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[QUOTE=Brock Poling] [QUOTE=Larry Davis] Consistent thickness in test comparison is essential also. Deflection "test" three pieces, one .250, one .125 and one .140 and your results are almost meaningless for comparison points. Keep in mind that species variances are plus or minus 20% making any two same species pieces of wood on the workbench up to 40% different in properties. Deflection testing will give a builder a leg up at least in picking. It seems well experienced builders intuitively can hand flex and just "know".[/QUOTE]

Of course... the idea is to test deflection and thin the plate until you hit a target deflection. The thickness of the plate is not the main concern.

Or, did I miss your point?

[/QUOTE]

I could hve made that more clear looking back at my post. I meant in the early phase of purchasing raw stock. Browsing thru a stack of rough spruce sets deciding which ones are "the best" and not in the shop working a set down with tools and purpose. No reason one couldn't take a "deflector jig" to the wood store keeping in mind thickness varience. It's the builder who's done hundreds of instruments and can pick up a piece of spruce, flex by hand and "understand" without the deflection test I'm refering to.

Yes. Definitely... I do that too..

This process is a way to determine final thickness before you build, not how to select which plate is "the best"

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Any measurement only has a meaning in the context of the other measurements you've made, and the results you got. We all have our own systems, and what works for me or Ervin, in terms of specific numbers, might not work for you with your different system.

What I've found I get from my measurements is a rising 'standard of mediocrity': my average instrument is better, and I make fewer 'bad' ones. The improvements are slow, in part because, IMO, the 'average' production instrument is pretty darn good. We're fighting for small percentages here, but they are important nonetheless. How this relates to 'great' instruments is anybody's guess.

I like to use a vibration test to find the 'Young's modulus' of the wood. I use my signal generator to drive the plate, and find the exact resonant frequency, as well as the 'half power bandwidth', which tells me something about how much energy the wood is absorbing. Then, with measurements of the plate size and mass I can calculate (well, my computer can: it's much more reliable than I am) the Young's modulus. This allows me to figure out how thick to make this top to match the stiffness of another one that worked.

I agree that we're probably not generally building very close to the limits. One issue with that is the 'weakest link' problem: the thing that fails first breaks the whole chain, and we can't really tell whether the rest is only just a little stronger or massively over built. Eventually somebody will build the 'one hoss shay' of guitars: everything fall to dust all at once. Until then I follow the 'McReady rule': anything that you've never had fail is overbuilt. Then again, customers sure like things that don't fail....

Gentlemen--

This thread has been an incredible eye opener for me. My personal goal is to become as consistent as possible with building so that I can aim for the tone that I hear in my head. This comes, I'm sure, from a background (long ago and far away) in civil engineering materials testing.

Wood is a much more organic material than, say concrete, but I am coming to understand it in a manner that is similar to my understanding of using Michigan field stone in a concrete mix rather than Great Basin basalt -- only on a level several orders of magnitude more complex.

Brock's general method would allow one to arrive at a relative benchmark for their own work. Although a lot of my current effort is devoted to gaining the necessary carpentry skills, I suspect that it is time to start building the personal database.

Thank you all so much for this very enlightening thread. There is much to digest here!

--Jim

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Jim Howell
Charlotte, NC