Thinned or thick-wall heads more prone to cracking?

[Peter Duggan]

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:

http://jnovoheadjoints.com/a-thinned-headjoint/

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?

[Nanohedron]

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.

[Peter Duggan]

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?

[Peter Duggan]

when I read the thread's title

There wasn't room to get in both 'wood' and 'lined' before I ran up against the (fairly arbitrary?) max. character limit. I did try!

[Nanohedron]

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?

[psychodonald]

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.

[Peter Duggan]

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?

Hmmm, yes, it's not clear. But could he really be doing 2mm wall thickness as just wood?

Another thought, what about fully lined head joints versus a partially lined head joint. 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.

Terry McGee says partial linings produce partial cracks, or something to that effect!

As previously mentioned (Nano), and I would agree, the humidity certainly must figure into the equation, big time.

Of course, but let's take appropriate care as read.

[paddler]

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.

Here are the pics of the F. Riley flute.

[Peter Duggan]

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?

Hmmm, yes, it's not clear. But could he really be doing 2mm wall thickness as just wood?

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!

[Gromit]

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.

[Loren]

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:

1. Abuse (dropped, sat on or otherwise crushed)

2. Under humidified (Also abuse, really)

3. 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.

[dcopley]

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/fplgt ... 3/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.

[dubrosa22]

Thank you for that informative reply Dave. Very interesting.

V

[Peter Duggan]

Interesting question.

And interesting answers, Dave. (You'll be relieved to know it's nothing to do with my beloved Copley & Boegli custom four-key!)

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!

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!

[benhall.1]

Are ye thinking of setting up a moot about that flute Peter?