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Hello, my name is Jose Antonio and I´m from Spain. I´m building my first acoustic guitar and I have got my first question : ). My guitar is to cedar top and indian rosewood for the back and sides. My top has got 2,5mm of thickness but a lot people say me that it is not good that 3mm is better, my question is, is there any problem if I use the 2,5mm thickness?, is dangerous for the guitar??. Thnaks so much. Some pictures of the guitar : )

Steel string or nylon string? Pals, vic.

steel string!

Hello, Jose. Vic asked an appropriate question, because the members here will need to know how you plan to string and brace your guitar in order to give you helpful answers. Your top seems to have very close, tight, straight grain and that's good. Your rosette design suggests to me that you are planning a steel string instrument. I am sure many experienced builders have dealt with cedar tops of this thickness, and they will be able to advise you about strengthening your top with slightly taller, stronger braces than those in your plan or perhaps other modifications to the bracing plan. Your guitar is already showing a beautiful shape. I love continuous, fair curves and full-waisted instruments. I particularly like guitars that are round at the tail, with little or no flat in that area. Best regards to you.
Patrick

2.5 is quite thin for cedar, it will probably be OK as long as it is braced well and only light strings are used. I am doing a sinker red cedar 00 size guitar and it will be around .120" and has red spruce bracing.

Fred

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IMHO, unless you are going for a latice bracing, that is too thin for conventional X bracing
You risk both excess soundboard distortion and a flabby bass heavy sound
Cut your losses now and get a new soundboard, leave this one for future experimentation when you have some experience.

It'll be fine. The braces might telegraph a little, but that';s okay. Cedar tops almost always distort a little earlier than spruce tops. It doesn't look like you have an especially large soundboard area, smaller soundboards are a little stiffer to begin with anyway. Normally for this type of shape I would use one tone bar and one finger, but in this case I would use two fingers and two tone bars (if you're doing x braces) and you won't get the flubby bass that Jeff's concerned about.
You could also increase the bridge patch area as well.
Also, you could do 5/16th's wide (or wider) braces instead of the usual 1/4 inch and they won't telegraph as much.

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Old growth, shmold growth!

Ok.thanks so much. Jeff in that case which is rhe correct thickness for this cedar top for this project? : )

If you're going to scrap it, send it to me!
No need to waste a perfectly good top, and I like that rosette....

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Old growth, shmold growth!

For an average cedar top, without testing,I'd be leaving it at around 3.2mm initially and approching 3.0mm during final sanding.

OK, thanks so much : )

If it is a smaller body guitar, and a stiff piece of cedar, I think that 2.5mm will be OK. I built a dread with a cedar top and it ended up at .105" or 2.667mm. I used .25" braces and they are not telegraphing at all. It currently has 11 or 12 gauge strings on it.

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PMoMC

The body is like a Lowden guitar shape.more or less like a jumbo.is big

I should look at the pics.


Posted using letters to form words, sentences and thoughts.

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PMoMC

Hello Jose,
Let me add to what those more experienced than myself have said. Make sure the wings of your bridge are well placed over the X- braces. Also consider using a larger footprint bridge plate.
I had a similar experience on my first build, with a cedar top of about the same thickness. Unfortunately my braces were also cedar and fairly light. The guitar bellied significantly. I replaced my original bridge plate with a large area spruce plate, something like this one used by Petros Guitars http://www.petrosguitars.com/features.htm (click on the Bridgeplate tab) and have been pleased with the results.
Gerry

I'd say that was pretty thin for a cedar top, if it's typical cedar. The problem is that, although cedar is usually lower in density and stiffness than most spruces, all woods vary a lot, so there's a lot of overlap in properties. Fortunately, there are ways to get more exact indications.

