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tl;dr - what is a good thickness for a Redwood top and Primavera B&S for a classical ?

Hiya guys - my first post here and first guitar build: a classical. I took Robbie's comment about "expensive box of firewood" to heart, and basically bought the cheapest wood money can buy! Turns out that 2nd grade Redwood tops are on sale at LMI for ~$25, so I got one. That, and I got a Primavera B&S set, also on sale. Reveling in the cost savings, it hit me that I have no idea how to thickness these woods properly for a classical.

In Robbie's Tips du Jour video about Redwood tops, he states to thickness them 10-15% thicker. Please correct me if I missed it, but I don't think he says thicker than spruce or cedar? I have his Build Your Own Classical DVD and he specifies thicknesses for both spruce and cedar, so that's a good start, at least.

I can find no info, so far, about thicknessing B&S using Primavera on a classical.

Thanks for any help you can offer !

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If thee meddle with dragon kin, thou will become crunchy and good with ketchup.

I've never used Primavera but I tend to go for about 3/32nd ~ .1in on backs and sides. 10-15% thicker for redwood is probably a good general rule of thumb. Once the box is build and before the binding goes on many luthiers like to thin out the perimeter of the top from 2-3 inches in to the edge around the lower bout.

I bought a B&S set of that LMI primavera too. Haven't used it yet, but I can tell you that for all practical purposes it is mahogany, and I'd suggest thicknessing it the same as you would mahogany. I don't have any expertise in classicals. The best thickness it's going to depend on the plans.

RE the redwood, I'm sure Robbie means thicker than spruce. Most people consider redwood in between spruce and cedar, perhaps a bit closer to cedar, and thickness accordingly. Soundboard dimensions on classicals vary widely, depending on design philosophy.

Even though you saved some money, there's no reason you can't build a first rate instrument with those materials. I had been intending to try primavera for some time, irrespective of the sale. It's a pretty cool wood, like naturally bleached 'hog.

Caveat, redwood is very soft and very brittle an 1/4 sawn splits very readily and dents easily 2.7 -3mm depending on weight an stiffness

Virtually all the redwood I've ever used has been on the hard and dense side, so I'm curious as to where folks get the idea that its soft and light. It does split easily, and usually has very low damping, and in both of those characteristics it's like WRC.

I did get some redwood a year or two ago (from LMI in fact) that was not much like the other redwood I've used. It was fairly dense, but had low long-grain stiffness, and very high damping. I have read (Archer: 'Growth Stresses and Strains in Trees', Springer-Verlag) that wood from the center of a large tree near the butt can have built-in compression stress high enough to crush the cells on a microscopic level. One sign of that is that the wood 'brash fractures': it breaks cleanly across the piece when flexed along the grain. I didn't try that on those pieces, since I was going to send them back, but it sure acted and looked like it had some such problem. They said it was 'salvaged' wood. Some of that sort works out really well; the famous 'Lucky Strike' log that Craig Carter got was salvaged, and that's really nice wood. However, in some cases I think there are good reasons why you might leave something in the stump. This is one of those cases where it's nice to be able to put some numbers on things, so that you can see how the wood you have compares with other pieces.

I think maybe a little surprise here:

Janka Hardness

http://www.wood-database.com/wood-artic ... -hardness/

Sitka Spruce 510
Red Spruce 490
Redwood 450
Englemann Spruce 390
Western Red Cedar 350

For reference Hard Maple 1450

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Ken Cierp

http://www.kennethmichaelguitars.com/

Wow! This is excellent info! Redwood has hardness (essentially) in between Spruce and Cedar. So, it seems like common sense to thickness my Redwood top in between Spruce and Cedar. I have not done the arithmetic for precise numbers, but I'll use the dimensions specified for Spruce and Cedar thicknesses as stated in Robbie's Classical DVD.

Coolness! Thank you all for your contribution to this thread! And, I'm seriously going to adjust the build so my Redwood top can defelct BB fire from those rowdy Janka zealots who frequently attend classical gigs. You know who you are!

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If thee meddle with dragon kin, thou will become crunchy and good with ketchup.

Hardness is NOT the thing that determines how thick to make the top, although it may be related. What you need to know is the Young's modulus: the resistance to stretching or compression of the wood. Most of the stiffness when you try to bend a piece comes from the stretching and compression of the wood on the surfaces, so the Young's modulus, especially along the grain, determines the stiffness at a given thickness. There are several ways to test this, depending on how accurate you need to be, and what sort of apparatus you have or can rig up. Do a search on 'deflection testing' for some pointers to a simple method.

Note also that those numbers are averages. All woods vary a lot; you can easily see samples in a range of +/- 20% from the 'average' in long-grain Young's modulus, and cross grain varies a lot more.

On my first Redwood topped classical, I thought it should be handled much like Cedar. Boy was I wrong. The guitar was not very responsive at all. I believe my original thickness under the bridge was in the 2.7 mm range, and thinned about 10 to 15% in the wings and at the tail. Ultimately I ended up removing the bridge from a finished guitar, thinning the top until it sounded like it had some pop and sustain, replaced the bridge and re-finished the guitar. Now it's a respectable instrument with great sound and power. Current thickness under the bridge as much as I can tell with the tools I have is about 2.3 - 2.4 mm, which is near Spruce dimensions.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Depends on the particular top you have.
Mc

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Mike Collins

First, let me thank everyone for your contributions to this thread!

So, doing a little surfing, I found info on "defelction testing", and more specifcally, Chaldni testing. See attached photo. I also found Dogwood guitars, who go through an elaborate testing and adjustment process based upon stiffness to weight ratios and computer analysis. All this is great.

