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I thought I would introduce myself and ask a question as I'm inching closer to actually working on a guitar. I've made a few attempts, a couple were successful, at making some electric guitars and decided to turn my attention to trying my hand at making an acoustic after finishing a workshop at the North Bennett School in Boston. That was three plus years ago. Since then I've been surfing the net, reading books, making a jig here and there and setting up shop.

The shop is almost ready to go and I've read every thing I can find at least three times so its about time I actually get started. I'm following Cumpiano's book and I'm building the steel string guitar as described in the book, I guess that's an OM????? Anyway, what I'm trying to get my head around is top stiffness and how to translate that to bracing.

From what I've read here it seems that brace thickness is generally recommended at 1/4 inch by 5/8 inch, at least for the X-braces. Cumpiano recommends 5/16 x 3/4 for the X-brace. I realize that the final dimensions have a lot to do with top stiffness, but can anyone give me a method to gauge stiffness and how to apply that to brace sizing.

I'm thinking I should support my top on edge and measure deflection with a standard weight. But do I do this after the top is glued and how much weight do I use? Also what amount of deflection equals a stiff top versus an loose top and how much do you adjust the brace width and height to compensate for a stiff orloose top?

I know this is a pretty broad topic, but any help you can give me in narrowing it dow would be appreciated.

John

http://www.lmii.com/CartTwo/Video.asp
You might find this helpful.

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Ron

Welcome to OLF John,

I can't answer you're question. I'm just chiming in to say I think it's an interesting topic and that I'd love to hear from the OLF's deflection testers, especially on this point:



Oh, I can at least tell you that the steel string in Cumpiano's book is a Dread.

Best,

Flori

Not to be an a**, but the guitar in the Cumpiano book is neither an OM or a dreadnaught. It's OM in shape but 1" wider at the lower bout (waist and upper bout are wider than an OM too). The waists and bouts are tighter than a Dread, also most dreads are based off Martin specs which are 15-5/8" at the lower bout, tail depth is deeper and the waist is not as tight.

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Hi John,
Not to be too nit picky but language is important when talking about technical things.

"I'm thinking I should support my top on edge"

I think you mean....by.... the edges which would mean the top was flat and you had supports out at the edges. On edge would mean that the top was on it's edge. Sitting on it's edge. So that you would have a very hard time deflecting it at all.
You might say, "what should I do with this piece of wood" ? And the answer to stand it on edge is very different than lay it flat or lay it flat and support it by 2 edges.

Your question is a good one. There is no definitive answer but some good starting points. Search the archives. You will find lots of great info from real experts that post here. A lot of these guys here could write a book themselves. I think this forum is better than anybook on guitar making. Not saying one shouldn't have guitar making books ( I have the usual ones, Cumpiano, Kinkead ) but just making a point as to how good this forum is.
Link

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Cut to size.....Beat to fit.....Paint to match.

For deflection testing you support the ends for long grain deflection testing. using a 5lb hand weight placed in the center. Measure the amount of deflection the weight produces. There are tons on info on this in the archives.

In my opinion this tells you very little about how to brace. but tells you a lot about the strength of the top and how to thin it. in most mid and small body guitars the trend is to brace lighter but with equal strength. the taller the brace the thinner you can go and still gain strength. Here again there is tons in archive on this topic. Put simply you gain much more strength in a brace with height than width.

The guitar in T&T is closer to a Grand Concert than an OM. but would still fall under the category of a mid size.

On all my mid size guitars I start with a 5/16" x 11/16" after radius brace stock for the X and all others are 1/4" x 11/16" after radius. the finger and tone bars will loose a good bit of height after carving and the all loose close to 20-30% of their mass during carving.

For someone just starting to build acoustics. I have to say you will learn more faster with better success by following developed and proven bracing designs for at least several builds rather tan trying to figure everything out during a first. you will have enough to learn on the first acoustic build without trying to build the perfect braced guitar straight out of the gate. Keep notes on every build. take time and builds to learn what your predecessors’ have found to be reliable before trying to improve on their success your first time out.

Let me give a totally un-biased recommendation that you try building from the OLF-SJ, OM or MJ plans available only at Stewart MacDonald’s web sight. Not only are they among the best plans on the market but a large part of the cost help supports this forum.

here is a link to the OM plan I spoke of http://www.stewmac.com/shop/Books,_plans/Plans/OM-style_Guitar_Plans.html