The first thing you need to know is the stiffness of the wood. This can be measured directly, and is particularly easy to find if you start by measuring the properties of the top half before you join it or cut it to shape. One way is to support it off the bench at both ends, put a weight in the middle, and measure the deflection as it bends along it's length. Many people simply use the same weight every time, on the same rig, and thickness the top until they get a deflection that they know will work for them. This is good if you have some datsa on what has worked for you in the past.If you know the distance between the supports, the thickness and width of the plate, the deflection and the weight you used you can calculate the Young's modulus, and this will allow you to figure out the thickness you'll need. Gore/Gilet give the formula for a rectangular beam as:

E= ( 9.81 F*(L^3))/4*b*d^3*y where:

E = Young's modulus in Pascals
F= the mass load in kg.
L= the span between supports, in meters
b= the width of the beam in meters ( a top plate can be thought of as a wide, thin beam, if the width and thickness are uniform)
d= the depth or thickness of the beam, also in meters
y= the defection, in meters

I have not used this method, but I'm pretty sure there are threads on 'deflection testing' in the archive of this forum. You can. of course, use off cuts from the plate to get this information, with the caveat that they may not be representative of the top as a whole.

Another way to do this is to find the vibration frequency of the first 'bar' mode of a rectangular beam, and use that and other information to calculate the Young's modulus. The formula there is:

E= (0.996*rho*f^2*L^4)/d^2 where:

E= Young's modulus in Pascals
rho = density in kg/cubic meter
f= frequency
L= the length of the beam
d= the thickness or depth of the beam.

You can sometimes get a usable indication of pitch from tapping a plate that is held about 1/5 of the way in from the end and tapped in the center. It's a pain to convert from pitch to frequency, though. There are lot of computer programs available these days that are either free or cheap that will give you the frequency of a recorded tap. This is not without pitfalls, but is pretty reliable, and easy to do. Note that the E value you get either way is probably not going to be perfectly exact: aside from inaccuracies of measurement, there are simplifications built into the formulas.

Once you have the young's modulus for bending along the grain, you can get a ball park thickness pretty easily. I use a formula that is based on a fairly simple model that is 'close enough', given the number of uncertainties. The idea is that you want to make the top as thin as you can (to keep the weight down) while still being stiff enough. The limit for stiffness is set by long term deformation under bridge torque. Lengthwise stiffness is far more important here than cross wise stiffness, which is usually lower than lengthwise stiffness anyway; so I just use the lengthwise E value. The stiffness of a beam is proportional to the Young's modulus and the cube of the depth or thickness, all else equal. I have come up with an 'index number' that is simply: I=d^3 * E, with d in _millimeters_, and E in _megaPascals_. I find that, for steel string guitars, an I of around 250,000 works well. For classicals I use I=160,000.

In use, then, suppose I find that a particular piece of cedar has an E value along the grain of 8000 mPa, which is pretty typical for cedar. I would divide that into the index number of 250,000, to get 31.25. The cube root of that is 3.14, and that is the thickness I'd go for, in millimeters.

You can go the other way, too. Your top is now 2.5mm thick. Cube that to get 16.625. 250,000/16.625= 15,000 mPa. I have never measured a piece of cedar with an E value that high, so I think that top is indeed too thin. I'll note, however, that the numbers I 've given are ones that work for me, using the methods and designs I'm accustomed to. You might be able to use that top with a different bracing system than I use: I'd suggest some sort of lattice.

I'll also note that there's a 'quick and dirty' way to get a pretty good idea of the Young's modulus along the grain of soft woods. It turns out that all soft woods havevery much the samer structure on a microscopic scale. As a result of this, the E value along the grain tracks the density quite closely in the same way for all soft woods. Soft woods with a density of 300 kg/m^3 will tend to have E values around 6000 mPa, and samples that have a density of 500 kg/m^3 will be around 16000 mPa. About 2/3 of the samples I've measured fall within 10% of that straight line. Given all the uncertainties involved, a measurement of density is probably the easiest way to find the E value 'close enough'.

Hi Alan, thank you so much for this good information. But I perhaps to save the top for another project and to use another top taller, but thanks so much for the same way, this a great information for me, i´m a beginner and i appreciate it. The new top is more clean than old, tomorrow I want to make the new rossete and i upload the new pictures. thanks so much : )