On a more pragmatic note, I still don't see see how to get from vibrating dust and killer apps to actual dimensions of top and brace thickness and the geometery/topology that would accompany those numbers. And in the photo, I sure see many test frequencies and the zones of vibration they create - but it says nothing about the sweet spots of vibration that bracing and thickness should be adjusted to create. For example, three or four large oval vibration zones in the lower bout. What connects that data to awesome tone? Do I even want 4 oval zones or is this something to be avoided ???

For example, is the formula that Dogwood uses a known mathematical algorithm? Or is it their own proprietary magic ? Those guys state boldly that they know how to match the data to the wood with killer tone as a result.

So I started out asking how thick to make my redwood top and primavera B&Ss. Now it seems like I'm in a deep rabbit hole... Maybe I should try to solve world hunger; it might be easier. OR maybe I can extrapoltae some generic numbers and aim for "definitely not terrible".

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If thee meddle with dragon kin, thou will become crunchy and good with ketchup.

Andrew, if you interested in that type of thing, as many of us are. Trevor Gore has published a 2 book set that covers that extensively (among other things) in volume 1 and volume 2 is a build volume. He derives all the formulas for you if you care to see where it's all coming from. They are very good and while they may seem a little expensive, depending on how long it's been since you've purchased a text book, they are a bargain considering the wealth of information contained with in them.
just search for Trevor Gore Guitars and his site should come up.

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Jim Watts
http://jameswattsguitars.com

Or keep an eye on the classifieds. They show up there for bargain prices from time to time.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Deflection testing and Chladni testing are two different things, with different aims. Deflection testing will tell you how thick to make the top to get the required stiffness. Chladni testing tells you something about how the top and bracing are working together, and helps (IMO) in getting a good tone.

The two are related, of course. The bracing is there to add stiffness to the top without adding as much weight as leaving the top thick enough to take the load by itself would. You can use a thin top with heavy bracing to take the load, and it will work, but maybe not as well as making it somewhat thicker with marginally lighter braces. There are lots of different opinions about the 'best' way to do this. It often comes down to what sort of sound you're looking for, and what sort of longevity you expect to get too.

There is a simple, but less reliable, way to get a rough reading on the stiffness of the wood along the grain. It turns out that for softwoods the Young's modulus tracks the density pretty well, with about 60% of the tops I've tested falling within 10% +/- of the predicted Young's modulus based on density. THIS IS NOT GUARANTEED! Those redwood tops I mentioned earlier were 'outliers', with much lower Young's modulus values than the density would have predicted, so there's no real substitute for an actual measurement. If you want more reliable values, then you have to learn how to do the measurements, either by deflection testing or a vibration test. I use the vibration test in part because I already have the equipment to do it, but deflection testing is simpler in some ways.

For a deflection test you'll need some way to support the top at either end, a weight to load it, and some way to measure the deflection, such as a dial gauge. Many makers simply start out comparing tops, and don't worry too much about getting absolute numbers such as the Young's modulus. You can start out by taking the top to some thickness that you think might work, and measuring he deflection under a load. When the guitar is done you decide whether the to was too thick or too thin, or just right, and make adjustments on the next one accordingly. That's fine if you plan on making a lot of guitars, and can afford some duds along the way that have tops that are too thick or too thin. There are ways to get the Young's modulus from a deflection test, if you know the thickness of the top , the span between supports and the mass of the weight you're using to load it. David Hurd gives equations for this in his 'Left Brain Lutherie', along with suggestions as to what to do with the information once you've gotten it.

Gore talks about using a simple 'tap test', along with measurements of the dimensions and weight of the top to find the Young's modulus of the wood. You record the tap on your computer, and use software to find the resonant frequency of the fundamental bending mode, which, in turn, gives you values that can be plugged into an equation to find the Young's modulus.

Be aware that there are pitfalls to any method of measuring, and limitations inherent in the results. Still, these tests are more objective than the usual flex and feel, and far more reliable than simply following guidelines based on the species or whatever. I use both the vibration measurement to determine the thickness to make the top so that it will be stiff enmough, and, later, use Chladni testing to help trim the braces to get the sound I want.

So, I'm thinking that I need some theory, but also a lot of practical cookie cutter info for my first instrument. I have considered the expensive texts, but what about Siminoff's books?

I am looking at The Art of Tap Tuning and the Luthier's Handbook. Both have received mixed reviews. I either own, or have access to, lots of recording and analysis gear, so the lab bench complexity is not a problem. But some reviewers have stated that his methods are controversial, at best, more akin to voodoo than repeatable science, at worst.

So, I guess I need an opinion if these books will get me what I need: dimensioning top, B&S, and tuning braces. I can certainly afford those two, compared the Gore's text set...

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If thee meddle with dragon kin, thou will become crunchy and good with ketchup.

Unless you are an engineer, and even if you are, I'd suggest for your first that you simply attempt to build a guitar that sounds decent, looks pretty good, and doesn't collapse. IMO just start with some off the shelf numbers or you will drive yourself nuts.

If you don't have it, get the Cumpiano & Natelson book, as it goes into more detail than any other that I know of. C&N recommend .10 to .11" for spruce classical tops, and .11-.12" for cedar. You might try .11" for redwood. Sides are .080". Backs (for beginners) should be: "an even 3/32 inch".

If you are game to be a little more scientific, starting with some simple deflection testing would be prudent. Wood varies immensely even within one species. You could compare an off cut strip from you redwood soundboard to spruce to make sure it's not extra mushy redwood. Check out David Hurd's site for a how to (he has a book too): http://ukuleles.com/

Have you checked out Hurd's book? It gives a full treatment of simple methods of stiffness testing, and it's not all that expensive. For that matter, there are discussions on line about deflection testing that go into it in some detail.

I disagree in a number of respects with Siminoff, and would not recommend his books. There is a lot better information out there.