A debate I’ve already started with Jem off-forum, but I’m curious and Google’s not helped much so far…
My first thoughts were that a thinned wood head over metal lining would be more prone to cracking than thick-wall, but then I found myself wondering whether the physics (less wood to shrink + possibly more flexibility?) might actually suggest the other way round. But the only clear opinion I’ve found so far is that of Juan Novo, who’s very much in the latter camp:
In addition, Novo’s ultra-thin design is also superior because his ultra-thin headjoint is less likely to crack than a thick-wall model due to the vast difference in mass. The thicker the wall, the more the mass; the greater the mass the more chances of cracking. The thickness of the Novo wall is slightly less than half of the thick-wall design. Therefore, it is fair to say that the thick-wall headjoint has twice the chances of cracking compared to Novo’s ultra-thin design.
So… anyone want to agree/disagree with that and, if so, why?
Well, I can’t help but think that Novo’s explanation is just so much armchair reasoning. I’m no physicist, but it seems to me one could just as well argue that greater mass (thickness, as Novo seems to mean) fortifies against cracking. I’ll tell you what - when I read the thread’s title, two things came immediately to mind: not thick or thin, but A) lined heads with B) poor humidification above all. Anything else strikes me as avoiding those realities. That’s my two cents.
Sure, mass sounds like the wrong term and I’m not convinced by a linear relationship between thickness and crack risk either way but, supposing what Novo’s trying to say is more along the lines of my (also armchair-reasoned) ‘less wood to shrink + possibly more flexibility’, could he be effectively right with dubious terminology?
I think you’d have to sacrifice a truckload of flutes in order to come to any reliable conclusions. Traditional flutewoods being both dense and hard, it seems to me that blackwood would be more crackable thin than thick, rigidity being the issue. The softer the wood, the more forgiving, would be my assumption. But I could be surprised by a more scientific study.
But I have to clarify and say that all my blather presupposes a lined head. At a glance Novo’s heads don’t appear to be lined, nor can I say I was able to find anything about that one way or the other. In his head design he mentions a “tenon tube” which he doesn’t compare or contrast against full lining in case one would wonder. He does speak of lined heads in other flutes, but his wording about his own isn’t so clear. If his heads are unlined they’re going to be far less prone to cracking no matter what. Anyone here have a cracked unlined headjoint?
Another thought, what about fully lined head joints versus a partially lined head joint (perhaps that was the point that Nano was making). I read somewhere that a fully lined head joint is more prone to cause the wood to crack, where as the partial head joint is less inclined. I realize this is a little off topic, as the op wondered about the thickness of the wood as being the issue. Still, I can’t help but wonder, is the question about head liners true or not? As previously mentioned (Nano), and I would agree, the humidity certainly must figure into the equation, big time.
I would expect a thicker head to be more prone to cracking than a thinned one. The following pieces of practical experience lead me to this opinion. First, I have restored several antique flutes that incorporate an extremely thin (little more than 1 mm thick) covering of cocuswood around the outer part of their metal tuning slide, and they have all been crack free. I’ve included a couple of pictures of such a flute, in cocuswood and ivory by F. Riley.
Its quite rare to find a lined head that is crack-free, and similarly, the thickened parts of sockets on French antiques (which have metal linings) often have cracks, as do nearly all barrels. So, this seems like the limit case for this experiment. If 1 mm thick cocuswood can stay intact for over 150 years, while being wrapped around a metal tube, then I think the thinness must have something to do with it. One factor that may contribute to the success is that such a thin piece of wood probably starts out almost perfectly seasoned, since seasoning depends on thickness as well as time.
Second, I collect and season my own wood, both for bowl turning and for flute making, and I have noticed that thick blocks are much more prone to cracking thank thin ones. Third, if I turn a bowl to finished form from green wood, it will crack as it dries if its left too thick. On the other hand, if its thin enough is won’t crack, but it will warp as it dries. So maybe this last point is inconclusive given that we are talking about wood wrapped around metal, and so it can’t warp.
Two pics from Google image search where the ones on Novo’s own site don’t really offer the right angles…
While the blackwood one still doesn’t help, the other suggests that particular headjoint might be lined?
But please understand that my question’s a general one and not really about Novo’s headjoints at all, with his blurb getting included simply because it was all I’d found in attempting to answer it!
As a woodworker I would tend to agree with Paddler - my gut feeling is that thinner is less likely to crack on the basis that cracks and splits appear where the wood dries out and shrinks at an uneven rate.
To air dry (season) wood it’s ripped down into planks and stacked so that air can get to all surfaces rather than left in thick boards which are more likely to split.
From a practical standpoint, I believe thickness is irrelevant. By that I mean, specifically as thickness applies to wood in the construction of working and practical flutes, as opposed to impractical thicknesses for flute making or non-flute applications: say blocks of wood or guitar tops.
Thickness makes no significant difference when using properly seasoned wood. Now poorly seasoned wood used for flute making might be a different story.
I base my opinion on this: I over the course of several years and exposure to thousands of used (and often abused) wooden recorders and flutes, I saw no more or less cracking based on thickness. Generally cracks were obviously due to either:
Abuse (dropped, sat on or otherwise crushed)
Under humidified (Also abuse, really)
Poor manufacture, including improperly seasoned wood. Rare among established makers, not so rare among makers in business less than 10 years or so who haven’t had wood seasoning that long and or who haven’t worked out the resting times during production.
Well seasoned wood flutes shouldn’t have problems if properly taken care of, regardless of wall thickness. That’s been my direct experience. FWIW.
Interesting question. First I had to blow the dust off my forty year old degree in materials science, but even that was not a lot of help since they did not teach us much about wood. There are two issues to be looked at in order to answer Peter’s question about the metal lined head joint.
