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I'm building a guitar that is 15-3/4 lower bout....9-1/2 waist...11-1/2 upper bout....The top I'm going to use is Western Red Cedar...my first time using this wood. I have read to keep the thickness alittle thicker than spruce....
I did a deflection test on it with 18" gap between supports....5 lb 3oz weight....I was shooting for a .250 deflection. I thicknessed the top down to .108 an got a deflection of .220 an stopped at that point. The density is 6.76g/in3...Does this seem to thin a top for this wood and size of guitar? Jus like to hear ya"lls opinion based on the info I have provided...

Thanks,
Jeff

I just finished the body on a WRC OM with a 15ish" lower bout and my top thickness is .127 in the middle and slightly thinner around the edges. Of course all pieces of wood have to be evaluated on a case-by-case basis. But my top is thicker than yours by nearly .020 and my span for an OM is less....

Your top may be on the thin side since your lower bout is a greater span. OTOH again each piece of wood is an individual.

Remember too that we all have to do some final sanding and in the case of WRC it's notorious for denting if you look at it the wrong way.... If it were me I would think that the top is too thin but YMMV.

Jeff-
Is this a steel-string or classical guitar project?

I'm trying to collect some info on deflection testing for myself- where did you get your target deflection, weight, etc info?

Thanks
John

I agree with Hesh. Sounds a bit too thin to me. I just tuned up a nice Italian spruce top for an OM last night - came out at 104 mil thick. I typically find my spruce tops end up between 95 and 120 mil thick. I prepared a sinker redwood top last week too - it's very similar to cedar in stiffness and that it is easily damaged. I stopped at 130mil and thought that was too thin in the end! I don't measure stiffness, but go by the tap tone of the jointed and rough-shaped soundboard.

Cheers,
Dave F.

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Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

To my way of thinking, you certainly have a stiff piece of cedar Jeff.

I would actually go thinner with this piece of wood. I look for a deflection of 3 - 3.5 MM under a 1 LB weight across 22 inches. I tend to build light tops.

If you decide to go thinner measure the deflection after each pass of the sander as cedar IMHO quickly becomes increasingly more flexible as it thins, if that makes sense.

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

I have built with cedar tops thinner than that, and I tend to brace lighter than most folks do. There is certainly no structural concern at that thickness, it is a question of whether it meets your tonal goals.

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David White, Toronto

"All my favourite singers can't sing."

Michael and David, do you use lattice bracing or unusually tall bracing and large bridgeplates? I am curious. A WRC top that thin would never make it in one of my builds. I finished an A-2 (OM size) a few weeks ago with a very stiff WRC top and it felt "right" at .124". And I consider my builds to be extremely light…

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Two points:
All wood speceis vary quite a bit in things like density.

The Young's modulus along the grain, and therefore the stiffness of the plate at a given thickness, scales linearly for softwoods.

I hate those mixed units (gm/cu. in.?), but if I did my math right, that works out to a specific gravity of .407, or a density of 407 kg/cu. meter. That's as high as any WRC I've tested, and up in the normal range for Euro, or the low end of the Red spruce range. That's reasonable thinkness for a top of that density, I think, although I might have gone just a tad thicker.

WRC, dense or not, still has lower splitting resistance than spruce. I like to make my bridges a bit wider on WRC tops, to help resist peeling up. You also have to be _really careful_ when scribing around the bridge to remove the finish that you don't cut wood fibers, especially along the back edge of the bridge. We had a Lowden in the shop that lost it's bridge for that reason: there was 100% wood shear, but the break was clean all the way around where somebody had scribed the top a little too deeply.

I agree, except I've never seen WRC comparable in stiffness and especially density to any spruce, save perhaps some Englemann.
The other thing I would add is that WRC, like redwood, sands real quick. Care must be taken not to remove too much material during finish prep.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Thank you for your responses!....I'm going to go forward with this top and see what happens. I do use adi bracing on all my builds and do deflection on them as well to get the stiffest pieces for the X.
This is the second guitar which I have used the deflection test....I'm pleased with the first one that I built useing the method so hopefully this one will turn out nice too.
And to the other question...this is a steel string..
Thanks again for your help!!

Jeff

Laurent, I beef up the bracing for cedar tops and add a slightly larger bridge plate.

I have not done lattice bracing on cedar but I’m about to with a current cedar top that deflects 6.1 mm under a 1 lb weight. This is an experiment and not something I'll likely do again. I want to see how the bracing will hold up with a very light top. I’m also looking for some data regarding stress around the perimeter of the top with respect to bracing and the top's thinness.

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

Would it be worth the extra effort to tape the bridge area off before finishing or would you still remove the finish?

You still need to cut through the finish on the tape edges with an x-acto knife in order not to lift the finish around the bridge. I always use a fresh x-acto blade when cutting the finish for the bridge. There's no control nor tracking with a dull, if ever so slightly, blade.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Edward-
Not the answer to your question, but a tip I picked up at Sergei deJonge's...
Use a sewing needle (you can glue it into a small wood handle) ,not a knife, to scribe around the bridge.
(Leave a fair bit of the needle (1"?) exposed.)
You can 'feel' pretty quickly when the needle hits the wood (and then stop scribing).
This was scribing a FP (thin) finish- with lacquer you would need more 'passes' with the needle, I'd think.

I've never had much luck with masking- I found that I still had to scribe around the tape, and I didn't like the result.

Cheers
John

Thanks for the tip John, sound like it would work well.
It might not of been the best idea to start my first with a cedar top, but I reaallly like cedar.

Ed

What Laurent said.

I tape off a bit under size, pin and scribe around the bridge, then peel the tape and finish scraping the surface. If you tape just to the edge of the bridge there will be finish buildup, which can look bad. Alternatively if you're even a little bit off there will be a bare area of the top next to the bridge, and that looks even worse.