believe I'm right in thinking that a smaller diameter tube will be stronger (against compression or shrinkage) than a larger one with the same wall-thickness?

[david_h]

jem, please can you remind us roughly how hard we are supposed to pull when wrapping a tenon. I thought it was just supposed to be a wodge of something with a bit more 'give' than wood.

Did you folks do about Hook's Law at school ?

[Rob Sharer]

so ya want to flaunt yer lack of imagination

I know, I'm getting duller by the day. Trying to stay "dry" for once.


Rob

[jemtheflute]

jem, please can you remind us roughly how hard we are supposed to pull when wrapping a tenon. I thought it was just supposed to be a wodge of something with a bit more 'give' than wood.

Did you folks do about Hook's Law at school ?

Don't think I'd ever heard of Hook's Law..... not that I remember.

How hard to pull? Well, to some extent that is defined by the breaking strain of your thread! - Less than that, obviously!!! As I whip it on by hand, I pull as hard as is comfortable to the tip of my thumb over which the thread passes, give or take (as mentioned previously) losses of tension when pausing/setting the work aside mid-wind.... I have no idea what actual average tension I may achieve in my chosen thread. I certainly don't achieve a constant, even tension. One reason I might (continue to) argue against use of hard-spun (and thicker) threads is that, per thickness, they take higher tension than a similar thickness loose-spun thread, as well as having a harder, more abrasive surface. Pipers hemp is usually pretty loose-spun and pulls apart (breaking strain!) quite readily. "Button thread" would be at the other extreme.

[david_h]

OK, Hooke

Thanks. I am fairly sure that for the effect of the winding tension to add up the way Rob is concerned about (and for anything 'per cm' to matter)) the thread has to be stretching elastically, which natural fibre threads don't, much. I suspect pulling hard on a 'soft' natural fibre thread is mainly making it a bit more like a 'harder' thread.

[George]

What I cannot wrap my own head around is how anyone could read the physics report as provided by Terry and still maintain that it's not possible to apply thread in such a way as to exert a crushing force. Everyone read this again (minus Terry's asides this time to shorten it up):

... [And since there are 2pi radius-lengths of string around the tube, that will mean our 700gm tension actually applies a force of

You left in the bit where Terry assumes you add the radii together, the brackets are his voice. That's going to raise the force.

I don't remember one person in this thread ever saying it's ***impossible*** for a thread wrap to deform a tenon. You are currently arguing a position no disagrees with on the one hand and somehow conflating that to understanding the physics involved on the other. To be clear I'm not arguing that it's *impossible* for a thread wrap to warp a tenon. For example you could use a much stronger/thicker thread and something to aid in keeping the tension on the thread (like wooden handles) as you wrap, that would significantly raise the force applied.

My position is that with the materials normally used, the manner in which they're normally applied and the medium's ability hold energy the wrap won't be pushing down with enough force to warp a tenon.

What exactly is your position? I ask because you've said a couple of times that a *properly* wrapped tenon won't hurt a flute, how do you define 'properly' in this situation?

To answer your question at the top, there hasn't been much arguing over this latest "physics report" I think because it sounds a lot more correct. Before, where you had outrageous figures for the force being applied to a tenon, that's the stuff I didn't buy for a second. More than going along with them you were questioning why someone would doubt them. Seems you had as little understanding of the physics involved as the rest of us, took an absurd position (going along with the 1.5 tons of pressure) only now I get the impression you feel you've been correct this whole time.

[Rob Sharer]

What exactly is your position? I ask because you've said a couple of times that a *properly* wrapped tenon won't hurt a flute, how do you define 'properly' in this situation?

I cannot be responsible for lazy reading. However, if you must have a succinct version, here it is:

When threading a tenon, don't feck up.

To answer your question at the top, there hasn't been much arguing over this latest "physics report" I think because it sounds a lot more correct. Before, where you had outrageous figures for the force being applied to a tenon, that's the stuff I didn't buy for a second. More than going along with them you were questioning why someone would doubt them. Seems you had as little understanding of the physics involved as the rest of us, took an absurd position (going along with the 1.5 tons of pressure) only now I get the impression you feel you've been correct this whole time.

What figures did I present, exactly? Careful reading, slow down, check the names. 'Twarn't me.


Rob

[david_h]

So far as its possible to compare like with like what happens to a wrapping with age ? What are old wrappings like ?

[George]

I cannot be responsible for lazy reading. However, if you must have a succinct version, here it is:

When threading a tenon, don't feck up.

No lazy reading on my part, you haven't defined "properly" until the above. Good advice, really helpful and totally adds weight to your argument. Keep'em coming ...

What figures did I present, exactly? Careful reading, slow down, check the names. 'Twarn't me.

I didn't say you presented figures. You think I'm the lazy reader!

More than going along with those figures you were questioning why I didn't believe them ...

09 Jan 2011 06:23

There is something fundamentally wrong with the models you all are coming up with producing these figures. There's no way a thread wrap is putting 580lbs of pressure on a tenon, let alone nearly 3,000lbs ...

And here it is again. What is wrong? Where is the backup for this statement, other than incredulity? As I said before, I think there's a fundamental misunderstanding of the physics here.

[Rob Sharer]

Well, answer the question: WHAT is fundamentally wrong with the idea? Explain, please.


Rob

[Denny]

So far as its possible to compare like with like what happens to a wrapping with age ? What are old wrappings like ?

like a shroud

[NicoMoreno]

Not just a luddite, but also a poor reader! Sigh, I had such high expectations for the generous man.

[highwood]

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...

[benhall.1]

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 ...