Official Luthiers Forum!

Hi everyone.

The title tells everything...It's my first acoustic build. It's a torres inspired guitar, redwood on EIR. My top is glued and cleaned, now i'm around 0.135''. I still have to inlay the rosette. I've been told around 110 to 115 for final thickness, does it sounds good to you? What are you experiences with redwood?

Thanks
Francis

_________________
Francis Richer, Montréal
Les Guitares F&M Guitars

I'm a newb relatively, so arm yourself with salt grains.

I've never used redwood, but I wouldn't think that redwood would be much different from WRC? and I don't go that much thicker with WRC than with spruce. I'd say maybe 2.6mm to start, so down around .10". I probably adjust the bracing more than the top thickness in switching from one top to another. I'd start with spruce around 2.4mm, or .095". I'm assuming you mean classical/flamenco, since you said Torres. If it is flamenco, I'd take it down another 0.2mm.

_________________
Jim Kirby
kirby@udel.edu

I think it all depends on how well 1/4d it is. The cross grain stiffness falls off pretty quick when the cut is off 1/4. I have a 000 box together with a sinker top that I left close to .130 using the deflection # normally used on spruce. Same deflection # gave me .105 on an Adi top. When the box was together it sounded real stiff and I thinned the profile quite a bit to loosen it up. Not sure if that helps.

Tim

I have built two 660mm scale redwood top classicals with a redwood bridge patch of 1mm, a thickness of 2.4mm, and a Hauser type fan brace style. I think graduating from 5 -3mm high as you go out from the center. All 4mm wide. They both have been strung for two years w/ high tension strings and are holding up fine. The tops are not perfectly quartered but I think this may -add- to their successful sound. if you take your thumb and index finger and push on the wings in a side to side rocking motion there is a lot of motion from the lack of cross grain stiffness. Both instruments started out w/braces all the same 5mm tall. after they were strung they both improved fairly dramatically when the outer fans got shaved down noticeably increasing this side to side flex between the ends of the bridge and the side.

I just learned to post pics. Here is #2 redwood

Looks pretty nice so far. I'm trying to figure out from the photo what part of this build is Torres inspired? I can see lots of departures.

_________________
Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

That's not the guitar of the OP It is a Hauserish thing. The open UTB appears in some of his guitars, like the one Romanillos used as inspiration for his open UTB.

_________________
Build log

I've used a lot of redwood over the years. Some of it is much like WRC, light and soft, and other pieces are harder and denser. You can't really go by the species; you have to go by the piece.

IMO, since it's the bridge torque that sets the limit as to how thin you can make the top, and that's mostly bending the top along the length, it's the lengthwise stiffness that counts most in determining the thickness. The stiffness, in turn, will be proportional to the product of Young's modulus along the grain and the cube of the thickness. If the Young's modulus is low, you make the top a bit thicker and get the stiffness that way.

It turns out that for almost all the softwoods I've tested (the exception so far has been Eastern Hemlock) the Young's modulus along the grain varies lineraly with the density: two pieces of wood with the same density will have the same lengthwise Young's modulus, within about 10% plus or minus, something like 2/3 of the time. Wood that has a lot of 'compression grain', with wide hard latewood lines, tends be less stiff along the grain than the density would indicate. This is not uncommon with redwood, since it grows to such large size.

Redwood makes a really nice classical top, since it tends to have very low damping (actually about as low as Brazilian rosewood, in many cases). This helps give the guitar both 'warmth' and 'clarity' if it's used right. It is, of course, all too easy to make a lousy guitar from nice wood, alas.

In light of what Al said, I left out one of the most important qualities of the wood in this guitar.
I measured the density of this top at 40-45% rh at close to 390g/m3.
The fan bracing is Hauserish, the rest has many departures that make me laugh now. for example I ordered a set of Hauser plans but was sent a Richard Shneider Kasha set by mistake. I just used the Shneider body outline. How is that for departure!
I'm always on the lookout for examples of top thickness/ brace designs that failed as these are the most instructive about where the line is. Although I imagine some may not want to talk about them.
Anyone care to share a classical structural failure story?
Is this a thread highjack Francis?

David, that is rather heavy, a lot more like light/medium spruce than WRC. I would have likely thinned it some more if it had enough stiffness, but then again my braces are stiffer. The depth of the doming matters too.

So Francis, you can see we are not very eager to jump and say "make it x mm" since the stuff varies greatly. At least try to find the density.

_________________
Build log

OOps! I just realized that David's guitar is NOT the one under consideration by the OP! And I just figured out what Alex was saying about it, too. Sorry for any confusion this may have brought.

David, your guitar is unlikely to fail as a result of your departures, but the Schneider/Kasha body is very large.

_________________
Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

I actually think it fits well over a Hauser in terms of widths, but it is indeed unusually long (over 50cm) - at least the two plans sold by LMI. The extra length should go very well with a thicker top, so i wouldn't complain about the mixup too much

_________________
Build log

More important info I left out;
the braces were sanded to a 25' dome and glued down in the radius form.
it was assembled on a work board scooped out but w/a flat rim for the top/side intersection.
this must have produced a slight re-curve tension in the top but was very slight and undetectable visually.
10 years+ ago when I built this instrument I was just learning classical playing. I was struggling to find a comfortable position and am fairly tall with big hands. That's why I opted for the long scale and decided the extra long body size might be a plus.