First question is whether the drying-induced shrinking of the wood creates enough force to compress the liner tube appreciably. If it does, then the reduction in diameter of the liner tube will relieve the stress on the wood and make it less likely to crack, particularly for thicker sections of wood. To answer that question you need to know the elastic modulus of the materials. Information is hard to find, but with a few assumptions and the information in http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf it looks like the modulus for a tropical hardwood, in the radial and tangential directions should be around 100,000 to 200,000 pounds per square inch. The modulus for silver is around 10 million pounds per square inch, so silver is 50 to 100 times stiffer than wood. In a typical head joint the wood is about 10 times thicker than the metal, but even with that difference the silver would not compress very much at all. If that is the case and you can treat the silver as incompressible, then the thickness of the wood should not make much difference to whether it cracks or not.
The other question concerns how the wood dries out, and the moisture gradient in the wood. As Paddler observed, thicker pieces of wood are more likely to crack as they dry. This is because the outside dries and shrinks while the inside remains moist (and therefore not shrunken) for much longer. This sets up stresses in the piece of wood, with a tensile stress at the outer surface which causes the cracks. The thicker the wood, the higher the stress, which is why flutemakers like to pre-drill and rough turn their wood as soon as possible. This effect could contribute to cracking of a lined head joint, especially if it was improperly seasoned or stored in a very dry place, which underlines the points Loren makes.
I don’t pretend that any of the above is a rigorous analysis, though it would be great if someone with time on their hands could do some real calculations. I think that in the end I would have to agree with Loren that the most important things are getting a flute made from well seasoned wood and taking proper care of it.
And interesting answers, Dave. (You’ll be relieved to know it’s nothing to do with my beloved Copley & Boegli custom four-key!)
It was actually (as Jem knows) because I was considering taking an unblemished 90-year-old thinned-head Rudall Carte Boehm on trial. But now a moot point re. that particular flute because I’ve also just been told that both head and foot have cracked sometime between Christmas and now!
“It was actually (as Jem knows) because I was considering taking an unblemished 90-year-old thinned-head Rudall Carte Boehm on trial. But now a moot point re. that particular flute because I’ve also just been told that both head and foot have cracked sometime between Christmas and now!”
Could this be the answer to your original question ? If the flute has a thinned head and standard thickness walled foot and both crack during the same period due to external conditions - presumably humidity/temperature, and not being damaged by accident ?
Just a thought raised by Dave Copley’s excellent contribution - when considering the resistance of a metal liner inside a wooden tube, the physical form is I think likely to be a relevant consideration in trying to work out the balance of forces. The liner is not solid metal, but a cylindrical tube. Such a tube is, so far as I understand these things, a strong shape, quite resistant to lateral forces, especially to all-round compression such as one would assume a shrinking outer wooden tube to exert. The metal may be relatively soft and malleable and easily squashed or dented or bent outside the wooden cladding, but inside it its tubular form is very strong, more than adequately resistant to compression by timber shrinking around it.
Not that that really helps answer Peter’s question, but since we’ve got onto the underlying processes…
Like Dave I’ve had to dig out my old college books on stresses in compound tubes (from the 60s so probably even older than his) though they never seem to consider wood as the outer material. I agree with Dave that there are different mechanisms at play here.
I did some calcs based on thermal expansion of the inner metal tube and, with the assumptions below, conclude that the stresses induced in the wood are so low that cracking should not occur, even with fatigue or the wooden equivalent,due to repeated cycles. The approach I took was simplistic but to me it seems reasonable.
The free expansion of the tube with 10C temperature rise would cause 0.0038mm increase in diameter but because of the restrictive effect of the Blackwood outer this is only 0.0013mm. The resulting hoop stress in the Blackwood is 1.18MPa, it’s hardly being tickled. Thinning the Blackwood wall causes the stress to rise but not substantially.
I would have to conclude that the principal mechanism for cracking is repeated unequal wetting & drying out of the wood.
I’ll happily send the spreadsheet to anyone who’s interested.
Assumptions :
The wood is properly seasoned & completely dry & does not experience any change in temperature.
The silver liner experiences a 10C rise.
No credit is taken from the retraining effects of the end rings.
Stresses in a Flute Headjoint
Wood properties African Blackwood
O/D 28 mm
I/D 19.92 mm
Modulus of Elasticity Ew 17950 MPa
Modulus of Rupture 213.6 MPa
Tube properties Sterling Silver
O/D 19.92 mm
I/D 19 mm
Modulus of Elasticity Et 8.30E+04 MPa
Yield strength 124 MPa
Therm expans coeff 1.9E-05 mm/mm/degC
Temperature difference 10 C
Free diam expans of tube 0.0038 mm
Assume negligible thermal expansion of wood
Assume total strain is shared between tube & wood
thk tubeEt 3.82E+04 mmMpa
thk woodEw 7.25E+04 mmMPa
ie actual expans of tube 0.0013 mm
Hoop stress tube =єtEt 10.33 MPa compressive
Hoop stress wood = єwEw 1.18 MPa tensile
Without benefit of calculations, I was reliably informed many years ago that the only dimension of a metal tube which changes significantly even under considerably greater heating than a flute liner will experience in use is its length, and that poses little structural risk to the wooden outer. That information was given (I forget by whom) in response to my quoting what I then believed, having been taught it by Paul Davies and/or read it in old flute books (maybe Bate?), that the cause of metal lined wooden tubes’ propensity to crack was the different coefficient of expansion, metal being more reactive than wood. I/we now know better. “We” have known for many years that it is the ongoing dessiccation shrinkage of even well seasoned timber which is the primary mechanism of cracking over a metal liner. Peter’s question about the behaviour of different thicknesses of wood is, in terms of detailed exploration, a new one to me.
