Official Luthiers Forum!

I am ready to thickness the back and top on first dreadnaught. Would you please explain the method that you use to determine your thickness on your backs and sides. Thanx, Vic.

Vic: Would suggest that you get the Stew-Mac dread plan by Don MacRostie. If you want to get more into it get the Gore-Gilet books.
Tom

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

Thanx- I have several setsof plans and a few books that state a general thickness. But what I would like to know how you determine thickness for a specific pieceof wood. I suspect that the thickness for aheavy piece of rosewood would different for a piece of mahogany. Thanx,again, Vic.

Look up deflection testing and you'll get some ideas.
However, you'll need to find your own numbers by building a few and collecting data.
As a guideline to start, .110 is safe for Sitka spruce, .090 for rosewood, .100 for mahogany. Of course these may not be ideal for the individual pieces you have, but you need to collect a few data points and adjust to your taste.
For what it's worth, a common default for deflection testing seems to be a 5lb weight across and 18" span.
There is a formula in the Gore/Gilet book for finding the perfect top thickness, but it is very very math heavy.

It's not that heavy! Just add/subtract/multiply/divide - a few times! Works for backs, too.

Regarding sides, it depends what acoustic effects you're after. "Floppy" sides are another way of lowering the main air resonant frequency (gives you more bass) if that's of interest to you. "Thick" solid sides are about 2.3mm thick. Above that thickness, bending them gets problematic. With care, most of the recognised side woods can be bent at that thickness and the easy bend woods will go quite a bit thicker. The bottom end of the range can be as low as 1mm (some classicals). The typical range is ~1.8mm to ~2.2mm.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Well, maybe only one very then;)
But to be honest, you're looking at it from the perspective of someone who can read that formula. Unless your math is at a certain level, it is entirely impenetrable, as if it were Sanskrit or Korean. I can recognize that it's a language but that's as far as I get. Like handing a sheet of music to a non musician.
I will eventually just pay someone to build me a spreadsheet....

meddlingfool, I programmed the formula in an Excel spreadsheet if you want a copy (since I know you've bought the book). Just enter the measured frequencies, thickness, body size, etc. and it calculates the thickness. Even has a spot to enter an adjustment factor so it calculates the thickness you desire for a given stiffness and body size.

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Formerly known as Adaboy.......

Oh yes please!
I would appreciate that very much!

Darryl, if you wouldn't mind sharing it with me as well, that would be great! I'm still digesting the books, haven't gotten around to entering stuff into an Excel sheet (or R scripts...occupational hazard of working with it daily)...