Indeed, we just don’t know at this stage. It’s possible that these flutes were strangled either over a very long time, or over a short time some time in the past, and the current thread simply wrapped around the neck of the dessicated corpse. We need to be on the lookout for modern strangulations.
My primary reason for using thread stems from the simple observation that when cork is applied to the flutes I make (despite my recommendations not to and stern warning that this voids the warranty), the sockets usually suffer a crack. Its perhaps because the cork is fit too tightly.
I can’t think of another reason.
In terms of numbers I did a few experiments that might give an idea of the compression force applied when thread wrapping a tenon. I compared this with the wet and dry compressibility of boxwood.
One was to measure the compressive force directly, when wrapping a tenon. Cork actually works well for this. I fashioned a piece of 1/16" thick cork 1/4" wide and 1/2" long, and wrapped over this using my regular thread wrapping technique and amount of thread. I then removed the thread and measured the thickness of the cork. It had compressed .16mm. I fashioned another piece of cork of the same size and placed it on an electronic food scale, and compressed it with a piece of steel, measuring the amount of force required to compressed it to the same degree as the one that had been thread wrapped over (.16mm). The scale measured 10kg.
Multiplying that by 8 gives the amount of compressive force per square inch, which is 80kg or 176 psi, which is far below the figure that Terry calculated. On my head joint tenons, there is probably about 3.3 square inches, so the total compressive force would be 580 pounds.
Hmmm, I know that is a lot less than my “theoretical maximum” (thread at breaking strain) calculation, but it still seems a lot. 3.2 standard American males standing on your tenon. Are you sure your tenon amounts to 3.3 sq ins - I’d have thought they’d be less than 1. And that 1 standard American male was plenty.
I decided to measure how much boxwood would compress if a measurable force was applied to it, using my 3 ton arbor press. I fashioned a 1" X 1" X 2mm thick piece of boxwood, measured its thickness and set my calipers at “0” so I would only be measuring the deviation. I then set it between 2 thick steel plates on the arbor press, and pressed down at full force (3 tons on 1 square inch - thus 6000 psi) and held it there for a minute or two until my arms were tired. I then immediately measured it and found it had been compressed only .02mm at the most. 6000 psi is 34 times the 176 psi that I measured on the tenon, yet it only deformed the wood .02mm.
I then soaked another similar piece of wood in water and repeated the same experiment. The wood was considerably more compressible, deforming by .05mm.
Now if your arbor press is the same as mine, it is labelled 3 ton, but I don’t reckon there’s any way you could get it to press that hard. Mine has a handle such that when fully extended, your hand falls about 500mm from the centre of the axle. The axle diameter, into which a pinion is cut, is 60mm diameter. So the mechanical advantage will be 500 divided by the radius (30mm) or 16.7 times. To get 6000 lb force, you would need to apply 360lb.
Secondly, your force was only applied for a minute. With flute tenons we’re worried about months and years. We’d really need to set this up for a long-term test.
I suspect that its the moisture deformability combined with the inward pressure of both wrappings (either cork or thread) and socket that compresses the tenon over a long period of time. A solution to this would be to seal the socket so that moisture doesn’t get into the wood.
My argument here is that:
a) the socket is not capable of applying much compressive force (as you said in relation to corking your own flutes, the socket wood gives way), and
b) the thread is applying pressure all the time, the cork only when the flute is assembled. That’s a 24:1 ratio even for those good folk who practice an hour daily!
It would be interesting to see how much the cork in the corked tenons compresses with use, and compare it with my measurement above (176psi) to determine if there is a significant difference in compression on the tenon, as a result of the thread being wrapped. Does cork get compressed thinner with use?
I really don’t know. I’ve certainly replaced cork on flutes, but whether it’s because the cork has compressed, or been worn away, of the wood has changed diameter, or what, I can’t be sure. I’d have guessed that it compresses with time, but then you get old flutes where the tenons are scarily tight you have to sand some cork off before you’d dare assemble them. I frankly don’t know what’s going on there!
It probably does warrant a long-term test. How would this go? We make two test tenons, using the same dimensions as normal, excepting that the ID is a cylinder whose diameter is whatever the diameter at the centre of the thread trough is normally. By using a cylinder, it will be easier to detect and measure any compression.
One we mount between centres on the lathe, and lap at high tension. The other we lap by whatever process you normally use. Both are lapped until they fit a standard head socket nicely. We record and graph the minimum ID of each every week until they stabilise.
Or we could even do the artificial aging thing - heat the tenons or even boil or steam them to simulate the passage of time. From memory, 105°C for three days is supposed to simulate 25 years. Hmmm, tricky though, dry heat would shrink the wood and take the pressure off, and steam or boiling would swell the wood, exaggerating the effect. Might just have to let time take its time.
Terry
