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I've just finished thicknessing and deflection testing 3 tops; I think they're where I want them but I'd sure like to hear some of your thoughts on these numbers.

My tester is not set up as well as it should be, here's a link: http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24464. My supports are 17" between centers with a 5 lb weight.

1. Sitka for a size 0: density 385 kg/m^3, Deflection 0.210" at 0.103" thick. Pretty floppy.

2. Sitka for an OM: density 539 kg/m^3, Deflection 0.140" at 0.105" thick. Stiff.

3. Adi for a Dred: density 437 kg/m^3, Deflection 0.140" at 0.114" thick. Very stiff.

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Steve Smith
"Music is what feelings sound like"

We can have that large difference of density between 2 pieces of the same species?

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Francis Richer, Montréal
Les Guitares F&M Guitars

Those two pieces came from totally different sources and I've had them for maybe 7 or 8 years. The piece for the O has a lot of streaking and could by something else.

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Steve Smith
"Music is what feelings sound like"

Steve, why do say # 3 is very stiff and #2 is only stiff?
According to the data you show #2 is your stiffest top.

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Jim Watts
http://jameswattsguitars.com

I should have been more clear, I'm referring to the apparent cross-grain stiffness.

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Steve Smith
"Music is what feelings sound like"

Those densities seem to me to be near the extremes for Sitka, from the few that I've measured, but not out of all reason. Yes; ALL woods can vary widely in density.

I'll note that for all of the softwoods I've tested density is a reasonably strong predictor of Young's modulus along the grain. The stiffness, the ability to resist bending under load, will be proportional to the Young's modulus times the cube of the thickenss. You need to have a certain amount of stiffness to resist the bridge torque over time, and, while you can get it from the bracing, I like to have the top taking it's share of the load. Under the circumstances, I'd have made that low density top a bit thicker than the others, even though it's for a smaller guitar. This really illustrates the fact that you can't just go by the species and guitar size to determine top thickness: you've got to measure the stiffness in some way to really know, preferably before you get it thinned out.

The two pieces that I think are Sitka may be something else. I don't have enough experience yet to be sure. I bought them long before I ever built anything so I wouldn't put those numbers in anyone's database for sure.

Alan, thanks for responding. I thicknessed the O top based on what I thought might be a reasonable deflection but it just doesn't feel stiff enough lengthwise (as if I actually have a clue). I think I'll put this top aside and use a euro top I got in the Swap-a-palooza.

I read all I can find about this stuff and think I understand but there's no substitute for just getting in there and trying it out.

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Steve Smith
"Music is what feelings sound like"

Those densities are also near the extremes for my samples of Sitka brace stock. I have seen as low as 22 pcf (350 kg/m^3) -- that board was very flexy and actually seemed somewhat weaker than its low density would suggest. The highest I've seen is 35 pcf (560 kg/m^3), which felt more like VG fir.

Actually, we could consider ourselves lucky -- the range for balsa is 4 to 20 pcf.

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David Malicky

Steve: Wonder if the three tops were the same size IE 17x22 or whatever? If so it would seem that #2 sitka was much stiffer than the red?

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A person who has never made a mistake has never made anything!!!

The width is very close on #2 and #3 and you are correct that #2 is easily the stiffest with the grain. However, #3 is by far the stiffest cross-grain.

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Steve Smith
"Music is what feelings sound like"

I've got Engelmann that runs from 335 kg/m^3 to about 510. My range for Sitka so far is 410 to 500.

Personally, I wouldn't count on the wood being anything other than what it was sold to you as.... There is a whole lot of wishful thinking when some folks get a wide-grained, stripey, and super hard Sitka top without particularly prominent med rays... Ooh Ooohh Maybe it's Adi!...

Nope.. Don't count on it! If they sold it as Sitka.. it is most likely Sitka... There really is that much variability!

On to the next thing...

Be very careful with your perception of "Stiffness" when samples are different thicknesses... because the "Cube Rule" is in full force here... A top that is 10% thicker will be 33% stiffer in deflection.... and you will notice this crossgrain quite a bit... which is probably accounting for some of your perception of "Stiffness"

You really need to calculate Moment of Elasticity X and Y numbers (With and across grain).. This helps correct for the Thickness of the sample...

If you search Al Carruth's posts, he goes into this in gory detail.

Thanks

John

John, you make some good points.

My biggest problem is I don't remember what the wood was sold as and at the time I was not labeling my wood as I do now.

I suspect you're right about my perceived cross-grain stiffness. I need to remember that the "Cube Rule" relates to tops just as well as to braces and other structural members.

So it looks like that was a nice experiment but I now realize I have more work to do. Onward to Ex, Ey, and Eave.

Thanks all for your comments and suggestions.

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Steve Smith
"Music is what feelings sound like"